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Assessing salt tolerance in plants under field conditions is a challenging task. The objective of this research was to assess the effectiveness of different methods (leaf disc assay and pot experiment) for evaluating salt tolerance in rice. Using two varieties with different salt tolerance, Changmaogu (CM) and 9311, under three NaCl levels (0, 0.3%, and 1.0%), we evaluated the photosynthetic performance in terms of chlorophyll content in leaf disc assays, as well as the photosynthetic rate (Pn), chlorophyll content, linear electron flow (LEF), and non-photochemical quenching (NPQ), in a semi-controlled pot experiment. In the leaf disc assay, CM showed a smaller decrease in chlorophyll content compared to 9311, especially under 1.0% salinity. Simultaneously, in the pot experiment, the CM variety employed flexible photosynthetic strategies, actively decreasing LEF and Pn after 5 days of salt stress (day 5) and then increasing photosynthetic capacity (chlorophyll content, LEF, and Pn) on day 10. Notably, the total chlorophyll content for the CM variety under 1.0% salinity was significantly higher than in the control, showing a 25.0% increase. Additionally, CM demonstrated NPQt sensitivity under 0.3% salinity, requiring an LEF of 150 to achieve an NPQt value of 3.0, compared to an LEF of 180 in the control. These results suggest that a simple leaf disc assay may not fully capture the adaptive mechanisms of rice plants under salinity stress. Therefore, we advocate for the use of more comprehensive methods, such as outdoor pot or field experiments, to gain a deeper understanding and more accurate evaluation of salt tolerance in rice. [ABSTRACT FROM AUTHOR]

Salinity is an abiotic stress restricting agricultural crop production globally, in which salts inhibit plants' ability to absorb water and nutrients. Pinto beans (Phaseolus vulgaris L.) are very important in human nutrition and are sensitive to salinity. The objective of this study was to assess the salinity tolerance of six pinto bean varieties by evaluating the effect of different salt types on germination and growth. In the germination experiment, varieties were arranged in a randomized complete block design with five replications and three saline solutions (NaCl, CaCl2, MgSO4·7H2O) at 0, 0.05 M, 0.1 M, and 0.15 M concentrations each. For the greenhouse experiment, saline solutions with the same EC (5 dS m−1), control (distilled water), and six pinto bean varieties were organized in a Complete Random Design with 10 replicates. The results demonstrated that germination percentage, speed of germination, and hypocotyl length decreased as salt concentrations increased. Othello's vegetative and reproductive parameters were significantly higher compared to the other varieties under saline conditions; its early maturity may have enabled it to perform better under salt stress. In addition to soil and water management, selecting salt-tolerant crops and varieties is essential to maintaining agricultural sustainability in regions undergoing salinization. [ABSTRACT FROM AUTHOR]

The development of methods to detect plant stress is not only a scientific challenge, but is also of great importance for agriculture and forestry. However, at present, stress diagnostics based on plant spectral characteristics has several limitations: (1) the high dependence of stress assessment on plant species identity; (2) the poor differentiation of different types of stress; and (3) the difficulty of detecting stress before visible symptoms appear. In this regard, the development of plant spectral metrics represents a significant area of research. Ficus elastica plants were exposed under the photosynthetic photon flux density (PPFD) from 0 to 1200 μmol photons m−2s−1. Exposure of F. elastica leaves to excess light (EL) (≥400 μmol photons m−2s−1) resulted in an increase in reflectance in the yellow-green region (522–594 nm) and a decrease in reflectance in the red region (666–682 nm) of the spectrum, accompanied by a shift of the red edge point toward the longer wavelength. These changes were revealed using the previously proposed light stress index (LSI = mean(R666:682)/mean(R522:594)). Based on the results obtained, two new vegetation indices (VIs) were proposed: LSIRed = R674/R654 and LSINorm = (R674 − R654)/(R674 + R654), indicating light stress by changes in the red region of the spectrum. The results of the study showed that LSI, LSIRed, and LSINorm have a high degree of coupling strength with maximal quantum yields of photosystem II values. The plant response to EL exposure, as assessed by the values of these three VIs, was well expressed regardless of the PPFD levels. The effect of EL at non-stressful PPFDs (50–200 μmol photons m−2s−1) was found to disappear within one hour after cessation of exposure. In contrast, the effect of the stressful PPFD (800 μmol photons m−2s−1) was found to persist for at least 80 h after cessation of exposure. The results of the study indicate the need to consider light history in spectral monitoring of vegetation. [ABSTRACT FROM AUTHOR]

Plant seedling morphogenesis is considerably related to photosynthesis, pigment synthesis, and circadian periodicity during seedling development. We identified and cloned a maize zebra or crossbanding leaves mutant wk3735, which produces pale white kernels and was identified and plays a role in the equilibrium of the Redox state the in/out of ETC by active oxygen scavenging. Interestingly, it produces the zebra leaves during the production of the first seven leaves, which is apparently different from the mutation of homologs AtPTOX in Arabidopsis. It is intriguing to investigate how and why yellow crossbands (zebra leaf phenotype) emerge on leaves. As expected, chlorophyll concentration and photosynthetic efficiency both significantly declined in the yellow sector of wk3735 leaves. Meanwhile, we observed the circadian expression pattern of ZmPTOX1, which was further validated by protein interaction assays of the circadian clock protein TIM1 and ZmPTOX1. The transcriptome data of yellow (muW) and green (muG) sectors of knock-out lines and normal leaves of overexpression lines (OE) at the 5th-leaf seedling stage were analyzed. Zebra leaf etiolated sections exhibit a marked defect in the expression of genes involved in the circadian rhythm and rhythmic stress (light and cold stress) responses than green sections. According to the analysis of co-DEGs of muW vs. OE and muG vs. OE, terms linked to cell repair function were upregulated while those linked to environmental adaptability and stress response were downregulated due to the mutation of ZmPTOX1. Further gene expression level analyses of reactive oxygen species (ROS) scavenging enzymes and detection of ROS deposition indicated that ZmPTOX1 played an essential role in plant stress resistance and ROS homeostasis. The pleiotropic roles of ZmPTOX1 in plant ROS homeostasis maintenance, stress response, and circadian rhythm character may collectively explain the phenotype of zebra leaves during wk3735 seedling development. [ABSTRACT FROM AUTHOR]

The effects of antimycin A on the redox state of plastoquinone and on electron donation to photosystem I (PS I) were studied in sulfur-deprived Chlamydomonas reinhardtii cells of the strains cc406 and 137c. We found that this reagent suppresses cyclic electron flow around PS I in the cc406 strain, whereas this inhibitory effect was completely absent in the 137c strain. In the latter strain, antimycin A induced rapid reduction of plastoquinone in the dark and considerably enhanced the rate of electron donation to P in the dark. Importantly, neither myxothiazol, an inhibitor of mitochondrial respiration, FCCP, a protonophore, nor propyl gallate, an inhibitor of the plastid terminal oxidase, induced such a strong effect like antimycin A. The results indicate that in the chloroplast of the 137c strain, antimycin A has a site of action outside of the machinery of cyclic electron flow. [ABSTRACT FROM AUTHOR]

The green alga Haematococcus pluvialis produces large amounts of the pink carotenoid astaxanthin under high photon flux density (PFD) and other oxidative stress conditions. However, the regulation and physiological role of carotenogenesis leading to astaxanthin formation is not well understood. Comparative transcriptional expression of five carotenoid genes along with growth and pigment composition as a function of PFD was studied using a wild-type and an astaxanthin-overproduction mutant of H. pluvialis NIES144. The results indicate that astaxanthin biosynthesis was mainly under transcriptional control of the gene encoding carotenoid hydroxylase, and to a lesser extent, the genes encoding isopentenyl isomerase and phytoene desaturase, and to the least extent, the genes encoding phytoene synthase and carotenoid oxygenase. The expression of a plastid terminal oxidase (PTOX) gene ptox2 underwent transient up-regulation under elevated PFDs, suggesting that PTOX may be functionally coupled with phytoene desaturase through the plastoquinone pool and may play a role in reducing redox-potential-dependent and oxygen-concentration-dependent formation of reactive oxygen species in the chloroplast. Over-expression of both the carotenogenic and PTOX genes confers to the astaxanthin-overproduction mutant more effective photoprotective capability than that of the wild type under photooxidative stress. [ABSTRACT FROM AUTHOR]

The plastid terminal oxidase (PTOX) is a plastoquinol oxidase involved in carotenoid biosynthesis in higher plants, and may also represent the elusive oxidase in chlororespiration. Haematococcus pluvialis is a green alga that has the ability to synthesize and accumulate large amounts of the red carotenoid astaxanthin (ca. 2% of dry weight) under various stress conditions. However, the occurrence and function of PTOX in astaxanthin synthesis and the stress response in this organism is unknown. In this study, two ptox cDNAs were cloned and sequenced from H. pluvialis and were designated as ptox1 and ptox2. Genome sequence analysis and database searching revealed that duplication of PTOX gene occurred in certain eukaryotic algae, but not in cyanobacteria and higher plants. The physiological and biochemical evidence indicated that PTOX is involved in astaxanthin synthesis and plays a critical protective role against stress. Analysis of the transcriptional expression of the PTOXs and phytoene desaturase gene further suggests that it may be PTOX1 rather than PTOX2 that is co-regulated with astaxanthin synthesis. The fact that the changes in transcripts of ptoxs in response to high light and other stressors and the differential expression of ptox1 and ptox2, suggests that PTOX, coupled with astaxanthin synthesis pathway, exerts broad, yet undefined functions in addition to those identified in higher plants. [ABSTRACT FROM AUTHOR]

Abstract: Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The ΔrbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between QA and QB, whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of ΔrbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F 0 ‘dark rise’ in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O2 in ΔrbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the ΔrbcL mutant under growth conditions. This protective capacity was rapidly exceeded in ΔrbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes. [Copyright &y& Elsevier]

The aim of this study is to investigate whether exogenous application of salicylic acid (SA) could modulate the photosynthetic capacity of soybean seedlings in water stress tolerance, and to clarify the potential functions of terminal oxidase (plastid terminal oxidase (PTOX) and alternative oxidase (AOX)) in SA′ s regulation on photosynthesis. The effects of SA and water stress on gas exchange, pigment contents, chlorophyll fluorescence, enzymes (guaiacol peroxidase (POD; EC 1.11.1.7), superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11) and NADP-malate dehydrogenase (NADP-MDH; EC1.1.1.82)) activity and transcript levels of PTOX , AOX1 , AOX2a , AOX2b were examined in a hydroponic cultivation system. Results indicate that water stress significantly decreased the photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E), pigment contents (Chla + b, Chla/b, Car), maximum quantum yield of PSⅡphotochemistry (Fv/Fm), efficiency of excitation capture of open PSⅡcenter (Fv’/Fm’), quantum efficiency of PSⅡphotochemistry (ΦPSⅡ), photochemical quenching (qP), and increased malondialdehyde (MDA) content and the activity of all the enzymes. SA pretreatment led to significant decreases in Ci and MDA content, and increases in Pn, Gs, E, pigment contents, Fv/Fm, Fv’/Fm’, ΦPSⅡ, qP, and the activity of all the enzymes. SA treatment and water stress alone significantly up-regulated the expression of PTOX , AOX1 and AOX2b . SA pretreatment further increased the transcript levels of PTOX and AOX2b of soybean seedling under water stress. These results indicate that SA application alleviates the water stress-induced decrease in photosynthesis may mainly through maintaining a lower reactive oxygen species (ROS) level, a greater PSⅡefficiency, and an enhanced alternative respiration and chlororespiration. PTOX and AOX may play important roles in SA-mediated resistance to water stress. [ABSTRACT FROM AUTHOR]

Flower colour and colour patterns are crucial traits for ornamental species; thus, a comprehensive understanding of their genetic basis is extremely significant for plant breeders. The tulip tree (Liriodendron tulipifera Linn.) is well known for its flowers, odd leave shape and tree form. However, the genetic basis of its colour inheritance remains unknown. In this study, a putative plastid terminal oxidase gene (LtuPTOX) was identified from L. tulipifera based on multiple databases of differentially expressed genes at various developmental stages. Then, the full-length cDNA of LtuPTOX was derived from tepals and leaves using RACE (rapid amplification of cDNA ends) approaches. Furthermore, gene structure and phylogenetic analyses of PTOX as well as AOXs (alternative oxidases), another highly similar homologue in the AOX family, were used to distinguish between the two subfamilies of genes. In addition, transient transformation and qPCR methods were used to determine the subcellular localization and tissue expression pattern of the LtuPTOX gene. Moreover, the expression of LtuPTOX as well as pigment contents was investigated to illustrate the function of this gene during the formation of orange bands on petals. The results showed that the LtuPTOX gene encodes a 358-aa protein that contains a complete AOX domain (PF01786). Accordingly, the LiriodendronPTOX and AOX genes were identified as only paralogs since they were rather similar in sequence. LtuPTOX showed chloroplast localization and was expressed in coloured organs such as petals and leaves. Additionally, an increasing pattern of LtuPTOX transcripts leads to carotenoid accumulation on the orange-band during flower bud development. Taken together, our results suggest that LtuPTOX is involved in petal carotenoid metabolism and orange band formation in L. tulipifera. The identification of this potentially involved gene will lay a foundation for further uncovering the genetic basis of flower colour in L. tulipifera. [ABSTRACT FROM AUTHOR]

Variable fluorescence measurements provide automatic and rapid estimates of electron-based photosynthesis. However, electron transport rates (ETR) to phytoplankton production vary greatly due to biological physiology and environmental stress along the Changjiang River to the adjacent seawater, where the turbid riverside, nutrient-rich river plume, and seawater are divided by the sediment front and plume front. We assessed the light responses of ETR, 18O-labeled gross primary production (GPP), and O2-based net community production (NCP) during the bloom season at the Changjiang River Estuary. The distribution of phytoplankton and potential photosynthesis efficiency were positively correlated, exhibiting offshore decreasing trends and peaking around the upwelling areas. The conversion factors of ΦGPP (mol e- per mol O2 production) ranged from 0.8 to 31.8, and ΦNCP ranged from 0.9 to 36.9, respectively. Φe: O increased from the nutrient-rich Changjiang diluted water (CDW) to the nutrient-poor seawater. Although ΦGPP and ΦNCP were linearly related to non-photochemical quenching (R² > 0.5) in the CDW, this relationship was decoupled in offshore seawater due to P-limitation, light stress, and shifts in the phytoplankton community. A better relationship between ΦGPP and ΦNCP was quantified usingmultiple linear analyses with the light attenuation coefficient (Kd), euphotic depth (Zeu), salinity, nutrients, and temperature (p< 0.001). This improvement in accuracy represents a major step forward for widespread and accurate fluorometry-based GPP and NCP applications at high temporal and spatial resolutions in the future. [ABSTRACT FROM AUTHOR]

Using Chlamydomonas as a model organism, we attempted to eliminate mitochondrial DNA (mtDNA) similar to rho0 or rho− cells (completely or partially mtDNA-eliminated cells) in yeast. We successfully generated partially mtDNA-eliminated cells named as crm- cells, causing the inactivation of mitochondrial activity. We used three different chemicals to eliminate mtDNA including acriflavine (AF), ethidium bromide (EB) and dideoxycytidine (ddC) which prevents replication, inhibits POLG (DNA polymerase gamma) and terminates the mtDNA chain, respectively. The qPCR method was used to detect the mtDNA copy number and the selected rrnL6 gene for the detection of mitochondria, as well as the selected Chlamydomonas CC-124 strain. A reduction in the mitochondrial copy number led to a higher expression of AOX1, UCP1, PGRL1 and ICL1, which indicates the disturbance of the mitochondria–chloroplast ATP and NADPH balance. We selected AOX genes to further study this family and carried out a genome-wide search to identify AOX genes in green algae (C. reinhardtii). Our results revealed that C. reinhardtii contains four AOX genes, i.e., CrAOX1, CrAOX2, CrAOX3 and CrAOX4, which are distributed on Chr 3, Chr7 and Chr9. All CrAOX genes were predicted to localize in mitochondria using bioinformatics tools. Phylogenetic analysis suggests that these CrAOXs are subdivided into four groups and genes existing in the same group could perform identical functions. Collinearity analysis describes the strong evolutionary relationships of AOXs between the unicellular green algae Chlamydomonas reinhardtii and the multicellular green algae Volvox carteri. GO (gene ontology) annotation analysis predicted that CrAOXs played an integral part in carrying out alternate oxidative and respirative activities. Three putative miRNAs, cre-miR1162-3p, cre-miR1171 and cre-miR914, targeting the CrAOX2 gene were identified. Our studies have laid a foundation for the further use of partially mtDNA-eliminated cells and elucidating the functional characteristics of the AOX gene family. [ABSTRACT FROM AUTHOR]

The global ecosystem relies on the metabolism of photosynthetic organisms, featuring the ability to harness light as an energy source. The most successful type of photosynthesis utilizes a virtually inexhaustible electron pool from water, but the driver of this oxidation, sunlight, varies on time and intensity scales of several orders of magnitude. Such rapid and steep changes in energy availability are potentially devastating for biological systems. To enable a safe and efficient light-harnessing process, photosynthetic organisms tune their light capturing, the redox connections between core complexes and auxiliary electron mediators, ion passages across the membrane, and functional coupling of energy transducing organelles. Here, microalgal species are the most diverse group, featuring both unique environmental adjustment strategies and ubiquitous protective mechanisms. In this review, we explore a selection of regulatory processes of the microalgal photosynthetic apparatus supporting smooth electron flow in variable environments. [ABSTRACT FROM AUTHOR]

We honor here Professor David Mauzerall, a pioneer in the fields of photochemistry and photobiology of porphyrins and chlorophylls in vitro and in vivo, on the occasion of his 95th birthday. Throughout his career at The Rockefeller University, he refined our understanding of how chlorophyll converts light energy into chemical energy. He exploited top-of-the-line laser technology in developing photoacoustics and a variety of other innovative experimental approaches. His experimental work and conceptual insights contributed greatly to our understanding of photosynthesis and the possible role of photosynthesis in the origin of life. His contributions include many landmark single-authored and collaborative papers, and his legacy includes the training of others who have become authorities themselves. After providing a brief description of his research accomplishments, we include tributes from several of his coworkers and his daughters highlighting their valuable experiences with David Mauzerall on this milestone birthday. [ABSTRACT FROM AUTHOR]

Introduction: The utilization of plant material for synthesizing nanoparticles effectively triggers physiological and biochemical responses in plants to combat abiotic stresses. Salt stress, particularly caused by NaCl, significantly affects plant morphology and physiology, leading to reduced crop yields. Understanding the mechanisms of salt tolerance is crucial for maintaining crop productivity. Methods: In this study, we examined the effects of 150 μM spinach-assisted gold nanoparticles (S-AuNPs) on various parameters related to seed germination, growth attributes, photosynthetic pigments, stomatal traits, ion concentrations, stress markers, antioxidants, metabolites, and nutritional contents of spinach plants irrigated with 50 mM NaCl. Results: Results showed that S-AuNPs enhanced chlorophyll levels, leading to improved light absorption, increased photosynthates production, higher sugar content, and stimulated plant growth under NaCl stress. Stomatal traits were improved, and partially closed stomata were reopened with S-AuNPs treatment, possibly due to K+/Na+ modulation, resulting in enhanced relative water content and stomatal conductance. ABA content decreased under S-AuNPs application, possibly due to K+ ion accumulation. S-AuNPs supplementation increased proline and flavonoid contents while reducing ROS accumulation and lipid peroxidation via activation of both non-enzymatic and enzymatic antioxidants. S-AuNPs also regulated the ionic ratio of K+/Na+, leading to decreased Na+ accumulation and increased levels of essential ions in spinach plants under NaCl irrigation. Discussion: Overall, these findings suggest that S-AuNPs significantly contribute to salt stress endurance in spinach plants by modulating various physiological attributes. [ABSTRACT FROM AUTHOR]

Drought stress is one of the most critical threats to crop productivity and global food security. This review addresses the multiple effects of drought on the process of photosynthesis in major food crops. Affecting both light-dependent and light-independent reactions, drought leads to severe damage to photosystems and blocks the electron transport chain. Plants face a CO2 shortage provoked by stomatal closure, which triggers photorespiration; not only does it reduce carbon fixation efficiency, but it also causes lower overall photosynthetic output. Drought-induced oxidative stress generates reactive oxygen species (ROS) that damage cellular structures, including chloroplasts, further impairing photosynthetic productivity. Plants have evolved a variety of adaptive strategies to alleviate these effects. Non-photochemical quenching (NPQ) mechanisms help dissipate excess light energy as heat, protecting the photosynthetic apparatus under drought conditions. Alternative electron pathways, such as cyclical electron transmission and chloroplast respiration, maintain energy balance and prevent over-reduction of the electron transport chain. Hormones, especially abscisic acid (ABA), ethylene, and cytokinin, modulate stomatal conductance, chlorophyll content, and osmotic adjustment, further increasing the tolerance to drought. Structural adjustments, such as leaf reordering and altered root architecture, also strengthen tolerance. Understanding these complex interactions and adaptive strategies is essential for developing drought-resistant crop varieties and ensuring agricultural sustainability. [ABSTRACT FROM AUTHOR]

Carbon capture and biochemical storage are some of the primary drivers of photosynthetic yield and productivity. To elucidate the mechanisms governing carbon allocation, we designed a photosynthetic light response test system for genetic and metabolic carbon assimilation tracking, using microalgae as simplified plant models. The systems biology mapping of high light-responsive photophysiology and carbon utilization dynamics between two variants of the same Picochlorum celeri species, TG1 and TG2 elucidated metabolic bottlenecks and transport rates of intermediates using instationary 13C-fluxomics. Simultaneous global gene expression dynamics showed 73% of the annotated genes responding within one hour, elucidating a singular, diel-responsive transcription factor, closely related to the CCA1/LHY clock genes in plants, with significantly altered expression in TG2. Transgenic P. celeri TG1 cells expressing the TG2 CCA1/LHY gene, showed 15% increase in growth rates and 25% increase in storage carbohydrate content, supporting a coordinating regulatory function for a single transcription factor. Researchers identify unique transcriptional regulation that controls photosynthetic response, growth and biochemical carbon storage in high light for two variants of the same algae species, offering a glimpse into diel control of plant and crop yields. [ABSTRACT FROM AUTHOR]

Fine-tuned and coordinated regulation of transport, metabolism and redox homeostasis allows plants to acclimate to osmotic and ionic stress caused by high salinity. Sugar beet is a highly salt tolerant crop plant and is therefore an interesting model to study sodium chloride (NaCl) acclimation in crops. Sugar beet plants were subjected to a final level of 300 mM NaCl for up to 14 d in hydroponics. Plants acclimated to NaCl stress by maintaining its growth rate and adjusting its cellular redox and reactive oxygen species (ROS) network. In order to understand the unusual suppression of ROS accumulation under severe salinity, the regulation of elements of the redox and ROS network was investigated at the transcript level. First, the gene families of superoxide dismutase (SOD), peroxiredoxins (Prx), alternative oxidase (AOX), plastid terminal oxidase (PTOX) and NADPH oxidase (RBOH) were identified in the sugar beet genome. Salinity induced the accumulation of Cu-Zn-SOD, Mn-SOD, Fe-SOD3, all AOX isoforms, 2-Cys-PrxB, PrxQ, and PrxIIF. In contrast, Fe-SOD1, 1-Cys-Prx, PrxIIB and PrxIIE levels decreased in response to salinity. Most importantly, RBOH transcripts of all isoforms decreased. This pattern offers a straightforward explanation for the low ROS levels under salinity. Promoters of stress responsive antioxidant genes were analyzed in silico for the enrichment of cis-elements, in order to gain insights into gene regulation. The results indicate that special cis-elements in the promoters of the antioxidant genes in sugar beet participate in adjusting the redox and ROS network and are fundamental to high salinity tolerance of sugar beet. [ABSTRACT FROM AUTHOR]

The cell transformation from green motile cells to non-motile cells and astaxanthin accumulation can be induced in the green alga Haematococcus pluvialis cultured outdoors. In the initial 3 d of incubation (cell transformation phase), light absorption and photosynthetic electron transport became more efficient. After five days of incubation (astaxanthin accumulation phase), the light absorption per active reaction center (ABS/RC) increased, but the efficiency of electron transport (Ψo) and the quantum yield of electron transport (φEo) decreased with increased time, indicating that the capacity of photosynthetic energy utilization decreased significantly during astaxanthin accumulation, leading to an imbalance between photosynthetic light absorption and energy utilization. It would inevitably aggravate photoinhibition under high light, e.g., at midday. However, the level of photoinhibition in H. pluvialis decreased as the incubation time increased, which is reflected by the fact that Fv/Fm determined at midday decreased significantly in the initial 3 d of incubation, but was affected very little after seven days of incubation, compared with that determined at predawn. This might be because the non-photochemical quenching, plastid terminal oxidase, photosystem I cyclic electron transport, defensive enzymes and the accumulated astaxanthin can protect cells against photoinhibition. [ABSTRACT FROM AUTHOR]

Incorrectly or simply not annotated data is largely increasing in most public databases, undoubtedly caused by the rise in sequence data and the more recent boom of genomic projects. Molecular biologists and bioinformaticists should join efforts to tackle this issue. Practical challenges have been experienced when studying the alternative oxidase (AOX) gene family, and hence the motivation for the present work. Commonly used databases were screened for their capacity to distinguish AOX from the plastid terminal oxidase (also called plastoquinol terminal oxidase; PTOX) and we put forward a simple approach, based on amino acids signatures, that unequivocally distinguishes these gene families. Further, available sequence data on the AOX family in plants was carefully revised to: (1) confirm the classification as AOX and (2) identify to which AOX family member they belong to. We bring forward the urgent need of misannotation awareness and re-annotation of public AOX sequences by highlighting different types of misclassifications and the large under-estimation of data availability. [ABSTRACT FROM AUTHOR]

With increase of temperature, Fo gradually rose in both WT and the mutant inactivated in the type 1 NAD(P)H dehydrogenase (NDH), a double mutant disrupted the genes of ndhJ and ndhK (AndhJK) or a triple mutant disrupted the genes of ndhC, ndhJ, and ndhK (AndhCJK). The temperature threshold of Fo rise was about 3-5°C lower in the mutants than in WT, indicating ΔndhJK and ΔndhCJK were more sensitive to elevated temperature. The Fo rise after the threshold was slower and the reached maximal level was lower in the mutants than in WT, implying the chlororespiratory pathway was suppressed when NDH was inactivated. Meanwhile, the maximum quantum efficiency of photosystem II (PS II) (Fv/Fm) decreased to a similar extent below 50°C in WT and mutants. However, the decline was sharper in WT when temperature rose above 55°C, indicating a down regulation of PS II photochemical activity by the chlororespiratory pathway in response to elevated temperature. On the other hand, in the presence of n-propyl gallate, an inhibitor of plastid terminal oxidase (PTOX), the less evident increase in Fo while the more decrease in Fv/Fm in ΔndhCJK than in WT after incubation at 50°C for 6 h suggest the increased sensitivity to heat stress when both NDH and chlororespiratory pathways are suppressed. Moreover, the net photosynthetic rate and photo-efficiency decreased more significantly in AndhJK than in WT under the heat stressed conditions. Compared to the light-oxidation of P700, the difference in the dark-reduction of P700+ between WT and ndhJK disruptant was much less under the heat stressed conditions, implying significantly enhanced cyclic electron flow in light and the competition for electron from PQ between PTOX and photosystem I in the dark at the elevated temperature. Heat-stimulated expression of both NdhK and PTOX significantly increased in WT, while the expression of PTOX was less in ΔndhJK than in WT. Meanwhile, the amount of active form of Rubisco activase decreased much more in the mutant. The results suggest that chlororespiration and cyclic electron flow mediated by NDH may coordinate to alleviate the over-reduction of stroma, thus to keep operation of CO2 assimilation at certain extent under heat stress condition. [ABSTRACT FROM AUTHOR]

The redox state of the plastoquinone (PQ) pool in thylakoids plays an important role in the regulation of chloroplast metabolism. In the light, the PQ pool is mostly reduced, followed by oxidation after light cessation. It has been believed for a long time that dark oxidation depends on oxygen, although the precise mechanisms of the process are still unknown and debated. In this work, we analyzed PQ pool oxidation kinetics in isolated pea (Pisum sativum) thylakoids by tracking the changes in the area above the OJIP fluorescence curve (Afl) over time intervals from 0.1 s to 10 min in the dark following illumination. Afl served as an indirect measure of the redox state of the PQ pool that enabled quantification of the rate of PQ pool oxidation. The results showed a two-phase increase in Afl. The “fast” phase appeared to be linked to electron flow from the PQ pool to downstream acceptors of the photosynthetic electron transport chain. The “slow” phase involved oxidation of PQH2 through oxygen-dependent mechanisms. Adding octyl gallate, an inhibitor of plastid terminal oxidase (PTOX), to isolated thylakoid suspensions decreased the rate of the “slow” phase of PQ pool oxidation in the dark after illumination. The addition of either H2O2 or catalase, an enzyme that decomposes H2O2, revealed that H2O2 accelerates oxidation of the PQ pool. This indicates that under conditions that favor H2O2 accumulation, H2O2 can contribute substantially to PQ pool oxidation in the dark after illumination. The contribution of PTOX and H2O2 to the modulation of the PQ pool redox state in plants in the dark after illumination is discussed.

Chlorophylls are essential cofactors in photosynthesis. All steps in the chlorophyll pathway are well characterized except for the cyclase reaction in which the fifth ring of the chlorophyll molecule is formed during conversion of Mg-protoporphyrin IX monomethyl ester into Protochlorophyllide. The only subunit of the cyclase identified so far, is AcsF (Xantha-l in barley and Chl27 in Arabidopsis). This subunit contains a typical consensus di-iron-binding sequence and belongs to a subgroup of di-iron proteins, such as the plastid terminal oxidase (PTOX) in the chloroplast and the alternative oxidase (AOX) found in mitochondria. In order to complete the catalytic cycle, the irons of these proteins need to be reduced from Fe 3+ to Fe 2+ and either a reductase or another form of reductant is required. It has been reported that the alternative oxidase (AOX) and the plastid terminal oxidase (PTOX) utilize the di-iron center to oxidise ubiquinol and plastoquinol, respectively. In this paper, we have used a specific inhibitor of di-iron proteins as well as Arabidopsis and barley mutants affected in regulation of photosynthetic electron flow, to show that the cyclase step indeed is directly coupled to the plastoquinone pool. Thus, plastoquinol might act as an electron donor for the cyclase reaction and thereby fulfil the role of a cyclase reductase. That would provide a functional connection between the redox status of the thylakoids and the biosynthesis of chlorophyll. [ABSTRACT FROM AUTHOR]

The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between photosystem I (PS I) and photosystem II (PS II) to provide balanced photosynthesis. Chlorophyll (Chl) a fluorescence induction (the so-called OJIPSMT transient) is a signature of several photosynthetic reactions. Here, we show that the slow (seconds to minutes) S to M fluorescence rise is reduced or absent in the stt7 mutant (which is locked in state 1) in C. reinhardtii. This suggests that the SM rise in wild type C. reinhardtii may be due to state 2 (low fluorescence state; larger antenna in PS I) to state 1 (high fluorescence state; larger antenna in PS II) transition, and thus, it can be used as an efficient and quick method to monitor state transitions in algae, as has already been shown in cyanobacteria (Papageorgiou et al. , ; Kaňa et al. ). We also discuss our results on the effects of (1) 3-(3,4-dichlorophenyl)-1,4-dimethyl urea, an inhibitor of electron transport; (2) n-propyl gallate, an inhibitor of alternative oxidase (AOX) in mitochondria and of plastid terminal oxidase in chloroplasts; (3) salicylhydroxamic acid, an inhibitor of AOX in mitochondria; and (4) carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an uncoupler of phosphorylation, which dissipates proton gradient across membranes. Based on the data presented in this paper, we conclude that the slow PSMT fluorescence transient in C. reinhardtii is due to the superimposition of, at least, two phenomena: qE dependent non-photochemical quenching of the excited state of Chl, and state transitions. [ABSTRACT FROM AUTHOR]

S pathiphyllum wallisii plants were used to study the effect of chilling stress under high illumination on photosynthesis and chlororespiration. Leaves showed different responses that depended on root temperature. When stem, but not root, was chilled, photosystem II ( PSII) was strongly photoinhibited. However, when the whole plant was chilled, the maximal quantum yield of PSII decreased only slightly below the normal values and cyclic electron transport was stimulated. Changes were also observed in the chlororespiration enzymes and PGR5. In whole plants chilled under high illumination, the amounts of NADH dehydrogenase ( NDH) complex and plastid terminal oxidase ( PTOX) remained similar to control and increased when only stem was chilled. In contrast, the amount of PGR5 polypeptide was higher in plants when both root and stem were chilled than in plants in which only stem was chilled. The results indicated that the contribution of chlororespiration to regulating photosynthetic electron flow is not relevant when the whole plant is chilled under high light, and that another pathway, such as cyclic electron flow involving PGR5 polypeptide, may be more important. However, when PSII activity is strongly photoinhibited in plants in which only stem is chilled, chlororespiration, together with other routes of electron input to the electron transfer chain, is probably essential. [ABSTRACT FROM AUTHOR]

In order to find some basis of salinity resistance in the chloroplastic metabolism, a halophytic Thellungiella salsuginea was compared with glycophytic Arabidopsis thaliana. In control T.s. plants the increased ratios of chlorophyll a/b and of fluorescence emission at 77 K (F730/F685) were documented, in comparison to A.t.. This was accompanied by a higher YII and lower NPQ (non‐photochemical quenching) values, and by a more active PSI (photosystem I). Another prominent feature of the photosynthetic electron transport (PET) in T.s. was the intensive production of H2O2 from PQ (plastoquinone) pool. Salinity treatment (0.15 and 0.30 M NaCl for A.t. and T.s., respectively) led to a decrease in ratios of chl a/b and F730/F685. In A.t., a salinity‐driven enhancement of YII and NPQ was found, in association with the stimulation of H2O2 production from PQ pool. In contrast, in salinity‐treated T.s., these variables were similar as in controls. The intensive H2O2 generation was accompanied by a high activity of PTOX (plastid terminal oxidase), whilst inhibition of this enzyme led to an increased H2O2 formation. It is hypothesized, that the intensive H2O2 generation from PQ pool might be an important element of stress preparedness in Thellungiella plants. In control T.s. plants, a higher activation state of carboxylase ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was also documented in concert with the attachment of Rubisco activase (RCA) to the thylakoid membranes. It is supposed, that a closer contact of RCA with PSI in T.s. enables a more efficient Rubisco activation than in A.t. [ABSTRACT FROM AUTHOR]

Astaxanthin (ASX), "king of carotenoids", is a xanthophyll carotenoid that is characterized by a distinct reddish-orange hue, procured from diverse sources including plants, microalgae, fungi, yeast, and lichens. It exhibits potent antioxidant and anti-ageing properties and has been demonstrated to mitigate ultraviolet-induced cellular and DNA damage, enhance immune system function, and improve cardiovascular diseases. Despite its broad utilization across nutraceutical, cosmetic, aquaculture, and pharmaceutical sectors, the large-scale production and application of ASX are constrained by the limited availability of natural sources, low production yields and stringent production requirements. This review provides a comprehensive analysis of ASX applications, emphasizing its dual roles in cosmetic and nutraceutical fields. It integrates insights into the qualitative differences of ASX from various natural sources and assesses biosynthetic pathways across organisms. Advanced biotechnological strategies for industrial-scale production are explored alongside innovative delivery systems, such as emulsions, films, microcapsules, nanoliposomes, and nanoparticles, designed to enhance ASX's bioavailability and functional efficacy. By unifying perspectives on its nutraceutical and cosmetic applications, this review highlights the challenges and advancements in formulation and commercialization. Prospective research directions for optimizing ASX's production and applications are also discussed, providing a roadmap for its future development., Competing Interests: Declarations. Conflict of interest: The authors declare no competing interests., (© 2025. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Nitrate reduction requires reducing equivalents produced by the photosynthetic electron transport chain. Therefore, it has been suggested that nitrate assimilation provides a sink for electrons under high light conditions. We tested this hypothesis by monitoring photosynthetic efficiency and the chloroplastic glutathione redox potential (chl-E GSH ) of plant lines with mutated glutamine synthetase 2 (GS2) and ferredoxin-dependent glutamate synthase 1 (GOGAT1). Mutant lines incorporated significantly less isotopically-labelled nitrate into amino acids than wild-type plants, demonstrating impaired nitrogen assimilation. When nitrate assimilation was compromised, photosystem II (PSII) proved more vulnerable to photodamage. The effect of the nitrate assimilation pathway on the chl- E GSH was monitored using the chloroplast-targeted roGFP2 biosensor (chl-roGFP2). Remarkably, while oxidation followed by reduction of chl-roGFP2 was detected in WT plants in response to high light, oxidation values were stable in the mutant lines, suggesting that chl-E GSH relaxation after high light-induced oxidation is achieved by diverting excess electrons to the nitrogen assimilation pathway. Importantly, similar ΦPSII and chl-roGFP2 patterns were observed at elevated CO 2, suggesting that mutant phenotypes are not associated with photorespiration activity. Together, these findings indicate that the nitrogen assimilation pathway serves as a sustainable energy dissipation route, ensuring efficient photosynthetic activity and fine-tuning redox metabolism under light-saturated conditions., (© 2025 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Background Leaf variegation is an intriguing phenomenon observed in many plant species. However, questions remain on its mechanisms causing patterns of diferent colours. In this study, we describe a tomato plant detected in an M2 population of EMS mutagenised seeds, showing variegated leaves with sectors of dark green (DG), medium green (MG), light green (LG) hues, and white (WH). Cells and tissues of these classes, along with wild-type tomato plants, were studied by light, fuorescence, and transmission electron microscopy. We also measured chlorophyll a/b and carotene and quantifed the variegation patterns with a machine-learning image analysis tool. We compared the genomes of pooled plants with wild-type-like and mutant phenotypes in a segregating F2 population to reveal candidate genes responsible for the variegation. Results A genetic test demonstrated a recessive nuclear mutation caused the variegated phenotype. Cross-sections displayed distinct anatomy of four-leaf phenotypes, suggesting a stepwise mesophyll degradation. DG sectors showed large spongy layers, MG presented intercellular spaces in palisade layers, and LG displayed deformed palisade cells. Electron photomicrographs of those mesophyll cells demonstrated a gradual breakdown of the chloroplasts. Chlorophyll a/b and carotene were proportionally reduced in the sectors with reduced green pigments, whereas white sectors have hardly any of these pigments. The colour segmentation system based on machine-learning image analysis was able to convert leaf variegation patterns into binary images for quantitative measurements. The bulk segregant analysis of pooled wild-type-like and variegated progeny enabled the identifcation of SNP and InDels via bioinformatic analysis. The mutation mapping bioinformatic pipeline revealed a region with three candidate genes in chromosome 4, of which the FtsH-like protein precursor (LOC100037730) carries an SNP that we consider the causal variegated phenotype mutation. Phylogenetic analysis shows the candidate is evolutionary closest to the Arabidopsis VAR1. The synonymous mutation created by the SNP generated a miRNA binding site, potentially disrupting the photoprotection mechanism and thylakoid development, resulting in leaf variegation. Conclusion We described the histology, anatomy, physiology, and image analysis of four classes of cell layers and chloroplast degradation in a tomato plant with a variegated phenotype. The genomics and bioinformatics pipeline revealed a VAR1-related FtsH mutant, the frst of its kind in tomato variegation phenotypes. The miRNA binding site of the mutated SNP opens the way to future studies on its epigenetic mechanism underlying the variegation. [ABSTRACT FROM AUTHOR]

Soybean is a very important food legume because of its high protein and oil concentrations. However, soybean is vulnerable to drought stress, which has become more severe and occasional in many regions worldwide. To alleviate drought's influence, the application of certain agents is increasingly gaining attention as it is economically affordable and practically applicable. Acetic acid (AA) is, by far, one of the cheapest agents that are reported to have potential benefits against drought; however, no accurate data on its influence on soybean genotypes differing in their drought tolerance are published. An experiment was conducted in a controlled environment to evaluate the effects of AA on the morpho-physiology of two soybean (Glycine max (L.) Merr.) genotypes: drought-tolerant 'Speeda' and drought-susceptible 'Coraline.' Chlorophyll-a and total carotenoids, stomatal conductance, and specific leaf area of both soybean genotypes decreased under water deprivation conditions. However, AA application enhanced these traits significantly. Drought reduced the optimal and the actual photochemical efficiency of PSII of 'Coraline,' but not 'Speeda.' The application of AA could not enhance the relative water content of both genotypes. Root and shoot morphology were negatively influenced by drought in both genotypes; however, AA helped in restoring these traits in 'Coraline,' but not 'Speeda,' indicating that AA application might be more beneficial in the case of drought-susceptible soybean genotypes. [ABSTRACT FROM AUTHOR]

Anthocyanin is an important pigment that prevents oxidative stress and mediates adaptation of plants to salt stress. Peanuts with dark red and black testa are rich in anthocyanin. However, correlation between salt tolerance and anthocyanin content in black and dark red testa peanuts is unknown. In this study, three peanut cultivars namely YZ9102 (pink testa), JHR1 (red testa) and JHB1 (black testa) were subjected to sodium chloride (NaCl) stress. The plant growth, ion uptake, anthocyanin accumulation, oxidation resistance and photosynthetic traits were comparatively analyzed. We observed that the plant height, leaf area and biomass under salt stress was highly inhibited in pink color testa (YZ9102) as compare to black color testa (JHB1). JHB1, a black testa colored peanut was identified as the most salt-tolerance cultivar, followed by red (JHR1) and pink(YZ9102). During salt stress, JHB1 exhibited significantly higher levels of anthocyanin and flavonoid accumulation compared to JHR1 and YZ9102, along with increased relative activities of antioxidant protection and photosynthetic efficiency. However, the K+/Na+ and Ca2+/Na+ were consistently decreased among three cultivars under salt stress, suggesting that the salt tolerance of black testa peanut may not be related to ion absorption. Therefore, we predicted that salt tolerance of JHB1 may be attributed to the accumulation of the anthocyanin and flavonoids, which activated antioxidant protection against the oxidative damage to maintain the higher photosynthetic efficiency and plant growth. These findings will be useful for improving salt tolerance of peanuts. [ABSTRACT FROM AUTHOR]

Mesotrione is a widely used post-emergence herbicide for maize. The toxicity of mesotrione to maize (especially sweet corn) has been widely reported, and some sweet corn varieties are highly sensitive to mesotrione, which affects subsequent plant growth periods. However, the molecular mechanisms responsible for the differences in susceptibility to mesotrione are not known. By comparing changes in the transcriptome of mesotrione-tolerant line 301 and mesotrione-sensitive line 276 after mesotrione treatment, we found that the genes coding light-harvesting chlorophyll protein complex were induced in 301, and the genes coding loosening cell walls were overrepresented in 276. The net photosynthetic rate, maximum photochemical efficiency of leaf PSII, photochemical quenching of chlorophyll fluorescence, and the electron transport rate were significantly higher in 301 than in 276 after mesotrione treatment, and these effects became more severe as time passed. In addition, oxidative balance was also affected by mesotrione. Compared with 301, SOD, POD, and CAT activities were significantly reduced with longer exposure time in 276. The results suggested that sweet corn can mitigate herbicide mesotrione toxicity by improving photosynthesis, ROS scavenging, and cell wall synthesis. [ABSTRACT FROM AUTHOR]

Chinese cabbage (Brassica rapa ssp. pekinensis) has a long cultivation history and is one of the vegetable crops with the largest cultivation area in China. However, salt stress severely damages photosynthesis and hormone metabolism, nutritional balances, and results in ion toxicity in plants. To better understand the mechanisms of salt-induced growth inhibition in Chinese cabbage, RNA-seq and physiological index determination were conducted to explore the impacts of salt stress on carbon cycle metabolism and photosynthesis in Chinese cabbage. Here, we found that the number of thylakoids and grana lamellae and the content of starch granules and chlorophyll in the leaves of Chinese cabbage under salt stress showed a time-dependent response, first increasing and then decreasing. Chinese cabbage increased the transcript levels of genes related to the photosynthetic apparatus and carbon metabolism under salt stress, probably in an attempt to alleviate damage to the photosynthetic system and enhance CO2 fixation and energy metabolism. The transcription of genes related to starch and sucrose synthesis and degradation were also enhanced; this might have been an attempt to maintain intracellular osmotic pressure by increasing soluble sugar concentrations. Soluble sugars could also be used as potential reactive oxygen species (ROS) scavengers, in concert with peroxidase (POD) enzymes, to eliminate ROS that accumulate during metabolic processes. Our study characterizes the synergistic response network of carbon metabolism and photosynthesis under salt stress. [ABSTRACT FROM AUTHOR]

The mottled leaf is one of the agronomic traits of zucchini and can be applied as a marker trait in aggregation breeding. However, the genetic mechanism responsible for mottled leaf has yet to be elucidated. In the present study, we used two inbred lines (line '19': silver mottled leaf; line '113': normal leaf) as parents for the physiological and genetic analysis of mottled leaf. The synthesis and net photosynthetic rate of chlorophyll were not significantly affected in the mottled areas of leaves. However, we detected a large space between the palisade parenchyma in the leaf mottle area of line '19', which may have caused the mottled leaf phenotype. Light also plays an important role in the formation of mottled leaf, and receiving light during the early stages of leaf development is a necessary factor. Genetic analysis has previously demonstrated that mottled leaf is a quantitative trait that is controlled by multiple genes. Based on the strategy of quantitative trait locus sequencing (QTL-seq), two QTLs were identified on chromosomes 1 and 17, named CpML1.1 and CpML17.1, respectively. Two major loci were identified using R/qtl software version 1.66 under greenhouse conditions in April 2019 (2019A) and April 2020 (2020A) and under open cultivation conditions in May 2020 (2020M). The major QTL, CpML1.1, was located in a 925.2-kb interval on chromosome 1 and explained 10.51%-24.15% of the phenotypic variation. The CpML17.1 was located in a 719.7-kb interval on chromosome 17 and explained 16.25%-38.68% of the phenotypic variation. Based on gene annotation, gene sequence alignment, and qRT–PCR analysis, the Cp4.1LG01g23790 at the CpML1.1 locus encoding a protein of the TPX2 family (target protein of Xklp2) may be a candidate gene for mottled leaf in zucchini. Our findings may provide a theoretical basis for the formation of mottled leaf and provide a foundation for the fine mapping of genes associated with mottled leaf. Molecular markers closely linked to mottled leaf can be used in molecular-assisted selection for the zucchini mottled leaf breeding. [ABSTRACT FROM AUTHOR]

Accumulated in large amounts in carrot, carotenoids are an important product quality attribute and therefore a major breeding trait. However, the knowledge of carotenoid accumulation genetic control in this root vegetable is still limited. In order to identify the genetic variants linked to this character, we performed an association mapping study with a candidate gene approach. We developed an original unstructured population with a broad genetic basis to avoid the pitfall of false positive detection due to population stratification. We genotyped 109 SNPs located in 17 candidate genes – mostly carotenoid biosynthesis genes – on 380 individuals, and tested the association with carotenoid contents and color components. Total carotenoids and β-carotene contents were significantly associated with genes zeaxanthin epoxydase (ZEP), phytoene desaturase (PDS) and carotenoid isomerase (CRTISO) while α-carotene was associated with CRTISO and plastid terminal oxidase (PTOX) genes. Color components were associated most significantly with ZEP. Our results suggest the involvement of the couple PDS/PTOX and ZEP in carotenoid accumulation, as the result of the metabolic and catabolic activities respectively. This study brings new insights in the understanding of the carotenoid pathway in non-photosynthetic organs. [ABSTRACT FROM AUTHOR]

The picoplanktonic cyanobacteria, Synechococcus spp., (Nägeli) are important contributors to global ocean primary production that can be stressed by solar radiation, both in the photosynthetically active ( PAR) and ultraviolet ( UV) range. We studied the responses of PSII quantum yield (active fluorescence), carbon fixation (14C assimilation) and oxygen evolution (membrane inlet mass spectrometry) in Synechococcus WH8102 under moderate UV and PAR. PSII quantum yield decreased during exposure to moderate UV and UV+ PAR, with response to the latter being faster (6.4 versus 2.8 min, respectively). Repair processes were also faster when UV+ PAR exposure was followed by moderate PAR (1.68 min response time) than when UV was followed by very low PAR (10.5 min response time). For the UV+ PAR treatment, the initial decrease in quantum yield was followed by a 50% increase ('rebound') after 7 min exposure, showing an apparent photoprotection induction. While oxygen uptake increased with PAR, it did not change under UV, suggesting that this oxygen-dependent mechanism of photoprotection, which may be acting as an electron sink, is not an important strategy against UV. We used propyl gallate, an antioxidant, to test for plastid terminal oxidase (ptox) or ptox-like enzymes activity, but it caused nonspecific and toxic effects on Synechococcus WH8102. [ABSTRACT FROM AUTHOR]

Chloroplast biogenesis is an essential light-dependent process involving the differentiation of photosynthetically competent chloroplasts from precursors that include undifferentiated proplastids in leaf meristems, as well as etioplasts in dark-grown seedlings. The mechanisms that govern these developmental processes are poorly understood, but entail the coordinated expression of nuclear and plastid genes. This coordination is achieved, in part, by signals generated in response to the metabolic and developmental state of the plastid that regulate the transcription of nuclear genes for photosynthetic proteins (retrograde signaling). Variegation mutants are powerful tools to understand pathways of chloroplast biogenesis, and over the years our lab has focused on immutans ( im) and variegated2 ( var2), two nuclear gene-induced variegations of Arabidopsis. im and var2 are among the best-characterized chloroplast biogenesis mutants, and they define the genes for plastid terminal oxidase (PTOX) and the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, respectively. To gain insight into the function of these proteins, forward and reverse genetic approaches have been used to identify second-site suppressors of im and var2 that replace or bypass the need for PTOX and AtFtsH2 during chloroplast development. In this review, we provide a brief update of im and var2 and the functions of PTOX and AtFtsH2. We then summarize information about second-site suppressors of im and var2 that have been identified to date, and describe how they have provided insight into mechanisms of photosynthesis and pathways of chloroplast development. [ABSTRACT FROM AUTHOR]

Oxygen plays an important role in photosynthesis by participating in a number of O2-consuming reactions. O2 inhibits CO2 fixation by stimulating photorespiration, thus reducing plant production. O2 interacts with photosynthetic electron transport in the chloroplasts’ thylakoids in two main ways: by accepting electrons from PSI (Mehler reaction); and by accepting electrons from reduced plastoquinone (PQ) mediated by the plastid terminal oxidase (PTOX). In this study, we show, using 101 plant species, that there is a difference in the potential for photosynthetic electron flow to O2 between angiosperms and gymnosperms. We found, from measurements of Chl fluorescence and leaf absorbance at 830 nm, (i) that electron outflow from PSII, as determined by decay kinetics of Chl fluorescence after application of a saturating light pulse, is more rapid in gymnosperms than in angiosperms; (ii) that the reaction center Chl of PSI (P700) is rapidly and highly oxidized in gymnosperms during induction of photosynthesis; and (iii) that these differences are dependent on oxygen. Finally, rates of O2 uptake measured by mass spectrometry in the absence of photorespiration were significantly promoted by illumination in dark-adapted leaves of gymnosperms, but not in those of angiosperms. The light-stimulated O2 uptake was around 10% of the maximum O2 evolution in gymnosperms and 1% in angiosperms. These results suggest that gymnosperms have increased capacity for electron leakage to oxygen in photosynthesis compared with angiosperms. The involvement of the Mehler reaction and PTOX in the electron flow to O2 is discussed. [ABSTRACT FROM AUTHOR]

Background: Cold acclimation in woody perennials is a metabolically intensive process, but coincides with environmental conditions that are not conducive to the generation of energy through photosynthesis. While the negative effects of low temperatures on the photosynthetic apparatus during winter have been well studied, less is known about how this is reflected at the level of gene and metabolite expression, nor how the plant generates primary metabolites needed for adaptive processes during autumn. Results: The MapMan tool revealed enrichment of the expression of genes related to mitochondrial function, antioxidant and associated regulatory activity, while changes in metabolite levels over the time course were consistent with the gene expression patterns observed. Genes related to thylakoid function were down-regulated as expected, with the exception of plastid targeted specific antioxidant gene products such as thylakoid-bound ascorbate peroxidase, components of the reactive oxygen species scavenging cycle, and the plastid terminal oxidase. In contrast, the conventional and alternative mitochondrial electron transport chains, the tricarboxylic acid cycle, and redox-associated proteins providing reactive oxygen species scavenging generated by electron transport chains functioning at low temperatures were all active. Conclusions: A regulatory mechanism linking thylakoid-bound ascorbate peroxidase action with "chloroplast dormancy" is proposed. Most importantly, the energy and substrates required for the substantial metabolic remodeling that is a hallmark of freezing acclimation could be provided by heterotrophic metabolism. [ABSTRACT FROM AUTHOR]

This work studies the effects of water deficit and heat, as well as the involvement of chlororespiration and the ferredoxin-mediated cyclic pathway, on the tolerance of photosynthesis to high light intensity in Hibiscus rosa-sinensis plants. Drought and heat resulted in the down--regulation of photosynthetic linear electron transport in the leaves, although only a slight decrease in variable fluorescence (Fv)/maximal fluorescence (Fm) was observed, indicating that the chloroplast was protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. The incubation of leaves from unstressed plants under high light intensity resulted in an increase of the activity of electron donation by nicotinamide adenine dinucleotide phosphate (NADPH) and ferredoxin to plastoquinone, but no increase was observed in plants exposed to water deficit, suggesting that cyclic electron transport was stimulated by high light only in control plants. In contrast, the activities of the chlororespiration enzymes (NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX)) increased after incubation under high light intensity in leaves of the water deficit plants, but not in control plants, suggesting that chlororespiration was stimulated in stressed plants. The results indicate that the relative importance of chlororespiration and the cyclic electron pathway in the tolerance of photosynthesis to high illumination differs under stress conditions. When plants were not subjected to stress, the contribution of chlororespiration to photosynthetic electron flow regulation was not relevant, and another pathway, such as the ferredoxin-mediated cyclic pathway, was more important. However, when plants were subjected to water deficit and heat, chlororespiration was probably essential. [ABSTRACT FROM AUTHOR]

Abstract: The physiological role of the plastid terminal oxidase (PTOX) involved in plastoquinol oxidation in chloroplasts has been investigated in vivo in tomato leaves. Enzyme activity was assessed by non-invasive methods based on the analysis of the kinetics of chlorophyll fluorescence changes. In the dark, the maximum PTOX rate was smaller than 1 electron per second per PSII. This value was further decreased upon light acclimation, and became almost negligible upon inhibition of the photosynthetic performances by reducing the CO2 availability. In contrast, prolonged exposure to high light resulted in an increase of the overall PTOX activity, which was paralleled by an increased protein accumulation. Under all the conditions tested the enzyme activity always remained about two orders of magnitude lower than that of electron flux through the linear photosynthetic electron pathway. Therefore, PTOX cannot have a role of a safety valve for photogenerated electrons, while it could be involved in acclimation to high light. Moreover, by playing a major role in the control of the stromal redox poise, PTOX is also capable of modulating the balance between linear and cyclic electron flow around PSI during the deactivation phase of carbon assimilation that follows a light to dark transition. [Copyright &y& Elsevier]

The immutans (im) variegation mutant of Arabidopsis has green and white-sectored leaves due to the absence of fully functional plastid terminal oxidase (PTOX), a plastoquinol oxidase in thylakoid membranes. PTOX appears to be at the nexus of a growing number of biochemical pathways in the plastid, including carotenoid biosynthesis, PSI cyclic electron flow, and chlororespiration. During the early steps of chloroplast biogenesis, PTOX serves as an alternate electron sink and is a prime determinant of the redox poise of the developing photosynthetic apparatus. Whereas a lack of PTOX causes the formation of photooxidized plastids in the white sectors of im, compensating mechanisms allow the green sectors to escape the effects of the mutation. This manuscript provides an update on PTOX, the mechanism of im variegation, and findings about im compensatory mechanisms. [ABSTRACT FROM AUTHOR]

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5 °C resulted in a decrease of both photosystem II (PSII) (45 %) and Photosystem I (PSI) (35 %) photochemical efficiencies compared to non-treated plants. In contrast, cold-acclimated (CA) leaves exhibited only 35 and 22 % decrease of PSII and PSI photochemistry, respectively, under the same conditions. This was accompanied by an accelerated rate of P700 re-reduction, indicating an up-regulation of PSI-dependent cyclic electron transport (CET). Interestingly, the expression of the NDH-H gene and the relative abundance of the Ndh-H polypeptide, representing the NDH-complex, decreased as a result of exposure to low temperatures. This indicates that the NDH-dependent CET pathway cannot be involved and the overall stimulation of CET in CA plants is due to up-regulation of the ferredoxin-plastoquinone reductase, antimycin A-sensitive CET pathway. The lower abundance of NDH complex also implies lower activity of the chlororespiratory pathway in CA plants, although the expression level and overall abundance of the other well-characterized component involved in chlororespiration, the plastid terminal oxidase (PTOX), was up-regulated at low temperatures. This suggests increased PTOX-mediated alternative electron flow to oxygen in plants exposed to low temperatures. Indeed, the estimated proportion of O-dependent linear electron transport not utilized in carbon assimilation and not directed to photorespiration was twofold higher in CA Arabidopsis. The possible involvement of alternative electron transport pathways in inducing greater resistance of both PSII and PSI to high light stress in CA plants is discussed. [ABSTRACT FROM AUTHOR]

The life cycle of the unicellular green alga Haematococcus pluvialis consists of motile and nonmotile stages under typical growing conditions. In this study, we observed that motile cells were more susceptible than nonmotile cells to high light, resulting in a decrease in population density and photo-bleaching. Using two Haematococcus strains, CCAP 34/12 (a motile cell dominated strain) and SAG 34/1b (a nonmotile cell dominated strain), as model systems we investigated the cause of cell death and the protective mechanisms of the cells that survived high light. The death of motile cells under high light was attributed to the generation of excess reactive oxygen species (ROS), which caused severe damage to the photosynthetic components and the membrane system. Motile cells were able to dissipate excess light energy by nonphotochemical quenching and to relax ROS production by a partially up-regulated scavenging enzyme system. However, these strategies were not sufficient to protect the motile cells from high light stress. In contrast, nonmotile cells were able to cope with and survive under high light by (i) relaxing the over-reduced photosynthetic electron transport chain (PETC), thereby effectively utilizing PETC-generated NADPH to produce storage starch, neutral lipid, and astaxanthin, and thus preventing formation of excess ROS; (ii) down-regulating the linear electron transport by decreasing the level of cytochrome f; and (iii) consuming excess electrons produced by PSII via a significantly enhanced plastid terminal oxidase pathway. [ABSTRACT FROM AUTHOR]

Summary In addition to linear electron transport from water to NADP+, alternative electron transport pathways are believed to regulate photosynthesis. In the two routes of photosystem I (PSI) cyclic electron transport, electrons are recycled from the stromal reducing pool to plastoquinone (PQ), generating additional ΔpH (proton gradient across thylakoid membranes). Plastid terminal oxidase (PTOX) accepts electrons from PQ and transfers them to oxygen to produce water. Although both electron transport pathways share the PQ pool, it is unclear whether they interact in vivo. To investigate the physiological link between PSI cyclic electron transport-dependent PQ reduction and PTOX-dependent PQ oxidation, we characterized mutants defective in both functions. Impairment of PSI cyclic electron transport suppressed leaf variegation in the Arabidopsis immutans ( im) mutant, which is defective in PTOX. The im variegation was more effectively suppressed in the pgr5 mutant, which is defective in the main pathway of PSI cyclic electron transport, than in the crr2-2 mutant, which is defective in the minor pathway. In contrast to this chloroplast development phenotype, the im defect alleviated the growth phenotype of the crr2-2 pgr5 double mutant. This was accompanied by partial suppression of stromal over-reduction and restricted linear electron transport. We discuss the function of the alternative electron transport pathways in both chloroplast development and photosynthesis in mature leaves. [ABSTRACT FROM AUTHOR]

The life of plants growing in cold extreme environments has been well investigated in terms of morphological, anatomical, and ecophysiological adaptations. In contrast, long-term cellular or metabolic studies have been performed by only a few groups. Moreover, a number of single reports exist, which often represent just a glimpse of plant behavior. The review draws together the literature which has focused on tissue and cellular adaptations mainly to low temperatures and high light. Most studies have been done with European alpine plants; comparably well studied are only two phanerogams found in the coastal Antarctic. Plant adaptation in northern polar regions has always been of interest in terms of ecophysiology and plant propagation, but nowadays, this interest extends to the effects of global warming. More recently, metabolic and cellular investigations have included cold and UV resistance mechanisms. Low-temperature stress resistance in plants from cold environments reflects the climate conditions at the growth sites. It is now a matter of molecular analyses to find the induced genes and their products such as chaperones or dehydrins responsible for this resistance. Development of plants under snow or pollen tube growth at 0°C shows that cell biology is needed to explain the stability and function of the cytoskeleton. Many results in this field are based on laboratory studies, but several publications show that it is not difficult to study cellular mechanisms with the plants adapted to a natural stress. Studies on high light and UV loads may be split in two parts. Many reports describe natural UV as harmful for the plants, but these studies were mainly conducted by shielding off natural UV (as controls). Other experiments apply additional UV in the field and have had practically no negative impact on metabolism. The latter group is supported by the observations that green overwintering plants increase their flavonoids under snow even in the absence of UV. Thus, their defense and antioxidant role dominates. Ultrastructural comparisons were unable to find special light adaptations in plants taken from polar regions vs. high alpine species. The only adaptation found at the subcellular level for most alpine and polar plants are protrusions of the chloroplast envelopes. They are seen as a demand for fast membrane transport requiring additional membrane surface area, whereby the increase in stroma volume may help to support carbohydrate formation. Plants forming such protrusions have to cope with a short vegetation time. These observations are connected to the question as to how photosynthesis works quite well even at or under zero temperatures. The interplay between plastids, mitochondria, and peroxisomes, known as photorespiration, seems to be more intense than in lowland plants. This organelle cooperation serves as a valve for a surplus in solar energy input under cold conditions. Additional metabolic acclimations are under investigation, such as the role of an alternative plastid terminal oxidase. Plants from cold environments may also be seen as ideal objects for studying the combined effects of high light plus cold resistance—from the molecular level to the whole plant adaptation. Modern instrumentation makes it possible to perform vital metabolic measurements under outdoor conditions, and research stations in remote polar and alpine areas provide support for scientists in the preparation of samples for later cellular studies in the home laboratory. [ABSTRACT FROM AUTHOR]

Summary: Haematococcus pluvialis, a unicellular green microalga, experiences photooxidative stress when exposed to excess photon flux density (PFD) relative to the capacity of photosynthesis, and particularly under other adverse environmental conditions (e.g., nutrient depletion, salinity, and excess heavy metals). Under stress, Haematococcus cells synthesize and accumulate large amounts of the secondary carotenoid astaxanthin stored in cytosolic lipid bodies. In this study, the transcriptional expression of five astaxanthin biosynthesis genes and two plastid terminal oxidase (PTOX) genes either in high PFD or in the presence of excessive sodium acetate and/or iron was determined by real-time reverse transcription PCR, and astaxanthin accumulation was measured by HPLC. Photosynthetic oxygen evolution, lipid peroxidation, and cell mortality were also investigated under these stress conditions. Our results indicate that the astaxanthin biosynthesis pathway may consume as much as 9.94% of the molecular oxygen evolved from photosynthesis under stress via at least two distinct routes: (1) extensive oxygen-dependent processes leading to astaxanthin formation, and (2) conversion of molecular oxygen into water using electrons derived from carotenogenic desaturation steps to PTOX via the photosynthetic plastoquinone (PQ) pool. Reduction of reactive oxygen species (ROS) production by reducing subcellular molecular oxygen substrates through the astaxanthin biosynthesis pathway may represent a novel protective mechanism to cope with oxidative stress. Reoxidation of the PQ pool by PTOX may further reduce photosynthetic electron transport chain-induced ROS formation. [Copyright &y& Elsevier]

Two species of Brassica were used to study their acclimation to heat and high illumination during the first stages of development. One, Brassica fruticulosa, is a wild species from south-east Spain and is adapted to both heat and high light intensity in its natural habitat, while the other, Brassica oleracea, is an agricultural species that is widely cultivated throughout the world. Growing Brassica plants under high irradiance and moderate heat was seen to affect the growth parameters and the functioning of the photosynthetic apparatus. The photosystem II (PSII) quantum yields and the capacity of photosynthetic electron transport, which were lower in B. fruticulosa than in B. oleracea, decreased in B. oleracea plants when grown under stress conditions, indicating inhibition of PSII. However, in B. fruticulosa, the values of these parameters were similar to the values of control plants. Photosystem I (PSI) activity was higher in B. fruticulosa than in B. oleracea, and in both species this activity increased in plants exposed to heat and high illumination. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH–K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed a higher amount of both proteins in B. fruticulosa than in B. oleracea. In addition, PTOX activity in plastoquinone oxidation, and NADH DH activity in thylakoid membranes were higher in the wild species ( B. fruticulosa) than in the agricultural species ( B. oleracea). The results indicate that tolerance to high illumination and heat of the photosynthetic activity was higher in the wild species than in the agricultural species, suggesting that plant adaptation to these stresses in natural conditions favours subsequent acclimation, and that the chlororespiration process is involved in adaptation to heat and high illumination in Brassica. [ABSTRACT FROM AUTHOR]

Abstract: Oat (Avena sativa L., cv. Prevision) plants grown under high irradiance and moderate heat showed a decrease in the quantum yield of photosystem (PS) II and the capacity of photosynthetic electron transport as consequence of the PS II inhibition. Acclimation to the same conditions increased the tolerance to subsequent stresses involving even higher heat and light levels, in young and mature leaves, but not in senescent leaves. PS I was more stable than PS II, and its activity was slightly higher in young and mature leaves from acclimated plants than in those from control plants, probably due to the protective role which PS I plays in several alternative PS I-driven routes of electron transport. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in young and mature leaves from plants acclimated to high light intensity and moderate heat. In addition, these stresses were seen to stimulate PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes from young and mature leaves. However, in senescent leaves chlororespiratory activities and NDH-K and PTOX levels were similar in stressed and unstressed plants, although the PTOX levels were lower in senescent than in young and mature leaves; the level and activity of the NADH DH complex increased during senescence. The upregulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the acclimation of oat leaves to high irradiance and moderate heat during development. [Copyright &y& Elsevier]