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Electron flow through the electron transport chain (ETC) is essential for oxidative phosphorylation in mitochondria and photosynthesis in chloroplasts. Electron fluxes depend on environmental parameters, e.g., ionic and osmotic conditions and endogenous factors, and this may cause severe imbalances. Plants have evolved alternative sinks to balance the reductive load on the electron transport chains in order to avoid overreduction, generation of reactive oxygen species (ROS), and to cope with environmental stresses. These sinks act primarily as valves for electron drainage and secondarily as regulators of tolerance-related metabolism, utilizing the excess reductive energy. High salinity is an environmental stressor that stimulates the generation of ROS and oxidative stress, which affects growth and development by disrupting the redox homeostasis of plants. While glycophytic plants are sensitive to high salinity, halophytic plants tolerate, grow, and reproduce at high salinity. Various studies have examined the ETC systems of glycophytic plants, however, information about the state and regulation of ETCs in halophytes under non-saline and saline conditions is scarce. This review focuses on alternative electron sinks in chloroplasts and mitochondria of halophytic plants. In cases where information on halophytes is lacking, we examined the available knowledge on the relationship between alternative sinks and gradual salinity resilience of glycophytes. To this end, transcriptional responses of involved components of photosynthetic and respiratory ETCs were compared between the glycophyte Arabidopsis thaliana and the halophyte Schrenkiella parvula, and the time-courses of these transcripts were examined in A. thaliana. The observed regulatory patterns are discussed in the context of reactive molecular species formation in halophytes and glycophytes., (© 2024 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

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]

Potassium ions enhance photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans‐thylakoid proton diffusion potential difference, can alleviate over‐reduction of the photosynthetic electron transport chain and maintain the functionality of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron‐transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings and caused an outflow of potassium ions from the chloroplast. It also lowered qP (indicator of the oxidation state of QA, the primary quinone electron acceptor in Photosystem II (PSII) and YPSII (average photochemical yield of PSII in the light‐adapted state) while increasing YNO+NF (nonregulatory energy dissipation in functional and nonfunctional PSII), accompanied by decreased expression of most light‐harvesting, energy‐transduction, and electron‐transport genes. However, exogenous potassium prevented these effects due to NaCl. Interestingly, lincomycin (an inhibitor of the synthesis of chloroplast‐encoded proteins in PSII) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl‐induced downregulation of transcription and photosynthetic activities to retrograde signaling induced by reactive oxygen species. There probably exist at least two types of retrograde signaling induced by reactive oxygen species, distinguished by their sensitivity to lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans‐thylakoid proton diffusion potential difference via a potassium channel, thereby accelerating ATP synthesis and carbon assimilation, alleviating over‐reduction of the photosynthetic electron transport chain, and maintaining the functionality of photosynthetic proteins. [ABSTRACT FROM AUTHOR]

Recent studies of chloroplast‐localized Sec14‐like protein (CPSFL1, also known as phosphatidylinositol transfer protein 7, PITP7) showed that CPSFL1 is necessary for photoautotropic growth and chloroplast vesicle formation in Arabidopsis (Arabidopsis thaliana). Here, we investigated the functional roles of CPSFL1/PITP7 using two A. thaliana mutants carrying a putative null allele (pitp7‐1) and a weak allele (pitp7‐2), respectively. PITP7 transcripts were undetectable in pitp7‐1 and less abundant in pitp7‐2 than in the wild‐type (WT). The severity of mutant phenotypes, such as plant developmental abnormalities, levels of plastoquinone‐9 (PQ‐9) and chlorophylls, photosynthetic protein complexes, and photosynthetic performance, were well related to PITP7 transcript levels. The pitp7‐1 mutation was seedling lethal and was associated with significantly lower levels of PQ‐9 and major photosynthetic proteins. pitp7‐2 plants showed greater susceptibility to high‐intensity light stress than the WT, attributable to defects in nonphotochemical quenching and photosynthetic electron transport. PITP7 is specifically bound to phosphatidylinositol phosphates (PIPs) in lipid‐binding assays in vitro, and the point mutations R82, H125, E162, or K233 reduced the binding affinity of PITP7 to PIPs. Further, constitutive expression of PITP7H125Q or PITP7E162K in pitp7‐1 homozygous plants restored autotrophic growth in soil but without fully complementing the mutant phenotypes. Consistent with a previous study, our results demonstrate that PITP7 is essential for plant development, particularly the accumulation of PQ‐9 and photosynthetic complexes. We propose a possible role for PITP7 in membrane trafficking of hydrophobic ligands such as PQ‐9 and carotenoids through chloroplast vesicle formation or direct binding involving PIPs. [ABSTRACT FROM AUTHOR]

Desiccation and low temperatures inhibit photosynthetic carbon reduction and, in combination with light, result in severe oxidative stress, thus, tolerant organisms must utilize enhanced photoprotective mechanisms to prevent damaging reactions from occurring. We sought to characterize the desiccation tolerance of the fern Polypodium virginianum and to assess the role of the xanthophyll cycle and sustained forms of thermal dissipation in its response to desiccation, as well as to low temperatures during winter. Our results demonstrate that P. virginianum is desiccation‐tolerant and that it increases its utilization of sustained forms of zexanthin (Z)‐dependent thermal dissipation in response to desiccation and low temperatures during winter. Experiments with detached fronds were conducted in dark and natural light conditions and demonstrated that some dark‐formation of Z occurs in this species. In addition, desiccation in the light resulted in more pronounced declines in maximal photochemical efficiency (Fv/Fm) and higher Z levels than desiccation in the dark, indicating a substantial fraction of the sustained reduction in Fv/Fm is due to Z‐dependent sustained dissipation. Recovery from desiccation and from low temperatures exhibited biphasic kinetics with a more rapid phase (1–4 h), which was accompanied by an increase in minimal fluorescence yield (Fo) but no change in Z, and a slower phase (up to 24 h) correlating with reconversion of Z to violaxanthin. These data suggest that two mechanisms of sustained thermal dissipation occur in response to desiccation and low temperatures, possibly corresponding to sustained forms of the energy‐dependent and zeaxanthin‐dependent mechanisms of dynamic thermal dissipation. [ABSTRACT FROM AUTHOR]

Photosynthetic production of molecular hydrogen (H2) by cyanobacteria and green algae is a potential source of renewable energy. These organisms are capable of water biophotolysis by taking advantage of photosynthetic apparatus that links water oxidation at Photosystem II and reduction of protons to H2 downstream of Photosystem I. Although the process has a theoretical potential to displace fossil fuels, photosynthetic H2 production in its current state is not yet efficient enough for industrial applications due to a number of physiological, biochemical, and engineering barriers. This article presents a short overview of the metabolic pathways and enzymes involved in H2 photoproduction in cyanobacteria and green algae and our present understanding of the mechanisms of this process. We also summarize recent advances in engineering photosynthetic cell factories capable of overcoming the major barriers to efficient and sustainable H2 production. [ABSTRACT FROM AUTHOR]

Cyanobacteria and microalgae perform oxygenic photosynthesis where light energy is harnessed to split water into oxygen and protons. This process releases electrons that are used by the photosynthetic electron transport chain to form reducing equivalents that provide energy for the cell metabolism. Constant changes in environmental conditions, such as light availability, temperature, and access to nutrients, create the need to balance the photochemical reactions and the metabolic demands of the cell. Thus, cyanobacteria and microalgae evolved several auxiliary electron transport (AET) pathways to disperse the potentially harmful over‐supply of absorbed energy. AET pathways are comprised of electron sinks, e.g. flavodiiron proteins (FDPs) or other terminal oxidases, and pathways that recycle electrons around photosystem I, like NADPH‐dehydrogenase‐like complexes (NDH) or the ferredoxin‐plastoquinone reductase (FQR). Under controlled conditions the need for these AET pathways is decreased and AET can even be energetically wasteful. Therefore, redirecting photosynthetic reducing equivalents to biotechnologically useful reactions, catalyzed by i.e. innate hydrogenases or heterologous enzymes, offers novel possibilities to apply photosynthesis research. [ABSTRACT FROM AUTHOR]

Snowmelt in alpine ecosystems brings ample water, and together with above‐freezing temperatures, initiates plant growth. In this scenario, rapid activation of photosynthesis is essential for a successful life‐history strategy. But, strong solar radiation in late spring enhances the risk of photodamage, particularly before photosynthesis is fully functional. We compared the photoprotective strategy of five alpine forbs: one geophyte not particularly specialised in subnival life (Crocus albiflorus) and four wintergreens differing in their degree of adaptation to subnival life, from least to most specialised: Gentiana acaulis, Geum montanum, Homogyne alpina and Soldanella alpina. We used distance to the edge of snow patches as a proxy to study time‐dependent changes after melting. We postulated that the photoprotective response of snowbed specialists would be stronger than of more‐generalist alpine meadow species. Fv/Fm was relatively low across wintergreens and even lower in the geophyte C. albiflorus. This species also had the largest xanthophyll‐cycle pool and lowest tocopherol and flavonoid glycoside contents. After snow melting, all the species progressively activated ETR, but particularly the intermediate snowbed species G. acaulis and G. montanum. The photoprotective responses after snowmelt were idiosyncratic: G. montanum rapidly accumulated xanthophyll‐cycle pigments, tocopherol and flavonoid glycosides; while S. alpina showed the largest increase in plastochromanol‐8 and chlorophyll contents and the greatest changes in optical properties. Climate warming scenarios might shift the snowmelt date and consequently alter the effectiveness of photoprotection mechanisms, potentially changing the fitness outcome of the different strategies adopted by alpine forbs. [ABSTRACT FROM AUTHOR]

Plants are increasingly exposed to events of elevated temperature and water deficit, which threaten crop productivity. Understanding the ability to rapidly recover from abiotic stress, restoring carbon assimilation and biomass production, is important to unravel crop climate resilience. This study compared the photosynthetic performance of two Triticum aestivum L. cultivars, Sokoll and Paragon, adapted to the climate of Mexico and UK, respectively, exposed to 1-week water deficit and high temperatures, in isolation or combination. Measurements included photosynthetic assimilation rate, stomatal conductance, in vitro activities of Rubisco (EC 4.1.1.39) and invertase (INV, EC 3.2.1.26), antioxidant capacity and chlorophyll a fluorescence. In both genotypes, under elevated temperatures and water deficit (WD38°C), the photosynthetic limitations were mainly due to stomatal restrictions and to a decrease in the electron transport rate. Chlorophyll a fluorescence parameters clearly indicate differences between the two genotypes in the photoprotection when subjected to WD38°C and showed faster recovery of Paragon after stress relief. The activity of the cytosolic invertase (CytINV) under these stress conditions was strongly related to the fast photosynthesis recovery of Paragon. Taken together, the results suggest that optimal sucrose export/utilization and increased photoprotection of the electron transport machinery are important components to limit yield fluctuations due to water shortage and elevated temperatures. [ABSTRACT FROM AUTHOR]

The afterglow (AG) photosynthetic luminescence is a long‐lived chlorophyll fluorescence emitted from PSII after the illumination of photosynthetic materials by FR or white light and placed in darkness. The AG emission corresponds to the fraction of PSII centers in the S2/3QB non‐radiative state immediately after pre‐illumination, in which the arrival of an electron transferred from stroma along cyclic/chlororespiratory pathway(s) produces the S2/3QB− radiative state that emits luminescence. This emission can be optimally recorded by a linear temperature gradient as sharp thermoluminescence (TL) band peaking at about 45°C. The AG emission recorded by TL technique has been proposed as a simple non‐invasive tool to investigate the chloroplast energetic state and some of its metabolism processes as cyclic transport of electrons around PSI, chlororespiration or photorespiration. On the other hand, this emission has demonstrated to be a useful probe to study the effect of various stress conditions in photosynthetic materials. [ABSTRACT FROM AUTHOR]

Drought induces stomata closure and lowers the CO2 concentration in the mesophyll, limiting CO2 assimilation and favoring photorespiration. The photosynthetic apparatus is protected under drought conditions by a number of downregulation mechanisms like photosynthetic control and activation of cyclic electron transport leading to the generation of a high proton gradient across the thylakoid membrane. Here, we studied photosynthetic electron transport by chlorophyll fluorescence, thermoluminescence (TL), and P700 absorption measurements in spinach exposed to moderate drought stress. Chlorophyll fluorescence induction and decay kinetics were slowed down. Under drought conditions, an increase of the TL AG‐band and a downshift of the maximum temperatures of both, the B‐band and the AG‐band, were observed when leaves were illuminated under conditions that maintained the proton gradient. When leaves were frozen prior to the TL measurements, the maximum temperature of the B‐band was upshifted in drought‐stressed leaves. This shows a stabilization of the QB/QB•− redox couple in accordance with the slower fluorescence decay kinetics. We propose that during drought stress, photorespiration exerts a feedback control on photosystem II via the binding of a photorespiratory metabolite at the non‐heme iron at the acceptor side of photosystem II. According to our hypothesis, an exchange of bicarbonate at the non‐heme iron by a photorespiratory metabolite such as glycolate would not only affect the midpoint potential of the QA/QA•− couple, as shown previously, but also that of the QB/QB•− couple. [ABSTRACT FROM AUTHOR]

Arabidopsis thaliana has been recognized as a chilling tolerant species based on analysis of resistance to low temperature stress, however, the mechanisms involved in this tolerance are not yet clarified. The low temperature‐induced effects are exacerbated when plants are exposed to low temperatures in the presence of high light irradiance but the experimental data on the impact of light intensity during cold stress and its influence during recovery from stress are rather limited. The main objective of this study was to re‐examine the photosynthetic responses of A. thaliana plants to short term (6 days) low temperature stress (12/10°C) under optimal (150 μmol m−2 s−1) and high light (500 μmol m−2 s−1) intensity and the subsequent recovery from the stress. Simultaneous measurements of the in vivo and in vitro functional performance of both photosystem II (PSII) and photosystem I (PSI), as well as, net photosynthesis, low temperature (77 K) chlorophyll fluorescence and immunoblot analysis of the relative abundance of PSII and PSI reaction center proteins were used to evaluate the role of light in the development of possible protective mechanisms during low temperature stress and the consequent recovery from exposure to low temperature and different light intensities. The results presented clearly suggest that Arabidopsis plants can employ a number of highly dynamic photoprotective strategies depending on the light intensity. These strategies include one based on LHCII quenching and two other quenching mechanisms localized within the PSII and PSI reaction centers, which are all expressed to different extent depending on the severity of the photoinhibitory treatments under low temperature stress conditions. [ABSTRACT FROM AUTHOR]

The huge internodal cells of the characean green algae are a convenient model to study long‐range interactions between organelles via cytoplasmic streaming. It has been shown previously that photometabolites and reactive oxygen species released by illuminated chloroplasts are transmitted to remote shaded regions where they interfere with photosynthetic electron transport and the differential activity of plasma membrane transporters, and recent findings indicated the involvement of organelle trafficking pathways. In the present study, we applied pulse amplitude‐modulated microscopy and pH‐sensitive electrodes to study the effect of brefeldin A (BFA), an inhibitor of vesicle trafficking, on long‐distance interactions in Chara australis internodal cells. These data were compared with BFA‐induced changes in organelle number, size and distribution using fluorescent dyes and confocal laser scanning microscopy. We found that BFA completely and immediately inhibited endocytosis in internodal cells and induced the aggregation of organelles into BFA compartments within 30–120 min of treatment. The comparison with the physiological data suggests that the early response, the arrest of endocytosis, is related to the attenuation of differences in surface pH, whereas the longer lasting formation of BFA compartments is probably responsible for the acceleration of the cyclosis‐mediated interaction between chloroplasts. These data indicate that intracellular turnover of membrane material might be important for the circulation of electric currents between functionally distinct regions in illuminated characean internodes and that translational movement of metabolites is delayed by transient binding of the transported substances to organelles. [ABSTRACT FROM AUTHOR]

In the French Alps, Soldanella alpina (S. alpina) grow under shade and sun conditions during the vegetation period. This species was investigated as a model for the dynamic acclimation of shade leaves to the sun under natural alpine conditions, in terms of photosynthesis and leaf anatomy. Photosynthetic activity in sun leaves was only slightly higher than in shade leaves. The leaf thickness, the stomatal density and the epidermal flavonoid content were markedly higher, and the chlorophyll/flavonoid ratio was significantly lower in sun than in shade leaves. Sun leaves also had a more oxidised plastoquinone pool, their PSII efficiency in light was higher and their non‐photochemical quenching (NPQ) capacity was higher than that of shade leaves. Shade‐sun transferred leaves increased their leaf thickness, stomatal density and epidermal flavonoid content, while their photosynthetic activity and chlorophyll/flavonoid ratio declined compared to shade leaves. Parameters indicating protection against high light and oxidative stress, such as NPQ and ascorbate peroxidase, increased in shade‐sun transferred leaves and leaf mortality increased. We conclude that the dynamic acclimation of S. alpina leaves to high light under alpine conditions mainly concerns anatomical features and epidermal flavonoid acclimation, as well as an increase in antioxidative protection. However, this increase is not large enough to prevent damage under stress conditions and to replace damaged leaves. [ABSTRACT FROM AUTHOR]

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The review covers data representing the plastoquinone pool as the component integrated in plant antioxidant defense and plant signaling. The main goal of the review is to discuss the evidence describing the plastoquinone‐involved biochemical reactions, which are incorporated in maintaining the sustainability of higher plants to stress conditions. In this context, the analysis of the reactions of various redox forms of plastoquinone with oxygen species is presented. The review describes how these reactions can constitute both the antioxidant and signaling functions of the pool. Special attention is paid to the reaction of superoxide anion radicals with plastohydroquinone molecules, producing hydrogen peroxide as signal molecules. Attention is also given to the processes affecting the redox state of the plastoquinone pool because the redox state of the pool is of special importance for antioxidant defense and signaling. [ABSTRACT FROM AUTHOR]

Senescence is the last stage of leaf development preceding the death of the organ, and it is important for nutrient remobilization and for feeding sink tissues. There are many reports on leaf senescence, but the mechanisms initiating leaf senescence are still poorly understood. Leaf senescence is affected by many environmental factors and seems to vary in different species and even varieties of plants, which makes it difficult to generalize the mechanism. Here, we give an overview on studies reporting about alterations in the composition of the photosynthetic electron transport chain in chloroplasts during senescence. We hypothesize that alternative electron flow and related generation of the proton motive force required for ATP synthesis become increasingly important during progression of senescence. We address the generation of reactive oxygen species (ROS) in chloroplasts in the initiation of senescence, retrograde signaling from the chloroplast to the nucleus and ROS‐dependent signaling associated with leaf senescence. Finally, a few ideas for increasing crop yields by increasing the chloroplast lifespan are presented. [ABSTRACT FROM AUTHOR]

In a changing environment, plants are able to acclimate to new conditions by regulating their metabolism through the antioxidant and redox systems involved in the stress response. Here, we studied a mitochondrial thioredoxin in wild‐type (WT) Arabidopis thaliana and two Attrxo1 mutant lines grown in the absence or presence of 100 mM NaCl. Compared to WT plants, no evident phenotype was observed in the mutant plants under control condition, although they had higher number of stomata, loss of water, nitric oxide and carbonyl protein contents as well as higher activity of superoxide dismutase (SOD) and catalase enzymes than WT plants. Under salinity, the mutants presented lower water loss and higher stomatal closure, H2O2 and lipid peroxidation levels accompanied by higher enzymatic activity of catalase and the different SOD isoenzymes compared to WT plants. These inductions may collaborate in the maintenance of plant integrity and growth observed under saline conditions, possibly as a way to compensate the lack of TRXo1. We discuss the potential of TRXo1 to influence the development of the whole plant under saline conditions, which have great value for the agronomy of plants growing under unfavorable environment. [ABSTRACT FROM AUTHOR]

In the 20th century, annual mean temperatures in the European Alps rose by almost 1 K and are predicted to rise further, increasing the impact of temperature on alpine plants. The role of light in the heat hardening of plants is still not fully understood. Here, the alpine dwarf shrub Vaccinium gaultherioides was exposed in situ to controlled short‐term heat spells (150 min with leaf temperatures 43–49°C) and long‐term heat waves (7 days, 30°C) under different irradiation intensities. Lethal leaf temperatures (LT50) were calculated. Low solar irradiation [max. 250 photosynthetic photon flux density (PPFD)] during short‐term heat treatments mitigated the heat stress, shown by reduced leaf tissue damage and higher Fv/Fm (potential quantum efficiency of photosystem 2) than in darkness. The increase in xanthophyll cycle activity and ascorbate concentration was more pronounced under low light, and free radical scavenging activity increased independent of light conditions. During long‐term heat wave exposure, heat tolerance increased from 3.7 to 6.5°C with decreasing mean solar irradiation intensity (585–115 PPFD). Long‐term exposure to heat under low light enhanced heat hardening and increased photosynthetic pigment, dehydroascorbate and violaxanthin concentration. In conclusion, V. gaultherioides is able to withstand temperatures of around 50°C, and its heat hardening can be enhanced by low light during both short‐ and long‐term heat treatment. Data showing the specific role of light during short‐ and long‐term heat exposure and the potential risk of lethal damage in alpine shrubs as a result of rising temperature are discussed. [ABSTRACT FROM AUTHOR]

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO2 concentrations in the chloroplasts (Cc) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low Cc, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars. [ABSTRACT FROM AUTHOR]

Photosynthesis and respiration are the hubs of energy metabolism in plants. Drought strongly perturbs photosynthesis as a result of both diffusive limitations resulting from stomatal closure, and in some cases biochemical limitations that are associated with a reduced abundance of key photosynthetic components. The effects of drought on respiration, particularly respiration in the light ( RL), are less understood. The plant mitochondrial electron transport chain includes a non-energy conserving terminal oxidase called alternative oxidase ( AOX). Several studies have shown that drought increases AOX transcript, protein and maximum capacity. Here we review recent studies comparing wild-type ( WT) tobacco to transgenic lines with altered AOX protein amount. Specifically during drought, RL was compromised in AOX knockdown plants and enhanced in AOX overexpression plants, compared with WT. Significantly, these differences in RL were accompanied by dramatic differences in photosynthetic performance. Knockdown of AOX increased the susceptibility of photosynthesis to drought-induced biochemical limitations, while overexpression of AOX delayed the development of such biochemical limitations, compared with WT. Overall, the results indicate that AOX is essential to maintaining RL during drought, and that this non-energy conserving respiration maintains photosynthesis during drought by promoting energy balance in the chloroplast. This review also outlines several areas for future research, including the possibility that enhancement of non-energy conserving respiratory electron sinks may be a useful biotechnological approach to increase plant performance during stress. [ABSTRACT FROM AUTHOR]

Water quality, soil and climate can interact to limit photosynthesis and to increase photooxidative damage in sensitive plants. This research compared diffusive and non-diffusive limitations to photosynthesis as well as photorespiration of leaves of grapefruit trees in heavy clay and sandy soils having a previous history of treated wastewater (TWW) irrigation for >10 years, with different water qualities [fresh water (FW) vs TWW and sodium amended treated wastewater (TWW + Na)] in two arid climates (summer vs winter) and in orchard and lysimeter experiments. TWW irrigation increased salts (Na+ and Cl−), membrane leakage, proline and soluble sugar content, and decreased osmotic potentials in leaves of all experiments. Reduced leaf growth and higher stomatal and non-stomatal (i.e. mesophyll) limitations were found in summer and on clay soil for TWW and TWW + Na treatments in comparison to winter, sandy soil and FW irrigation, respectively. Stomatal closure, lower chlorophyll content and altered Rubisco activity are probable causes of higher limitations. On the other hand, non-photochemical quenching, an alternative energy dissipation pathway, was only influenced by water quality, independent of soil type and season. Furthermore, light and CO2 response curves were investigated for other possible causes of higher non-stomatal limitation. A higher proportion of non-cyclic electrons were directed to the O2 dependent pathway, and a higher proportion of electrons were diverted to photorespiration in summer than in winter. In conclusion, both diffusive and non-diffusive limitations contribute to the lower photosynthetic performance of leaves following TWW irrigation, and the response depends on soil type and environmental factors. [ABSTRACT FROM AUTHOR]

Overwintering plants face a pronounced imbalance between light capture and use of that excitation for photosynthesis. In response, plants upregulate thermal dissipation, with concomitant reductions in photochemical efficiency, in a process characterized by a slow recovery upon warming. These sustained depressions of photochemical efficiency are termed winter photoinhibition (WPI) here. WPI has been extensively studied in conifers and in few overwintering crops, but other plant species have received less attention. Furthermore, the literature shows some controversies about the association of WPI with xanthophylls and the environmental conditions that control xanthophylls conversion. To overview current knowledge and identify knowledge gaps on WPI mechanisms, we performed a comprehensive meta-analysis of literature published over the period 1991-2011. All publications containing measurements of Fv/Fm for a cold period and a corresponding warm control were included in our final database of 190 studies on 162 species. WPI was estimated as the relative decrease in Fv/Fm. High WPI was always accompanied by a high (A+Z)/(V+A+Z). Activation of lasting WPI was directly related to air temperature, with a threshold of around 0°C. Tropical plants presented earlier (at a temperature of >0°C) and higher WPI than non-tropical plants. We conclude that (1) activation of a xanthophyll-dependent mechanism of WPI is a requisite for maintaining photosynthetic structures at sub-zero temperatures, while (2) absence (or low levels) of WPI is not necessarily related to low (A+Z)/(V+A+Z); and (3) the air temperature that triggers lasting WPI, and the maximum level of WPI, do not depend on plant growth habit or bioclimatic origin of species. [ABSTRACT FROM AUTHOR]

During their lifetime, plants need to adapt to a changing environment, including light and temperature. To understand how these factors influence plant growth, we investigated the physiological and antioxidant responses of two Arabidopsis accessions, Shahdara (Sha) from the Shahdara valley (Tajikistan, Central Asia) in a mountainous area and Lovvik-5 (Lov-5) from northern Sweden to different light and temperature conditions. These accessions originate from different latitudes and have different life strategies, both of which are known to be influenced by light and temperature. We showed that both accessions grew better in high-light and at a lower temperature (16°C) than in low light and at 23°C. Interestingly, Sha had a lower chlorophyll content but more efficient non-photochemical quenching than Lov-5. Sha, also showed a higher expression of vitamin E biosynthetic genes. We did not observe any difference in the antioxidant prenyllipid level under these conditions. Our results suggest that the mechanisms that keep the plastoquinone (PQ)-pool in more oxidized state could play a role in the adaptation of these accessions to their local climatic conditions. [ABSTRACT FROM AUTHOR]

Sorghum is one of the most drought tolerant crops but surprisingly, little is known about the mechanisms achieving this. We have compared physiological and biochemical responses to drought in two sorghum cultivars with contrasting drought tolerance. These closely related cultivars have starkly contrasting responses to water deficit. In the less tolerant Samsorg 40, drought induced progressive loss of photosynthesis. The more drought tolerant Samsorg 17 maintained photosynthesis, transpiration and chlorophyll content until the most extreme conditions. In Samsorg 40, there was a highly specific down-regulation of selected proteins, with loss of PSII and Rubisco but maintenance of PSI and cytochrome b6f, allowing plants to maintain ATP synthesis. The nitrogen released allows for accumulation of glycine betaine and proline. To the best of our knowledge, this is the first example of specific reengineering of the photosynthetic apparatus in response to drought. In contrast, in Samsorg 17 we detected no substantial change in the photosynthetic apparatus. Rather, plants showed constitutively high soluble sugar concentration, enabling them to maintain transpiration and photosynthesis, even in extremely dry conditions. The implications for these strikingly contrasted strategies are discussed in relation to agricultural and natural systems. [ABSTRACT FROM AUTHOR]

'Tardivo' mandarin is a mutant of 'Comune' Clementine with a delay in peel degreening and coloration, allowing late harvesting. In this work, we have explored if the late-harvesting phenotype of 'Tardivo' mandarin is related to altered perception and sensitivity to ethylene. The peel degreening rate was examined after a single ethephon treatment or during a continuous ethylene application in fruits at two maturation stages. In general, ethylene-induced peel degreening was considerably delayed and reduced in fruits of 'Tardivo', as well as the concomitant reduction of chlorophyll (Chl) and chloroplastic carotenoids, and the accumulation of chromoplastic carotenoids. Analysis of the expression of genes involved in Chl degradation, carotenoids, ABA, phenylpropanoids and ethylene biosynthesis revealed an impairment in the stimulation of most genes by ethylene in the peel of 'Tardivo' fruits with respect to 'Comune', especially after 5 days of ethylene application. Moreover, ethylene-induced expression of two ethylene receptor genes, ETR1 and ETR2, was also reduced in mutant fruits. Expression levels of two ethylene-responsive factors, ERF1 and ERF2, which were repressed by ethylene, were also impaired to a different extent, in fruits of both genotypes. Collectively, results suggested an altered sensitivity of the peel of 'Tardivo' to ethylene-induced physiological and molecular responses, including fruit degreening and coloration processes, which may be time-dependent since an early moderated reduction in the responses was followed by the latter inability to sustain ethylene action. These results support the involvement of ethylene in the regulation of at least some aspects of peel maturation in the non-climacteric citrus fruit. [ABSTRACT FROM AUTHOR]

Non-photochemical quenching (NPQ) plays a major role in photoprotection. Anastatica hierochuntica is an annual desert plant found in hot deserts. We compared A. hierochuntica to three other different species: Arabidopsis thaliana, Eutrema salsugineum and Helianthus annuus, which have different NPQ and photosynthetic capacities. Anastatica hierochuntica plants had very different induction kinetics of NPQ and, to a lesser extent, of photosystem II electron transport rate (PSII ETR), in comparison to all other plants species in the experiments. The major components of the unusual photosynthetic and photoprotective response in A. hierochuntica were: (1) Low NPQ at the beginning of the light period, at various light intensities and CO2 concentrations. The described low NPQ cannot be explained by low leaf absorbance or by low energy distribution to PSII, but was related to the de-epoxidation state of xanthophylls. (2) Relatively high PSII ETR at various CO2 concentrations in correlation with low NPQ. PSII ETR responded positively to the increase of CO2 concentrations. At low CO2 concentrations PSII ETR was mostly O2 dependent. At moderate and high CO2 concentrations the high PSII ETR in A. hierochuntica was accompanied by relatively high CO2 assimilation rates. We suggest that A. hierochuntica have an uncommon NPQ and PSII ETR response. These responses in A. hierochuntica might represent an adaptation to the short growing season of an annual desert plant. [ABSTRACT FROM AUTHOR]

Alternative oxidase (AOX) constitutes a nonenergy conserving branch of the mitochondrial electron transport chain. AOX activity may be important to avoid reactive oxygen species (ROS) generation by the chain, particularly during abiotic stress. We compared leaf AOX1a transcript and AOX protein amounts in wild‐type (WT) Nicotiana tabacum plants experiencing mild to severe drought. Mild to moderate drought resulted in a progressive and modest increase in AOX amount, accompanied by a progressive increased expression of different ROS‐scavenging components. Under these conditions, transgenic plants with suppressed AOX amount, due to an RNA interference construct, were not compromised in their ability to manage ROS load and prevent cellular damage. Severe drought stress, particularly when combined with increased irradiance, strongly increased AOX amount in WT tobacco and this coincided with an increase in total respiration and, despite further induction of ROS‐scavenging systems, some evidence of cellular damage. Under these severe conditions, plants lacking AOX suffered more cellular damage than WT and, at the most severe stage, were found to downregulate rather than upregulate the transcript level of several important ROS‐scavenging components. At this stage, WT plants could still recover rapidly after rewatering, but the recoverability of AOX knockdown plants was strongly compromised. [ABSTRACT FROM AUTHOR]

The significance of total glutathione content was investigated in two alpine plant species with highly differing antioxidative scavenging capacity. Leaves of Soldanella alpina and Ranunculus glacialis incubated for 48 h in the presence of buthionine-sulfoximine had 50% lower glutathione contents when compared with leaves incubated in water. The low leaf glutathione content was not compensated for by activation of other components involved in antioxidative protection or electron consumption. However, leaves with normal but not with low glutathione content increased their ascorbate content during high light (HL) treatment (S. alpina) or catalase activity at low temperature (LT) (R. glacialis), suggesting that the mere decline of the leaf glutathione content does not act as a signal to ameliorate antioxidative protection by alternative mechanisms. CO2-saturated oxygen evolution was not affected in glutathione-depleted leaves at various temperatures, except at 35°C, thereby increasing the high temperature (HT) sensitivity of both alpine species. Leaves with low and normal glutathione content were similarly resistant to photoinhibition and photodamage during HL treatment at ambient temperature in the presence and absence of paraquat or at LT. However, HL- and HT-induced photoinhibition increased in leaves with low compared to leaves with normal glutathione content, mainly because the recovery after heat inactivation was retarded in glutathione-depleted leaves. Differences in the response of photosystem II (PSII) activity and CO2-saturated photosynthesis suggest that PSII is not the primary target during HL inactivation at HT. The results are discussed with respect to the role of antioxidative protection as a safety valve for temperature extremes to which plants are not acclimated. [ABSTRACT FROM AUTHOR]

CO2 fixation in mosses saturates at moderate irradiances. Relative electron transport rate (RETR) inferred from chlorophyll fluorescence saturates at similar irradiance in shade species (e.g. Plagiomnium undulatum, Trichocolea tomentella), but many species of unshaded habitats (e.g. Andreaea rothii, Schistidium apocarpum, Sphagnum spp. and Frullania dilatata) show non-saturating RETR at high irradiance, with high non-photochemical quenching (NPQ). In P. undulatum and S. apocarpum, experiments in different gas mixtures showed O2 and CO2 as interchangeable electron sinks. Nitrogen + saturating CO2 gave high RETR and depressed NPQ. In S. apocarpum, glycolaldehyde (inhibiting photosynthesis and photorespiration) depressed RETR in air more at low than at high irradiance; in CO2-free air RETR was maintained at all irradiances. Non-saturating electron flow was not suppressed in ambient CO2 with 1% O2. The results indicate high capacity for oxygen photoreduction when CO2 assimilation is limited. Non-saturating light-dependent H2O2 production, insensitive to glycolaldehyde, suggests that electron transport is supported by oxygen photoreduction, perhaps via the Mehler-peroxidase reaction. Consistent with this, mosses were highly tolerant to paraquat, which generates superoxide at photosystem I (PSI). Protection against excess excitation energy in mosses involves high capacity for photosynthetic electron transport to oxygen and high NPQ, activated at high irradiance, alongside high reactive oxygen species (ROS) tolerance. [ABSTRACT FROM AUTHOR]

Reactive oxygen species (ROS) resulting from oxygen reduction, superoxide anion radical O2.- and hydrogen peroxide H 2O2 are very significant in the cell metabolism of aerobic organisms. They can be destructive and lead to apoptosis and they can also serve as signal molecules. In the light, chloroplasts are known to be one of the main sources of ROS in plants. However, the components involved in oxygen reduction and the detailed chemical mechanism are not yet well established. The present review describes the experimental data and theoretical considerations that implicate the plastoquinone pool (PQ-pool) in this process. The evidence indicates that the PQ-pool has a dual role: (1) the reduction of O2 by plastosemiquinone to superoxide and (2) the reduction of superoxide by plastohydroquinone to hydrogen peroxide. The second role represents not only the scavenging of superoxide, but also the generation of hydrogen peroxide as an important signaling molecule. The regulatory and protective functions of the PQ-pool are discussed in the context of these reactions. [ABSTRACT FROM AUTHOR]

Responses of males and females to salinity were studied in order to reveal sex-specific adaptation and evolution in Populus cathayana Rehd cuttings. This dioecious tree species plays an important role in maintaining ecological stability and providing commercial raw material in southwest China. Female and male cuttings of P. cathayana were treated for about 1 month with 0, 75 and 150 mM NaCl. Plant growth traits, gas exchange parameters, chlorophyll pigments, intrinsic water use efficiency (WUEi), membrane system injuries, ion transport and ultrastructural morphology were assessed and compared between sexes. Salt stress caused less negative effects on the dry matter accumulation, growth rate of height, growth rate of stem base diameter, total number of leaves and photosynthetic abilities in males than in females. Relative electrolyte leakage increased more in females than in males under salinity stress. Soil salinity reduced the amounts of leaf chlorophyll a, chlorophyll b and total chlorophyll, and the chlorophyll a/b ratio more in females than in males. WUEi decreased in both sexes under salinity. Regarding the ultrastructural morphology, thylakoid swelling in chloroplasts and degrading structures in mitochondria were more frequent in females than in males. Moreover, females exhibited significantly higher Na+ and Cl− concentrations in leaves and stems, but lower concentrations in roots than did males under salinity. In all, female cuttings of P. cathayana are more sensitive to salinity stress than males, which could be partially due to males having a better ability to restrain Na+ transport from roots to shoots than do females. [ABSTRACT FROM AUTHOR]

The function of cytochrome b559 (cyt b559) in photosystem II (PSII) was studied in a tobacco mutant in which the conserved phenylalanine at position 26 in the β-subunit was changed to serine. Young leaves of the mutant showed no significant difference in chloroplast ultra structure or in the amount and activity of PSII, while in mature leaves the size of the grana stacks and the amount of PSII were significantly reduced. Mature leaves of the mutant showed a higher susceptibility to photoinhibition and a higher production of singlet oxygen, as shown by spin trapping electron paramagnetic resonance (EPR) spectroscopy. Oxygen consumption and superoxide production were studied in thylakoid membranes in which the Mn cluster was removed to ensure that all the cyt b559 was present in its low potential form. In thylakoid membranes, from wild-type plants, the larger fraction of superoxide production was 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive. This type of superoxide formation was absent in thylakoid membranes from the mutant. The physiological importance of the plastoquinol oxidation by cyt b559 for photosynthesis is discussed. [ABSTRACT FROM AUTHOR]

All plants maintain a non-energy-conserving pathway of mitochondrial electron transport referred to as alternative oxidase (AOX) respiration. Here, we briefly review some of the most prevailing themes for the metabolic and physiological roles of this respiratory pathway. Many of these themes relate to the potential of AOX to provide metabolic homeostasis in response to fluctuating cellular conditions, such as is often seen during stress. We then review reverse genetic experiments that have been used to test these hypotheses. To date, such experiments have been limited to just two dicot species and have only targeted one member (a stress-induced member) of the AOX multigene family. Nonetheless, the experiments to date strongly reinforce the idea that AOX respiration is of particular importance during abiotic and biotic stress. Finally, we propose that another core role of AOX may be to modulate the strength of a stress-signaling pathway from the mitochondrion that controls cellular responses to stress. In this way, AOX could be acting to provide a degree of signaling homeostasis from the mitochondrion. This hypothesis may provide explanation for some of the disparate results seen in reverse genetic experiments regarding the impact of AOX on the reactive oxygen network and oxidative damage. [ABSTRACT FROM AUTHOR]

Growth rates of plant tissues depend on both the respiration rate and the efficiency with which carbon is incorporated into new structural biomass. Calorespirometric measurement of respiratory heat and CO2 rates, from which both efficiency and growth rate can be calculated, is a well established method for determining the effects of rapid temperature changes on the respiratory and growth properties of plant tissues. The effect of the alternative oxidase/cytochrome oxidase activity ratio on efficiency is calculated from first principles. Data on the temperature dependence of the substrate carbon conversion efficiency are tabulated. These data show that ε is maximum and approximately constant through the optimum growth temperature range and decreases rapidly as temperatures approach temperature limits to growth. The width of the maximum and the slopes of decreasing ε at high and low temperatures vary greatly with species, cultivars and accessions. [ABSTRACT FROM AUTHOR]

In addition to the conventional cytochrome c oxidase, mitochondria of all plants studied to date contain a second cyanide-resistant terminal oxidase or alternative oxidase (AOX). The AOX is located in the inner mitochondrial membrane and branches from the cytochrome pathway at the level of the quinone pool. It is non-protonmotive and couples the oxidation of ubiquinone to the reduction of oxygen to water. For many years, the AOX was considered to be confined to plants, fungi and a small number of protists. Recently, it has become apparent that the AOX occurs in wide range of organisms including prokaryotes and a moderate number of animal species. In this paper, we provide an overview of general features and current knowledge available about the AOX with emphasis on structure, the active site and quinone-binding site. Characterisation of the AOX has advanced considerably over recent years with information emerging about the role of the protein, regulatory regions and functional sites. The large number of sequences available is now enabling us to obtain a clearer picture of evolutionary origins and diversity. [ABSTRACT FROM AUTHOR]

The gymnosperm Pinus pinea L. (stone pine) is a typical Mediterranean pine used for nuts and timber production, and as an ornamental around the world. Pine genomes are large in comparison to other species. The hypothesis that retrotransposons, such as gymny, made a large contribution to this alteration in genome size was recently confirmed. However, P. pinea is unique in other various aspects. P. pinea demonstrates a different pattern of gymny organization than other Pinus subgenera. Additionally, P. pinea has a highly recalcitrant behaviour in relation to standard conifer protocols for the induction of somatic embryogenesis or rooting. Because such types of cell reprogramming can be explained as a reaction of plant cells to external stress, it is of special interest to study sequence peculiarities in stress-inducible genes, such as the alternative oxidase ( AOX). This is the first report containing molecular evidence for the existence of AOX in gymnosperms at the genetic level. P. pinea AOXs were isolated by a polymerase chain reaction (PCR) approach and three genes were identified. Two of the genes belong to the AOX1 subfamily and one belongs to the AOX2 subfamily. The existence of both AOX subfamilies in gymnosperms is reported here for the first time. This discovery supports the hypothesis that AOX1 and AOX2 subfamilies arose prior to the separation of gymnosperms and angiosperms, and indicates that the AOX2 is absent in monocots because of subsequent gene loss events. Polymorphic P. pinea AOX1 sequences from a selected genetic clone are presented indicating non-allelic, non-synonymous and synonymous translation products. [ABSTRACT FROM AUTHOR]

Olive ( Olea europaea L.) trees are mainly propagated by adventitious rooting of semi-hardwood cuttings. However, efficient commercial propagation of valuable olive tree cultivars or landraces by semi-hardwood cuttings can often be restricted by a low rooting capacity. We hypothesize that root induction is a plant cell reaction linked to oxidative stress and that activity of stress-induced alternative oxidase (AOX) is importantly involved in adventitious rooting. To identify AOX as a source for potential functional marker sequences that may assist tree breeding, genetic variability has to be demonstrated that can affect gene regulation. The paper presents an applied, multidisciplinary research approach demonstrating first indications of an important relationship between AOX activity and differential adventitious rooting in semi-hardwood cuttings. Root induction in the easy-to-root Portuguese cultivar ‘Cobrançosa’ could be significantly reduced by treatment with salicyl-hydroxamic acid, an inhibitor of AOX activity. On the contrary, treatment with H2O2 or pyruvate, both known to induce AOX activity, increased the degree of rooting. Recently, identification of several O. europaea (Oe) AOX gene sequences has been reported from our group. Here we present for the first time partial sequences of OeAOX2. To search for polymorphisms inside of OeAOX genes, partial OeAOX2 sequences from the cultivars ‘Galega vulgar’, ‘Cobrançosa’ and ‘Picual’ were cloned from genomic DNA and cDNA, including exon, intron and 3′-untranslated regions (3′-UTRs) sequences. The data revealed polymorphic sites in several regions of OeAOX2. The 3′-UTR was the most important source for polymorphisms showing 5.7% of variability. Variability in the exon region accounted 3.4 and 2% in the intron. Further, analysis performed at the cDNA from microshoots of ‘Galega vulgar’ revealed transcript length variation for the 3′-UTR of OeAOX2 ranging between 76 and 301 bp. The identified polymorphisms and 3′-UTR length variation can be explored in future studies for effects on gene regulation and a potential linkage to olive rooting phenotypes in view of marker-assisted plant selection. [ABSTRACT FROM AUTHOR]

Heat stress causes inhibition of photosynthetic CO2 assimilation, affects light photosynthetic reactions and accelerates alternative pathways of plastoquinone pool reduction (APPR). We have studied all these heat-sensitive processes after preheating to a broad range of physiological temperatures (24–46°C) to explore a role of these alternative pathways during heat stress. Primarily, the effective quantum yield of PSII photochemistry was reduced (at 40°C). This PSII downregulation was accompanied by the stimulation of APPR and preceded reduction of photosynthetic CO2 assimilation by 2°; it occurred after preheating at 42°C because of inhibition in Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) activation process. Thus, we suggest that the heat-induced stimulation of APPR is not associated with the heat-induced inhibition of Calvin cycle as it was reported for other types of stresses. A possible role of APPR in the compensation of PSII downregulation is briefly discussed. [ABSTRACT FROM AUTHOR]

Photoreduction of oxygen by the photosynthetic electron transport chain has been suggested to be an important process in protecting leaves from excess light under conditions of stress; however, there is little evidence that this process occurs significantly except when plants are exposed to conditions outside their normal tolerance range. We have examined the oxygen dependency of photosynthetic electron transport in the two vascular plants found growing in Antarctica – Colobanthus quitensis and Deschampsia antarctica. Photosynthetic electron transport in C. quitensis is insensitive to changes in oxygen concentration under non-photorespiratory conditions, indicating that electron transport to oxygen is negligible; however, it has a substantial capacity for non-photochemical quenching (NPQ) of chlorophyll fluorescence. In contrast, D. antarctica has up to 30% of its photosynthetic electron transport being linked to oxygen, but has a substantially lower capacity for NPQ. Thus, these plants rely on contrasting photoprotective mechanisms to cope with the Antarctic environment. Both plants seem to use cyclic electron flow associated with PSI, however, this is activated at a lower irradiance in C. quitensis than in D. antarctica. [ABSTRACT FROM AUTHOR]

Variegation in the immutans (im) mutant of Arabidopsis is induced by a nuclear recessive gene. The white leaf sectors of im contain abnormal plastids lacking pigments and organized lamellae, whereas the green leaf sectors possess normal-appearing chloroplasts. IMMUTANS codes for a thylakoid membrane terminal oxidase that functions as a safety valve to dissipate excess energy. Previous studies have shown that the green sectors of im, regardless of illumination conditions, have anatomical adaptations that are reminiscent of acclimation to high-light stress. It has been suggested that these adaptations provide a means of enhancing photosynthesis to feed the white sectors and maximize plant growth. We have utilized Chl fluorescence imaging to better understand these compensatory mechanisms using, as our experimental material, im leaves with predominantly green (img) or predominantly white (imw) tissues. The samples were examined under conditions of normal growth or high-light stress (photoinhibition). Steady-state fluorescence quenching revealed that the green sectors of the imw leaves had lower levels of 1 − qp than the img leaves, and that this was accompanied by increased electron transport rates. In response to short-term high-light exposure, the green sectors of the imw leaves displayed enhanced non-photochemical quenching (NPQ), which correlated with increased xanthophyll pool sizes and increased amounts of several different Lhcb polypeptides and the PsbS protein. In summary, our data show that, compared with primarily green leaves (img), the green sectors of predominantly white leaves (imw) have elevated rates of electron transport and an enhanced NPQ capacity. We conclude that, in the absence of IM, green sectors develop morphological and biochemical adaptations that allow them to maximize photosynthesis to feed the white sectors, and to protect against photodamage. [ABSTRACT FROM AUTHOR]

An Ndh-deficient mutant of tobacco (Nicotiana tabacum cv. Petit Havana) was prepared by disrupting the ndhF gene in a transplastomic approach. The mutant (ΔndhF) showed 10% of the Ndh complex activity (EC 1.6.5.3) and 8% of the NDH-F polypeptide of that of non-transformed plants. However, in ΔndhF, NDH-A, another Ndh polypeptide, was still present at 50% of the level in non-transformed plants. ΔndhF tobacco showed higher sensitivity than non-transformed plants to photo-oxidative stress (as judged by chlorophyll bleaching) caused by increased light intensity and paraquat applications. These photo-oxidative treatments increased the amount and activity of the Ndh complex, thylakoid peroxidase, post-illumination chlorophyll fluorescence and non-photochemical quenching (NPQ) of chlorophyll fluorescence in non-transformed but not in ΔndhF tobacco. Highly stressed non-transformed plants showed a rapid post-rise decline of chlorophyll fluorescence, probably indicating a re-oxidation of reduced plastoquinone. The results indicate that, in normal plants, the Ndh complex and thylakoid peroxidase (EC 1.11.1.7) provide and remove electrons, respectively, to balance the redox level of the intermediates of cyclic electron transport. In this way, they optimize the generation of the transmembrane H+ gradient of thylakoids and, as a consequence, increase the NPQ and the protection against photo-oxidative stress. [ABSTRACT FROM AUTHOR]

Variegation mutants offer excellent opportunities to study interactions between the nucleus-cytoplasm, the chloroplast, and the mitochondrion. Variegation in the immutans ( im) mutant of Arabidopsis is induced by a nuclear recessive gene and the extent of variegation can be modulated by light and temperature. Whereas the green sectors have morphologically normal chloroplasts, the white sectors are devoid of pigments and accumulate a colourless carotenoid, phytoene. The green sectors are hypothesized to arise from cells that have avoided irreversible photooxidative damage whereas the white sectors originate from cells that are photooxidized. Cloning of the IMMUTANS ( IM) gene has revealed that IMMUTANS (IM) is a plastid homologue of the mitochondrial alternative oxidase. This finding suggested a model in which IM functions as a redox component of the phytoene desaturation pathway, which requires phytoene desaturase activity. Consistent with this idea, IM has quinol oxidase activity in vitro. Recent studies have revealed that IM plays a more global role in plastid metabolism. For example, it appears to be the elusive terminal oxidase of chlororespiration and also functions as a light stress protein. [ABSTRACT FROM AUTHOR]

The redox-state is a critical determinate of cell function, and any major imbalances can cause severe damage or death. The cellular redox status therefore needs to be sensed and modulated before such imbalances occur. Various redox-active components are involved in these processes, including thioredoxins, glutaredoxins and other thiol/disulphide-containing proteins. The cellular reactions for cytoprotection and for signalling are integrated with physiological redox-reactions in photosynthesis, assimilation and respiration. They also determine the developmental fate of the cell and finally decide on proliferation or cell death. An international workshop on redox regulation, organized by the research initiative FOR 387 of the Deutsche Forschungsgemeinschaft, was held in Bielefeld, Germany in 2002. A selection of articles originating from the meeting is printed in this issue of Physiologia Plantarum. [ABSTRACT FROM AUTHOR]