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Summary: The coordinated interspecific variation in leaf traits and leaf lifespan is known as the leaf economic spectrum (LES). The limitation of CO2 diffusion to chloroplasts within the lamina is significant in C3 photosynthesis, resulting in a shortage of CO2 for Rubisco. Although Rubisco CO2/O2 specificity (SC/O) should be adaptively adjusted in response to the interspecific variation in CO2 concentrations [CO2] associated with Rubisco, SC/O variations across species along the LES remain unknown.We investigated the coordination among leaf traits, including SC/O, CO2 conductance, leaf protein content, and leaf mass area, across 23 woody C3 species coexisting on an oceanic island through phylogenetic correlation analyses.A high SC/O indicates a high CO2 specificity of Rubisco. SC/O was negatively correlated with [CO2] at Rubisco and total CO2 conductance within lamina, while it was positively correlated with leaf protein across species, regardless of phylogenetic constraint. A simulation analysis shows that the optimal SC/O for maximizing photosynthesis depends on both [CO2] at Rubisco sites and leaf protein per unit leaf area.SC/O is a key parameter along the LES axis and is crucial for maximizing photosynthesis across species and the adaptation of woody plants. [ABSTRACT FROM AUTHOR]

SUMMARY Photosynthetic and chemosynthetic extremophiles have evolved adaptations to thrive in challenging environments by finely adjusting their metabolic pathways through evolutionary processes. A prime adaptation target to allow autotrophy in extreme conditions is the enzyme Rubisco, which plays a central role in the conversion of inorganic to organic carbon. Here, we present an extensive compilation of Rubisco kinetic traits from a wide range of species of bacteria, archaea, algae, and plants, sorted by phylogenetic group, Rubisco type, and extremophile type. Our results show that Rubisco kinetics for the few extremophile organisms reported up to date are placed at the margins of the enzyme's natural variability. Form ID Rubisco from thermoacidophile rhodophytes and form IB Rubisco from halophile terrestrial plants exhibit higher specificity and affinity for CO2 than their non‐extremophilic counterparts, as well as higher carboxylation efficiency, whereas form ID Rubisco from psychrophile organisms possess lower affinity for O2. Additionally, form IB Rubisco from thermophile cyanobacteria shows enhanced CO2 specificity when compared to form IB non‐extremophilic cyanobacteria. Overall, these findings highlight the unique characteristics of extremophile Rubisco enzymes and provide useful clues to guide next explorations aimed at finding more efficient Rubiscos. [ABSTRACT FROM AUTHOR]

Autotrophy is the basis for complex life on Earth. Central to this process is rubisco—the enzyme that catalyzes almost all carbon fixation on the planet. Yet, with only a small fraction of rubisco diversity kinetically characterized so far, the underlying biological factors driving the evolution of fast rubiscos in nature remain unclear. We conducted a high-throughput kinetic characterization of over 100 bacterial form I rubiscos, the most ubiquitous group of rubisco sequences in nature, to uncover the determinants of rubisco's carboxylation velocity. We show that the presence of a carboxysome CO2 concentrating mechanism correlates with faster rubiscos with a median fivefold higher rate. In contrast to prior studies, we find that rubiscos originating from α-cyanobacteria exhibit the highest carboxylation rates among form I enzymes (≈10 s−1 median versus <7 s−1 in other groups). Our study systematically reveals biological and environmental properties associated with kinetic variation across rubiscos from nature. Synopsis: Form I rubisco, known for its slow kinetics in plants and algae, exhibits a great unexplored diversity in autotrophic bacteria. This article represents the first large-scale survey of bacterial form I rubisco kinetics and reveals unifying features of fast carboxylating rubiscos. Over 100 homologs were systematically screened, spanning the wide genetic diversity of form I rubisco enzymes across a variety of ecological niches and metabolic profiles. Phototrophy and carboxysome association are correlated with fast-carboxylating rubiscos. α-cyanobacteria emerges as the bacterial clade expressing the fastest form I rubiscos on Earth. Bacterial rubisco enzymes associated with carboxysomes have the fastest CO2-fixing rates. [ABSTRACT FROM AUTHOR]

Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCO) is the enzyme responsible for the first step of carbon dioxide (CO2) fixation in plants, which proceeds via the carboxylation of ribulose 1,5‐biphosphate. Because of the enormous importance of this reaction in agriculture and the environment, there is considerable interest in the mechanism of fixation of CO2 by RuBisCO. Here, a serial synchrotron crystallography structure of spinach RuBisCO is reported at 2.3 Å resolution. This structure is consistent with earlier single‐crystal X‐ray structures of this enzyme and the results are a good starting point for a further push towards time‐resolved serial synchrotron crystallography in order to better understand the mechanism of the reaction. [ABSTRACT FROM AUTHOR]

SUMMARY: The introduction of the carboxysome‐based CO2 concentrating mechanism (CCM) into crop plants has been modelled to significantly increase crop yields. This projection serves as motivation for pursuing this strategy to contribute to global food security. The successful implementation of this engineering challenge is reliant upon the transfer of a microcompartment that encapsulates cyanobacterial Rubisco, known as the carboxysome, alongside active bicarbonate transporters. To date, significant progress has been achieved with respect to understanding various aspects of the cyanobacterial CCM, and more recently, different components of the carboxysome have been successfully introduced into plant chloroplasts. In this Perspective piece, we summarise recent findings and offer new research avenues that will accelerate research in this field to ultimately and successfully introduce the carboxysome into crop plants for increased crop yields. Significance Statement: The efficacy of the cyanobacterial CO2‐concentrating mechanism (CCM) relies on its ability to concentrate CO2 around the Rubisco enzyme, which is natively encapsulated within a carboxysome. As our understanding of carboxysome biogenesis and functionality grows, there is optimism that this microcompartment and the cyanobacterial CCM as a whole, can contribute to addressing global food security. [ABSTRACT FROM AUTHOR]

Summary: The kinetic properties of Rubisco, the most important carbon‐fixing enzyme, have been assessed in a small fraction of the estimated existing biodiversity of photosynthetic organisms. Until recently, one of the most significant gaps of knowledge in Rubisco kinetics was marine macrophytes, an ecologically relevant group including brown (Ochrophyta), red (Rhodophyta) and green (Chlorophyta) macroalgae and seagrasses (Streptophyta). These organisms express various Rubisco types and predominantly possess CO2‐concentrating mechanisms (CCMs), which facilitate the use of bicarbonate for photosynthesis. Since bicarbonate is the most abundant form of dissolved inorganic carbon in seawater, CCMs allow marine macrophytes to overcome the slow gas diffusion and low CO2 availability in this environment. The present review aims to compile and integrate recent findings on the biochemical diversity of Rubisco and CCMs in the main groups of marine macrophytes. The Rubisco kinetic data provided demonstrate a more relaxed relationship among catalytic parameters than previously reported, uncovering a variability in Rubisco catalysis that has been hidden by a bias in the literature towards terrestrial vascular plants. The compiled data indicate the existence of convergent evolution between Rubisco and biophysical CCMs across the polyphyletic groups of marine macrophytes and suggest a potential role for oxygen in shaping such relationship. [ABSTRACT FROM AUTHOR]

SUMMARY: Plants grown under low magnesium (Mg) soils are highly susceptible to encountering light intensities that exceed the capacity of photosynthesis (A), leading to a depression of photosynthetic efficiency and eventually to photooxidation (i.e., leaf chlorosis). Yet, it remains unclear which processes play a key role in limiting the photosynthetic energy utilization of Mg‐deficient leaves, and whether the plasticity of A in acclimation to irradiance could have cross‐talk with Mg, hence accelerating or mitigating the photodamage. We investigated the light acclimation responses of rapeseed (Brassica napus) grown under low‐ and adequate‐Mg conditions. Magnesium deficiency considerably decreased rapeseed growth and leaf A, to a greater extent under high than under low light, which is associated with higher level of superoxide anion radical and more severe leaf chlorosis. This difference was mainly attributable to a greater depression in dark reaction under high light, with a higher Rubisco fallover and a more limited mesophyll conductance to CO2 (gm). Plants grown under high irradiance enhanced the content and activity of Rubisco and gm to optimally utilize more light energy absorbed. However, Mg deficiency could not fulfill the need to activate the higher level of Rubisco and Rubisco activase in leaves of high‐light‐grown plants, leading to lower Rubisco activation and carboxylation rate. Additionally, Mg‐deficient leaves under high light invested more carbon per leaf area to construct a compact leaf structure with smaller intercellular airspaces, lower surface area of chloroplast exposed to intercellular airspaces, and CO2 diffusion conductance through cytosol. These caused a more severe decrease in within‐leaf CO2 diffusion rate and substrate availability. Taken together, plant plasticity helps to improve photosynthetic energy utilization under high light but aggravates the photooxidative damage once the Mg nutrition becomes insufficient. Significance Statement: Plants grown under low Mg soils are highly susceptible to excessive light intensity, resulting in photooxidation and subsequent leaf chlorosis. The results indicated that plant plasticity, such as increasing Rubisco content and gm, helps to improve photosynthetic energy utilization under high light but aggravates the photooxidative damage once the Mg nutrition becomes insufficient. [ABSTRACT FROM AUTHOR]

Summary: Efficient plant acclimation to changing environmental conditions relies on fast adjustments of the transcriptome, proteome, and metabolome. Regulation of enzyme activity depends on the activity of specific chaperones, chemical post‐translational modifications (PTMs) of amino acid residues, and changes in the cellular and organellar microenvironment. Central to carbon assimilation, and thus plant growth and yield, Rubisco activity is regulated by its chaperone Rubisco activase (Rca) and by adjustments in the chloroplast stroma environment. Focused on crops, this review highlights the main PTMs and stromal ions and metabolites affecting Rubisco and Rca in response to environmental stimuli. Rca isoforms differ in regulatory properties and heat sensitivity, with expression changing according to the surrounding environment. Much of the physiological relevance of Rubisco and Rca PTMs is still poorly understood, though some PTMs have been associated with Rubisco regulation in response to stress. Ion and metabolite concentrations in the chloroplast change in response to variations in light and temperature. Some of these changes promote Rubisco activation while others inhibit activation, deactivate the enzyme, or change the rates of catalysis. Understanding these regulatory mechanisms will aid the development of strategies to improve carbon fixation by Rubisco under rapidly changing environments as experienced by crop plants. [ABSTRACT FROM AUTHOR]

The functionality of all metabolic processes in chloroplasts depends on a balanced integration of nuclear‐ and chloroplast‐encoded polypeptides into the plastid's proteome. The chloroplast chaperonin machinery is an essential player in chloroplast protein folding under ambient and stressful conditions, with a more intricate structure and subunit composition compared to the orthologous GroEL/ES chaperonin of Escherichia coli. However, its exact role in chloroplasts remains obscure, mainly because of very limited knowledge about the interactors. We employed the competition immunoprecipitation method for the identification of the chaperonin's interactors in Chlamydomonas reinhardtii. Co‐immunoprecipitation of the target complex in the presence of increasing amounts of isotope‐labelled competitor epitope and subsequent mass spectrometry analysis specifically allowed to distinguish true interactors from unspecifically co‐precipitated proteins. Besides known substrates such as RbcL and the expected complex partners, we revealed numerous new interactors with high confidence. Proteins that qualify as putative substrate proteins differ from bulk chloroplast proteins by a higher content of beta‐sheets, lower alpha‐helical conformation and increased aggregation propensity. Immunoprecipitations targeted against a subunit of the co‐chaperonin lid revealed the ClpP protease as a specific partner complex, pointing to a close collaboration of these machineries to maintain protein homeostasis in the chloroplast. Summary statement: We applied competition co‐immunoprecipitation to reveal the interactome of the chloroplast Cpn60 chaperonin in Chlamydomonas reinhardtii cells under ambient conditions and heat and characterised the physico‐chemical features of the proteins that are preferably bound by the folding apparatus. [ABSTRACT FROM AUTHOR]

A coordinated increase in the photosynthetic rate (A) and photosynthetic nitrogen use efficiency (PNUE) is an effective strategy for improving crop yield and nitrogen (N) utilization efficiency. PNUE tends to decrease with increasing N levels, but there are natural variations. Consequently, leaf functional N partitioning in Brassica napus genotypes under different N rates was measured to explore the optimized N allocation model for synchronously increasing A and PNUE values. The results showed that genotypes whose PNUE increased with increasing N supply (PNUE‐I) produced an approximate A value with a relatively low leaf N content, owing to reduced storage N (Nstore) and close photosynthetic N (Npsn) content. Partial least squares path modeling showed that A was dominated by the Npsn content, and PNUE was directly influenced by A and Nstore. The A value increased with the Npsn content until the Npsn content exceeded the threshold value. The boundary line of PNUE varied with the Npsn and Nstore proportions, indicating that the optimum Npsn and Nstore proportions were 51.6% and 40.3%, respectively. The Nstore proportion of PNUE‐I was closer to the thresholds and benefited from lower increments in Rubisco content and nonprotein form storage N content with improved N supply. Optimized Nstore and Npsn trade‐off by regulating increments in Nstore content with increased N supply, thereby promoting coordinated increases in A and PNUE. [ABSTRACT FROM AUTHOR]

We analysed whether Phacelia secunda populations from different elevations exhibit intrinsic traits associated with diffusive and biochemical components of photosynthesis, and if they differ in acclimation of photosynthesis to warmer temperatures. We hypothesized that P. secunda will have similar photosynthetic performance regardless of altitudinal provenance and that plants from high elevations will have a lower photosynthetic acclimation capacity to higher temperature than plants from low elevations.Plants from 1600, 2800 and 3600 m a.s.l. in the central Chilean Andes were collected and grown under two temperature regimes (20/16 °C and 30/26 °C day/night). The following photosynthetic traits were measured in each plant for the two temperature regimes: AN, gs, gm, Jmax, Vcmax, Rubisco carboxylation kcatc.Under a common growth environment, plants from the highest elevation had slightly lower CO2 assimilation rates compared to lower elevation plants. While diffusive components of photosynthesis increased with elevation provenance, the biochemical component decreased, suggesting compensation that explains the similar rates of photosynthesis among elevation provenances. Plants from high elevations had lower photosynthetic acclimation to warmer temperatures compared to plants from lower elevations, and these responses were related to elevational changes in diffusional and biochemical components of photosynthesis.Plants of P. secunda from different elevations maintain photosynthetic traits when grown in a common environment, suggesting low plasticity to respond to future climate changes. The fact that high elevation plants had lower photosynthetic acclimation to warmer temperature suggests higher susceptibility to increases in temperature associated with global warming. [ABSTRACT FROM AUTHOR]

Leaf photosynthetic nitrogen‐use efficiency (PNUE) diversified significantly among C3 species. To date, the morpho‐physiological mechanisms and interrelationships shaping PNUE on an evolutionary time scale remain unclear. In this study, we assembled a comprehensive matrix of leaf morpho‐anatomical and physiological traits for 679 C3 species, ranging from bryophytes to angiosperms, to comprehend the complexity of interrelationships underpinning PNUE variations. We discovered that leaf mass per area (LMA), mesophyll cell wall thickness (Tcwm), Rubisco N allocation fraction (PR), and mesophyll conductance (gm) together explained 83% of PNUE variations, with PR and gm accounting for 65% of those variations. However, the PR effects were species‐dependent on gm, meaning the contribution of PR on PNUE was substantially significant in high‐gm species compared to low‐gm species. Standard major axis (SMA) and path analyses revealed a weak correlation between PNUE and LMA (r2 = 0.1), while the SMA correlation for PNUE–Tcwm was robust (r2 = 0.61). PR was inversely related to Tcwm, paralleling the relationship between gm and Tcwm, resulting in the internal CO2 drawdown being only weakly proportional to Tcwm. The coordination of PR and gm in relation to Tcwm constrains PNUE during the course of evolution. Summary Statement: Leaf photosynthetic nitrogen‐use efficiency (PNUE) varies significantly among major phylogenetic clades and species. Numerous studies have examined the variations in PNUE among major plant groups, attributing these variations to leaf habits or height‐related economic spectra, microhabitat acclimations, or unknown fundamental differences. Results from a comprehensive matrix of leaf morpho‐anatomical and physiological traits for 679 C3 species discovered that cell wall thickness (Tmcw) has phylogenetically consistent effects on PNUE of terrestrial plants. [ABSTRACT FROM AUTHOR]

This document is a profile of Alistair McCormick, a Professor of Plant Engineering Biology at the University of Edinburgh. McCormick's interest in plant science began during his undergraduate biology degree, where he had the opportunity to engage in both outdoor and indoor plant science experiences. He decided to pursue a career in research after an internship at the South African Sugarcane Research Institute, where he focused on understanding the source-sink relationship in sugarcane. McCormick is motivated by the processes of planning and setting things up, as well as the collaborative nature of working with others. He considers his previous supervisors as role models and mentors, and his favorite New Phytologist paper is his own first-author paper. McCormick's research focuses on photosynthesis and growth in plants, algae, and cyanobacteria, with a particular interest in improving the efficiency of photosynthetic carbon capture and using synthetic biology approaches to generate high-value secondary products. [Extracted from the article]

Carbon isotope biosignatures preserved in the Precambrian geologic record are primarily interpreted to reflect ancient cyanobacterial carbon fixation catalyzed by Form I RuBisCO enzymes. The average range of isotopic biosignatures generally follows that produced by extant cyanobacteria. However, this observation is difficult to reconcile with several environmental (e.g., temperature, pH, and CO2 concentrations), molecular, and physiological factors that likely would have differed during the Precambrian and can produce fractionation variability in contemporary organisms that meets or exceeds that observed in the geologic record. To test a specific range of genetic and environmental factors that may impact ancient carbon isotope biosignatures, we engineered a mutant strain of the model cyanobacterium Synechococcus elongatus PCC 7942 that overexpresses RuBisCO across varying atmospheric CO2 concentrations. We hypothesized that changes in RuBisCO expression would impact the net rates of intracellular CO2 fixation versus CO2 supply, and thus whole‐cell carbon isotope discrimination. In particular, we investigated the impacts of RuBisCO overexpression under changing CO2 concentrations on both carbon isotope biosignatures and cyanobacterial physiology, including cell growth and oxygen evolution rates. We found that an increased pool of active RuBisCO does not significantly affect the 13C/12C isotopic discrimination (εp) at all tested CO2 concentrations, yielding εp of ≈ 23‰ for both wild‐type and mutant strains at elevated CO2. We therefore suggest that expected variation in cyanobacterial RuBisCO expression patterns should not confound carbon isotope biosignature interpretation. A deeper understanding of environmental, evolutionary, and intracellular factors that impact cyanobacterial physiology and isotope discrimination is crucial for reconciling microbially driven carbon biosignatures with those preserved in the geologic record. [ABSTRACT FROM AUTHOR]

The crystal structure of the complex of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) from Archaeoglobus fulgidus (afRubisco) with its products 3PGAs has been determined to a resolution of 1.7 Å and is of the closed form. Type III Rubiscos such as afRubisco have 18 out of the 19 essential amino acid residues of canonical Rubisco; the 19th is Tyr rather than Phe. Superposition with the structure of a complex of the similar tkRubisco with the six‐carbon intermediate analog 2CABP shows the same conformation of the 19 residues except for Glu46 and Thr51. Glu46 adopts a unique conformation different from that in other Rubiscos and makes two H‐bonds with the ligand 3PGA. Similar to other closed state Rubiscos, the backbone of Thr51 is rotated and the side chain makes an H‐bond with the ligand 3PGA. Two product 3PGA molecules are bound at the active site, overlapping well with the 2CABP of tkRubisco/2CABP. The positions of the P1 and P2 phosphate groups differ by 0.4 and 0.53 Å, respectively, between 2CABP and the two 3PGAs. This afRubisco/3PGA complex mimics an intermediate stage of the carboxylation reaction which occurs after the production of the two 3PGA products but before the reopening of the active site. The stability of this complex suggests that the Rubisco active site will not reopen before both 3PGA products are formed. [ABSTRACT FROM AUTHOR]

SUMMARY: Crop photosynthesis (A) and productivity are often limited by a combination of nutrient stresses, such that changes in the availability of one nutrient may affect the availability of another nutrient, in turn influencing A. In this study, we examined the synergistic effects of phosphorus (P) and potassium (K) on leaf A in a nutrient amendment experiment, in which P and K were added individually or in combination to Brassica napus grown under P and K co‐limitation. The data revealed that the addition of P gradually removed the dominant limiting factor (i.e. the limited availability of P) and improved leaf A. Strikingly, the addition of K synergistically improved the overall uptake of P, mainly by boosting plant growth, and compensated for the physiological demand for P by prioritizing investment in metabolic pools of P (P‐containing metabolites and inorganic phosphate, Pi). The enlarged pool of metabolically active P was partially associated with the upregulation of Pi regeneration through release from triose phosphates rather than replacement of P‐containing lipids. This process mitigated P restrictions on A by maintaining the ATP/NADPH and NADPH/NADP+ ratios and increasing the content and activity of Rubisco. Our findings demonstrate that sufficient K increased Pi‐limited A by enhancing metabolic P fractions and Rubisco activity. Thus, ionic synergism may be exploited to mitigate nutrient‐limiting factors to improve crop productivity. Significance Statement: The uptake and utilization of phosphorus (P) and potassium (K) by Brassica napus were found to be closely linked and mutually influential. Notably, the results provide a physiological overview of the role of K in cellular P investment to mitigate Pi‐limited photosynthesis under P and K co‐limitation, which demonstrates the synergistic effects of K in enlarging the metabolic pool of P and ensuring the maintenance of photosynthesis. [ABSTRACT FROM AUTHOR]

Plant‐based proteins are gaining a lot of attention for their health benefits and are considered as an alternative to animal proteins for developing sustainable food systems. Against the backdrop, ensuring a healthy diet supplemented with good quality protein will be a massive responsibility of governments across the globe. Increasing the yield of food crops has its limitations, including low acceptance of genetically modified crops, land availability for cultivation, and the need for large quantities of agrochemicals. It necessitates the sensible use of existing resources and farm output to derive the proteins. On average, the protein content of plant leaves is similar to that of milk, which can be efficiently tapped for food applications across the globe. There has been limited research on utilizing plant leaf proteins for food product development over the years, which has not been fruitful. However, the current global food production scenario has pushed some leading economies to reconsider the scope of plant leaf proteins with dedicated efforts. It is evident from installing pilot‐scale demonstration plants for protein extraction from agro‐food residues to cater to the protein demand with product formulation. The present study thoroughly reviews the opportunities and challenges linked to the production of plant leaf proteins, including its nutritional aspects, extraction and purification strategies, anti‐nutritional factors, functional and sensory properties in food product development, and finally, its impact on the environment. Practical Application: Plant leaf proteins are one of the sustainable and alternative source of proteins. It can be produced in most of the agroclimatic conditions without requiring much agricultural inputs. It's functional properties are unique and finds application in novel food product formulations. [ABSTRACT FROM AUTHOR]

Calcium oxalate (CaOx) crystals in plants are formed in crystal idioblasts cells and have specific geometric shapes. Their proposed functions include calcium homeostasis and carbon source, among others. Amaranth is a plant that presents high tolerance to abiotic stresses and accumulates considerable amounts of CaOx crystals; however, few studies have focused on characterizing the crystals ultrastructure and none is related to identifying proteins bound to them. This information is of great interest to understand the mechanisms related to CaOx crystal formation and to support their proposed functions. Thus, this work aimed to characterize CaOx crystals in amaranth leaves. Crystals were purified and the proteins bound to them were isolated and identified by nLC–MS/MS. Leaf sections were analyzed by light and electron microscopy. The identified proteins were related to the chloroplast such as ATPb synthase, RuBisCO large subunit, and cell wall‐related proteins, which were validated by immunohistochemistry and immunogold labeling. In addition, it was observed that CaOx crystal idioblasts were formed from parenchyma cells associated with mesophyll and veins, in which the thylakoid membranes of degraded chloroplasts turned into crystal chambers. These results significantly advance our understanding of the mechanisms of CaOx crystal formation and the potential function as an alternative carbon source in leaves. [ABSTRACT FROM AUTHOR]

Summary: Oxygenic photosynthesis evolved in cyanobacteria, primary producers of striking ecological importance. Like plants, cyanobacteria use the Calvin–Benson–Bassham cycle for CO2 fixation, fuelled by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCO). In a competitive reaction this enzyme also fixes O2 which makes it rather ineffective. To mitigate this problem, cyanobacteria evolved a CO2 concentrating mechanism (CCM) to pool CO2 in the vicinity of RuBisCO. However, the regulation of these carbon (C) assimilatory systems is understood only partially.Using the model Synechocystis sp. PCC 6803 we characterized an essential LysR‐type transcriptional regulator encoded by gene sll0998. Transcript profiling of a knockdown mutant revealed diminished expression of several genes involved in C acquisition, including rbcLXS, sbtA and ccmKL encoding RuBisCO and parts of the CCM, respectively.We demonstrate that the Sll0998 protein binds the rbcL promoter and acts as a RuBisCO regulator (RbcR). We propose ATTA(G/A)‐N5‐(C/T)TAAT as the binding motif consensus. Our data validate RbcR as a regulator of inorganic C assimilation and define the regulon controlled by it.Biological CO2 fixation can sustain efforts to reduce its atmospheric concentrations and is fundamental for the light‐driven production of chemicals directly from CO2. Information about the involved regulatory and physiological processes is crucial to engineer cyanobacterial cell factories. [ABSTRACT FROM AUTHOR]

SUMMARY: The green alga Chlamydomonas reinhardtii is one of the most studied microorganisms in photosynthesis research and for biofuel production. A detailed understanding of the dynamic regulation of its carbon metabolism is therefore crucial for metabolic engineering. Post‐translational modifications can act as molecular switches for the control of protein function. Acetylation of the ɛ‐amino group of lysine residues is a dynamic modification on proteins across organisms from all kingdoms. Here, we performed mass spectrometry‐based profiling of proteome and lysine acetylome dynamics in Chlamydomonas under varying growth conditions. Chlamydomonas liquid cultures were transferred from mixotrophic (light and acetate as carbon source) to heterotrophic (dark and acetate) or photoautotrophic (light only) growth conditions for 30 h before harvest. In total, 5863 protein groups and 1376 lysine acetylation sites were identified with a false discovery rate of <1%. As a major result of this study, our data show that dynamic changes in the abundance of lysine acetylation on various enzymes involved in photosynthesis, fatty acid metabolism, and the glyoxylate cycle are dependent on acetate and light. Exemplary determination of acetylation site stoichiometries revealed particularly high occupancy levels on K175 of the large subunit of RuBisCO and K99 and K340 of peroxisomal citrate synthase under heterotrophic conditions. The lysine acetylation stoichiometries correlated with increased activities of cellular citrate synthase and the known inactivation of the Calvin–Benson cycle under heterotrophic conditions. In conclusion, the newly identified dynamic lysine acetylation sites may be of great value for genetic engineering of metabolic pathways in Chlamydomonas. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Dong et al. (2022), 235: 1692–1700. [ABSTRACT FROM AUTHOR]

SUMMARY: Photosynthetic inefficiencies limit the productivity and sustainability of crop production and the resilience of agriculture to future societal and environmental challenges. Rubisco is a key target for improvement as it plays a central role in carbon fixation during photosynthesis and is remarkably inefficient. Introduction of mutations to the chloroplast‐encoded Rubisco large subunit rbcL is of particular interest for improving the catalytic activity and efficiency of the enzyme. However, manipulation of rbcL is hampered by its location in the plastome, with many species recalcitrant to plastome transformation, and by the plastid's efficient repair system, which can prevent effective maintenance of mutations introduced with homologous recombination. Here we present a system where the introduction of a number of silent mutations into rbcL within the model plant Nicotiana tabacum facilitates simplified screening via additional restriction enzyme sites. This system was used to successfully generate a range of transplastomic lines from wild‐type N. tabacum with stable point mutations within rbcL in 40% of the transformants, allowing assessment of the effect of these mutations on Rubisco assembly and activity. With further optimization the approach offers a viable way forward for mutagenic testing of Rubisco function in planta within tobacco and modification of rbcL in other crops where chloroplast transformation is feasible. The transformation strategy could also be applied to introduce point mutations in other chloroplast‐encoded genes. Significance Statement: A simplified transformation strategy was developed to perform site‐directed mutagenesis in the chloroplast‐encoded Rubisco large subunit of tobacco. This approach reduces unwanted mismatch repair events, enables rapid screening of transformed lines that possess desired mutations and can be applied to modify any chloroplast‐encoded gene in plant species amenable to plastid genome transformation. [ABSTRACT FROM AUTHOR]

Summary: Opening of stomata in plants with crassulacean acid metabolism (CAM) is mainly shifted to the night period when atmospheric CO2 is fixed by phosphoenolpyruvate carboxylase and stored as malic acid in the vacuole. As such, CAM plants ameliorate transpirational water losses and display substantially higher water‐use efficiency compared with C3 and C4 plants. In the past decade significant technical advances have allowed an unprecedented exploration of genomes, transcriptomes, proteomes and metabolomes of CAM plants and efforts are ongoing to engineer the CAM pathway in C3 plants. Whilst research efforts have traditionally focused on nocturnal carboxylation, less information is known regarding the drivers behind diurnal malate remobilisation from the vacuole that liberates CO2 to be fixed by RuBisCo behind closed stomata. To shed more light on this process, we provide a stoichiometric analysis to identify potentially rate‐limiting steps underpinning diurnal malate mobilisation and help direct future research efforts. Within this remit we address three key questions: Q1 Does light‐dependent assimilation of CO2 via RuBisCo dictate the rate of malate mobilisation? Q2: Do the enzymes responsible for malate decarboxylation limit daytime mobilisation from the vacuole? Q3: Does malate efflux from the vacuole set the pace of decarboxylation? [ABSTRACT FROM AUTHOR]

The regulation of photosynthesis and carbon gain of crassulacean acid metabolism (CAM) plants has not yet been disclosed to the extent of C3‐plants. In this study, the tropical epiphyte Phalaenopsis cv. "Sacramento" was subjected to different lighting regimes. Photosynthesis and biochemical measuring techniques were used to address four specific questions: (1) the response of malate decarboxylation to light intensity, (2) the malate carboxylation pathway in phase IV, (3) the response of diel carbon gain to the light integral and (4) the response of diel carbon gain to CO2. The four CAM‐phases were clearly discernable. The length of phase III and the malate decarboxylation rate responded directly to light intensity. In phase IV, CO2 was initially mainly carboxylated via Rubisco. However, at daylength of 16 h, specifically beyond ±12 h, it was mainly phosphoenolpyruvate carboxylase (PEP‐C) carboxylating CO2. Diel carbon gain appeared to be controlled by the light integral during phase III rather than the total daily light integral. Elevated CO2 further enhanced carbon gain both in phase IV and phase I. This establishes that neither malate storage capacity, nor availability of PEP as substrate for nocturnal CO2 carboxylation were limiting factors for carbon gain enhancement. These results advance our understanding of CAM‐plants and are also of practical importance for growers. In CAM‐species Phalaenopsis the malate decarboxylation rate responded to light intensity, but in the long term it was the light integral during phase III that regulated the malate pool. Elevated CO2 enhanced diel carbon gain, both in CAM‐phase IV and I, proving that neither storage capacity, nor substrate (PEP) were limiting factors. [ABSTRACT FROM AUTHOR]

The regulation of photosynthesis and carbon gain of crassulacean acid metabolism (CAM) plants has not yet been disclosed to the extent of C3‐plants. In this study, the tropical epiphyte Phalaenopsis cv. "Sacramento" was subjected to different lighting regimes. Photosynthesis and biochemical measuring techniques were used to address four specific questions: (1) the response of malate decarboxylation to light intensity, (2) the malate carboxylation pathway in phase IV, (3) the response of diel carbon gain to the light integral and (4) the response of diel carbon gain to CO2. The four CAM‐phases were clearly discernable. The length of phase III and the malate decarboxylation rate responded directly to light intensity. In phase IV, CO2 was initially mainly carboxylated via Rubisco. However, at daylength of 16 h, specifically beyond ±12 h, it was mainly phosphoenolpyruvate carboxylase (PEP‐C) carboxylating CO2. Diel carbon gain appeared to be controlled by the light integral during phase III rather than the total daily light integral. Elevated CO2 further enhanced carbon gain both in phase IV and phase I. This establishes that neither malate storage capacity, nor availability of PEP as substrate for nocturnal CO2 carboxylation were limiting factors for carbon gain enhancement. These results advance our understanding of CAM‐plants and are also of practical importance for growers. In CAM‐species Phalaenopsis the malate decarboxylation rate responded to light intensity, but in the long term it was the light integral during phase III that regulated the malate pool. Elevated CO2 enhanced diel carbon gain, both in CAM‐phase IV and I, proving that neither storage capacity, nor substrate (PEP) were limiting factors. [ABSTRACT FROM AUTHOR]

Summary: A significant resistance to CO2 diffusion is imposed by mesophyll tissue inside leaves. Mesophyll resistance, rm (or its reciprocal, mesophyll conductance, gm), reduces the rate at which Rubisco can fix CO2, increasing the water and nitrogen costs of carbon acquisition. gm varies in proportion to the surface area of chloroplasts exposed to intercellular airspace per unit leaf area. It also depends on the thickness and effective porosity of the cell wall and the CO2 permeabilities of membranes. As no measurements exist for the effective porosity of mesophyll cell walls, and CO2 permeability values are too low to account for observed rates of CO2 assimilation, conclusions from modelling must be treated with caution. There is great variation in the mesophyll resistance per unit chloroplast area for a given cell wall thickness, which may reflect differences in effective porosity. While apparent gm can vary with CO2 and irradiance, the underlying conductance at the cellular level may remain unchanged. Dynamic changes in apparent gm arise for spatial reasons and because chloroplasts differ in their photosynthetic composition and operate in different light environments. Measurements of the temperature sensitivity of membrane CO2 permeability are urgently needed to explain variation in temperature responses of gm. [ABSTRACT FROM AUTHOR]

New alternative protein sources are needed. Clover grasses as white clover (WC) and red clover (RC) may provide a novel protein source to achieve high‐quality food protein. To prevent protein oxidation, endogenous oxidative enzymes as polyphenol oxidase (PPO) and peroxidases were inhibited with sulphite. RC showed higher PPO activity than WC. Low sulphite inhibited the PPO activity in both species, but browning still occurred in RC. Sulphite did not affect the polyphenol (PP) content in WC, rather suggesting peroxidase than PPO activity. In RC juice, the PP content increased in a dose‐dependent manner with increasing sulphite. High sulphite impaired browning and increased the content of soluble rubisco in RC, mainly by increasing the content of native rubisco. In vitro digestibility of RC protein increased with increasing sulphite. In conclusion, sulphite inhibited oxidative enzymes and increased the quality of protein extracted from WC and RC. [ABSTRACT FROM AUTHOR]

Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors at Rubisco catalytic sites during CO2 fixation. Most plant species express two Rca isoforms, the larger Rca‐α and the shorter Rca‐β, either by alternative splicing from a single gene or expression from separate genes. The mechanism of Rubisco activation by Rca isoforms has been intensively studied in C3 plants. However, the functional role of Rca in C4 plants where Rubisco and Rca are located in a much higher [CO2] compartment is less clear. In this study, we selected four C4 bioenergy grasses and the model C4 grass setaria (Setaria viridis) to investigate the role of Rca in C4 photosynthesis. All five C4 grass species contained two Rca genes, one encoding Rca‐α and the other Rca‐β, which were positioned closely together in the genomes. A variety of abiotic stress‐related motifs were identified in the Rca‐α promoter of each grass, and while the Rca‐β gene was constantly highly expressed at ambient temperature, Rca‐α isoforms were expressed only at high temperature but never surpassed 30% of Rca‐β content. The pattern of Rca‐α induction on transition to high temperature and reduction on return to ambient temperature was the same in all five C4 grasses. In sorghum (Sorghum bicolor), sugarcane (Saccharum officinarum), and setaria, the induction rate of Rca‐α was similar to the recovery rate of photosynthesis and Rubisco activation at high temperature. This association between Rca‐α isoform expression and maintenance of Rubisco activation at high temperature suggests that Rca‐α has a functional thermo‐protective role in carbon fixation in C4 grasses by sustaining Rubisco activation at high temperature. [ABSTRACT FROM AUTHOR]

Summary: The wild relatives of modern wheat represent an underutilized source of genetic and phenotypic diversity and are of interest in breeding owing to their wide adaptation to diverse environments. Leaf photosynthetic traits underpin the rate of production of biomass and yield and have not been systematically explored in the wheat relatives.This paper identifies and quantifies the phenotypic variation in photosynthetic, stomatal, and morphological traits in up to 88 wheat wild relative accessions across five genera. Both steady‐state measurements and dynamic responses to step changes in light intensity are assessed.A 2.3‐fold variation for flag leaf light and CO2‐saturated rates of photosynthesis Amax was observed. Many accessions showing higher and more variable Amax, maximum rates of carboxylation, electron transport, and Rubisco activity when compared with modern genotypes. Variation in dynamic traits was also significant; with distinct genus‐specific trends in rates of induction of nonphotochemical quenching and rate of stomatal opening.We conclude that utilization of wild relatives for improvement of photosynthesis is supported by the existence of a high degree of natural variation in key traits and should consider not only genus‐level properties but variation between individual accessions. [ABSTRACT FROM AUTHOR]

Photosynthetic dinoflagellates are ecologically and biogeochemically important in marine and freshwater environments. However, surprisingly little is known of how this group acquires inorganic carbon or how these diverse processes evolved. Consequently, how CO2 availability ultimately influences the success of dinoflagellates over space and time remains poorly resolved compared to other microalgal groups. Here we review the evidence. Photosynthetic core dinoflagellates have a Form II RuBisCO (replaced by Form IB or Form ID in derived dinoflagellates). The in vitro kinetics of the Form II RuBisCO from dinoflagellates are largely unknown, but dinoflagellates with Form II (and other) RuBisCOs have inorganic carbon concentrating mechanisms (CCMs), as indicated by in vivo internal inorganic C accumulation and affinity for external inorganic C. However, the location of the membrane(s) at which the essential active transport component(s) of the CCM occur(s) is (are) unresolved; isolation and characterization of functionally competent chloroplasts would help in this respect. Endosymbiotic Symbiodiniaceae (in Foraminifera, Acantharia, Radiolaria, Ciliata, Porifera, Acoela, Cnidaria, and Mollusca) obtain inorganic C by transport from seawater through host tissue. In corals this transport apparently provides an inorganic C concentration around the photobiont that obviates the need for photobiont CCM. This is not the case for tridacnid bivalves, medusae, or, possibly, Foraminifera. Overcoming these long‐standing knowledge gaps relies on technical advances (e.g., the in vitro kinetics of Form II RuBisCO) that can functionally track the fate of inorganic C forms. [ABSTRACT FROM AUTHOR]

Interventions to increase crop radiation use efficiency rely on understanding of how biochemical and stomatal limitations affect photosynthesis. When leaves transition from shade to high light, slow increases in maximum Rubisco carboxylation rate and stomatal conductance limit net CO2 assimilation for several minutes. However, as stomata open intercellular [CO2] increases, so electron transport rate could also become limiting. Photosynthetic limitations were evaluated in three important Brassica crops: Brassica rapa, Brassica oleracea and Brassica napus. Measurements of induction after a period of shade showed that net CO2 assimilation by B. rapa and B. napus saturated by 10 min. A new method of analyzing limitations to induction by varying intercellular [CO2] showed this was due to co‐limitation by Rubisco and electron transport. By contrast, in B. oleracea persistent Rubisco limitation meant that CO2 assimilation was still recovering 15 min after induction. Correspondingly, B. oleracea had the lowest Rubisco total activity. The methodology developed, and its application here, shows a means to identify the basis of variation in photosynthetic efficiency in fluctuating light, which could be exploited in breeding and bioengineering to improve crop productivity. Slow, Rubisco‐ and stomata‐limited post‐shade induction was evaluated in three Brassica species. Net CO2 uptake in Brassica oleracea was persistently Rubisco‐limited and took >15 min to recover after shade; in Brassica napus and Brassica rapa, recovery was faster, <10 min, due to co‐limitation by electron transport. [ABSTRACT FROM AUTHOR]

Summary: Leaf photosynthetic properties, for example the maximum carboxylation velocity or Vc,max, change with leaf age due to ontogenetic processes. This study introduces an optimal dynamic allocation scheme to model changes in leaf‐level photosynthetic capacity as a function of leaf biochemical constraints (costs of synthesis and defence), nitrogen availability and other environmental factors (e.g. light).The model consists of a system of equations describing RuBisCO synthesis and degradation within chloroplasts, defence and ageing at leaf levels, nitrogen transfer and carbon budget at plant levels.Model results show that optimal allocation principles explained RuBisCO dynamics with leaf age. An approximated analytical solution can reproduce the basic pattern of RuBisCO and Vc,max in rice and in two tropical tree species. The model also reveals leaf life complementarities that remained unexplained in previous approaches, as the interplay between Vc,max at maturation, life span and the decline in photosynthetic capacity with age. Furthermore, it explores the role of defence, which is not implemented in current models.This framework covers some of the existing gaps in integrating multiple processes across plant organs (chloroplast, leaf and whole plant) and is a first‐step towards representing mechanistically leaf ontogenetic processes into physiological and ecosystem models. [ABSTRACT FROM AUTHOR]

The mechanisms responsible for photosynthetic acclimation are not well understood, effectively limiting predictability under future conditions. Least‐cost optimality theory can be used to predict the acclimation of photosynthetic capacity based on the assumption that plants maximize carbon uptake while minimizing the associated costs. Here, we use this theory as a null model in combination with multiple datasets of C3 plant photosynthetic traits to elucidate the mechanisms underlying photosynthetic acclimation to elevated temperature and carbon dioxide (CO2). The model‐data comparison showed that leaves decrease the ratio of the maximum rate of electron transport to the maximum rate of Rubisco carboxylation (Jmax/Vcmax) under higher temperatures. The comparison also indicated that resources used for Rubisco and electron transport are reduced under both elevated temperature and CO2. Finally, our analysis suggested that plants underinvest in electron transport relative to carboxylation under elevated CO2, limiting potential leaf‐level photosynthesis under future CO2 concentrations. Altogether, our results show that acclimation to temperature and CO2 is primarily related to resource conservation at the leaf level. Under future, warmer, high CO2 conditions, plants are therefore likely to use less nutrients for leaf‐level photosynthesis, which may impact whole‐plant to ecosystem functioning. [ABSTRACT FROM AUTHOR]

SUMMARY: Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors from the active sites of Rubisco and thereby plays a central role in initiating and sustaining Rubisco activation. In Arabidopsis, alternative splicing of a single Rca gene results in two Rca isoforms, Rca‐α and Rca‐β. Redox modulation of Rca‐α regulates the function of Rca‐α and Rca‐β acting together to control Rubisco activation. Although Arabidopsis Rca‐α alone less effectively activates Rubisco in vitro, it is not known how CO2 assimilation and plant growth are impacted. Here, we show that two independent transgenic Arabidopsis lines expressing Rca‐α in the absence of Rca‐β ('Rca‐α only' lines) grew more slowly in various light conditions, especially under low light or fluctuating light intensity, and in a short day photoperiod compared to wildtype. Photosynthetic induction was slower in the Rca‐α only lines, and they maintained a lower rate of CO2 assimilation during both photoperiod types. Our findings suggest Rca oligomers composed of Rca‐α only are less effective in initiating and sustaining the activation of Rubisco than when Rca‐β is also present. Currently there are no examples of any plant species that naturally express Rca‐α only but numerous examples of species expressing Rca‐β only. That Rca‐α exists in most plant species, including many C3 and C4 food and bioenergy crops, implies its presence is adaptive under some circumstances. Significance Statement: Alternative splicing of a single Arabidopsis Rubisco activase (Rca) gene yields two Rca isoforms, Rca‐α and Rca‐β, and we show Rca‐α only expression less effectively regulates Rubisco and reduces Arabidopsis plant growth and photosynthesis compared to the presence of both isoforms or Rca‐β only expression. A search of genomic databases revealed no plant species that contain only Rca‐α genes, indicating that, when present, Rca‐α must interact with Rca‐β to effectively sustain Rubisco activation. [ABSTRACT FROM AUTHOR]

Summary: Green algae expressing a carbon‐concentrating mechanism (CCM) are usually associated with a Rubisco‐containing micro‐compartment, the pyrenoid. A link between the small subunit (SSU) of Rubisco and pyrenoid formation in Chlamydomonas reinhardtii has previously suggested that specific RbcS residues could explain pyrenoid occurrence in green algae.A phylogeny of RbcS was used to compare the protein sequence and CCM distribution across the green algae and positive selection in RbcS was estimated. For six streptophyte algae, Rubisco catalytic properties, affinity for CO2 uptake (K0.5), carbon isotope discrimination (δ13C) and pyrenoid morphology were compared.The length of the βA–βB loop in RbcS provided a phylogenetic marker discriminating chlorophyte from streptophyte green algae. Rubisco kinetic properties in streptophyte algae have responded to the extent of inducible CCM activity, as indicated by changes in inorganic carbon uptake affinity, δ13C and pyrenoid ultrastructure between high and low CO2 conditions for growth.We conclude that the Rubisco catalytic properties found in streptophyte algae have coevolved and reflect the strength of any CCM or degree of pyrenoid leakiness, and limitations to inorganic carbon in the aquatic habitat, whereas Rubisco in extant land plants reflects more recent selective pressures associated with improved diffusive supply of the terrestrial environment. [ABSTRACT FROM AUTHOR]

The maximum carboxylation rate (Vcmax) is a key parameter in determining the plant photosynthesis rate per unit leaf area. However, most terrestrial biosphere models currently treat Vcmax as constants changing only with plant functional types, leading to large uncertainties in modeled carbon fluxes. Vcmax is tightly linked with Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco). Here we investigated the relationship between leaf chlorophyll and Rubisco (Chl‐Rub) contents within a winter wheat paddock. With chlorophyll as a proxy of Rubisco, a semimechanistic model was developed to model Vcmax25 (Vcmax normalized to 25°C). The Chl‐Rub relationship was validated using measurements in a temperate mixed deciduous forest in Canada. The results showed that Rubisco was strongly correlated with chlorophyll (R2 = 0.96, p < 0.001) for winter wheat since the absorption of light energy by chlorophyll and the amount of CO2 catalyzed by Rubisco are tightly coupled. Incorporating the Chl‐Rub relationship into the semimechanistic model, the root mean square error of modeled Vcmax25 was the lowest among all estimation models. The slopes of Chl‐Rub relations were almost consistent in the winter wheat and temperate forest, demonstrating the potential for using leaf chlorophyll content as a proxy of leaf Rubisco in modeling Vcmax25 at large spatial scales. We anticipate that improving Vcmax25 estimates over time and space will reduce uncertainties in global carbon budgets simulated by terrestrial biosphere models. Key Points: Similar linear chlorophyll‐Rubisco relationships were found between two vegetation typesEstimation of Vcmax25 was improved when considering chlorophyll‐Rubisco relationshipThe potential of mapping Vcmax25 with chlorophyll content was demonstrated [ABSTRACT FROM AUTHOR]

SUMMARY: The regulation of Rubisco, the gatekeeper of carbon fixation into the biosphere, by its molecular chaperone Rubisco activase (Rca) is essential for photosynthesis and plant growth. Using energy from ATP hydrolysis, Rca promotes the release of inhibitors and restores catalytic competence to Rubisco‐active sites. Rca is sensitive to moderate heat stress, however, and becomes progressively inhibited as the temperature increases above the optimum for photosynthesis. Here, we identify a single amino acid substitution (M159I) that fundamentally alters the thermal and regulatory properties of Rca in bread wheat (Triticum aestivum L.). Using site‐directed mutagenesis, we demonstrate that the M159I substitution extends the temperature optimum of the most abundant Rca isoform by 5°C in vitro, while maintaining the efficiency of Rubisco activation by Rca. The results suggest that this single amino acid substitution acts as a thermal and regulatory switch in wheat Rca that can be exploited to improve the climate resilience and efficiency of carbon assimilation of this cereal crop as temperatures become warmer and more volatile. [ABSTRACT FROM AUTHOR]

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature‐induced increase in psbA occurred only in nutrient‐replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient‐sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark‐reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments. [ABSTRACT FROM AUTHOR]

Summary: Many C4 plants, including maize, perform poorly under chilling conditions. This phenomenon has been linked in part to decreased Rubisco abundance at lower temperatures. An exception to this is chilling‐tolerant Miscanthus, which is able to maintain Rubisco protein content under such conditions. The goal of this study was to investigate whether increasing Rubisco content in maize could improve performance during or following chilling stress. Here, we demonstrate that transgenic lines overexpressing Rubisco large and small subunits and the Rubisco assembly factor RAF1 (RAF1‐LSSS), which have increased Rubisco content and growth under control conditions, maintain increased Rubisco content and growth during chilling stress. RAF1‐LSSS plants exhibited 12% higher CO2 assimilation relative to nontransgenic controls under control growth conditions, and a 17% differential after 2 weeks of chilling stress, although assimilation rates of all genotypes were ~50% lower in chilling conditions. Chlorophyll fluorescence measurements showed RAF1‐LSSS and WT plants had similar rates of photochemical quenching during chilling, suggesting Rubisco may not be the primary limiting factor that leads to poor performance in maize under chilling conditions. In contrast, RAF1‐LSSS had improved photochemical quenching before and after chilling stress, suggesting that increased Rubisco may help plants recover faster from chilling conditions. Relatively increased leaf area, dry weight and plant height observed before chilling in RAF1‐LSSS were also maintained during chilling. Together, these results demonstrate that an increase in Rubisco content allows maize plants to better cope with chilling stress and also improves their subsequent recovery, yet additional modifications are required to engineer chilling tolerance in maize. [ABSTRACT FROM AUTHOR]

Summary: The photorespiratory pathway, in short photorespiration, is a metabolic repair system that enables the CO2 fixation enzyme Rubisco to sustainably operate in the presence of oxygen, that is, during oxygenic photosynthesis of plants and cyanobacteria. Photorespiration is necessary because an auto‐inhibitory metabolite, 2‐phosphoglycolate (2PG), is produced when Rubisco binds oxygen instead of CO2 as a substrate and must be removed, to avoid collapse of metabolism, and recycled as efficiently as possible. The basic principle of recycling 2PG very likely evolved several billion years ago in connection with the evolution of oxyphotobacteria. It comprises the multi‐step combination of two molecules of 2PG to form 3‐phosphoglycerate. The biochemistry of this process dictates that one out of four 2PG carbons is lost as CO2, which is a long‐standing plant breeders' concern because it represents by far the largest fraction of respiratory processes that reduce gross‐photosynthesis of major crops down to about 50% and less, lowering potential yields. In addition to the ATP needed for recycling of the 2PG carbon, extra energy is needed for the refixation of liberated equal amounts of ammonia. It is thought that the energy costs of photorespiration have an additional negative impact on crop yields in at least some environments. This paper discusses recent advances concerning the origin and evolution of photorespiration, and gives an overview of contemporary and envisioned strategies to engineer the biochemistry of, or even avoid, photorespiration. Significance Statement: This paper discusses recent advances concerning the function, origin and evolution of photorespiration, and gives a critical overview of contemporary and envisioned strategies to engineer the biochemistry of, or even avoid, photorespiration. [ABSTRACT FROM AUTHOR]

Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) fixes atmospheric carbon dioxide into bioavailable sugar molecules. It is also well known that a kinetic isotope effect (KIE; CO2 carbon atoms) accompanies the carboxylation process. To describe the reaction and the KIE α, two different types of molecular dynamics (MD) simulations (ab initio MD and classical MD) have been performed with an Own N‐layered Integrated molecular Orbitals and molecular Mechanics (ONIOM)‐hybrid model. A channel structure for CO2 transport has been observed during the MD simulation in RubisCO, and assuming the reaction path from the inlet to the product through the coordinate complex with Mg2+, simulations have been performed on several molecular configuration models fixing several distances between CO2 and ribulose‐1,5‐bisphosphate along the channel. Free energy analysis and diffusion coefficient analysis have been evaluated for different phases of the process. It is confirmed that the isotopic fractionation effect for CO2 containing either 13C or 12C would appear through the transiting path in the channel structure identified in RubisCO. The estimated isotope fractionation constant was quite close to the experimental value. [ABSTRACT FROM AUTHOR]

Summary: Bundle Sheath Defective 2, BSD2, is a stroma‐targeted protein initially identified as a factor required for the biogenesis of ribulose 1,5‐bisphosphate carboxylase/oxygenase (RuBisCO) in maize. Plants and algae universally have a homologous gene for BSD2 and its deficiency causes a RuBisCO‐less phenotype. As RuBisCO can be the rate‐limiting step in CO2 assimilation, the overexpression of BSD2 might improve photosynthesis and productivity through the accumulation of RuBisCO. To examine this hypothesis, we produced BSD2 overexpression lines in Arabidopsis. Compared with wild type, the BSD2 overexpression lines BSD2ox‐2 and BSD2ox‐3 expressed 4.8‐fold and 8.8‐fold higher BSD2 mRNA, respectively, whereas the empty‐vector (EV) harbouring plants had a comparable expression level. The overexpression lines showed a significantly higher CO2 assimilation rate per available CO2 and productivity than EV plants. The maximum carboxylation rate per total catalytic site was accelerated in the overexpression lines, while the number of total catalytic sites and RuBisCO content were unaffected. We then isolated recombinant BSD2 (rBSD2) from E. coli and found that rBSD2 reduces disulfide bonds using reductants present in vivo, for example glutathione, and that rBSD2 has the ability to reactivate RuBisCO that has been inactivated by oxidants. Furthermore, 15% of RuBisCO freshly isolated from leaves of EV was oxidatively inactivated, as compared with 0% in BSD2‐overexpression lines, suggesting that the overexpression of BSD2 maintains RuBisCO to be in the reduced active form in vivo. Our results demonstrated that the overexpression of BSD2 improves photosynthetic efficiency in Arabidopsis and we conclude that it is involved in mediating RuBisCO activation. Significance Statement: Catalytic properties of RuBisCO are susceptible to oxidation of its thiols, but the underlying process resulting in a change in RuBisCO activation has been largely elusive. Here we identify BSD2, known as a chaperone for RuBisCO assembly, as a protein that takes an essential role in the redox homeostasis of mature RuBisCO. We show that overexpression of BSD2 improves RuBisCO carboxylation efficiency through manipulating the redox potential. [ABSTRACT FROM AUTHOR]

Summary: In this work, we review the physiological and molecular mechanisms that allow vascular plants to perform photosynthesis in extreme environments, such as deserts, polar and alpine ecosystems. Specifically, we discuss the morpho/anatomical, photochemical and metabolic adaptive processes that enable a positive carbon balance in photosynthetic tissues under extreme temperatures and/or severe water‐limiting conditions in C3 species. Nevertheless, only a few studies have described the in situ functioning of photoprotection in plants from extreme environments, given the intrinsic difficulties of fieldwork in remote places. However, they cover a substantial geographical and functional range, which allowed us to describe some general trends. In general, photoprotection relies on the same mechanisms as those operating in the remaining plant species, ranging from enhanced morphological photoprotection to increased scavenging of oxidative products such as reactive oxygen species. Much less information is available about the main physiological and biochemical drivers of photosynthesis: stomatal conductance (gs), mesophyll conductance (gm) and carbon fixation, mostly driven by RuBisCO carboxylation. Extreme environments shape adaptations in structures, such as cell wall and membrane composition, the concentration and activation state of Calvin–Benson cycle enzymes, and RuBisCO evolution, optimizing kinetic traits to ensure functionality. Altogether, these species display a combination of rearrangements, from the whole‐plant level to the molecular scale, to sustain a positive carbon balance in some of the most hostile environments on Earth. Significance Statement: The molecular and physiological mechanisms that drive photosynthesis in vascular species from extreme environments still remain largely unknown. Here we review the different adaptations (from the whole plant to the molecular level) that enable plants to succeed and sustain an annual positive carbon balance under the typical short and harsh growing seasons in these environments. [ABSTRACT FROM AUTHOR]

Summary: Photorespiratory metabolism is essential for plants to maintain functional photosynthesis in an oxygen‐containing environment. Because the oxygenation reaction of Rubisco is followed by the loss of previously fixed carbon, photorespiration is often considered a wasteful process and considerable efforts are aimed at minimizing the negative impact of photorespiration on the plant's carbon uptake. However, the photorespiratory pathway has also many positive aspects, as it is well integrated within other metabolic processes, such as nitrogen assimilation and C1 metabolism, and it is important for maintaining the redox balance of the plant. The overall effect of photorespiratory carbon loss on the net CO2 fixation of the plant is also strongly influenced by the physiology of the leaf related to CO2 diffusion. This review outlines the distinction between Rubisco oxygenation and photorespiratory CO2 release as a basis to evaluate the costs and benefits of photorespiration. Significance Statement: The overall effect of photorespiratory carbon loss on the net CO2 fixation of the plant is strongly influenced by the physiology of the leaf related to Rubisco kinetics and CO2 diffusion. This review outlines the distinction between Rubisco oxygenation and photorespiratory CO2 release as a basis to evaluate the costs and benefits of photorespiration. [ABSTRACT FROM AUTHOR]

Summary: In plants, rubisco activase (Rca) regulates rubisco by removing inhibitory molecules such as ribulose‐1,5‐bisphosphate (RuBP). In cyanobacteria, a homologous protein (activase‐like cyanobacterial protein, ALC), contains a distinctive C‐terminal fusion resembling the small‐subunit of rubisco. Although cyanobacterial rubisco is believed to be less sensitive to RuBP inhibition, the ALC is widely distributed among diverse cyanobacteria.Using microscopy, biochemistry and molecular biology, the cellular localization of the ALC, its effect on carboxysome/cell ultrastructure in Fremyella diplosiphon, and its function in vitro were studied. Bioinformatic analysis uncovered evolutionary relationships between the ALC and rubisco.ALC localizes to carboxysomes and exhibits ATPase activity. Furthermore, the ALC induces rubisco aggregation in a manner similar to that of another carboxysomal protein, M35, and this activity is affected by ATP. An alc deletion mutant showed modified cell morphology when grown under enriched CO2 and impaired regulation of carboxysome biogenesis, without affecting growth rate. Carbamylation of Fremyella recombinant rubisco was inhibited by RuBP, but this inhibition was not relieved by the ALC.The ALC does not appear to function like a canonical Rca; instead, it exerts an effect on the response to CO2 availability at the level of a metabolic module, the carboxysome, through rubisco network formation, and carboxysome organization. [ABSTRACT FROM AUTHOR]

Photosynthesis uses the energy of sunlight to convert water and atmospheric CO2 into sugars, providing food and oxygen for life. The fixation of atmospheric CO2 in this crucial biological process is mediated by the enzyme Rubisco. The inefficiencies of Rubisco have inspired researchers for decades to explore ways to improve its function with the goal of increasing crop yields [1–4], and more recently to combat global warming [5]. In this graphical review we highlight the challenges involved in engineering plant Rubisco, with a focus on the extensive chaperone requirement for its biogenesis. We discuss strategies for engineering the catalytic properties of Rubisco and for sequestering the enzyme in membraneless compartments to increase CO2 fixation. [ABSTRACT FROM AUTHOR]

Global temperatures are rising, and higher rates of temperature increase are projected over land areas that encompass the globe's major agricultural regions. In addition to increased growing season temperatures, heat waves are predicted to become more common and severe. High temperatures can inhibit photosynthetic carbon gain of crop plants and thus threaten productivity, the effects of which may interact with other aspects of climate change. Here, we review the current literature assessing temperature effects on photosynthesis in key crops with special attention to field studies using crop canopy heating technology and in combination with other climate variables. We also discuss the biochemical reactions related to carbon fixation that may limit crop photosynthesis under warming temperatures and the current strategies for adaptation. Important progress has been made on several adaptation strategies demonstrating proof‐of‐concept for translating improved photosynthesis into higher yields. These are now poised to test in important food crops. [ABSTRACT FROM AUTHOR]

Summary: Photorespiration is frequently considered a wasteful and inefficient process. However, mutant analysis demonstrated that photorespiration is essential for recycling of 2‐phosphoglycolate in C3 and C4 land plants, in algae, and even in cyanobacteria operating carboxysome‐based carbon (C) concentrating mechanisms. Photorespiration links photosynthetic C assimilation with other metabolic processes, such as nitrogen and sulfur assimilation, as well as C1 metabolism, and it may contribute to balancing the redox poise between chloroplasts, peroxisomes, mitochondria and cytoplasm. The high degree of metabolic interdependencies and the pleiotropic phenotypes of photorespiratory mutants impedes the distinction between core and accessory functions. Newly developed synthetic bypasses of photorespiration, beyond holding potential for significant yield increases in C3 crops, will enable us to differentiate between essential and accessory functions of photorespiration. [ABSTRACT FROM AUTHOR]

The maximum carboxylation capacity of Rubisco, Vc,max, is an important photosynthetic parameter that is key to accurate estimation of carbon assimilation. The gold‐standard technique for determining Vc,max is to derive Vc,max from the initial slope of an A–Ci curve (the response of photosynthesis, A, to intercellular CO2 concentration, Ci). Accurate estimates of Vc,max derived from an alternative and rapid "one‐point" measurement of photosynthesis could greatly accelerate data collection and model parameterization. We evaluated the practical application of the one‐point method in six species measured under standard conditions (saturating irradiance and 400 μmol CO2 mol−1) and under conditions that would increase the likelihood for successful estimation of Vc,max: (a) ensuring Rubisco‐limited A by measuring at 300 μmol CO2 mol−1 and (b) allowing time for acclimation to saturating irradiance prior to measurement. The one‐point method significantly underestimated Vc,max in four of the six species, providing estimates 21%–32% below fitted values. We identified ribulose‐1,5‐bisphosphate‐limited A, light acclimation, and the use of an assumed respiration rate as factors that limited the effective use of the one‐point method to accurately estimate Vc,max. We conclude that the one‐point method requires a species‐specific understanding of its application, is often unsuccessful, and must be used with caution. The "one‐point" rapid method for estimating the maximum carboxylation capacity of Rubisco (Vc,max) from a single measurement of photosynthesis, often requires prior acclimation to saturating light and/or measurements of day respiration in order to accurately estimate Vc,max. [ABSTRACT FROM AUTHOR]

SUMMARY In Chlamydomonas reinhardtii, plant‐specific serine/threonine kinases (SnRK2 kinases) play a central role in sulfur metabolism. However, their role in environmental stress has not been clearly understood. Cold stress is one of the most important factors that limit the growth, productivity, and development of photosynthetic organisms. In this report, the effect of cold stress on some physiological parameters were investigated in SnRK2.2 mutant and wild type C. reinhardtii culture. Our results showed that cold stress had significantly enhanced lipid peroxidation rate in wild type, while no significant change was observed in SnRK2.2 mutant culture. Our data also indicated a decline in Rubisco protein amount of wild type culture under low temperature exposure. Low temperature reduced glutathione reductase (GR) activity in the wild type, while GR activity was enhanced in SnRK2.2 mutant. This result indicated the potential of SnRK2 kinase in cold acclimation process. [ABSTRACT FROM AUTHOR]