Photorespiration secures oxygenic photosynthesis from metabolic constraints arising from the oxygenation reaction of Rubisco in today's oxygen-rich atmosphere. Beyond this protective function, it is vital for primary metabolism, cellular redox balance, and stomatal regulation. Importantly, photorespiration has emerged as a key target for improving CO2 assimilation and biomass production. Among proposed strategies, enhancing photorespiratory efficiency via upregulation of core enzymes has highlighted glycine decarboxylase (GDC) as a promising target, however, it remains unclear whether simultaneous upregulation of multiple GDC components would yield additive or synergistic improvements in plant environmental acclimation. Here, we systematically compared single and double GDC overexpression lines under varying photoperiods, elevated oxygen and high light. GDC upregulation consistently promoted growth across photoperiods and enhanced photosynthetic performance under high light and oxygen, indicating a severe benefit of increased photorespiratory metabolite conversion. Notably, double overexpression lines exhibited enhanced metabolic flexibility beyond that observed in single lines, supporting synergistic effects on carbon assimilation and allocation. Short-term high-light exposure further revealed increased Rubisco activity and efficient partitioning of carbon into sucrose and starch. Collectively, our results demonstrate that coordinated enhancement of GDC activity strengthens photorespiratory efficiency and can generate synergistic improvements in photosynthetic robustness, metabolic robustness, and environmental acclimation under stressful conditions. These findings position multicomponent GDC engineering as a promising strategy for developing crops with improved environmental acclimation capacity.

Genetic engineering of Arabidopsis thaliana reveals that mitochondrial serine hydroxymethyltransferase 1 (SHMT1) protein abundance does not limit the capacity of photorespiration, photosynthesis, or stomatal conductance. Photorespiration safeguards photosynthesis from negative metabolic feedback inhibition caused by Rubisco’s major oxygenation byproduct, 2-phosphoglycolate, and its downstream metabolites. Previous studies have shown that photosynthetic limitations can be alleviated by enhancing endogenous photorespiratory flux through position-specific enzyme overexpression. Here, we investigated whether overexpression of mitochondrial serine hydroxymethyltransferase 1 (SHMT1) influences this regulatory circuit. However, the 1.3-threefold increase in SHMT1 protein abundance did not affect the visual phenotype, growth, or photosynthetic gas-exchange parameters, including CO2 assimilation rates and stomatal conductance, under either standard or photorespiration-promoting conditions. Metabolomic analyses further revealed wild-type-like steady-state accumulation patterns of photorespiratory and other primary metabolites in transgenic lines during both, illumination and darkness, respectively. These results indicate that SHMT1 is already expressed at sufficient levels in Arabidopsis wild type, as substantial increases in protein abundance did not enhance photorespiratory flux and growth under laboratory conditions.

Photorespiration is an essential metabolic repair process in oxygenic photosynthesis, as it detoxifies Rubisco's inhibitory oxygenase byproduct, 2-phosphoglycolate (2-PG). It has been demonstrated that improving endogenous photorespiration in C3 plants through enzyme overexpression can enhance photosynthesis and promote plant growth. However, the potential impact of improved photorespiration in leaves on heterotrophic roots remained unexplored. To address this, we conducted a metabolome analysis of Arabidopsis leaves and roots using transgenic lines with enhanced glycine decarboxylase (GDC) activity, achieved by overexpressing the mitochondrial lipoamide dehydrogenase (mtLPD1) subunit. In the leaves, mtLPD1 overexpression primarily resulted in reduced steady-state levels of intermediates associated with photorespiration, the tricarboxylic acid (TCA) cycle, and soluble sugars, while intermediates related to nitrogen metabolism were elevated. In roots, where mtLPD1 expression was unchanged, we observed contrasting accumulation patterns in the transgenic lines compared to the wildtype, including increased levels of photorespiratory and TCA-cycle intermediates. Notably, we also detected elevated amounts of soluble sugars, nitrate, and starch. Phloem exudate analysis revealed altered metabolite profiles in the overexpressors, particularly with respect to photorespiratory intermediates linked to the GDC reaction, as well as soluble sugars and metabolites involved in cellular redox homeostasis. This suggested an increased transport of these metabolites from shoots to roots, thereby altering sink organ metabolism. In summary, we hypothesize that optimizing photorespiration enhances photosynthesis, which leads to an increased export of carbon surplus to heterotrophic tissues. Thus, improving photorespiration may hold potential for increasing yields in beet- and tuber-forming plants.

Photorespiratory serine hydroxymethyltransferases (SHMTs) are important enzymes of cellular one-carbon metabolism. In this study, we investigated the potential role of SHMT6 in Arabidopsis thaliana. We found that SHMT6 is localized in the nucleus and expressed in different tissues during development. Interestingly SHMT6 is inducible in response to avirulent, virulent Pseudomonas syringae and to Fusarium oxysporum infection. Overexpression of SHMT6 leads to larger flowers, siliques, seeds, roots, and consequently an enhanced overall biomass. This enhanced growth was accompanied by increased stomatal conductance and photosynthetic capacity as well as ATP, protein, and chlorophyll levels. By contrast, a shmt6 knockout mutant displayed reduced growth. When challenged with Pseudomonas syringae pv tomato (Pst) DC3000 expressing AvrRpm1, SHMT6 overexpression lines displayed a clear hypersensitive response which was characterized by enhanced electrolyte leakage and reduced bacterial growth. In response to virulent Pst DC3000, the shmt6 mutant developed severe disease symptoms and becomes very susceptible, whereas SHMT6 overexpression lines showed enhanced resistance with increased expression of defense pathway associated genes. In response to Fusarium oxysporum, overexpression lines showed a reduction in symptoms. Moreover, SHMT6 overexpression lead to enhanced production of ethylene and lignin, which are important components of the defense response. Collectively, our data revealed that SHMT6 plays an important role in development and defense against pathogens.

H-protein is part of the multi-protein glycine cleavage system found in both prokaryotes and eukaryotes. The Arabidopsis thaliana genome contains three loci for genes encoding H-protein. We show that in land plants, there are two clades of mitochondrial H-proteins; one that clusters with H1 and H3, and another that clusters with H2, an isoform mostly expressed in heterotrophic tissue and for which a null mutation results in a >99% impeded growth. After showing that all three isoforms fulfill the same metabolic function, we evidenced that the impaired root growth in h2 results from an altered cell cycle progression concomitant to a lower Target Of Rapamycin kinase activity. Subsequent metabolomic approaches revealed an accumulation of storage sugars and a significant decrease of the TCA intermediates and several vitamins in the h2 root cells as compared to the wild-type ones. Additional investigations evidenced that H2 acts as the main relay for lipoylation in root mitochondria, which diverges from the current lipoylation model proposed for photosynthetic tissues. Together, this work provides new insight on the control of the cell cycle by mitochondrial metabolism, and also challenges the current dogma for lipoylation in mitochondria of plant cells. ### Competing Interest Statement The authors have declared no competing interest.

The photorespiratory repair pathway (photorespiration in short) was set up from ancient metabolic modules about three billion years ago in cyanobacteria, the later ancestors of chloroplasts. These prokaryotes developed the capacity for oxygenic photosynthesis, i.e. the use of water as a source of electrons and protons (with O2 as a by-product) for the sunlight-driven synthesis of ATP and NADPH for CO2 fixation in the Calvin cycle. However, the CO2-binding enzyme, ribulose 1,5-bisphosphate carboxylase (known under the acronym Rubisco), is not absolutely selective for CO2 and can also use O2 in a side reaction. It then produces 2-phosphoglycolate (2PG), the accumulation of which would inhibit and potentially stop the Calvin cycle and subsequently photosynthetic electron transport. Photorespiration removes the 2-PG and in this way prevents oxygenic photosynthesis from poisoning itself. In plants, the core of photorespiration consists of ten enzymes distributed over three different types of organelles, requiring interorganellar transport and interaction with several auxiliary enzymes. It goes together with the release and to some extent loss of freshly fixed CO2. This disadvantageous feature can be suppressed by CO2-concentrating mechanisms, such as those that evolved in C4 plants thirty million years ago, which enhance CO2 fixation and reduce 2PG synthesis. Photorespiration itself provided a pioneer variant of such mechanisms in the predecessors of C4 plants, C3-C4 intermediate plants. This article is a review and update particularly on the enzyme components of plant photorespiration and their catalytic mechanisms, on the interaction of photorespiration with other metabolism and on its impact on the evolution of photosynthesis. This focus was chosen because a better knowledge of the enzymes involved and how they are embedded in overall plant metabolism can facilitate the targeted use of the now highly advanced methods of metabolic network modelling and flux analysis. Understanding photorespiration more than before as a process that enables, rather than reduces, plant photosynthesis, will help develop rational strategies for crop improvement.

The photorespiratory repair pathway (photorespiration) is a supplement to the Calvin-Benson cycle that allows photosynthetic fixation of CO2 to be based on the photochemical splitting of water, that is, in the presence of oxygen. Photorespiration is necessary because oxygen can replace CO2 at the CO2 fixation enzyme ribulose-1,5-bisphosphate carboxylase (Rubisco) and oxidize the CO2 acceptor molecule ribulose-1,5-bisphosphate (RuBP), generating the powerful enzyme inhibitor 2-phosphoglycolate (2PG). 2PG must be removed because its accumulation would block operation of the Calvin-Benson cycle and in turn halt photosynthesis. Photorespiratory removal and detoxification of 2PG occurs by recycling to 3-phosphoglycerate which can be used to generate fresh RuBP. This repair process efficiently recovers three out of four misdirected carbon atoms while one carbon is oxidized and lost as CO2.

Photorespiration is an integral component of plant primary metabolism. Accordingly, it has been often observed that impairing the photorespiratory flux negatively impacts other cellular processes. In this study, the metabolic acclimation of the Arabidopsisthaliana wild type was compared with the hydroxypyruvate reductase 1 (HPR1; hpr1) mutant, displaying only a moderately reduced photorespiratory flux. Plants were analyzed during development and under varying photoperiods with a combination of non-targeted and targeted metabolome analysis, as well as 13C- and 14C-labeling approaches. The results showed that HPR1 deficiency is more critical for photorespiration during the vegetative compared to the regenerative growth phase. A shorter photoperiod seems to slowdown the photorespiratory metabolite conversion mostly at the glycerate kinase and glycine decarboxylase steps compared to long days. It is demonstrated that even a moderate impairment of photorespiration severely reduces the leaf-carbohydrate status and impacts on sulfur metabolism. Isotope labeling approaches revealed an increased CO2 release from hpr1 leaves, most likely occurring from enhanced non-enzymatic 3-hydroxypyruvate decarboxylation and a higher flux from serine towards ethanolamine through serine decarboxylase. Collectively, the study provides evidence that the moderate hpr1 mutant is an excellent tool to unravel the underlying mechanisms governing the regulation of metabolic linkages of photorespiration with plant primary metabolism.

Photorespiration has been shown to be essential for all oxygenic phototrophs in the present-day oxygen-containing atmosphere. The strong similarity of the photorespiratory cycle in cyanobacteria and plants led to the hypothesis that oxygenic photosynthesis and photorespiration co-evolved in cyanobacteria, and then entered the eukaryotic algal lineages up to land plants via endosymbiosis. However, the evolutionary origin of the photorespiratory enzyme glycolate oxidase (GOX) is controversial, which challenges the common origin hypothesis. Here, we tested this hypothesis using phylogenetic and biochemical approaches with broad taxon sampling. Phylogenetic analysis supported the view that a cyanobacterial GOX-like protein of the 2-hydroxy-acid oxidase family most likely served as an ancestor for GOX in all eukaryotes. Furthermore, our results strongly indicate that GOX was recruited to the photorespiratory metabolism at the origin of Archaeplastida, because we verified that Glaucophyta, Rhodophyta, and Streptophyta all express GOX enzymes with preference for the substrate glycolate. Moreover, an “ancestral” protein synthetically derived from the node separating all prokaryotic from eukaryotic GOX-like proteins also preferred glycolate over l-lactate. These results support the notion that a cyanobacterial ancestral protein laid the foundation for the evolution of photorespiratory GOX enzymes in modern eukaryotic phototrophs.

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.

The capacity to assimilate carbon and nitrogen, to transport the resultant sugars and amino acids to sink tissues, and to convert the incoming sugars and amino acids into storage compounds in the sink tissues, are key determinants of crop yield. Given that all of these processes have the potential to co-limit growth, multiple genetic interventions in source and sink tissues, plus transport processes may be necessary to reach the full yield potential of a crop. We used biolistic combinatorial co-transformation (up to 20 transgenes) for increasing C and N flows with the purpose of increasing tomato fruit yield. We observed an increased fruit yield of up to 23%. To better explore the reconfiguration of metabolic networks in these transformants, we generated a dataset encompassing physiological parameters, gene expression and metabolite profiling on plants grown under glasshouse or polytunnel conditions. A Sparse Partial Least Squares regression model was able to explain the combination of genes that contributed to increased fruit yield. This combinatorial study of multiple transgenes targeting primary metabolism thus offers opportunities to probe the genetic basis of metabolic and phenotypic variation, providing insight into the difficulties in choosing the correct combination of targets for engineering increased fruit yield.

The multienzyme glycine cleavage system (GCS) converts glycine and tetrahydrofolate to the one-carbon compound 5,10-methylenetetrahydrofolate, which is of vital importance for most if not all organisms. Photorespiring plant mitochondria contain very high levels of GCS proteins organised as a fragile glycine decarboxylase complex (GDC). The aim of this study is to provide mass spectrometry-based stoichiometric data for the plant leaf GDC and examine whether complex formation could be a general property of the GCS in photosynthesizing organisms. The molar ratios of the leaf GDC component proteins are 1L2-4P2-8T-26H and 1L2-4P2-8T-20H for pea and Arabidopsis, respectively, as determined by mass spectrometry. The minimum mass of the plant leaf GDC ranges from 1,550-1,650 kDa, which is larger than previously assumed. The Arabidopsis GDC contains four times more of the isoforms GCS-P1 and GCS-L1 in comparison with GCS-P2 and GCS-L2, respectively, whereas the H-isoproteins GCS-H1 and GCS-H3 are fully redundant as indicated by their about equal amounts. Isoform GCS-H2 is not present in leaf mitochondria. In the cyanobacterium Synechocystis sp. PCC 6803, GCS proteins are present at low concentration but above the complex formation threshold reported for pea leaf GDC. Indeed, formation of a cyanobacterial GDC from the individual recombinant GCS proteins in vitro could be demonstrated. Presence and metabolic significance of a Synechocystis GDC in vivo remain to be examined but could involve multimers of the GCS H-protein that dynamically crosslink the three GCS enzyme proteins, facilitating glycine metabolism by the formation of multienzyme metabolic complexes.

Photorespiration metabolizes 2-phosphoglyolate (2-PG) to avoid inhibition of carbon assimilation and allocation. In addition to 2-PG removal, photorespiration has been shown to play a role in stress protection. Here, we studied the impact of faster 2-PG degradation through overexpression of 2-PG phosphatase (PGLP) on the abiotic stress-response of Arabidopsis thaliana (Arabidopsis). Two transgenic lines and the wild type were subjected to short-time high light and elevated temperature stress during gas exchange measurements. Furthermore, the same lines were exposed to long-term water shortage and elevated temperature stresses. Faster 2-PG degradation allowed maintenance of photosynthesis at combined light and temperatures stress and under water-limiting conditions. The PGLP-overexpressing lines also showed higher photosynthesis compared to the wild type if grown in high temperatures, which also led to increased starch accumulation and shifts in soluble sugar contents. However, only minor effects were detected on amino and organic acid levels. The wild type responded to elevated temperatures with elevated mRNA and protein levels of photorespiratory enzymes, while the transgenic lines displayed only minor changes. Collectively, these results strengthen our previous hypothesis that a faster photorespiratory metabolism improves tolerance against unfavorable environmental conditions, such as high light intensity and temperature as well as drought. In case of PGLP, the likely mechanism is alleviation of inhibitory feedback of 2-PG onto the Calvin-Benson cycle, facilitating carbon assimilation and accumulation of transitory starch.

The toxic cyanobacterium Nodularia spumigena regularly forms large surface blooms in the central Baltic Sea during the summer period. Bloom formation occurs when the water body is virtually free of the nutrients nitrate and phosphate. The capacity of Nodularia spp. for nitrogen fixation makes it independent from combined nitrogen sources; however, its adaptation to low phosphate levels is poorly investigated. We compared the behaviour of N. spumigena strain CCY9414 under high- and low-phosphate conditions in the laboratory. Cells cultivated under P-limited conditions showed no growth but higher expression of pho regulon genes, such as pstS, which are known to respond to P-starvation. However, electron microscopy showed a massive accumulation of polyphosphate granules in long-term P-starved cells and not in those grown in P-containing medium. The polyphosphate nature of the black granules was verified by electron energy-loss spectroscopy and subsequently by polyphosphate quantification in Nodularia cells. Polyphosphate accumulation was also observed in cells of Aphanizomenon flos-aquae obtained from a summer bloom, where only traces of phosphate were available in the surrounding water body. These findings indicate that bloom-forming cyanobacteria prefer to store the traces of available phosphate in the form of polyphosphates, instead of using it for further growth. This strategy could help in competition with other microorganisms for phosphate during bloom conditions, whilst allowing rapid cell division upon more favourable phosphate concentrations.

Despite the well-known biochemistry of the major pathways involved in central carbon and amino acid metabolism, there are still gaps regarding their regulation or regulatory interactions. Recent research demonstrated the physiological significance of the mitochondrial redox machinery, particularly thioredoxin o1 (TRXo1), for proper regulation of the tricarboxylic acid cycle, components of the mitochondrial electron transport chain and photorespiration. These findings imply that TRXo1 regulation contributes to the metabolic acclimation toward changes in the prevailing environmental conditions. Here, we analyzed if TRXo1 is involved in the light induction of photosynthesis. Our results show that the trxo1 mutant activates CO2 assimilation rates to a significantly lower extend than wild type in response to short-term light/dark changes. Metabolite analysis suggests that activation of glycine-to-serine conversion catalyzed through glycine decarboxylase in conjunction with serine hydroxymethyltransferase in trxo1 is slowed down at onset of illumination. We propose that redox regulation via TRXo1 is necessary to allow the rapid induction of mitochondrial steps of the photorespiratory cycle and, in turn, to facilitate light-induction of photosynthesis.

Since it is known that cancer cells exhibit a preference for increased glycine consumption, the respective glycine metabolizing enzymes are in focus of many research projects. However, no cancer associated studies are available for the Glycine Cleavage System Protein H (GCSH) to date. Our initial analysis revealed a GCSH-overexpression of the protein-coding transcript variant 1 (Tv1) in breast cancer cells and tissue. Furthermore, a shorter (391 bp) transcript variant (Tv*) was amplified with an increased expression in healthy breast cells and a decreased expression in breast cancer samples. The Tv1/Tv* transcript ratio is 1.0 in healthy cells on average, and between 5–10 in breast cancer cells. Thus, a GCSH-equilibrium at the transcript level is likely conceivable for optimal glycine degradation. A possible regulative role of Tv* was proven by Tv1-Tv*-RNA-binding and overexpression studies which consequently led to serious physiological alterations: decreased metabolic activity, release of the lactate dehydrogenase, increased extracellular acidification, and finally necrosis as a result of impaired plasma membranes. In contrast, Tv1-overexpression led to an additional increase in cellular vitality of the tumor cells, primarily due to the acceleration of the mitochondrial glycine decarboxylation activity. Ultimately, we provide the first evidence of a sensitive GCSH-antisense regulation which determines cancerous cell viability.

Bicarbonate removal from the nonheme iron at the acceptor side of photosystem II (PSII) was shown recently to shift the midpoint potential of the primary quinone acceptor Q(A) to a more positive potential and lowers the yield of singlet oxygen (O-1(2)) production. The presence of Q(A) - results in weaker binding of bicarbonate, suggesting a redox-based regulatory and protective mechanism where loss of bicarbonate or exchange of bicarbonate by other small carboxylic acids may protect PSII against O-1(2) in vivo under photorespiratory conditions. Here, we compared the properties of Q(A) in the Arabidopsis (Arabidopsis thaliana) photorespiration mutant deficient in peroxisomal HYDROXYPYRUVATE REDUCTASE1 (hpr1-1), which accumulates glycolate in leaves, with the wild type. Photosynthetic electron transport was affected in the mutant, and chlorophyll fluorescence showed slower electron transport between Q(A) and Q(B) in the mutant. Glycolate induced an increase in the temperature maximum of thermoluminescence emission, indicating a shift of the midpoint potential of Q(A) to a more positive value. The yield of O-1(2) production was lowered in thylakoid membranes isolated from hpr1-1 compared with the wild type, consistent with a higher potential of Q(A)/Q(A) -. In addition, electron donation to photosystem I was affected in hpr1-1 at higher light intensities, consistent with diminished electron transfer out of PSII. This study indicates that replacement of bicarbonate at the nonheme iron by a small carboxylate anion occurs in plants in vivo. These findings suggested that replacement of the bicarbonate on the nonheme iron by glycolate may represent a regulatory mechanism that protects PSII against photooxidative stress under low-CO2 conditions.

Photorespiratory phosphoglycolate (2PG) metabolism is essential for cyanobacteria, algae, and plants. The first enzyme of the pathway, 2PG phosphatase (PGPase), is known from plants and algae but was scarcely investigated in cyanobacteria. In silico analysis revealed four candidate genes (slr0458, slr0586, sll1349, and slr1762) in the genome of the model cyanobacterium Synechocystis sp. PCC 6803 that all belong to the 2-haloacid dehalogenase (HAD) superfamily and could possibly encode PGPase proteins. However, in contrast to known algal and plant PGPases, the putative cyanobacterial PGPases belong to another HAD subfamily implying that PGPases in eukaryotic phototrophs did not originate from cyanobacterial PGPases. To verify their function, these four genes were inactivated both individually and in combination. A mild high-CO2-requiring (HCR) growth phenotype typical for photorespiratory mutants was observed only in Δsll1349. Combinatorial inactivation enhanced the HCR phenotype in specific double and triple mutants. Heterologous expression of the putative cyanobacterial PGPases in E. coli led to higher PGPase activities in crude cell extracts, but only the purified Slr0458 protein showed PGPase activity. Hence, we propose that a consortium of up to four photorespiratory PGPases may initiate photorespiratory 2PG metabolism in Synechocystis. We suggest that redundancy of this essential enzyme activity could be related to the highly adaptive lifestyle of cyanobacteria such as Synechocystis sp. PCC 6803, which allows them to grow under very diverse conditions.

Photorespiration is indispensable for oxygenic photosynthesis since it detoxifies and recycles 2-phosphoglycolate (2PG), which is the primary oxygenation product of Rubisco. However, C4 plant species typically display very low rates of photorespiration due to their efficient biochemical carbon-concentrating mechanism. Thus, the broader relevance of photorespiration in these organisms remains unclear. In this study, we assessed the importance of a functional photorespiratory pathway in the C4 plant Flaveria bidentis using knockdown of the first enzymatic step, namely 2PG phosphatase (PGLP). The isolated RNAi lines showed strongly reduced amounts of PGLP protein, but distinct signs of the photorespiratory phenotype only emerged below 5% residual PGLP protein. Lines with this characteristic were stunted in growth, had strongly increased 2PG content, exhibited accelerated leaf senescence, and accumulated high amounts of branched-chain and aromatic amino acids, which are both characteristics of incipient carbon starvation. Oxygen-dependent gas-exchange measurements consistently suggested the cumulative impairment of ribulose-1,5-bisphosphate regeneration with increased photorespiratory pressure. Our results indicate that photorespiration is essential for maintaining high rates of C4 photosynthesis by preventing the 2PG-mediated inhibition of carbon utilization efficiency. However, considerably higher 2PG accumulation can be tolerated compared to equivalent lines of C3 plants due to the differential distribution of specific enzymatic steps between the mesophyll and bundle sheath cells.

AbstractThe photorespiratory repair pathway is an indispensable component of the photosynthetic CO2assimilation apparatus and a major highway of plant primary metabolism. The photorespiratory pathway interacts with the tricarboxylic acid (TCA) cycle in at least three ways: synthesis of 2‐oxoglutarate (2OG) for nitrogen metabolism, inhibition of TCA cycle enzymes by ammonia and NADH, and competition for NAD+. The preference given to NADH oxidation interlinks photorespiratory glycine oxidation with oxidative phosphorylation to provide adenosine triphosphate (ATP) to the cytosol. Use of water for photosynthesis, together with the generation of molecular oxygen, triggered a major side reaction at the key CO2fixation enzyme ribulose 1,5‐bisphosphate carboxylase (Rubisco): addition of dioxygen cleaves the CO2acceptor molecule ribulose 1,5‐bisphosphate (RuBP), generating the powerful enzyme inhibitor glycolate 2‐phosphate (2PG). Central for photorespiration is the conversion of the two‐carbon skeleton of glycolate into a three‐carbon skeleton that is compatible with the Calvin‐Benson cycle.