Reactivity of glyoxylate with hydrogen perioxide and simulation of the glycolate pathway of C3 plants and Euglena

The nonenzymatic reaction of glyoxylate and H₂O₂ was measured under physiological conditions of the pH and concentrations of reactants. The reaction of glyoxylate and H₂O₂ was secondorder, with a rate constant of 2.27 l mol^-1 s^-1 at pH 8.0 and 25° C. The rate constant increased by 4.4 times in the presence of Zn²⁺ and doubled at 35°C. We propose a mechanism for the reaction between glyoxylate and H₂O₂. From a comparison of the rates of H₂O₂ decomposition by catalase and the reaction with glyoxylate, we conclude that H₂O₂ produced during glycolate oxidation in peroxisomes is decomposed by catalase but not by the reaction with glyoxylate, and that photorespiratory CO₂ originates from glycine, but not from glyoxylate, in C₃ plants. Simulation using the above rate constant and reported kinetic parameters leads to the same conclusion, and also makes it clear that alanine is a satisfactory amino donor in the conversion of glyoxylate to glycine. Some serine might be decomposed to give glycine and methylene-tetrahydrofolate; the latter is ultimately oxidized to CO₂. In the simulation of the glycolate pathway of Euglena, the rate constant was high enough to ensure the decarboxylation of glyoxylate by H₂O₂ to produce photorespiratory CO₂ during the glycolate metabolism of this organism.Abbreviations Chl chlorophyll - GGT glutamate: glyoxylate aminotransferase (EC 2.6.1.4) - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid - SGT serine: glyoxylate aminotransferase (EC 2.6.1.45) This is the ninth in a series on the metabolism of glycolate in Euglena gracilis. The eighth is Yokota et al. (1982)     

Photorespiratory rates in wheat and maize as determined by o-labeling

A method was devised to quantify short-term photorespiratory rates in terrestrial plants using ¹⁸O-intermediates of the glycolate pathway, specifically glycolate, glycine, and serine. The pathway intermediates were isolated and analyzed on a GC/MS to determine molecular percent ¹⁸O-enrichment. Rates of glycolate synthesis were determined from ¹⁸O-labeling kinetics of the intermediates, derived rate equations, and nonlinear regression techniques. Glycolate synthesis in wheat (Triticum aestivum L.), a C₃ plant, and maize (Zea mays L.), a C₄ plant, was stimulated by high O₂ concentrations and inhibited by high CO₂ concentrations. The synthesis rates were 7.3, 2.1, and 0.7 micromoles per square decimeter per minute under a 21% O₂ and 0.035% CO₂ atmosphere for leaf tissue of wheat, maize seedlings, and 3-month-old maize, respectively. Photorespiratory CO₂ evolution rates were estimated to be 27, 6, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue support the hypothesis that C₄ plants photorespire, albeit at a reduced rate in comparison to C₃ plants, and that the CO₂/O₂ ratio in the bundle sheath of maize is higher in mature tissue than in seedling tissue. The pool size of the three photorespiratory intermediates remained constant and were unaffected by changes in either CO₂ or O₂ concentrations throughout the 10-minute labeling period. This suggests that photorespiratory metabolism is regulated by other mechanism besides phosphoglycolate synthesis by ribulose-1,5-bisphosphate carboxylase/oxygenase, at least under short-term conditions. Other mechanisms could be alternate modes of synthesis of the intermediates, regulation of some of the enzymes of the photorespiratory pathway, or regulation of carbon flow between organelles involved in photorespiration. The glycolate pool became nearly 100% ¹⁸O-labeled under an atmosphere of 40% O₂. This pool failed to become 100% ¹⁸O-enriched under lower O₂ concentrations.     

Metabolic regulation of carbon flux during C4 photosynthesis

The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO₂ in leaves of C₄ plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C₃ and C₄ Panicum species. CO₂ evolution by leaf slices of P. bisulcatum, a C₃ species, fed glycolate or glycine was light-independent and O₂-sensitive. The C₄ P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO₂ in the dark but not in the light. In C₄ species, dark CO₂ evolution was abolished by the addition of phosphoenolpyruvate (PEP)⁴. The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO₂ efflux by C₄ leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O₂ uptake by the C₄ leaf slices or on glycolate and glycine metabolism in C₃ leaf slices. The rate of photorespiratory CO₂ evolution in the C₃ Panicum species was 3 times higher than that observed with the C₄ species. The ratio of glycolate-dependent CO₂ evolution to O₂ uptake in both groups was 1:2. Isolated C₄ mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C₃ mesophyll protoplasts and C₄ bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O₂-sensitive manner, and the addition of PEP or maleate had no effect. C₄ bundle sheath- and C₃-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C₄ bundle sheath- and C₃-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C₄ mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C₄ plants bundle sheath cells are the primary site of O₂-sensitive photorespiratory CO₂ evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO₂ before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO₂ pump and reassimilation of photorespiratory CO₂ are discussed in relation to the apparent lack of photorespiration in leaves of C₄ species.Abbreviations BSA bovine serum albumin - Chl chlorophyll - PEP phosphoenolpyruvate - Rbu-P ₂ ribulose 1,5-bisphosphate - Rib-5-P ribose-5-phosphate - Ru-5-P ribuluse-5-phosphate - FCCP carbonyl cyanide p-trifluoromethoxyphenylhydrazone Journal Series Paper, New Jersey Agricultural Experiment Station     

Affixing the O to Rubisco: discovering the source of photorespiratory glycolate and its regulation

The source of glycolate in photorespiration and its control, a particularly active and controversial research topic in the 1970s, was resolved in large part by several discoveries and observations described here. George Bowes discovered that the key carboxylation enzyme Rubisco (ribulosebisphosphate carboxylase/oxygenase) is competitively inhibited by O₂ and that O₂ substitutes for CO₂ in the initial `dark' reaction of photosynthesis to yield glycolate-P, the substrate for photorespiration. William Laing derived an equation from basic enzyme kinetics that describes the CO₂, O₂, and temperature dependence of photosynthesis, photorespiration, and the CO₂ compensation point in C₃ plants. Jerome Servaites established that photosynthesis cannot be increased by inhibiting the photorespiratory pathway prior to the release of photorespiratory CO₂, and Douglas Jordan discovered substantial natural variation in the Rubisco oxygenase/carboxylase ratio. A mutant Arabidopsis plant with defective glycolate-P phosphatase, isolated by Chris Somerville, definitively established the role of O₂ and Rubisco in providing photorespiratory glycolate. Selection techniques to isolate photorespiration-deficient plants were devised by Jack Widholm and by Somerville, but no plants with reduced photorespiration were found. Somerville's approach, directed mutagenesis of Arabidopsis plants, was subsequently successful in the isolation of numerous other classes of mutants and revolutionized the science of plant biology. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photorespiratory glycine metabolism in corn leaf discs

The total glycine pool in Zea mays L. Mo17×B73 leaf discs was measured after steady state photosynthesis in 50%, 21% and 1% O₂. The glycine pool was a function of O₂ concentration; it was largest in 50% O₂ and smallest in 1% O₂. Incubation of discs with methyl hydroxybutynoic acid in 21% O₂ in the light caused an accumulation of carbon in glycolate. This accumulation was O₂ sensitive, as subsequent photosynthetic periods in 50%, 21%, and 1% O₂ resulted in the largest glycolate pool in 50% O₂ and the smallest in 1% O₂. At the same time, the O₂-dependent increase in the glycine pool was eliminated. After untreated leaf discs reached steady state photosynthesis in 21% O₂, measurements made subsequently in darkness, or in 1% O₂ in the light, showed that the glycine pool decreased. On the basis of these results, we conclude that a major portion of the total glycine pool in corn is an intermediate in the photorespiratory glycolate pathway. Considering both the rate of decay of the glycine pool in the dark and the rate of decay of the glycine pool after changing from 21% to 1% O₂, we conclude that this glycine pool is turning over slowly.     

Erratum

Glycolate synthesis was inhibited 40–50% in illuminated tobacco leaf disks, which have rapid rates of photorespiration, when floated on 20 mm potassium glycidate (2,3-epoxypropionate), an epoxide similar in structure to glycolate. The inhibitor also decreased the release of photorespiratory CO₂ about 40%, and the specificity of glycidate was demonstrated by the 40–50% increase in rate of photosynthetic CO₂ uptake observed in its presence. The importance of glycolate synthesis and metabolism in the production of photorespiratory CO₂ and the role of glycolate in diminishing net photosynthesis in species with rapid rates of photorespiration was thus further confirmed. L-(or 2S)-Glycidate was slightly more active than DL-glycidate, but glycidate was more effective as a specific inhibitor in leaf tissue than several other epoxide analogs of glycolate examined. The products of photosynthetic ¹⁴O₂ fixation after 3 or 4 min of uptake were proportionately altered in the presence of glycidate, and the specific radioactivity of the [¹⁴C]glycolate produced was closer to that of the ¹⁴CO₂ supplied. Glycidate inhibited glycolate synthesis in tobacco leaf disks irreversibly, since the degree of inhibition was the same for at least 2 hr after the inhibitor solution was removed. Glycidate also blocked glycolate synthesis in maize leaf disks, tissue with low rates of photorespiration, but large increases in net photosynthesis were not observed in maize with glycidate, because glycolate synthesis is normally only about 10% as rapid in maize as in tobacco. The demonstration of increases in net photosynthesis of 40–50% when glycolate synthesis (and photorespiration) is blocked with glycidate indicates in an independent manner that the biochemical or genetic control of photorespiration should permit large increases in plant productivity in plant species possessing rapid rates of photorespiration.     

The ancestors of diatoms evolved a unique mitochondrial dehydrogenase to oxidize photorespiratory glycolate

Like other oxygenic photosynthetic organisms, diatoms produce glycolate, a toxic intermediate, as a consequence of the oxygenase activity of Rubisco. Diatoms can remove glycolate through excretion and through oxidation as part of the photorespiratory pathway. The diatom Phaeodactylum tricornutum encodes two proteins suggested to be involved in glycolate metabolism: PtGO1 and PtGO2. We found that these proteins differ substantially from the sequences of experimentally characterized proteins responsible for glycolate oxidation in other species, glycolate oxidase (GOX) and glycolate dehydrogenase. We show that PtGO1 and PtGO2 are the only sequences of P. tricornutum homologous to GOX. Our phylogenetic analyses indicate that the ancestors of diatoms acquired PtGO1 during the proposed first secondary endosymbiosis with a chlorophyte alga, which may have previously obtained this gene from proteobacteria. In contrast, PtGO2 is orthologous to an uncharacterized protein in Galdieria sulphuraria, consistent with its acquisition during the secondary endosymbiosis with a red alga that gave rise to the current plastid. The analysis of amino acid residues at conserved positions suggests that PtGO2, which localizes to peroxisomes, may use substrates other than glycolate, explaining the lack of GOX activity we observe in vitro. Instead, PtGO1, while only very distantly related to previously characterized GOX proteins, evolved glycolate-oxidizing activity, as demonstrated by in gel activity assays and mass spectrometry analysis. PtGO1 localizes to mitochondria, consistent with previous suggestions that photorespiration in diatoms proceeds in these organelles. We conclude that the ancestors of diatoms evolved a unique alternative to oxidize photorespiratory glycolate: a mitochondrial dehydrogenase homologous to GOX able to use electron acceptors other than O₂.     

Inhibition of dark‐induced stomatal closure by fusicoccin involves a removal of hydrogen peroxide in guard cells of Vicia faba

Fusicoccin (FC) treatment prevents dark‐induced stomatal closure, the mechanism of which is still obscure. By using pharmacological approaches and laser‐scanning confocal microscopy, the relationship between FC inhibition of dark‐induced stomatal closure and the hydrogen peroxide (H₂O₂) levels in guard cells in broad bean was studied. Like ascorbic acid (ASA), a scavenger of H₂O₂ and diphenylene iodonium (DPI), an inhibitor of H₂O₂‐generating enzyme NADPH oxidase, FC was found to inhibit stomatal closure and reduce H₂O₂ levels in guard cells in darkness, indicating that FC‐caused inhibition of dark‐induced stomatal closure is related to the reduction of H₂O₂ levels in guard cells. Furthermore, like ASA, FC not only suppressed H₂O₂‐induced stomatal closure and H₂O₂ levels in guard cells treated with H₂O₂ in light, but also reopened the stomata which had been closed by darkness and reduced the level of H₂O₂ that had been generated by darkness, showing that FC causes H₂O₂ removal in guard cells. The butyric acid treatment simulated the effects of FC on the stomata treated with H₂O₂ and had been closed by dark, and on H₂O₂ levels in guard cells of stomata treated with H₂O₂ and had been closed by dark, and both FC and butyric acid reduced cytosol pH in guard cells of stomata treated with H₂O₂ and had been closed by dark, which demonstrates that cytosolic acidification mediates FC‐induced H₂O₂ removal. Taken together, our results provide evidence that FC causes cytosolic acidification, consequently induces H₂O₂ removal, and finally prevents dark‐induced stomatal closure.     

Does glycolate decarboxylation occur in intact leaf peroxisomes?

Release of ¹⁴CO₂ during metabolism of [1-¹⁴C]glycolate was studied in purified, intact and Triton X-100-solublized peroxisomes isolated from leaves of Secale cereale. Decarboxylation, apparently resulting from H₂O₂ attack on glyoxylate, was stimulated in both intact and solubilized peroxisomal preparations by the catalase inhibitors aminotriazole and sodium azide. CO₂ evolution was also observed in solubilized peroxisomes in the absence of inhibitor when an amino donor was not provided for conversion of glyoxylate to glycine. Loss of CO₂ from labelled glycolate in the absence of amino donors was reduced by addition of exogenous catalase to the reaction medium.Intact peroxisomes showed no glycolate decarboxylation whether or not amino donors were supplied. It appears that the CO₂ release from glycolate observed in previous investigations may be an artifact of the peroxisomal preparations and the assay systems used, and not a significant factor in photorespiratory metabolism in vivo.     

Experimental Evidence for a Hydride Transfer Mechanism in Plant Glycolate Oxidase Catalysis

In plants, glycolate oxidase is involved in the photorespiratory cycle, one of the major fluxes at the global scale. To clarify both the nature of the mechanism and possible differences in glycolate oxidase enzyme chemistry from C₃ and C₄ plant species, we analyzed kinetic parameters of purified recombinant C₃ (Arabidopsis thaliana) and C₄ (Zea mays) plant enzymes and compared isotope effects using natural and deuterated glycolate in either natural or deuterated solvent. The ¹²C/¹³C isotope effect was also investigated for each plant glycolate oxidase protein by measuring the ¹³C natural abundance in glycolate using natural or deuterated glycolate as a substrate. Our results suggest that several elemental steps were associated with an hydrogen/deuterium isotope effect and that glycolate α-deprotonation itself was only partially rate-limiting. Calculations of commitment factors from observed kinetic isotope effect values support a hydride transfer mechanism. No significant differences were seen between C₃ and C₄ enzymes.     

The effect of ocean acidification on symbiont photorespiration and productivity in Acropora formosa

Ocean acidification is expected to lower the net accretion of coral reefs yet little is known about its effect on coral photophysiology. This study investigated the effect of increasing CO₂ on photosynthetic capacity and photoprotection in Acropora formosa. The photoprotective role of photorespiration within dinoflagellates (genus Symbiodinium) has largely been overlooked due to focus on the presence of a carbon‐concentrating mechanism despite the evolutionary persistence of a Form II Rubisco. The photorespiratory fixation of oxygen produces phosphoglycolate that would otherwise inhibit carbon fixation though the Calvin cycle if it were not converted to glycolate by phosphoglycolate phosphatase (PGPase). Glycolate is then either excreted or dealt with by enzymes in the photorespiratory glycolate and/or glycerate pathways adding to the pool of carbon fixed in photosynthesis. We found that CO₂ enrichment led to enhanced photoacclimation (increased chlorophyll a per cell) to the subsaturating light levels. Light‐enhanced dark respiration per cell and xanthophyll de‐epoxidation increased, with resultant decreases in photosynthetic capacity (P_nmax) per chlorophyll. The conservative CO₂ emission scenario (A1B; 600–790 ppm) led to a 38% increase in the P_nmax per cell whereas the ‘business‐as‐usual’ scenario (A1F1; 1160–1500 ppm) led to a 45% reduction in PGPase expression and no change in P_nmax per cell. These findings support an important functional role for PGPase in dinoflagellates that is potentially compromised under CO₂ enrichment.     

Enhancement of Ethylene Release from Leaf Tissue during Glycolate Decarboxylation : A Possible Role for Photorespiration

When leaf discs of Xanthium strumarium L. and Salvia splendens L. are incubated in sealed flasks in the light, more C₂H₄ gas is released in the presence of added CO₂ (30-200 millimolar NaHCO₃) than without CO₂. In Salvia, the maximum rate of C₂H₄ release occurs when sufficient CO₂ (above 125 millimolar NaHCO₃) is added to saturate photosynthesis confirming previous studies. The maximum rate of C₂H₄ release from illuminated discs is similar to the rate in the dark with or without CO₂ in both species. Glycolate enhances a CO₂-dependent C₂H₄ evolution from illuminated leaf discs. However, the maximum rate of C₂H₄ release with glycolate is the same as that observed with saturating CO₂. When photosynthesis is inhibited by darkness or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, glycolate has no effect.

Studies with [2,3-¹⁴C]-1-aminocyclopropane-1-carboxylic acid (ACC) show that the pattern of C₂H₄ release and the specific activity of the ¹⁴C₂H₄ in the presence and absence of glycolate is similar to that described above, indicating that glycolate does not alter uptake of the exogenously supplied precursor (ACC) or stimulate C₂H₄ release from an endogenous source at appreciable rates. Glycolate oxidase in vitro generates H₂O₂ which stimulates a slow breakdown of ACC to C₂H₄, but since exogenous glycolate is oxidized to CO₂ in both the light and the dark it is argued that the glycolate-dependent increase in C₂H₄ release from illuminated leaf discs is not mediated directly by the action of enzymes of glycolate catabolism. The effects of glycolate and CO₂ are not easily explained by changes in stomatal resistance. The data support the view that glycolate decarboxylation at subsaturating levels of CO₂ in the light stimulates C₂H₄ release by raising the CO₂ level in the tissue.

     

Down‐regulation of catalase activity allows transient accumulation of a hydrogen peroxide signal in Chlamydomonas reinhardtii

In photosynthetic organisms, excess light is a stress that induces production of reactive oxygen species inside the chloroplasts. As a response, the capacity of antioxidative defence mechanisms increases. However, when cells of Chlamydomonas reinhardtii were shifted from dark to high light, a reversible partial inactivation of catalase activity was observed, which correlated with a transient increase in the level of H₂O₂ in the 10 μm range. This concentration range seems to be necessary to activate H₂O₂‐dependent signalling pathways stimulating the expression of H₂O₂ responsive genes, such as the heat shock protein HSP22C. Catalase knock‐down mutants had lost the transient accumulation of H₂O₂, suggesting that a decrease in catalase activity was the key element for establishing a transient H₂O₂ burst. Catalase was inactivated by a one‐electron event consistent with the reduction of a single cysteine. We propose that under high light intensity, the redox state of the photosynthetic electron transport chain is sensed and transmitted to the cytosol to regulate the catalase activity. This allows a transient accumulation of H₂O₂, inducing a signalling event that is transmitted to the nucleus to modulate the expression of chloroplast‐directed protection enzymes.     

iTRAQ‐based quantitative proteomic analysis reveals new metabolic pathways of wheat seedling growth under hydrogen peroxide stress

As an abundant ROS, hydrogen peroxide (H₂O₂) plays pivotal roles in plant growth and development. In this work, we conducted for the first time an iTRAQ‐based quantitative proteomic analysis of wheat seedling growth under different exogenous H₂O₂ treatments. The growth of seedlings and roots was significantly restrained by increased H₂O₂ concentration stress. Malondialdehyde, soluble sugar, and proline contents as well as peroxidase activity increased with increasing H₂O₂ levels. A total of 3 425 proteins were identified by iTRAQ, of which 157 showed differential expression and 44 were newly identified H₂O₂‐responsive proteins. H₂O₂‐responsive proteins were mainly involved in stress/defense/detoxification, signal transduction, and carbohydrate metabolism. It is clear that up‐regulated expression of signal transduction and stress/defence/detoxification‐related proteins under H₂O₂ stress, such as plasma membrane intrinsic protein 1, fasciclin‐like arabinogalactan protein, and superoxide dismutase, could contribute to H₂O₂ tolerance of wheat seedlings. Increased gluconeogenesis (phosphoenol‐pyruvate carboxykinase) and decreased pyruvate kinase proteins are potentially related to the higher H₂O₂ tolerance of wheat seedlings. A metabolic pathway of wheat seedling growth under H₂O₂ stress is presented.     

A phosphopantetheinyl transferase that is essential for mitochondrial fatty acid biosynthesis

In this study we report the molecular genetic characterization of the Arabidopsis mitochondrial phosphopantetheinyl transferase (mtPPT), which catalyzes the phosphopantetheinylation and thus activation of mitochondrial acyl carrier protein (mtACP) of mitochondrial fatty acid synthase (mtFAS). This catalytic capability of the purified mtPPT protein (encoded by AT3G11470) was directly demonstrated in an in vitro assay that phosphopantetheinylated mature Arabidopsis apo‐mtACP isoforms. The mitochondrial localization of the AT3G11470‐encoded proteins was validated by the ability of their N‐terminal 80‐residue leader sequence to guide a chimeric GFP protein to this organelle. A T‐DNA‐tagged null mutant mtppt‐1 allele shows an embryo‐lethal phenotype, illustrating a crucial role of mtPPT for embryogenesis. Arabidopsis RNAi transgenic lines with reduced mtPPT expression display typical phenotypes associated with a deficiency in the mtFAS system, namely miniaturized plant morphology, slow growth, reduced lipoylation of mitochondrial proteins, and the hyperaccumulation of photorespiratory intermediates, glycine and glycolate. These morphological and metabolic alterations are reversed when these plants are grown in a non‐photorespiratory condition (i.e. 1% CO₂ atmosphere), demonstrating that they are a consequence of a deficiency in photorespiration due to the reduced lipoylation of the photorespiratory glycine decarboxylase.     

Functional comparison of catalase genes in the elimination of photorespiratory H2O2 using promoter‐ and 3′‐untranslated region exchange experiments in the Arabidopsis cat2 photorespiratory mutant

Photorespiration‐associated production of H₂O₂ accounts for the majority of total H₂O₂ in leaves of C₃ plants and is mainly eliminated by catalases. In Arabidopsis, lack of CAT2, but not CAT1 or CAT3, results in growth suppression and a marked accumulation of H₂O₂ in leaves. To evaluate the contribution of individual catalase genes and their promoters to catalase function, we investigated the growth suppression and H₂O₂ accumulation phenotypes of Arabidopsis derivatives expressing catalase genes from heterologous CAT promoters in a cat2 mutant background. The expression of CAT2 from the CAT2 promoter restored the wild‐type phenotype in a cat2‐1 mutant, while CAT1 and CAT3 promoter‐driven expression of CAT2 did not. Ectopic expression of CAT3 from the CAT2 promoter also restored the normal phenotype, unlike that of CAT1 which required replacement of the CAT1 3′‐untranslated region (UTR) with that of CAT2. These results demonstrated that the photorespiratory role of CAT2 is determined mainly by the regulation of its promoter activity. The 3′‐UTR of CAT2 was vital for controlling CAT2 protein levels under photorespiratory conditions. Identification of component of heterotetramers catalase isoforms suggested that there is some functional redundancy between CAT2 and CAT1 and CAT3.     

Further studies on o(2)-resistant photosynthesis and photorespiration in a tobacco mutant with enhanced catalase activity

The increase in net photosynthesis in M₄ progeny of an O₂-resistant tobacco (Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O₂ has been confirmed and further investigated. Self-pollination of an M₃ mutant produced M₄ progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O₂-resistant photosynthesis. About 25% of the F₁ progeny of reciprocal crosses between the same M₃ mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO₂ as a percent of net photosynthesis in CO₂-free 21% O₂ and 36% less in CO₂-free 42% O₂ compared with wild type. The mutant leaf tissue also released less ¹⁴CO₂ per [1-¹⁴C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O₂-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O₂ where the stoichiometry of CO₂ release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H₂O₂.