Photorespiratory Carbon Oxidation Research Articles

Rubisco and abiotic stresses in plants: Current assessment

Abiotic stresses are serious environmental factors militating against the production of many crops around the world. The consequence of this, is the difficulty of meeting the demands of the increasing world population. Aside from other negative effects, reduction in photosynthesis is an important feature of abiotic stresses. Abiotic stresses limit photosynthesis in a number of ways. The reduction in ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) content and activity is one of the paramount ways through which abiotic stresses affect photosynthesis. Rubisco is the CO2 fixing enzyme of photosynthesis and also catalyses the photo-respiratory carbon oxidation. The enzyme has low turnover and also copes with competitive inhibition by O2 . Hence, manipulating the enzyme in order to boost photosynthesis has been the target of scientists, especially in stressed environments. Based on recent studies, the mechanism of the harmful effects of abiotic stresses on Rubisco is examined in this review. In addition, the prevalent ways through which Rubisco can be made to thrive well despite the various abiotic stresses are evaluated. This review paper also outlines practicable approaches to promote existing ways of enhancing Rubisco tolerance to abiotic stresses in order to produce more crops with higher stress resilience.

Higher Atmospheric CO2 Levels Favor C3 Plants Over C4 Plants in Utilizing Ammonium as a Nitrogen Source.

Photosynthesis of wheat and maize declined when grown with NH4+ as a nitrogen (N) source at ambient CO2 concentration compared to those grown with a mixture of NO3– and NH4+, or NO3– as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO2 concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO2 levels. Hydroponic experiments were carried out using a C3 plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C4 plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO3– (NN), sole NH4+ (AN) and a mixture of both NO3– and NH4+ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO2 (Ca) concentration could be maintained: 280 μmol mol–1 (representing the pre-Industrial Revolution CO2 concentration of the 18th century), 400 μmol mol–1 (present level) and 550 μmol mol–1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (Pn) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [Je(PCR)] and electron flux for photorespiratory carbon oxidation [Je(PCO)] were also observed under AN for both species. However, the intercellular (Ci) and chloroplast (Cc) CO2 concentration increased with increasing atmospheric CO2 in C3 wheat but not in C4 maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of Pn under AN was relieved in wheat through higher CO2 levels, but that was not the case in maize. In conclusion, elevating atmospheric CO2 concentration increased Ci and Cc in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH4+ involvement in N nutrition of crops growing under different levels of CO2.

Enhanced Rubisco activation associated with maintenance of electron transport alleviates inhibition of photosynthesis under low nitrogen conditions in winter wheat seedlings.

Studying the response of photosynthesis to low nitrogen (N) and the underlying physiological mechanism can provide a theoretical basis for breeding N-efficient cultivars and optimizing N management. We conducted hydroponic experiments using two wheat (Triticum aestivum) cultivars, Zaoyangmai (low N sensitive) and Yangmai158 (low N tolerant), with either 0.25 mM N as a low N (LN) treatment or 5 mM N as a control. Under LN, a decrease in net photosynthetic rate (Pn) was attributed to reduction in the maximum Rubisco carboxylation rate, which then accelerated a reduction in the maximum ribulose-1,5-bisphosphate regeneration rate, and the reduction in Pn was 5-35% less in Yangmai158 than in Zaoyangmai. Yangmai158 maintained a 10-25% higher Rubisco concentration, especially in the upper leaves, and up-regulated Rubisco activase activity compared with Zaoyangmai to increase the Rubisco activation to sustain Rubisco carboxylation under LN conditions. In addition, Yangmai158 increased electron flux to the photorespiratory carbon oxidation cycle and alternative electron flux to maintain a faster electron transport rate and avoid photodamage. In conclusion, the LN-tolerant cultivar showed enhanced Rubisco activation and sustained electron transport to maintain a greater photosynthetic capacity under LN conditions.

Response of carbon assimilation and chlorophyll fluorescence to soybean leaf phosphorus across CO2: Alternative electron sink, nutrient efficiency and critical concentration

To evaluate the response of CO2 assimilation rate (PN) and various chlorophyll fluorescence (CF) parameters to phosphorus (P) nutrition, soybean plants were grown in controlled environment with sufficient (0.50mM) and deficient (0.10 and 0.01mM) phosphate (P) supply under ambient and elevated CO2 (aCO2, 400 and eCO2, 800μmolmol−1, respectively). Measurements were made at ambient (21%) and low (2%) O2 concentrations. Results showed strong correlation of leaf P concentration with PN and CF parameters. The P deficiency showed parallel decreases in PN, and CF parameters including quantum efficiency (Fv′/Fm′), quantum yield of photosystem II (ΦPSII), electron transport rate (JF), and photochemical quenching (qP). The Fv′/Fm′ decreased as a result of greater decline in maximal (Fm′) than minimal (Fo′) fluorescence. The eCO2 stimulated PN especially under higher leaf P concentrations. Low O2 also stimulated PN but only at aCO2. The photosynthetic carbon reduction (PCR, signified by PN) and photorespiratory carbon oxidation cycles (PCO, signified photorespiration as indicated by ratio of JF to gross PN and % increase in PN at 2% O2) was the major electron sinks. However, the presence of alternative electron sink was also evident as determined by the difference between the electron transport calculated from chlorophyll fluorescence and gas exchange measurements. Alternative electron sink declined at lower leaf P concentration suggesting its minor role in photochemical energy consumption, thus dissipation of the excess excitation pressure of PSII reaction center under P deficiency. The JF/PG and % increase in PN at 2 versus 21% O2 remained consistent across leaf P concentration suggesting PCO cycle as an important mechanism to dissipate excess excitation energy in P deficient leaves. The severe decline of Fv′/Fm′, ΦPSII, JF and qP under P deficiency also suggested the occurrences of excess radiant energy dissipation by non-photochemical quenching mechanisms. Critical leaf P concentration (CLPC) needed to achieve 90% of the maximum value was greater for PN than CF parameters. Moreover, CLPC was always higher at eCO2 suggesting increased sensitivity of soybean to P deficiency under eCO2. An increased phosphorus utilization efficiency of PN and CF parameters was also achieved but with the expense of net CO2 assimilation in P-deficient leaves.

Alternative electron sinks are crucial for conferring photoprotection in field-grown cotton under water deficit during flowering and boll setting stages.

To clarify the photoprotective mechanisms of cotton leaves under water deficit in the field, leaf gas exchange, chlorophyll a fluorescence as well as the corresponding physiological responses were examined in cotton (Gossypium hirsutum L.) to evaluate electron flux distribution. With increasing water deficit, net photosynthetic rate (Pn) significantly decreased, the total electron flux through PSII [Je(PSII)] gradually decreased and the fraction of electron flux required to sustain CO2 assimilation [Je(PCR)] markedly declined. Simultaneously, the ratio of quantum efficiency of PSII [Φ(PSII)] to the quantum efficiency of CO2 fixation [Φ(CO2)] increased, accompanied by an increase in the alternative electron flux (Ja). The enhanced alternative electron flux of O2-dependent Ja(O2-dependent) indicated that electrons had been transported to O2 in the Mehler-peroxide reaction (MPR) and that the remaining alternative electron flux Ja(O2-independent) had been used for nitrate reduction, as indicated by an increase in nitrate reductase (NR) and glutathinone reductase (GR) activities. In addition, mild water deficit increased the proportion of electron flux for the photorespiratory carbon oxidation [Je(PCO)]. Water deficit significantly increased surperoxide radical production rate (O2-•) and hydrogen peroxide content (H2O2), and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD) and catalase (CAT) in cotton leaves also increased under water deficit. Therefore, the Mehler-peroxidation reaction, photorespiration and nitrate reduction helped to dissipated excess light energy, being important photoprotective mechanisms for adapting the photosynthetic apparatus to mild and moderate water deficit in cotton.

Interactive effects of CO 2 enrichment and brassinosteroid on CO 2 assimilation and photosynthetic electron transport in Cucumis sativus

CO 2 enrichment and brassinosteroids (BR) both have positive impacts on photosynthesis and plant growth. To examine the interactive effect of CO 2 enrichment and BR on photosynthesis and plant growth, CO 2 assimilation, chlorophyll fluorescence quenching, carbohydrate metabolism, photosynthetic gene transcript and enzyme activity were analyzed in leaves of young plants of cucumber ( Cucumis sativus L.) in response to a doubling of growth CO 2 level, foliar BR application alone or in combination. Both CO 2 elevation and application of BR increased shoot biomass, leaf area, CO 2 assimilation, total soluble sugar and starch contents, transcript for photosynthetic gene and activity for enzymes involved in Benson–Calvin cycle but a combination of the two treatments resulted in a more significant effect. Although an elevation of CO 2 level had little effects on quantum efficiency of PSII ( Ф PSII), it significantly increased the electron flux for photosynthetic carbon reduction [ J e(PCR)] but decreased electron flux for photorespiratory carbon oxidation [ J e(PCO)]. In contrast, BR treatment increased Ф PSII and this increase in Ф PSII was associated with increased J e(PCR) and J e(PCO). Furthermore, a combined treatment of CO 2 elevation and BR resulted in an additive effect on PSII electron flux. However, alternative electron flux was almost unaltered after CO 2 enrichment and BR treatment. Thus, short term CO 2 elevation did not induce a down-regulation of photosynthesis and there was an additive effect between BR and CO 2 on the enhancement of CO 2 assimilation in leaves of young cucumber plants.

Alternative Electron Flows (Water–Water Cycle and Cyclic Electron Flow Around PSI) in Photosynthesis: Molecular Mechanisms and Physiological Functions

An electron flow in addition to the major electron sinks in C(3) plants [both photosynthetic carbon reduction (PCR) and photorespiratory carbon oxidation (PCO) cycles] is termed an alternative electron flow (AEF) and functions in the chloroplasts of leaves. The water-water cycle (WWC; Mehler-ascorbate peroxidase pathway) and cyclic electron flow around PSI (CEF-PSI) have been studied as the main AEFs in chloroplasts and are proposed to play a physiologically important role in both the regulation of photosynthesis and the alleviation of photoinhibition. In the present review, I discuss the molecular mechanisms of both AEFs and their functions in vivo. To determine their physiological function, accurate measurement of the electron flux of AEFs in vivo are required. Methods to assay electron flux in CEF-PSI have been developed recently and their problematic points are discussed. The common physiological function of both the WWC and CEF-PSI is the supply of ATP to drive net CO(2) assimilation. The requirement for ATP depends on the activities of both PCR and PCO cycles, and changes in both WWC and CEF-PSI were compared with the data obtained in intact leaves. Furthermore, the fact that CEF-PSI cannot function independently has been demonstrated. I propose a model for the regulation of CEF-PSI by WWC, in which WWC is indispensable as an electron sink for the expression of CEF-PSI activity.

Binding of 2CA1P (nocturnal inhibitor) to the active site of RubisCO using genetic algorithm (GA).

Ribulose-1, 5- Bisphosphate carboxylase/ oxygenase (RubisCO) catalyzes the first step in net photosynthetic assimilation and photorespiratory carbon oxidation. The activity of this enzyme is modulated in response to changes in light intensity as suggested in a number of early reports. Several studies found that the natural inhibitor 2CA1P is involved in the inhibition of the enzyme under reduced light intensity in rice (Oryza sativa). Due to the lack of studies and information on the interaction between this inhibitor and the active site of the enzyme, we attempted to predict the interaction between the amino acids in the active site and the inhibitor using both Hyperchem7.5 and GOLD software. After the docking; three possibilities having the highest fitness score were found (65.71, 64.72, 62.04), in these possibilities the inhibitor was bound to the enzyme, the phosphate and carboxylate groups in the same positions with a clear difference in the position of OH. In order to confirm the accuracy of the genetic algorithm, the artificial inhibitor 2CABP was docked back in the active site of the enzyme using the same parameters used in the case of the 2CA1P and the algorithm's predictions were compared with the experimentally observed binding mode. The results showed that the difference in the active sites before and after the docking was in the range of 0.93 A which indicated that the results were very accurate. Depending on this result it was concluded that the results obtained in the case of the 2CA1P were close to the experimental results.

Impact of light variation on development of photoprotection, antioxidants, and nutritional value in Lactuca sativa L.

Lettuce plants were grown at low (LL), middle (ML), and high light (HL) conditions to examine the relationship between photoacclimatory plasticity, light energy utilization, and antioxidant capacity. With the increase in light intensity from LL to ML, the energy flux via DeltapH- and xanthophylls-regulated thermal dissipation, fluorescence and constitutive thermal dissipation, and electron transport for photorespiratory carbon oxidation all increased significantly. However, plants at HL exhibited reduced electron transport for photosynthetic carbon reduction and decreased maximal photochemical efficiency of photosytem II (PSII) as compared to that at ML. Increasing light level significantly increased the alternative electron transport, O(2)(*-) production rate, and H(2)O(2) and malondialdehyde (MDA) contents followed by increased ferric reducing antioxidant power (FRAP) and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR). Moreover, plants exposed to HL showed higher nutritional value as indicated by the high contents of ascorbate, glutathione, carotenoids, and alpha-tocopherol. It was concluded that absorption of excess photon energy at high light was associated with increased antioxidant capacity and that produce quality could be improved by short-term exposure to suboptimum irradiance.

Compensatory acclimated mechanisms of photoprotection in a Xa mutant of Lycopersicon esculentum Mill.

To probe the role of xanthophylls in non-photochemical quenching (NPQ) and the compensatory acclimated photoprotection mechanisms, a tomato (Lycopersicon esculentum Mill. cv. Ailsa Craig) Xa mutant with deficit in lutein (L) and neoxanthin (N) contents was used. The Xa mutant showed lowered NPQ, an increased degree of de-epoxidation state [(A+Z)/(V+A+Z)], and decreases of photosystem 2 (PS2) antenna size. Although the Xa mutant had a CO2 assimilation rate similar to that of Ailsa Craig, it exhibited a much larger stomatal conductance (g s) than Ailsa Craig. Decreased electron flux in PS2 (J PS2) for the Xa mutant was associated with electron flux for photorespiratory carbon oxidation (J o) and alternative electron flux in PS2 (J a) while electron flux for photosynthetic carbon reduction (J c) was not different from Ailsa Craig. Moreover, the Xa mutant also exhibited higher activities of antioxidant enzymes, higher contents of ascorbate and glutathione, and lower contents of reactive oxygen species. Hence some compensatory acclimated mechanisms of photoprotection operated properly in the lack of NPQ and xanthophylls.

Widespread positive selection in the photosynthetic Rubisco enzyme.

BackgroundRubisco enzyme catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation and is responsible for almost all carbon fixation on Earth. The large subunit of Rubisco is encoded by the chloroplast rbcL gene, which is widely used for reconstruction of plant phylogenies due to its conservative nature. Plant systematicists have mainly used rbcL paying little attention to its function, and the question whether it evolves under Darwinian selection has received little attention. The purpose of our study was to evaluate how common is positive selection in Rubisco among the phototrophs and where in the Rubisco structure does positive selection occur.ResultsWe searched for positive selection in rbcL sequences from over 3000 species representing all lineages of green plants and some lineages of other phototrophs, such as brown and red algae, diatoms, euglenids and cyanobacteria. Our molecular phylogenetic analysis found the presence of positive selection in rbcL of most analyzed land plants, but not in algae and cyanobacteria. The mapping of the positively selected residues on the Rubisco tertiary structure revealed that they are located in regions important for dimer-dimer, intradimer, large subunit-small subunit and Rubisco-Rubisco activase interactions, and that some of the positively selected residues are close to the active site.ConclusionOur results demonstrate that despite its conservative nature, Rubisco evolves under positive selection in most lineages of land plants, and after billions of years of evolution Darwinian selection still fine-tunes its performance. Widespread positive selection in rbcL has to be taken into account when this gene is used for phylogenetic reconstructions.

An evolution-based analysis scheme to identify CO2/O2 specificity-determining factors for ribulose 1,5-bisphosphate carboxylase/oxygenase

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo) catalyzes a rate-limiting step in photosynthetic carbon assimilation (reacting with CO2) and its competitive photo-respiratory carbon oxidation (reacting with O2). RuBisCo enzyme with an enhanced CO2/O2 specificity would boost the ability to make great progress in agricultural production and environmental management. RuBisCos in marine non-green algae, resulting from an earlier endo-symbiotic event, diverge greatly from those in green plants and cyanobacteria and, further, have the highest CO2/O2 specificity whereas RuBisCos in cyanobacteria have the lowest. We assumed that there exist different levels of CO2/O2 specificity-determining factors, corresponding to different evolutionary events and specificity levels. Based on this assumption, we devised a scheme to identify these substrate-determining factors. From this analysis, we are able to discover different categories of the CO2/O2 specificity-determining factors that show which residue substitutions account for (relatively) small specificity changes, as happened in green plants, or a tremendous enhancement, as observed in marine non-green algae. Therefore, the analysis can improve our understanding of molecular mechanisms in the substrate specificity development and prioritize candidate specificity-determining surface residues for site-directed mutagenesis.

Modeling scalar and heat sources, sinks, and fluxes within a forest canopy during and after rainfall events

A one‐dimensional, multilayer model that infers the fluxes of heat, water vapor, and CO2 in a biosphere‐atmosphere system, as well as their sources and sinks, was described and evaluated in a secondary broad‐leaved forest. Coupling of a third‐order closure model allows the model to infer profiles of scalar fluxes, sinks, and sources with the computed velocity statistics. Furthermore, the model combines a leaf water process model and makes it possible to capture the impacts of rainfall events on the fluxes, sinks, and sources. The model test was conducted for the two periods (rain and nonrain events) to investigate whether the proposed model can reproduce the profiles of scalar fluxes, sinks, and sources in different weather conditions in the experimental forest. Modeled scalar and heat fluxes for the nonrainfall period showed generally good agreement with measurements. Model calculation showed that CO2 sink of the canopy during rainfall decreased, since the deep canopy layers work as a source due to least radiation transferred to the deep layers. After rainfall, evaporation from wetted leaves caused low vapor pressure deficit (VPD) between the leaf and air, creating favorable conditions for stomatal conductance and intercellular CO2 concentration, which consequently increases carboxylation rates in the photorespiratory carbon oxidation cycle. These results suggest that photosynthesis rates during and after rainfall events are primarily influenced by radiation as well as a variety of environmental factors, particularly VPD. The model results also indicated that the maximum source/sink strengths of heat and H2O were distributed within the top 30% of the canopy and that the CO2 distribution had the feature of the vegetation shape. These differences imply that the roughness heights of individual components are different, causing different effective exchange heights.

Rubisco: structure, regulatory interactions, and possibilities for a better enzyme.

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.

Crystal structure of activated ribulose-1,5-bisphosphate carboxylase/oxygenase from green alga Chlamydomonas reinhardtii complexed with 2-carboxyarabinitol-1,5-bisphosphate

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the initial steps of photosynthetic carbon reduction and photorespiratory carbon oxidation cycles by combining CO 2 and O 2, respectively, with ribulose-1,5-bisphosphate. Many photosynthetic organisms have form I rubiscos comprised of eight large (L) and eight small (S) subunits. The crystal structure of the complex of activated rubisco from the green alga Chlamydomonas reinhardtii and the reaction intermediate analogue 2-carboxyarabinitol-1,5-bisphosphate (2-CABP) has been solved at 1.84 Å resolution ( R cryst of 15.2 % and R free of 18.1 %). The subunit arrangement of Chlamydomonas rubisco is the same as those of the previously solved form I rubiscos. Especially, the present structure is very similar to the activated spinach structure complexed with 2-CABP in the L-subunit folding and active-site conformation, but differs in S-subunit folding. The central insertion of the Chlamydomonas S-subunit forms the longer βA-βB loop that protrudes deeper into the solvent channel of rubisco than higher plant, cyanobacterial, and red algal (red-like) βA-βB loops. The C-terminal extension of the Chlamydomonas S-subunit does not protrude into the solvent channel, unlike that of the red algal S-subunit, but lies on the protein surface anchored by interactions with the N-terminal region of the S-subunit. Further, the present high-resolution structure has revealed novel post-translational modifications. Residue 1 of the S-subunit is N α-methylmethionine, residues 104 and 151 of the L-subunit are 4-hydroxyproline, and residues 256 and 369 of the L-subunit are S γ-methylcysteine. Furthermore, the unusual electron density of residue 471 of the L-subunit, which has been deduced to be threonine from the genomic DNA sequence, suggests that the residue is isoleucine produced by RNA editing or O γ-methylthreonine.

Growth in Elevated CO2 Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition

Biochemically based models of C(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO(2) partial pressure (pCO(2)) will increase light-saturated linear electron flow through photosystem II (J(t)). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J(c)) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J(o)). Where elevated pCO(2) increases J(t), then the ratio of absorbed energy dissipated photochemically to that dissipated non-photochemically will rise. These predictions were tested on Dactylis glomerata grown in fully controlled environments, at either ambient (35 Pa) or elevated (65 Pa) pCO(2), and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J(t) was significantly higher in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). This was due to a significant increase in J(c) exceeding any suppression of J(o). This increase in photochemistry at elevated pCO(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J(t) was significantly lower in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). Elevated pCO(2) again suppressed J(o); however growth in elevated pCO(2) resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO(2).

A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2 -grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3− and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3– of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3− channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3−, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.

A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2-grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3− and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3– of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3− channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3−, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.

Determination of the rate of photoreduction of O2 in the water-water cycle in watermelon leaves and enhancement of the rate by limitation of photosynthesis.

A study was performed to determine how the electron fluxes for the photosynthetic carbon reduction (PCR) and the photorespiratory carbon oxidation (PCO) cycles affect the photoreduction of O2 at PSI, which is the limiting step in the water-water cycle. Simultaneous measurements were made of CO2-gas exchange, transpiration and quantum yield of PSII [phi(PSII)] using leaves of watermelon (Citrullus lanatus). The total electron flux in PSII[Je(PSII)], as estimated from phi(PSII), was always larger than the total electron flux required for the PCR and PCO cycles at various partial pressures of CO2 and O2 and 1,100 micromol photons m(-2)s(-1). This observation suggested the existence of an alternative electron flux (Ja). Ja was divided into O2-dependent [Ja(O2-depend)] and O2-independent [Ja(O2-independ)] components. The magnitude of half Ja(O2-depend), 7.5 to 9.5 micromol e- m(-2)s(-1), and its apparent Km for O2, about 8.0 kPa, could be accounted for by the photoreduction of O2 at PSI either mediated by ferredoxin or catalyzed by monodehydroascorbate reductase. The results indicated that Ja(O2-depend) was driven by the water-water cycle. A decrease in the intercellular partial pressure of CO2 from 23 to 5.0 Pa at 21 kPa O2 enhanced Ja(O2-depend) by a factor of 1.3. Saturation of the activities of both the PCR and PCO cycles by increasing the photon flux density induced Ja. These results indicate the electron flux in PSII that exceeds the flux required for the PCR and PCO cycles induces the photoreduction of O2 in the water-water cycle.

Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction.

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of the small subunit of Rubisco were used to investigate the role of O2 as an electron acceptor during photosynthesis. The reduction in Rubisco has reduced the capacity for CO2-fixation in these plants without a similar reduction in electron transport capacity. Concurrent measurements of chlorophyll fluorescence and CO2 assimilation at different CO2 and O2 partial pressures showed close linear relationships between chloroplast electron transport rates calculated from chlorophyll fluorescence and those calculated from CO2-fixation. These relationships were similar for wild-type and transgenic plants, indicating that the reduced capacity for CO2 fixation in the transgenic plants did not result in extra electron transport not associated with the photosynthetic carbon reduction (PCR) or photorespiratory carbon oxidation (PCO) cycle. This was further investigated with mass spectrometric measurements of 16O2 and 18O2 exchange made concurrently with measurements of chlorophyll fluorescence. In all tobacco lines the rates of 18O2 uptake in the dark were similar to the 18O2 uptake rates at very high CO2 partial pressures in the light. Rates of oxygenase activity calculated from 18O2 uptake at the compensation point were linearly related to the Rubisco content of leaves. The ratios of oxygenase to carboxylase rates were calculated from measurements of 16O2 evolution and 18O2 uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO2 compensation points. It is concluded that although there may be some electron transport to O2 to balance conflicting demands of NADPH to ATP requirements, this flux must decrease in proportion with the reduced demand for ATP and NADPH consumption in the transgenic lines. The altered balance between electron transport and Rubisco capacity, however, does not result in rampant electron transport to O2 or other electron transport acceptors in the absence of PCR and PCO cycle activity.