Search

The influence of reduced glycine decarboxylase complex (GDC) activity on leaf atmosphere CO2 and 13CO2 exchange was tested in transgenic Oryza sativa with the GDC H - subunit knocked down in leaf mesophyll cells. Leaf measurements on transgenic gdch knockdown and wild-type plants were carried out in the light under photorespiratory and low photorespiratory conditions (i.e. 18.4 kPa and 1.84 kPa atmospheric O2 partial pressure, respectively), and in the dark. Under approximately current ambient O2 partial pressure (18.4 kPa p O2), the gdch knockdown plants showed an expected photorespiratory-deficient phenotype, with lower leaf net CO2 assimilation rates (A) than the wild-type. Additionally, under these conditions, the gdch knockdown plants had greater leaf net discrimination against 13CO2 (Δo) than the wild-type. This difference in Δo was in part due to lower 13C photorespiratory fractionation (f) ascribed to alternative decarboxylation of photorespiratory intermediates. Furthermore, the leaf dark respiration rate (R d) was enhanced and the 13CO2 composition of respired CO2 (δ13CRd) showed a tendency to be more depleted in the gdch knockdown plants. These changes in R d and δ13CRd were due to the amount and carbon isotopic composition of substrates available for dark respiration. These results demonstrate that impairment of the photorespiratory pathway affects leaf 13CO2 exchange, particularly the 13C decarboxylation fractionation associated with photorespiration. [ABSTRACT FROM AUTHOR]

The glycine decarboxylase complex (GDC) plays a critical role in the photorespiratory C2 cycle of C3 species by recovering carbon following the oxygenation reaction of ribulose-1,5-bisphosphate carboxylase/oxygenase. Loss of GDC from mesophyll cells (MCs) is considered a key early step in the evolution of C4 photosynthesis. To assess the impact of preferentially reducing GDC in rice MCs, we decreased the abundance of OsGDCH (Os10g37180) using an artificial microRNA (amiRNA) driven by a promoter that preferentially drives expression in MCs. GDC H- and P-proteins were undetectable in leaves of gdch lines. Plants exhibited a photorespiratory-deficient phenotype with stunted growth, accelerated leaf senescence, reduced chlorophyll, soluble protein and sugars, and increased glycine accumulation in leaves. Gas exchange measurements indicated an impaired ability to regenerate ribulose 1,5-bisphosphate in photorespiratory conditions. In addition, MCs of gdch lines exhibited a significant reduction in chloroplast area and coverage of the cell wall when grown in air, traits that occur during the later stages of C4 evolution. The presence of these two traits important for C4 photosynthesis and the non-lethal, down-regulation of the photorespiratory C2 cycle positively contribute to efforts to produce a C4 rice prototype. [ABSTRACT FROM AUTHOR]

The CO2 compensation point in the absence of day respiration (Γ*) is a key parameter for modelling leaf CO2 exchange. Γ* links the kinetics of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) with the stoichiometry of CO2 released per Rubisco oxygenation from photorespiration (α), two essential components of biochemical models of photosynthesis. There are two main gas-exchange methods for measuring Γ*: (i) the Laisk method, which requires estimates of mesophyll conductance to CO2 (gm) and (ii) measurements of O2 isotope exchange, which assume constant values of α and a fixed stoichiometry between O2 uptake and Rubisco oxygenation. In this study, the temperature response of Γ* measured using the Laisk and O2-exchange methods was compared under ambient (25 °C) and elevated (35 °C) temperatures to determine whether both methods yielded similar results. Previously published temperature responses of Γ* estimated with the Laisk and O2-exchange methods in Nicotiana tabacum demonstrated that the Laisk-derived model of Γ* was more sensitive to temperature compared with the O2-exchange model. Measurements in Arabidopsis thaliana indicated that the Laisk and O2-exchange methods produced similar Γ* at 25 °C; however, Γ* values from O2 exchange were lower at 35 °C compared with the Laisk method. Compared with a photorespiratory mutant (pmdh1pmdh2hpr) with increased α, wild-type (WT) plants had lower Laisk values of Γ* at 25 °C but were not significantly different at 35 °C. These differences between Laisk and O2 exchange values of Γ* at 35 °C could be explained by temperature sensitivity of α in WT and/or errors in the assumptions of O2 exchange. The differences between Γ* measured using the Laisk and O2-exchange method with temperature demonstrate that assumptions used to measure Γ*, and possibly the species-specific validity of these assumptions, need to be considered when modelling the temperature response of photosynthesis. [ABSTRACT FROM PUBLISHER]

ABSTRACT The efficiency of C4 photosynthesis in Zea mays, Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, 13CO2 photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C4 photosynthesis and leaf 13CO2 discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C4 and C3 cycles across photosynthetically active radiation (PAR). Leakiness ( φ), modelled using C4 light-limited photosynthesis equations ( φmod), matched values from the isotope method without simplifications ( φis) and increased slightly from high to low PAR in all species. However, simplifications of bundle-sheath [CO2] and respiratory fractionation lead to large overestimations of φ at low PAR with the isotope method. These species used different strategies to maintain similar φ. For example, Z. mays had large rates of the C4 cycle and low bundle-sheath cells CO2 conductance ( gbs). While F. bidentis had larger gbs but lower respiration rates and M. giganteus had less C4 cycle capacity but low gbs, which resulted in similar φ. This demonstrates that low gbs is important for efficient C4 photosynthesis but it is not the only factor determining φ. Additionally, these C4 species are able to optimize photosynthesis and minimize φ over a range of PARs, including low light. [ABSTRACT FROM AUTHOR]

ABSTRACT Photosynthetic carbon gain in plants using the C3 photosynthetic pathway is substantially inhibited by photorespiration in warm environments, particularly in atmospheres with low CO2 concentrations. Unlike C4 plants, C3 plants are thought to lack any mechanism to compensate for the loss of photosynthetic productivity caused by photorespiration. Here, for the first time, we demonstrate that the C3 plants rice and wheat employ a specific mechanism to trap and reassimilate photorespired CO2. A continuous layer of chloroplasts covering the portion of the mesophyll cell periphery that is exposed to the intercellular air space creates a diffusion barrier for CO2 exiting the cell. This facilitates the capture and reassimilation of photorespired CO2 in the chloroplast stroma. In both species, 24-38% of photorespired and respired CO2 were reassimilated within the cell, thereby boosting photosynthesis by 8-11% at ambient atmospheric CO2 concentration and 17-33% at a CO2 concentration of 200 µmol mol−1. Widespread use of this mechanism in tropical and subtropical C3 plants could explain why the diversity of the world's C3 flora, and dominance of terrestrial net primary productivity, was maintained during the Pleistocene, when atmospheric CO2 concentrations fell below 200 µmol mol−1. [ABSTRACT FROM AUTHOR]

Leakiness (φ), the proportion of carbon fixed by phosphoenolpyruvate carboxylation that leaks out of the bundle-sheath cells, determines C4 photosynthetic efficiency. Large increases in φ have been described at low irradiance. The underlying mechanisms for this increase remain uncertain, but changes in photorespiration or the energy partitioning between the C4 and C3 cycles have been suggested. Additionally, values of φ at low light could be magnified from assumptions made when comparing measured photosynthetic discrimination against 13C (Δ) with the theoretical formulation for Δ. For example, several simplifications are often made when modelling Δ to predict φ including: (i) negligible fractionation during photorespiration and dark respiration; (ii) infinite mesophyll conductance; and (iii) CO2 inside bundle-sheath cells (Cs) is much larger than values in mesophyll cells (Cm). Theoretical models for C4 photosynthesis and C4 Δ were combined to evaluate how these simplifications affect calculations of Δ and φ at different light intensities. It was demonstrated that the effects of photorespiratory fractionations and mesophyll conductance were negligible at low light. Respiratory fractionation was relevant only when the magnitude of the fractionation factor was artificially increased during measurements. The largest error in estimating φ occurred when assuming Cs was much larger than Cm at low light levels, when bundle-sheath conductance was large (gs), or at low O2 concentrations. Under these conditions, the simplified equation for Δ overestimated φ, and compromised comparisons between species with different gs, and comparisons across O2 concentrations. [ABSTRACT FROM PUBLISHER]

Summary • To determine the response of C4 plants to elevated CO2 it is necessary to establish whether young leaves have a fully developed C4 photosynthetic apparatus, and whether photosynthesis in these leaves is responsive to elevated CO2 . • The effect of free-air CO2 enrichment (FACE) on the photosynthetic development of the C4 crop Sorghum bicolor was monitored. Simultaneous measurements of chlorophyll a fluorescence and carbon assimilation were made to determine energy utilization, quantum yields of carbon fixation (φCO2 ) and photosystem II (φPSII), as well as photorespiration. • Assimilation in the second leaf of FACE plants was 37% higher than in control plants and lower apparent rates of photorespiration at growth CO2 concentrations were exhibited. In these leaves, φPSII : φCO2 was high at low atmospheric CO2 concentration (Ca) due to overcycling of the C4 pump and increased leakiness. As Ca increased, φPSII : φCO2 decreased as a greater proportion of energy derived from linear electron transfer was used by the C3 cycle. • The stimulation of C4 photosynthesis at elevated Ca in young leaves was partially due to suppressed photorespiration. Additionally, elevated Ca enhanced energy-use efficiency in young leaves, possibly by decreasing CO2 leakage from bundle sheath cells, and by decreasing overcycling of the C4 pump. [ABSTRACT FROM AUTHOR]

ABSTRACT Photosynthetic carbon gain in plants using the C3 photosynthetic pathway is substantially inhibited by photorespiration in warm environments, particularly in atmospheres with low CO2 concentrations. Unlike C4 plants, C3 plants are thought to lack any mechanism to compensate for the loss of photosynthetic productivity caused by photorespiration. Here, for the first time, we demonstrate that the C3 plants rice and wheat employ a specific mechanism to trap and reassimilate photorespired CO2. A continuous layer of chloroplasts covering the portion of the mesophyll cell periphery that is exposed to the intercellular air space creates a diffusion barrier for CO2 exiting the cell. This facilitates the capture and reassimilation of photorespired CO2 in the chloroplast stroma. In both species, 24-38% of photorespired and respired CO2 were reassimilated within the cell, thereby boosting photosynthesis by 8-11% at ambient atmospheric CO2 concentration and 17-33% at a CO2 concentration of 200 µmol mol−1. Widespread use of this mechanism in tropical and subtropical C3 plants could explain why the diversity of the world's C3 flora, and dominance of terrestrial net primary productivity, was maintained during the Pleistocene, when atmospheric CO2 concentrations fell below 200 µmol mol−1. [ABSTRACT FROM AUTHOR]