Characterization of factors affecting the activity of photosystem I cyclic electron transport in chloroplasts

PSI cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the antimycin A-sensitive pathway is the main route of electrons in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, its function is still unclear, and there are numerous debates on the rate of electron transport in vivo and its regulation. To assess how PGR5-dependent PSI cyclic electron transport is regulated in vivo, we characterized its activity in ruptured chloroplasts isolated from Arabidopsis thaliana. The activity of ferredoxin (Fd)-dependent plastoquinone (PQ) reduction in the dark is impaired in the pgr5 mutant. Alkalinization of the reaction medium enhanced the activity of Fd-dependent PQ reduction in the wild type. Even weak actinic light (AL) illumination also markedly activated PGR5-dependent PSI cyclic electron transport in ruptured chloroplasts. Even in the presence of linear electron transport [11 mumol O2 (mg Chl)(-1) h(-1)], PGR5-dependent PSI electron transport was detected as a difference in Chl fluorescence levels in ruptured chloroplasts. In the wild type, PGR5-dependent PSI cyclic electron transport competed with NADP+ photoreduction. These results suggest that the rate of PGR5-dependent PSI cyclic electron transport is high enough to balance the production ratio of ATP and NADPH during steady-state photosynthesis, consistently with the pgr5 mutant phenotype. Our results also suggest that the activity of PGR5-dependent PSI cyclic electron transport is regulated by the redox state of the NADPH pool.     

PROTON GRADIENT REGULATION5 Is Essential for Proper Acclimation of Arabidopsis Photosystem I to Naturally and Artificially Fluctuating Light Conditions

In nature, plants are challenged by constantly changing light conditions. To reveal the molecular mechanisms behind acclimation to sometimes drastic and frequent changes in light intensity, we grew Arabidopsis thaliana under fluctuating light conditions, in which the low light periods were repeatedly interrupted with high light peaks. Such conditions had only marginal effect on photosystem II but induced damage to photosystem I (PSI), the damage being most severe during the early developmental stages. We showed that PROTON GRADIENT REGULATION5 (PGR5)-dependent regulation of electron transfer and proton motive force is crucial for protection of PSI against photodamage, which occurred particularly during the high light phases of fluctuating light cycles. Contrary to PGR5, the NAD(P)H dehydrogenase complex, which mediates cyclic electron flow around PSI, did not contribute to acclimation of the photosynthetic apparatus, particularly PSI, to rapidly changing light intensities. Likewise, the Arabidopsis pgr5 mutant exhibited a significantly higher mortality rate compared with the wild type under outdoor field conditions. This shows not only that regulation of PSI under natural growth conditions is crucial but also the importance of PGR5 in PSI protection.     

Overproduction of PGR5 enhances the electron sink downstream of photosystem I in a C4 plant,Flaveria bidentis

C₄ plants can fix CO₂ efficiently using CO₂‐concentrating mechanisms (CCMs), but they require additional ATP. To supply the additional ATP, C₄ plants operate at higher rates of cyclic electron transport around photosystem I (PSI), in which electrons are transferred from ferredoxin to plastoquinone. Recently, it has been reported that the NAD(P)H dehydrogenase‐like complex (NDH) accumulated in the thylakoid membrane in leaves of C₄ plants, making it a candidate for the additional synthesis of ATP used in the CCM. In addition, C₄ plants have higher levels of PROTON GRADIENT REGULATION 5 (PGR5) expression, but it has been unknown how PGR5 functions in C₄ photosynthesis. In this study, PGR5 was overexpressed in a C₄ dicot, Flaveria bidentis. In PGR5‐overproducing (OP) lines, PGR5 levels were 2.3‐ to 3.0‐fold greater compared with wild‐type plants. PGR5‐like PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1), which cooperates with PGR5, increased with PGR5. A spectroscopic analysis indicated that in the PGR5‐OP lines, the acceptor side limitation of PSI was reduced in response to a rapid increase in photon flux density. Although it did not affect CO₂ assimilation, the overproduction of PGR5 contributed to an enhanced electron sink downstream of PSI.     

Cooperation of photosystems II and I in leaves as analyzed by simultaneous measurements of chlorophyll fluorescence and transmittance at 800 nm

Parallel measurements of CO2 assimilation, Chl fluorescence and 800 nm transmittance were carried out on intact leaves of wild type and cytochrome b6/f deficient transgenic tobacco grown at two different light intensities and temperatures, with the aim to diagnose processes limiting quantum yield of photosynthesis and investigate their adaptations to growth conditions. Relative optical cross-sections of PSII and PSI antennae were calculated from measured gas exchange rates and fluorescence-related losses at PSII and P700 oxidation-related losses at PSI. In nonstress conditions (high light grown wild type and low light grown antisense type) optimal relative optical cross-section of PSII (aII) was 0.48-0.51 and that of PSI (aI) was 0.38-0.40, leaving a non-photosynthetic absorption cross-section (a0) of 0.09-0.14 for nitrite assimilation and absorption in PSII beta and other photosynthetically inactive pigments. Stress conditions (low light grown wild type and high light grown antisense type, elevated growth temperatures) tend to increase a0 and decrease PSII antenna cross-section more than that of PSI antenna, but this rule is reversed during senescence.     

Consequences of photosystem‐I damage and repair on photosynthesis and carbon use in Arabidopsis thaliana

Natural growth environments commonly include fluctuating conditions that can disrupt the photosynthetic energy balance and induce photoinhibition through inactivation of the photosynthetic apparatus. Photosystem II (PSII) photoinhibition is efficiently reversed by the PSII repair cycle, whereas photoinhibited photosystem I (PSI) recovers much more slowly. In the current study, treatment of the Arabidopsis thaliana mutant proton gradient regulation 5 (pgr5) with excess light was used to compromise PSI functionality in order to investigate the impact of photoinhibition and subsequent recovery on photosynthesis and carbon metabolism. The negative impact of PSI photoinhibition on CO₂ fixation was especially deleterious under low irradiance. Impaired starch accumulation after PSI photoinhibition was reflected in reduced respiration in the dark, but this was not attributed to impaired sugar synthesis. Normal chloroplast and mitochondrial metabolisms were shown to recover despite the persistence of substantial PSI photoinhibition for several days. The results of this study indicate that the recovery of PSI function involves the reorganization of the light‐harvesting antennae, and suggest a pool of surplus PSI that can be recruited to support photosynthesis under demanding conditions.     

The role of PGR5 in the redox poising of photosynthetic electron transport

The pgr5 mutant of Arabidopsis thaliana has been described as being deficient in cyclic electron flow around photosystem I, however, the precise role of the PGR5 protein remains unknown. To address this issue, photosynthetic electron transport was examined in intact leaves of pgr5 and wild type A. thaliana. Based on measurements of the kinetics of P700 oxidation in far red light and re-reduction following oxidation in the presence of DCMU, we conclude that this mutant is able to perform cyclic electron flow at a rate similar to the wild type. The PGR5 protein is therefore not essential for cyclic flow. However, cyclic flow is affected by the pgr5 mutation under conditions where this process is normally enhanced in wild type leaves, i.e. high light or low CO(2) concentrations resulted in enhancement of cyclic electron flow. This suggests a different capacity to regulate cyclic flow in response to environmental stimuli in the mutant. We also show that the pgr5 mutant is affected in the redox poising of the chloroplast, with the electron transport chain being substantially reduced under most conditions. This may result in defective feedback regulation of photosynthetic electron transport under some conditions, thus providing a rationale for the reduced efficiency of cyclic electron flow.     

Contribution of NDH-dependent cyclic electron transport around photosystem I to the generation of proton motive force in the weak mutant allele of pgr5

In angiosperms, cyclic electron transport (CET) around photosystem I (PSI) consists of two pathways, depending on PGR5/PGRL1 proteins and the chloroplast NDH complex. In single mutants defective in chloroplast NDH, photosynthetic electron transport is only slightly affected at low light intensity, but in double mutants impaired in both CET pathways photosynthesis and plant growth are severely affected. The question is whether this strong mutant phenotype observed in double mutants can be simply explained by the additive effect of defects in both CET pathways. In this study, we used the weak mutant allele of pgr5-2 for the background of double mutants to avoid possible problems caused by the secondary effects due to the strong mutant phenotype. In two double mutants, crr2-2 pgr5-2 and ndhs-1 pgr5-2, the plant growth was unaffected and linear electron transport was only slightly affected. However, NPQ induction was more severely impaired in the double mutants than in the pgr5-2 single mutant. A similar trend was observed in the size of the proton motive force. Despite the slight reduction in photosystem II parameters, PSI parameters were severely affected in the pgr5-2 single mutant, the phenotype that was further enhanced by adding the NDH defects. Despite the lack of ?pH-dependent regulation at the cytochrome b₆f complex (donor-side regulation of PSI), the plastoquinone pool was more reduced in the double mutants than in the pgr5-2 single mutants. This phenotype suggests that both PGR5/PGRL1- and NDH-dependent CET contribute to supply sufficient acceptors from PSI by balancing the ATP/NADPH production ratio.     

Expressing an RbcS Antisense Gene in Transgenic Flaveria bidentis Leads to an Increased Quantum Requirement for CO2 Fixed in Photosystems I and II

It was previously shown with concurrent measurements of gas exchange and carbon isotope discrimination that the reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase by an antisense gene construct in transgenic Flaveria bidentis (a C4 species) leads to reduced CO2 assimilation rates, increased bundle-sheath CO2 concentration, and leakiness (defined as the ratio of CO2 leakage to the rate of C4 acid decarboxylation; S. von Caemmerer, A. Millegate, G.D. Farquhar, R.T. Furbank [1997] Plant Physiol 113: 469-477). Increased leakiness in the transformants should result in an increased ATP requirement per mole of CO2 fixed and a change in the ATP-to-NADPH demand. To investigate this, we compared measurements of the quantum yield of photosystem I and II ([phi]PSI and [phi]PSII) with the quantum yield of CO2 fixation ([phi]CO2) in control and transgenic F. bidentis plants in various conditions. Both [phi]PSI/[phi]CO2 and [phi]PSII/[phi]CO2 increased with a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase content, confirming an increase in leakiness. In the wild type the ratio of [phi]PSI to [phi]PSII was constant at different irradiances but increased with irradiance in the transformants, suggesting that cyclic electron transport may be higher in the transformants. To evaluate the relative contribution of cyclic or linear electron transport to extra ATP generation, we developed a model that links leakiness, ATP/NADP requirements, and quantum yields. Despite some uncertainties in the light distribution between photosystem I and II, we conclude from the increase of [phi]PSII/[phi]CO2 in the transformants that cyclic electron transport is not solely responsible for ATP generation without NADPH production.     

The Absence of Alternative Oxidase AOX1A Results in Altered Response of Photosynthetic Carbon Assimilation to Increasing CO2 in Arabidopsis thaliana

In higher plants, the mitochondrial electron transport chain has non-phosphorylating alternative pathways that include the alternative terminal oxidase (AOX). This alternative pathway has been suggested to act as a sink for dissipating excess reducing power, minimizing oxidative stress and possibly optimizing photosynthesis in response to changing conditions. The expression patterns of the AOX genes have been well characterized under different growth conditions, particularly in response to light and temperature stress. Additionally, it has been suggested that mitochondrial electron transport is important for avoiding chloroplast over-reduction and balancing energy partitioning among photosynthesis, photorespiration and respiration. Nonetheless, the role AOX plays in optimizing photosynthetic carbon metabolism is unclear. Therefore, the response of photosynthesis to the disruption of AOX was investigated in the Arabidopsis thaliana T-DNA mutant aox1a (SALK_084897). Gas exchange analysis revealed a lower net CO(2) assimilation rate (A) at high CO(2) concentrations in the aox1a mutant compared to wild type. This decrease in A was accompanied by a lower maximum electron transport rate and quantum yield of PSII, and higher excitation pressure on PSII and non-photochemical quenching. The aox1a mutant also exhibited a lower estimated rate of ribulose 1,5-bisphosphate regeneration, and the ribulose 1,5-bisphosphate content was lower at high CO(2) concentrations, suggesting an ATP limitation of the Calvin-Benson cycle. Additionally, the activity of the malate-oxaloacetate shuttle was lower in the mutant compared to wild type. These results indicate that AOX is important for optimizing rates of photosynthetic CO(2) assimilation in response to rising CO(2) concentration by balancing the NAD(P)H/ATP ratio and rates of ribulose 1,5-bisphosphate regeneration within the chloroplast.     

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b₆f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI – contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b₆f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.     

Photosynthetic parameters of leaves of wild type and Cyt b6/f deficient transgenic tobacco studied by CO2 uptake and transmittance at 800 nm

Parallel measurements of CO2 assimilation and 800 nm transmission were carried out on intact leaves of wild type and cytochrome b6/f deficient transgenic tobacco grown at different light intensities and temperatures, with the aim to diagnose rate-limiting processes in photosynthesis and investigate their adaptations to growth conditions. Maximum CO2- and light-saturated photosynthetic rate, mesophyll conductance, assimilatory charge and specific carboxylation efficiency were determined from CO2 fixation measurements and postillumination P700 rereduction time constant was measured from the transient of the 800 nm signal. Results show that growth conditions continue to modulate the expression of genes in transgenic plants, interfering with the antisense modulation, but under all environmental conditions the antisense treatment to decrease Cyt b6/f complexes ensured that the control of electron/proton transport rate by proton backpressure on the PSI donor side was stronger than the control by electron backpressure on the PSI acceptor side. Coordinated control of gene expression and enzyme activation ensures that different parts of the photosynthetic machinery--components of the electron transport chain, ribulose-1,5-bisphosphate carboxylase/oxygenase, enzymes of the sucrose and starch synthesis chains-are synthesized more or less proportionally under different environmental conditions and in case of mild genetic interference.     

A complex containing PGRL1 and PGR5 is involved in the switch between linear and cyclic electron flow in Arabidopsis

During photosynthesis, two photoreaction centers located in the thylakoid membranes of the chloroplast, photosystems I and II (PSI and PSII), use light energy to mobilize electrons to generate ATP and NADPH. Different modes of electron flow exist, of which the linear electron flow is driven by PSI and PSII, generating ATP and NADPH, whereas the cyclic electron flow (CEF) only generates ATP and is driven by the PSI alone. Different environmental and metabolic conditions require the adjustment of ATP/NADPH ratios and a switch of electron distribution between the two photosystems. With the exception of PGR5, other components facilitating CEF are unknown. Here, we report the identification of PGRL1, a transmembrane protein present in thylakoids of Arabidopsis thaliana. Plants lacking PGRL1 show perturbation of CEF, similar to PGR5-deficient plants. We find that PGRL1 and PGR5 interact physically and associate with PSI. We therefore propose that the PGRL1-PGR5 complex facilitates CEF in eukaryotes.     

Commonalities and specialties in photosynthetic functions of PROTON GRADIENT REGULATION5 variants in Arabidopsis

The PROTON GRADIENT REGULATION5 (PGR5) protein is required for trans-thylakoid proton gradient formation and acclimation to fluctuating light (FL). PGR5 functionally interacts with two other thylakoid proteins, PGR5-like 1 (PGRL1) and 2 (PGRL2); however, the molecular details of these interactions are largely unknown. In the Arabidopsis (Arabidopsis thaliana) pgr5-1 mutant, the PGR5_G130S protein accumulates in only small amounts. In this work, we generated a knockout allele of PGR5 (pgr5-Cas) using CRISPR-Cas9 technology. Like pgr5-1, pgr5-Cas is seedling-lethal under FL, but photosynthesis and particularly cyclic electron flow, as well as chlorophyll content, are less severely affected in both pgr5-Cas and pgrl1ab (which lacks PGRL1 and PGR5) than in pgr5-1. These differences are associated with changes in the levels of 260 proteins, including components of the Calvin–Benson cycle, photosystems II and I, and the NDH complex, in pgr5-1 relative to the wild type (WT), pgr5-Cas, and pgrl1ab. Some of the differences between pgr5-1 and the other mutant lines could be tentatively assigned to second-site mutations in the pgr5-1 line, identified by whole-genome sequencing. However, others, particularly the more pronounced photosynthetic defects and PGRL1 depletion (compared to pgr5-Cas), are clearly due to specific negative effects of the amino-acid substitution in PGR5_G130S, as demonstrated by complementation analysis. Moreover, pgr5-1 and pgr5-Cas plants are less tolerant to long-term exposure to high light than pgrl1ab plants. These results imply that, in addition to the previously reported necessity of PGRL1 for optimal PGR5 function, PGR5 is required alongside PGRL1 to avoid harmful effects on plant performance.

Plants completely devoid of the PROTON GRADIENT REGULATION5 (PGR5) protein are less affected in chloroplast proteome and photosynthesis than plants with the mutated version of the protein PGR5_G130S.     

The function of chloroplastic NAD(P)H dehydrogenase in tobacco during chilling stress under low irradiance

The function of chloroplastic NAD(P)H dehydrogenase (NDH) was examined by comparing a tobacco transformant (DeltandhB) in which the ndhB gene had been disrupted with its wild type, upon exposure to chilling temperature (4 degrees C) under low irradiance (100 micro mol m(-2) s(-1) PFD). During the chilling stress, the maximum photochemical efficiency of PSII (F(v)/F(m)) decreased markedly in both the wild type and DeltandhB. However, both F(v)/F(m) and P700(+), as well as the PSII-driven electron transport rate (ETR), in DeltandhB were lower than that in the wild type, implying that NDH-dependent cyclic electron flow around PSI functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under the stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (qf) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), were distinguishable in DeltandhB from those in the wild type. The lower NPQ in DeltandhB might be related to an inefficient proton gradient across thylakoid membranes (DeltapH) because of lacking an NDH-dependent cyclic electron flow around PSI at chilling temperature under low irradiance.     

Compensation for PSII photoinactivation by regulated non-photochemical dissipation influences the impact of photoinactivation on electron transport and CO2 assimilation

The extent to which PSII photoinactivation affects electron transport (PhiPSII) and CO2 assimilation remains controversial, in part because it frequently occurs alongside inactivation of other components of photosynthesis, such as PSI. By manipulating conditions (darkness versus low light) after a high light/low temperature treatment, we examined the influence of different levels of PSII inactivation at the same level of PSI inactivation on PhiPSII and CO2 assimilation for Arabidopsis. Furthermore, we compared PhiPSII at high light and optimum temperature for wild-type Arabidopsis and a mutant (npq4-1) with impaired capacities for energy dissipation. Levels of PSII inactivation typical of natural conditions (< 50%) were not associated with decreases in PhiPSII and CO2 assimilation at photon flux densities (PFDs) above 150 micromol m(-2) s(-1). At higher PFDs, the light energy being absorbed was in excess of the energy that could be utilized by downstream processes. Arabidopsis plants downregulate PSII activity to dissipate such excess in accordance with the level of PSII photoinactivation that also serves to dissipate absorbed energy. Therefore, the overall levels of non-photochemical dissipation and the efficiency of photochemistry were not affected by PSII inactivation at high PFD. Under low PFD conditions, such compensation is not necessary, because the amount of light energy absorbed is not in excess of that needed for photochemistry, and inactive PSII complexes are dissipating energy. We conclude that moderate photoinactivation of PSII complexes will only affect plant performance when periods of high PFD are followed by periods of low PFD.     

Chlororespiration and cyclic electron flow around PSI during photosynthesis and plant stress response

Besides major photosynthetic complexes of oxygenic photosynthesis, new electron carriers have been identified in thylakoid membranes of higher plant chloroplasts. These minor components, located in the stroma lamellae, include a plastidial NAD(P)H dehydrogenase (NDH) complex and a plastid terminal plastoquinone oxidase (PTOX). The NDH complex, by reducing plastoquinones (PQs), participates in one of the two electron transfer pathways operating around photosystem I (PSI), the other likely involving a still uncharacterized ferredoxin-plastoquinone reductase (FQR) and the newly discovered PGR5. The existence of a complex network of mechanisms regulating expression and activity of the NDH complex, and the presence of higher amounts of NDH complex and PTOX in response to environmental stress conditions the phenotype of mutants, indicate that these components likely play a role in the acclimation of photosynthesis to changing environmental conditions. Based on recently published data, we propose that the NDH-dependent cyclic pathway around PSI participates to the ATP supply in conditions of high ATP demand (such as high temperature or water limitation) and together with PTOX regulates cyclic electron transfer activity by tuning the redox state of intersystem electron carriers. In response to severe stress conditions, PTOX associated to the NDH and/or the PGR5 pathway may also limit electron pressure on PSI acceptor and prevent PSI photoinhibition.     

The role of PGR5 in the redox poising of photosynthetic electron transport

The pgr5 mutant of Arabidopsis thaliana has been described as being deficient in cyclic electron flow around photosystem I, however, the precise role of the PGR5 protein remains unknown. To address this issue, photosynthetic electron transport was examined in intact leaves of pgr5 and wild type A. thaliana. Based on measurements of the kinetics of P700 oxidation in far red light and re-reduction following oxidation in the presence of DCMU, we conclude that this mutant is able to perform cyclic electron flow at a rate similar to the wild type. The PGR5 protein is therefore not essential for cyclic flow. However, cyclic flow is affected by the pgr5 mutation under conditions where this process is normally enhanced in wild type leaves, i.e. high light or low CO₂ concentrations resulted in enhancement of cyclic electron flow. This suggests a different capacity to regulate cyclic flow in response to environmental stimuli in the mutant. We also show that the pgr5 mutant is affected in the redox poising of the chloroplast, with the electron transport chain being substantially reduced under most conditions. This may result in defective feedback regulation of photosynthetic electron transport under some conditions, thus providing a rationale for the reduced efficiency of cyclic electron flow.     

Does Decrease in Ribulose-1,5-Bisphosphate Carboxylase by Antisense RbcS Lead to a Higher N-Use Efficiency of Photosynthesis under Conditions of Saturating CO2 and Light in Rice Plants?

Rice (Oryza sativa L.) plants with decreased ribulose-1,5-bisphosphate carboxylase (Rubisco) were obtained by transformation with the rice rbcS antisense gene under the control of the rice rbcS promoter. The primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO2 partial pressures for photosynthesis under conditions of high irradiance and 25[deg]C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. The R1 selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R1 population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO2), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO2, (100-115 Pa CO2) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO2 and high irradiance.     

A balanced PGR5 level is required for chloroplast development and optimum operation of cyclic electron transport around photosystem I

PSI cyclic electron transport contributes markedly to photosynthesis and photoprotection in flowering plants. Although the thylakoid protein PGR5 (Proton Gradient Regulation 5) has been shown to be essential for the main route of PSI cyclic electron transport, its exact function remains unclear. In transgenic Arabidopsis plants overaccumulating PGR5 in the thylakoid membrane, chloroplast development was delayed, especially in the cotyledons. Although photosynthetic electron transport was not affected during steady-state photosynthesis, a high level of non-photochemical quenching (NPQ) was transiently induced after a shift of light conditions. This phenotype was explained by elevated activity of PSI cyclic electron transport, which was monitored in an in vitro system using ruptured chloroplasts, and also in leaves. The effect of overaccumulation of PGR5 was specific to the antimycin A-sensitive pathway of PSI cyclic electron transport but not to the NAD(P)H dehydrogenase (NDH) pathway. We propose that a balanced PGR5 level is required for efficient regulation of the rate of antimycin A-sensitive PSI cyclic electron transport, although the rate of PSI cyclic electron transport is probably also regulated by other factors during steady-state photosynthesis.