Equisetum praealtum and E. hyemale have abundant Rubisco with a high catalytic turnover rate and low CO2 affinity.

The kinetic properties of Rubisco, a key enzyme for photosynthesis, have been examined in numerous plant species. However, this information on some plant groups, such as ferns, is scarce. This study examined Rubisco carboxylase activity and leaf Rubisco levels in seven ferns, including four Equisetum plants (E. arvense, E. hyemale, E. praealtum, and E. variegatum), considered living fossils. The turnover rates of Rubisco carboxylation (kcatc) in E. praealtum and E. hyemale were comparable to those in the C4 plants maize (Zea mays) and sorghum (Sorghum bicolor), whose kcatc values are high. Rubisco CO2 affinity, estimated from the percentage of Rubisco carboxylase activity under CO2 unsaturated conditions in kcatc in these Equisetum plants, was low and also comparable to that in maize and sorghum. In contrast, kcatc and CO2 affinities of Rubisco in other ferns, including E. arvense and E. variegatum were comparable with those in C3 plants. The N allocation to Rubisco in the ferns examined was comparable to that in the C3 plants. These results indicate that E. praealtum and E. hyemale have abundant Rubisco with high kcatc and low CO2 affinity, whereas the carboxylase activity and abundance of Rubisco in other ferns were similar to those in C3 plants. Herein, the Rubisco properties of E. praealtum and E. hyemale were discussed regarding their evolution and physiological implications.

Behavior of Photosystems II and I Is Modulated Depending on N Partitioning to Rubisco in Mature Leaves Acclimated to Low N Levels and Senescent Leaves in Rice.

In mature leaves acclimated to low N levels and in senescent leaves, photosystems II and I (PSII and PSI, respectively) show typical responses to excess light energy. As CO2 assimilation is not transiently suppressed in these situations, the behavior of PSII and PSI is likely caused by endogenous biochemical changes in photosynthesis. In this study, this subject was studied in rice (Oryza sativa L.). Analysis was performed on mature and senescent leaves of control and N-deficient plants. Total leaf-N, Rubisco and chlorophyll (Chl) levels and their ratios were determined as biochemical parameters of photosynthesis. Total leaf-N, Rubisco and Chl levels decreased in the mature leaves of N-deficient plants and senescent leaves. The percentage of Rubisco-N in the total leaf-N decreased in these leaves, whereas that of Chl-N tended to remain almost constant in mature leaves but increased in senescent leaves. Changes in PSII and PSI parameters were best accounted for by the Rubisco-N percentage, strongly suggesting that the behavior of PSII and PSI is modulated depending on changes in N partitioning to Rubisco in mature leaves acclimated to low N levels and in senescent leaves. It is likely that a decrease in N partitioning to Rubisco leads to a decrease in Rubisco capacity relative to other photosynthetic capacities that inevitably generate excess light energy and that the operation of PSII and PSI is modulated in such situations.

Suppression of chloroplast triose phosphate isomerase evokes inorganic phosphate-limited photosynthesis in rice.

The availability of inorganic phosphate (Pi) for ATP synthesis is thought to limit photosynthesis at elevated [CO2] when Pi regeneration via sucrose or starch synthesis is limited. We report here another mechanism for the occurrence of Pi-limited photosynthesis caused by insufficient capacity of chloroplast triose phosphate isomerase (cpTPI). In cpTPI-antisense transgenic rice (Oryza sativa) plants with 55%-86% reductions in cpTPI content, CO2 sensitivity of the rate of CO2 assimilation (A) decreased and even reversed at elevated [CO2]. The pool sizes of the Calvin-Benson cycle metabolites from pentose phosphates to 3-phosphoglycerate increased at elevated [CO2], whereas those of ATP decreased. These phenomena are similar to the typical symptoms of Pi-limited photosynthesis, suggesting sufficient capacity of cpTPI is necessary to prevent the occurrence of Pi-limited photosynthesis and that cpTPI content moderately affects photosynthetic capacity at elevated [CO2]. As there tended to be slight variations in the amounts of total leaf-N depending on the genotypes, relationships between A and the amounts of cpTPI were examined after these parameters were expressed per unit amount of total leaf-N (A/N and cpTPI/N, respectively). A/N at elevated [CO2] decreased linearly as cpTPI/N decreased before A/N sharply decreased, owing to further decreases in cpTPI/N. Within this linear range, decreases in cpTPI/N by 80% led to decreases up to 27% in A/N at elevated [CO2]. Thus, cpTPI function is crucial for photosynthesis at elevated [CO2].

Overexpression of Chloroplast Triosephosphate Isomerase Marginally Improves Photosynthesis at Elevated CO2 Levels in Rice.

We recently suggested that chloroplast triosephosphate isomerase (cpTPI) has moderate control over the rate of CO2 assimilation (A) at elevated CO2 levels via the capacity for triose phosphate utilization (TPU) in rice (Oryza sativa L.) from its antisense-suppression study. In the present study, the effects of cpTPI overexpression on photosynthesis were examined in transgenic rice plants overexpressing the gene encoding cpTPI. The amounts of cpTPI protein in the two lines of transgenic plants were 4.8- and 12.1-folds higher than in wild-type plants, respectively. The magnitude of the increase approximately corresponded to the increase in transcript levels of cpTPI. A at CO2 levels of 100 and 120 Pa increased by 6-9% in the transgenic plants, whereas those at ambient and low CO2 levels were scarcely affected. Similar increases were observed for TPU capacity estimated from the CO2 response curves of A. These results indicate that the overexpression of cpTPI marginally improved photosynthesis at elevated CO2 levels via improvement in TPU capacity in rice. However, biomass production at a CO2 level of 120 Pa did not increase in transgenic plants, suggesting that the improvement in photosynthesis by cpTPI overexpression was not sufficient to improve biomass production in rice.

Co-overproducing Rubisco and Rubisco activase enhances photosynthesis in the optimal temperature range in rice.

Rubisco limits C3 photosynthesis under some conditions and is therefore a potential target for improving photosynthetic efficiency. The overproduction of Rubisco is often accompanied by a decline in Rubisco activation, and the protein ratio of Rubisco activase (RCA) to Rubisco (RCA/Rubisco) greatly decreases in Rubisco-overproducing plants (RBCS-ox). Here, we produced transgenic rice (Oryza sativa) plants co-overproducing both Rubisco and RCA (RBCS-RCA-ox). Rubisco content in RBCS-RCA-ox plants increased by 23%-44%, and RCA/Rubisco levels were similar or higher than those of wild-type plants. However, although the activation state of Rubisco in RBCS-RCA-ox plants was enhanced, the rates of CO2 assimilation at 25°C in RBCS-RCA-ox plants did not differ from that of wild-type plants. Alternatively, at a moderately high temperature (optimal range of 32°C-36°C), the rates of CO2 assimilation in RBCS-ox and RBCS-RCA-ox plants were higher than in wild-type plants under conditions equal to or lower than current atmospheric CO2 levels. The activation state of Rubisco in RBCS-RCA-ox remained higher than that of RBCS-ox plants, and activated Rubisco content in RCA overproducing, RBCS-ox, RBCS-RCA-ox, and wild-type plants was highly correlated with the initial slope of CO2 assimilation against intercellular CO2 pressures (A:Ci) at 36°C. Thus, a simultaneous increase in Rubisco and RCA contents leads to enhanced photosynthesis within the optimal temperature range.

Effects of suppression of chloroplast phosphoglycerate kinase on photosynthesis in rice.

As chloroplast phosphoglycerate kinase (cpPGK) is one of the enzymes which has the highest capacity among the Calvin-Benson cycle enzymes, it has not been regarded as a determinant for photosynthetic capacity. However, it was reported that the rate of CO2 assimilation decreased under high irradiance and normal [CO2] levels in the Arabidopsis cpPGK-knockdown mutant, implying that cpPGK has a control over photosynthetic capacity at a normal [CO2] level. In the present study, the contribution of cpPGK to photosynthetic capacity was evaluated in transgenic rice plants with decreased amounts of cpPGK protein under high irradiance and various [CO2] levels. The gene encoding cpPGK was suppressed using RNA interference techniques. Two lines of transgenic plants, Pi3 and Pi5, in which the amount of cpPGK protein decreased to 21% and 76% of that in wild-type plants, respectively, were obtained. However, there was no substantial difference in the rates of CO2 assimilation between wild-type and transgenic plants. The rates of CO2 assimilation decreased only slightly at elevated [CO2] levels in the transgenic line Pi3 and did not differ between wild-type plants and the transgenic line Pi5, irrespective of [CO2] level. These results clearly indicate that cpPGK does not have a strong control over photosynthetic capacity at various [CO2] levels in rice.

Expression of flavodiiron protein rescues defects in electron transport around PSI resulting from overproduction of Rubisco activase in rice.

Fragility of photosystem I has been observed in transgenic rice plants that overproduce Rubisco activase (RCA). In this study, we examined the effects of RCA overproduction on the sensitivity of PSI to photoinhibition in three lines of plants overexpressing RCA (RCA-ox). In all the RCA-ox plants the quantum yield of PSI [Y(I)] decreased whilst in contrast the quantum yield of acceptor-side limitation of PSI [Y(NA)] increased, especially under low light conditions. In the transgenic line with the highest RCA content (RCA-ox 1), the quantum yield of PSII [Y(II)] and CO2 assimilation also decreased under low light. When leaves were exposed to high light (2000 µmol photon m-2 s-1) for 60 min, the maximal activity of PSI (Pm) drastically decreased in RCA-ox 1. These results suggested that overproduction of RCA disturbs PSI electron transport control, thus increasing the susceptibility of PSI to photoinhibition. When flavodiiron protein (FLV), which functions as a large electron sink downstream of PSI, was expressed in the RCA-ox 1 background (RCA-FLV), PSI and PSII parameters, and CO2 assimilation were recovered to wild-type levels. Thus, expression of FLV restored the robustness of PSI in RCA-ox plants.

Contribution of the grain size QTL GS3 to yield properties and physiological nitrogen-use efficiency in the large-grain rice cultivar 'Akita 63'.

The development of crop varieties with high nitrogen-use efficiency (NUE) is thought to be important in achieving sustainable cereal crop production. The high yield large-grain rice cultivar Oryza sativa L. 'Akita 63' (temperate japonica) has high physiological NUE (PNUE) for grain yield (GY). Our previous study revealed that a large-grain allele of GS3 is present in 'Akita 63'. Here, we verified the influence of GS3 on the yield properties and PNUE for GY in 'Akita 63'. The frequency distribution of brown rice length in F2 crosses of 'Iwate 75' and 'Akita 63' showed a continuous distribution that could be explained by GS3. A near-isogenic line was developed to substitute the GS3 segment of 'Koshihikari', which harbours a normal-sized grain allele, in the genetic background of 'Akita 63' and the line was designated as Akita63NILGS3-Koshihikari. Compared with Akita63NILGS3-Koshihikari, 'Akita 63' exhibited a significantly increased grain length, single brown grain weight and GY, although no significant differences were observed in the nitrogen content and above-ground biomass per unit of cultivated area. These results indicate that the GS3 large-grain allele is a contributing factor to high PNUE for GY in 'Akita 63'. These findings will facilitate the development of nitrogen-efficient rice varieties.

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.

Photochemistry of Photosystems II and I in Rice Plants Grown under Different N Levels at Normal and High Temperature.

Although N levels affect leaf photosynthetic capacity, the effects of N levels on the photochemistry of photosystems II and I (PSII and PSI, respectively) are not well-understood. In the present study, we examined this aspect in rice (Oryza sativa L. 'Hitomebore') plants grown under three different N levels at normal or high temperatures that can occur during rice culture and do not severely suppress photosynthesis. At both growth temperatures, the quantum efficiency of PSII [Y(II)] and the fraction of the primary quinone electron acceptor in its oxidized state were positively correlated with the amount of total leaf-N, whereas the quantum yields of non-photochemical quenching and donor-side limitation of PSI [Y(ND)] were negatively correlated with the amount of total leaf-N. These changes in PSII and PSI parameters were strongly correlated with each other. Growth temperatures scarcely affected these relationships. These results suggest that the photochemistry of PSII and PSI is coordinately regulated primarily depending on the amount of total leaf-N. When excess light energy occurs in low N-acclimated plants, oxidation of the reaction center chlorophyll of PSI is thought to be stimulated to protect PSI from excess light energy. It is also suggested that PSII and PSI normally operate at high temperature used in the present study. In addition, as the relationships between Y(II) and Y(ND) were found to be almost identical to those observed in osmotically stressed rice plants, common regulation is thought to be operative when excess light energy occurs due to different causes.

GFS9 Affects Piecemeal Autophagy of Plastids in Young Seedlings of Arabidopsis thaliana.

Chloroplasts, and plastids in general, contain abundant protein pools that can be major sources of carbon and nitrogen for recycling. We have previously shown that chloroplasts are partially and sequentially degraded by piecemeal autophagy via the Rubisco-containing body. This degradation occurs during plant development and in response to the environment; however, little is known about the fundamental underlying mechanisms. To discover the mechanisms of piecemeal autophagy of chloroplasts/plastids, we conducted a forward-genetics screen following ethyl-methanesulfonate mutagenesis of an Arabidopsis (Arabidopsis thaliana) transgenic line expressing chloroplast-targeted green fluorescent protein (CT-GFP). This screen allowed us to isolate a mutant, gfs9-5, which hyperaccumulated cytoplasmic bodies labeled with CT-GFP of up to 1.0 µm in diameter in the young seedlings. We termed these structures plastid bodies (PBs). The mutant was defective in a membrane-trafficking factor, green fluorescent seed 9 (GFS9), and PB accumulation in gfs9-5 was promoted by darkness and nutrient deficiency. Transmission electron microscopy indicated that gfs9-5 hyperaccumulated structures corresponding to autophagosomes and PBs. gfs9-5 hyperaccumulated membrane-bound endogenous ATG8 proteins, transgenic yellow fluorescent protein (YFP)-ATG8e proteins and autophagosome-like structures labeled with YFP-ATG8e. The YFP-ATG8e signal was associated with the surface of plastids and their protrusions in gfs9-5. Double mutants of gfs9 and autophagy-defective 5 did not accumulate PBs. In gfs9-5, the YFP-ATG8e proteins and PBs could be delivered to the vacuole and autophagic flux was increased. We discuss a possible connection between GFS9 and autophagy and propose a potential use of gfs9-5 as a new tool to study piecemeal plastid autophagy.

Overproduction of Chloroplast Glyceraldehyde-3-Phosphate Dehydrogenase Improves Photosynthesis Slightly under Elevated [CO2] Conditions in Rice.

Chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) limits the regeneration of ribulose 1,5-bisphosphate (RuBP) in the Calvin-Benson cycle. However, it does not always limit the rate of CO2 assimilation. In the present study, the effects of overproduction of GAPDH on the rate of CO2 assimilation under elevated [CO2] conditions, where the capacity for RuBP regeneration limits photosynthesis, were examined in transgenic rice (Oryza sativa). GAPDH activity was increased to 3.2- and 4.5-fold of the wild-type levels by co-overexpression of the GAPDH genes, GAPA and GAPB, respectively. In the transgenic rice plants, the rate of CO2 assimilation under elevated [CO2] conditions increased by approximately 10%, whereas that under normal and low [CO2] conditions was not affected. These results indicate that overproduction of GAPDH is effective in improving photosynthesis under elevated [CO2] conditions, although its magnitude is relatively small. By contrast, biomass production of the transgenic rice plants was not greater than that of wild-type plants under elevated [CO2] conditions, although starch content tended to increase marginally.

Overexpression of both Rubisco and Rubisco activase rescues rice photosynthesis and biomass under heat stress.

Global warming threatens food security by decreasing crop yields through damage to photosynthetic systems, especially Rubisco activation. We examined whether co-overexpression of Rubisco and Rubisco activase improves the photosynthetic and growth performance of rice under high temperatures. We grew three rice lines-the wild-type (WT), a Rubisco activase-overexpressing line (oxRCA) and a Rubisco- and Rubisco activase-co-overexpressing line (oxRCA-RBCS)-and analysed photosynthesis and biomass at 25 and 40°C. Compared with the WT, the Rubisco activase content was 153% higher in oxRCA and 138% higher in oxRCA-RBCS, and the Rubisco content was 27% lower in oxRCA and similar in oxRCA-RBCS. The CO2 assimilation rate (A) of WT was lower at 40°C than at 25°C, attributable to Rubisco deactivation by heat. On the other hand, that of oxRCA and oxRCA-RBCS was maintained at 40°C, resulting in higher A than WT. Notably, the dry weight of oxRCA-RBCS was 26% higher than that of WT at 40°C. These results show that increasing the Rubisco activase content without the reduction of Rubisco content could improve yield and sustainability in rice at high temperature.

Phosphorus toxicity disrupts Rubisco activation and reactive oxygen species defence systems by phytic acid accumulation in leaves.

Phosphorus (P) is an essential mineral nutrient for plants. Nevertheless, excessive P accumulation in leaf mesophyll cells causes necrotic symptoms in land plants; this phenomenon is termed P toxicity. However, the detailed mechanisms underlying P toxicity in plants have not yet been elucidated. This study aimed to investigate the molecular mechanism of P toxicity in rice. We found that under excessive inorganic P (Pi) application, Rubisco activation decreased and photosynthesis was inhibited, leading to lipid peroxidation. Although the defence systems against reactive oxygen species accumulation were activated under excessive Pi application conditions, the Cu/Zn-type superoxide dismutase activities were inhibited. A metabolic analysis revealed that excessive Pi application led to an increase in the cytosolic sugar phosphate concentration and the activation of phytic acid synthesis. These conditions induced mRNA expression of genes that are activated under metal-deficient conditions, although metals did accumulate. These results suggest that P toxicity is triggered by the attenuation of both photosynthesis and metal availability within cells mediated by phytic acid accumulation. Here, we discuss the whole phenomenon of P toxicity, beginning from the accumulation of Pi within cells to death in land plants.

Effects of Overproduction of Rubisco Activase on Rubisco Content in Transgenic Rice Grown at Different N Levels.

It has been reported that overproduction of Rubisco activase (RCA) in rice (Oryza sativa L.) decreased Rubisco content, resulting in declining photosynthesis. We examined the effects of RCA levels on Rubisco content using transgenic rice with overexpressed or suppressed RCA under the control of different promoters of the RCA and Rubisco small subunit (RBCS) genes. All plants were grown hydroponically with different N concentrations (0.5, 2.0 and 8.0 mM-N). In RCA overproduced plants with > 2-fold RCA content (RCA-HI lines), a 10%-20% decrease in Rubisco content was observed at 0.5 and 2.0 mM-N. In contrast, at 8.0 mM-N, Rubisco content did not change in RCA-HI lines. Conversely, in plants with 50%-60% increased RCA content (RCA-MI lines), Rubisco levels remained unchanged, regardless of N concentration. Such effects on Rubisco content were independent of the promoter that was used. In plants with RCA suppression to < 10% of the wild-type RCA content, Rubisco levels were increased at 0.5 mM-N, but were unchanged at 2.0 and 8.0 mM-N. Thus, the effects of the changes in RCA levels on Rubisco content depended on N supply. Moreover, RCA overproduction was feasible without a decrease in Rubisco content, depending on the degree of RCA production.

Flavodiiron Protein Substitutes for Cyclic Electron Flow without Competing CO2 Assimilation in Rice.

Flavodiiron protein (FLV) mediates photoreduction of O2 to H2O. It is conserved from cyanobacteria to gymnosperms but not in angiosperms. The introduction of a moss (Physcomitrella patens) FLV (PpFLV) gene into Arabidopsis (Arabidopsis thaliana) made photosystem I (PSI) resistant to fluctuating light. Here, we used the same strategy with three rice (Oryza sativa) genotypes. PpFLV in the wild-type rice background functioned as an efficient PSI electron sink and increased resistance to PSI photodamage under fluctuating light. The introduction of PpFLV into the PGR5-RNAi mutant [defective in PROTON GRADIENT REGULATION5 (PGR5)-dependent cyclic electron transport around PSI, CET-PSI], the crr6 mutant [defective in chloroplast NAD(P)H-dehydrogenase-like complex (NDH)-dependent CET-PSI], and the PGR5-RNAi crr6 double mutant (double defective in CET-PSI activity) alleviated PSI photodamage under fluctuating light. Furthermore, PpFLV substituted for the function of PGR5- and NDH-dependent CET-PSI without competing for CO2 assimilation under constant light, as there was no difference in CO2 assimilation per Rubisco content and biomass production was recovered to the wild-type level. Thus, the exogenous FLV system could act not only as a safety valve under fluctuating light, but also generate a proton motive force for balancing the ATP/NADPH production ratio during steady-state photosynthesis.

Responses of the Photosynthetic Electron Transport Reactions Stimulate the Oxidation of the Reaction Center Chlorophyll of Photosystem I, P700, under Drought and High Temperatures in Rice.

It is of interest how photosynthetic electron transport (PET) reactions respond to excess light energy caused by the combination of drought stress and high temperatures. Since such information is scarcely available for photosystem I (PSI), this question was explored in rice (Oryza sativa L.) plants subjected to drought stress, using culture solutions that contain poly(ethylene glycol) at different concentrations under two day/night temperature regimes. At 27/22 °C (day/night), drought stress led to the oxidation of the reaction center of the chlorophyll of PSI (P700), and also led to decreases in the quantum efficiencies of photosystem II (PSII) and PSI, and a reduction of the primary quinone electron acceptor of PSI. Such drought stress responses were wholly stimulated at 35/30 °C. These parameters were strongly correlated with each other and were minimally affected by temperature. These results indicate that the drought stress responses of the respective PET reactions are closely associated with each other in the oxidization of P700 and that such responses are stimulated at high temperatures. The underlying mechanisms of these phenomena were discussed. While P700 oxidation is thought to suppress reactive oxygen species (ROS) production, PSI photoinhibition was observed under severe stress conditions, implying that P700 oxidation is not sufficient for the protection of PSI under drought stress.

Vacuolar Protein Degradation via Autophagy Provides Substrates to Amino Acid Catabolic Pathways as an Adaptive Response to Sugar Starvation in Arabidopsis thaliana.

The vacuolar lytic degradation of proteins releases free amino acids that plants can use instead of sugars for respiratory energy production. Autophagy is a major cellular process leading to the transport of proteins into the vacuole for degradation. Here, we examine the contribution of autophagy to the amino acid metabolism response to sugar starvation in mature leaves of Arabidopsis thaliana. During sugar starvation arising from the exposure of wild-type (WT) plants to darkness, autophagic transport of chloroplast stroma, which contains most of the proteins in a leaf, into the vacuolar lumen was induced within 2 d. During this time, the level of soluble proteins, primarily Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), decreased and the amount of free amino acid increased. In dark-treated autophagy-defective (atg) mutants, the decrease of soluble proteins was suppressed, which resulted in the compromised release of basic amino acids, branched-chain amino acids (BCAAs) and aromatic amino acids. The impairment of BCAA catabolic pathways in the knockout mutants of the electron transfer flavoprotein (ETF)/ETF:ubiquinone oxidoreductase (etfqo) complex and the electron donor protein isovaleryl-CoA dehydrogenase (ivdh) caused a reduced tolerance to dark treatment similar to that in the atg mutants. The enhanced accumulation of BCAAs in the ivdh and etfqo mutants during the dark treatment was reduced by additional autophagy deficiency. These results indicate that vacuolar protein degradation via autophagy serves as an adaptive response to disrupted photosynthesis by providing substrates to amino acid catabolic pathways, including BCAA catabolism mediated by IVDH and ETFQO.

Effects of genetic manipulation of the activity of photorespiration on the redox state of photosystem I and its robustness against excess light stress under CO2-limited conditions in rice.

Under CO2-limited conditions such as during stomatal closure, photorespiration is suggested to act as a sink for excess light energy and protect photosystem I (PSI) by oxidizing its reaction center chlorophyll P700. In this study, this issue was directly examined with rice (Oryza sativa L.) plants via genetic manipulation of the amount of Rubisco, which can be a limiting factor for photorespiration. At low [CO2] of 5 Pa that mimicked stomatal closure condition, the activity of photorespiration in transgenic plants with decreased Rubisco content (RBCS-antisense plants) markedly decreased, whereas the activity in transgenic plants with overproduction of Rubisco (RBCS-sense plants) was similar to that in wild-type plants. Oxidation of P700 was enhanced at [CO2] of 5 Pa in wild-type and RBCS-sense plants. PSI was not damaged by excess light stress induced by repetitive saturated pulse-light (rSP) in the presence of strong steady-state light. On the other hand, P700 was strongly reduced in RBCS-antisense plants at [CO2] of 5 Pa. PSI was also damaged by rSP illumination. These results indicate that oxidation of P700 and the robustness of PSI against excess light stress are hampered by the decreased activity of photorespiration as a result of genetic manipulation of Rubisco content. It is also suggested that overproduction of Rubisco does not enhance photorespiration as well as CO2 assimilation probably due to partial deactivation of Rubisco.

New insights into the cellular mechanisms of plant growth at elevated atmospheric carbon dioxide concentrations.

Rising atmospheric carbon dioxide concentration ([CO2 ]) significantly influences plant growth, development, and biomass. Increased photosynthesis rate, together with lower stomatal conductance, has been identified as the key factors that stimulate plant growth at elevated [CO2 ] (e[CO2 ]). However, variations in photosynthesis and stomatal conductance alone cannot fully explain the dynamic changes in plant growth. Stimulation of photosynthesis at e[CO2 ] is always associated with post-photosynthetic secondary metabolic processes that include carbon and nitrogen metabolism, cell cycle functions, and hormonal regulation. Most studies have focused on photosynthesis and stomatal conductance in response to e[CO2 ], despite the emerging evidence of e[CO2 ]'s role in moderating secondary metabolism in plants. In this review, we briefly discuss the effects of e[CO2 ] on photosynthesis and stomatal conductance and then focus on the changes in other cellular mechanisms and growth processes at e[CO2 ] in relation to plant growth and development. Finally, knowledge gaps in understanding plant growth responses to e[CO2 ] have been identified with the aim of improving crop productivity under a CO2 rich atmosphere.