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CO2-responsive CCT protein (CRCT) is suggested to be a positive regulator of starch biosynthesis in the leaf sheaths of rice, regulating the expression levels of starch biosynthesis-related genes. In this study, the effects of CRCT expression levels on the expression of starch biosynthesis-related enzymes and the quality of starch were studied. Using native-PAGE/activity staining and immunoblotting, we found that the protein levels of starch synthase I, branching enzyme I, branching enzyme IIa, isoamylase 1 and phosphorylase 1 were largely correlated with the CRCT expression levels in the leaf sheaths of CRCT transgenic lines. In contrast, the CRCT expression levels largely did not affect the expression levels and/or activities of starch biosynthesis-related enzymes in the leaf blades and endosperm tissues. The analysis of the chain-length distribution of starch in the leaf sheaths showed that short chains with a degree of polymerization from 5 to 14 were increased in the overexpression lines but decreased in the knockdown lines. The amylose content of starch in the leaf sheath was greatly increased in the overexpression lines. In contrast, the molecular weight of the amylopectin of starch in the leaf sheath of overexpression lines did not change compared with those of the non-transgenic rice. These results suggest that CRCT can control the quality and the quantity of starch in the leaf sheath by regulating the expression of particular starch biosynthesis-related enzymes., (� The Author(s) 2019. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Long-term high CO2 exposure accelerates photosynthetic induction response due to rapid light increase. However, it is unclear whether the acceleration is caused by acclimation of photosynthetic components (long-term CO2 effect) and/or by the sufficient substrate under high CO2 at the measurement (short-term CO2 effect). Populus koreana × trichocarpa cv. Peace has wide-open stomata almost not responding to changes of photon flux density. Using this species, we examined the long- and short-term CO2 effects on photosynthetic induction by focusing on biochemical components. We grew the plants under [CO2] of 380, 700 and 1020 μmol CO2 mol-1 air and measured the photosynthetic induction response under [CO2] of 380 and 1020 μmol CO2 mol-1 air. Despite significant reduction in Rubisco content and light-saturated photosynthetic rate in the leaves from the high growth CO2, the photosynthetic induction time was similar in leaves from different growth CO2 plants when measurement [CO2] was the same. The induction, however, was significantly fast at the higher than at the lower measurement [CO2], regardless of growth CO2 of the plants. These results demonstrate that the acceleration of apparent photosynthetic induction under high CO2 environment was mainly contributed by a short-term CO2 effect rather than by a long-term acclimation effect when stomatal limitation is not the major factor., (© The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Sugar metabolism pathways such as photosynthesis produce dicarbonyls, e.g. methylglyoxal (MG), which can cause cellular damage. The glyoxalase (GLX) system comprises two enzymes GLX1 and GLX2, and detoxifies MG; however, this system is poorly understood in the chloroplast, compared with the cytosol. In the present study, we determined GLX1 and GLX2 activities in spinach chloroplasts, which constituted 40% and 10%, respectively, of the total leaf glyoxalase activity. In Arabidopsis thaliana, five GFP-fusion GLXs were present in the chloroplasts. Under high CO 2 concentrations, where increased photosynthesis promotes the MG production, GLX1 and GLX2 activities in A. thaliana increased and the expression of AtGLX1-2 and AtGLX2-5 was enhanced. On the basis of these findings and the phylogeny of GLX in oxygenic phototrophs, we propose that the GLX system scavenges MG produced in chloroplasts during photosynthesis.

CO 2 -responsive CCT protein (CRCT) is the suggested positive regulator of starch synthesis in vegetative organs, particularly the leaf sheath of rice. In this study, we analyzed the effects of the starch level in the leaf sheath on the photosynthetic rate in the leaf blade using CRCT overexpression and RNA interference (RNAi) knockdown transgenic rice grown under ambient (38 Pa) or elevated (100 Pa) CO 2 conditions. In leaf sheath, the starch content was markedly changed in relation to CRCT expression levels under both CO 2 conditions. In contrast, the soluble sugar and starch contents of the leaf blade were markedly increased in the knockdown line grown under elevated CO 2 conditions. The overexpression or RNAi knockdown of CRCT did not cause large effects on the photosynthetic rate of the transgenic lines grown under ambient CO 2 condition. However, the photosynthetic rate of the overexpression line was enhanced, while that of the knockdown line was substantially decreased under elevated CO 2 conditions. These photosynthetic rates were weakly correlated with the nitrogen contents and negatively correlated with the total non-structural carbohydrate contents. Thus, the capacity for starch synthesis in leaf sheath, which is controlled by CRCT, can indirectly affect the carbohydrate content, and then the photosynthetic rate in the leaf blade of rice grown under elevated CO 2 conditions., (© The Author 2016. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

A unique CO2-Responsive CONSTANS, CONSTANS-like, and Time of Chlorophyll a/b Binding Protein1 (CCT) Protein (CRCT) containing a CCT domain but not a zinc finger motif is described, which is up-regulated under elevated CO2 in rice (Oryza sativa). The expression of CRCT showed diurnal oscillation peaked at the end of the light period and was also increased by sugars such as glucose and sucrose. Promoter β-glucuronidase analysis showed that CRCT was highly expressed in the phloem of various tissues such as leaf blade and leaf sheath. Overexpression or RNA interference knockdown of CRCT had no appreciable effect on plant growth and photosynthesis except that tiller angle was significantly increased by the overexpression. More importantly, starch content in leaf sheath, which serves as a temporary storage organ for photoassimilates, was markedly increased in overexpression lines and decreased in knockdown lines. The expressions of several genes related to starch synthesis, such as ADP-glucose pyrophospholylase and α-glucan phospholylase, were significantly changed in transgenic lines and positively correlated with the expression levels of CRCT. Given these observations, we suggest that CRCT is a positive regulator of starch accumulation in vegetative tissues, regulating coordinated expression of starch synthesis genes in response to the levels of photoassimilates., (© 2015 American Society of Plant Biologists. All Rights Reserved.)

Under field conditions, the leaves of plants are exposed to fluctuating light, as observed in sunfleck. The duration and frequency of sunfleck, which is caused by the canopy being blown by the wind, are in the ranges from 0.2 to 50 s, and from 0.004 to 1 Hz, respectively. Furthermore, >60% of the sunfleck duration ranges from 0.2 to 0.8 s. In the present research, we analyzed the effects of repetitive illumination by short-pulse (SP) light of sunflower leaves on the photosynthetic electron flow. The duration of SP light was set in the range from 10 to 300 ms. We found that repetitive illumination with SP light did not induce the oxidation of P700 in PSI, and mainly inactivated PSI. Increases in the intensity, duration and frequency of SP light enhanced PSI photoinhibition. PSI photoinhibition required the presence of O2. The inactivation of PSI suppressed the net CO2 assimilation. On the other hand, the increase in the oxidized state of P700 suppressed PSI inactivation. That is, PSI with a reduced reaction center would produce reactive oxygen species (ROS) by SP light, leading to PSI photodamage. This mechanism probably explains the PSI photodamage induced by constant light., (© The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

To enable prediction of future rice production in a changing climate, we need to understand the interactive effects of temperature and elevated [CO2] (E[CO2]). We therefore examined if the effect of E[CO2] on the light-saturated leaf photosynthetic rate (Asat) was affected by soil and water temperature (NT, normal; ET, elevated) under open-field conditions at the rice free-air CO2 enrichment (FACE) facility in Shizukuishi, Japan, in 2007 and 2008. Season-long E[CO2] (+200 µmol mol(-1)) increased Asat by 26%, when averaged over two years, temperature regimes and growth stages. The effect of ET (+2°C) on Asat was not significant at active tillering and heading, but became negative and significant at mid-grain filling; Asat in E[CO2]-ET was higher than in ambient [CO2] (A[CO2])-NT by only 4%. Photosynthetic down-regulation at E[CO2] also became apparent at mid-grain filling; Asat compared at the same [CO2] in the leaf cuvette was significantly lower in plants grown in E[CO2] than in those grown in A[CO2]. The additive effects of E[CO2] and ET decreased Asat by 23% compared with that of A[CO2]-NT plants. Although total crop nitrogen (N) uptake was increased by ET, N allocation to the leaves and to Rubisco was reduced under ET and E[CO2] at mid-grain filling, which resulted in a significant decrease (32%) in the maximum rate of ribulose-1,5-bisphosphate carboxylation on a leaf area basis. Because the change in N allocation was associated with the accelerated phenology in E[CO2]-ET plants, we conclude that soil and water warming accelerates photosynthetic down-regulation at E[CO2].

The role of Rubisco activase in steady-state and non-steady-state photosynthesis was analyzed in wild-type (Oryza sativa) and transgenic rice that expressed different amounts of Rubisco activase. Below 25°C, the Rubisco activation state and steady-state photosynthesis were only affected when Rubisco activase was reduced by more than 70%. However, at 40°C, smaller reductions in Rubisco activase content were linked to a reduced Rubisco activation state and steady-state photosynthesis. As a result, overexpression of maize Rubisco activase in rice did not lead to an increase of the Rubisco activation state, nor to an increase in photosynthetic rate below 25°C, but had a small stimulatory effect at 40°C. On the other hand, the rate at which photosynthesis approached the steady state following an increase in light intensity was rapid in Rubisco activase-overexpressing plants, intermediate in the wild-type, and slowest in antisense plants at any leaf temperature. In Rubisco activase-overexpressing plants, Rubisco activation state at low light was maintained at higher levels than in the wild-type. Thus, rapid regulation by Rubisco activase following an increase in light intensity and/or maintenance of a high Rubisco activation state at low light would result in a rapid increase in Rubisco activation state and photosynthetic rate following an increase in light intensity. It is concluded that Rubisco activase plays an important role in the regulation of non-steady-state photosynthesis at any leaf temperature and, to a lesser extent, of steady-state photosynthesis at high temperature., (© 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd.)

The effects of overexpression of Rubisco activase on photosynthesis were studied in transgenic rice expressing barley or maize Rubisco activase. Immunoblot and SDS-PAGE analyses showed that transgenic lines from both gene constructs expressed the foreign Rubisco activase at high levels. The activation state of Rubisco in transgenic lines was slightly higher than that in non-transgenic plants (NT). In addition, light activation of Rubisco was significantly more rapid in transgenic lines compared with NT. These findings indicate that the overexpression of Rubisco activase can enhance Rubisco activation. However, despite enhanced activation of Rubisco in these transgenic plants, the CO(2) assimilation rate at ambient CO(2) conditions was decreased. This decrease in CO(2) assimilation rate was observed in both young developing and mature leaves independent of nitrogen nutrition. The contents of nitrogen and Chl did not differ significantly between transformants and NT; however, Rubisco content was substantially decreased in transgenic lines. There was no evidence for reduced transcription of RbcS or RbcL in these transgenic lines; in fact, transcript levels were marginally increased compared with NT. These results indicate that the overexpression of Rubisco activase leads to a decrease in Rubisco content, possibly due to post-transcriptional mechanisms.

Abstract: Rising atmospheric CO2 concentration is accompanied by global warming. However, interactive effects of elevated CO2 and soil temperature have not been well studied in plants. Here, we studied the effects of free air CO2 enrichment (FACE, ambient +20Pa), elevated soil temperature (+2°C), or a combination of both on gene expression in developing 10th and fully expanded flag leaves of rice by microarray. Gene expression was significantly affected by FACE and 400–600 up-regulated or down-regulated genes were identified in each experimental condition. Overall, elevated soil temperature was likely to increase the number of differently expressed genes. Among them, only 31 up-regulated and 83 down-regulated genes were common irrespective of the leaf stage and soil temperature. These included up-regulation of sucrose transporter and fructosyltransferase and down-regulation of chlorophyll a/b binding protein, nitrate transporter and aquaporin, though most of differently expressed genes were functionally unknown. Although there were some exceptions, the expression of most of primary metabolism genes was not significantly altered by FACE. The expression of genes related to CO2 fixation such as Rubisco and carbonic anhydrase were down-regulated, whereas a large proportion of genes related to sucrose synthesis, glycolysis, TCA cycle and N fixation were up-regulated under FACE, though changes were small in all cases. These results might reflect the transcriptional adaptation of metabolism to FACE, reducing the CO2 fixation capacity and stimulating the dark respiration. [Copyright &y& Elsevier]

We investigated the regulation mechanism of cyclic electron flow around photosystem I (CEF-PSI) in the rice leaves which suffered from photosystem II (PSII) photoinhibition. High-light (HL) treatment [2000 µmol photon m–2 s–1 at 0% carbon dioxide (CO2), 2% oxygen (O2) and 25°C] of rice leaves decreased both the maximum quantum efficiency of PSII (Fv/Fm) and the light-dependent O2-evolution rate [V(O2)]. High-light treatment did not affect the relative electron flux in PSI [Φ(PSI) × PFD]. In non-treated leaves, increasing in the photon flux density (PFD) enhanced V(O2), Φ(PSI) × PFD and the ratio of oxidized P700 to total P700 [(P700+)/(P700)total]. Φ(PSI) × PFD continued to increase even after the saturation of V(O2) against PFD. These results suggested that the electrons not used for the major electron sink, photosynthetic carbon reduction-cycle, did turnover in PSI, that is, CEF-PSI functioned at a higher PFD. High-light treatments did not affect the activity of CEF-PSI and increased (P700+)/(P700)total in the lower PFDs, compared to non-treated leaves. The activity of CEF-PSI depends on the amount of oxidized PQ. Photoinhibition of PSII suppressed electron influx from PSII to photosynthetic linear electron transport. The enhanced (P700+)/(P700)total suggested the increase in the ratio of oxidized plastoquinone (PQ) to total PQ, which supported the activity of CEF-PSI in the photoinhibited leaves. [ABSTRACT FROM PUBLISHER]

Four enzymes, namely, the maize C4-specific phosphoenolpyruvate carboxylase (PEPC), the maize C4-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C3-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO2 assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO2 assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO2 assimilation, however, was more marked at higher CO2 concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO2 inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C4-like photosynthetic pathway into C3 plants, are discussed. [ABSTRACT FROM PUBLISHER]