Sunlight represents the energy source for photosynthesis and plant growth. When growing in the field, plant photosynthesis has to manage strong fluctuations in light intensities. Regulation based on the thio- redoxin ...Sunlight represents the energy source for photosynthesis and plant growth. When growing in the field, plant photosynthesis has to manage strong fluctuations in light intensities. Regulation based on the thio- redoxin （Trx） system is believed to ensure light-responsive control of photosynthetic reactions in the chlo- roplast. However, direct evidence for a role of this system in regulating dynamic acclimation of photosyn- thesis in fluctuating conditions is largely lacking. In this report we show that the ferredoxin-dependent Trxs ml and m2 as well as the NADPH-dependent NTRC are both indispensable for photosynthetic acclimation in fluctuating light intensities. Arabidopsis mutants with combined deficiency in Trxs ml and m2 show wild- type growth and photosynthesis under constant light condition, while photosynthetic parameters are strongly modified in rapidly alternating high and low light. Two independent trxmlm2 mutants show lower photosynthetic efficiency in high light, but surprisingly significantly higher photosynthetic efficiency in low light. Our data suggest that a main target of Trx ml and m2 is the NADP-malate dehydrogenase involved in export of excess reductive power from the chloroplast. The decreased photosynthetic efficiency in the high-light peaks may thus be explained by a reduced capacity of the trxm lm2 mutants in the rapid light acti-vation of this enzyme. In the ntrc mutant, dynamic responses of non-photochemical quenching of excita- tion energy and plastoquinone reduction state both were strongly attenuated in fluctuating light intensities, leading to a massive decrease in PSII quantum efficiency and a specific decrease in plant growth under these conditions. This is likely due to the decreased ability of the ntrc mutant to control the stromal NADP（H） redox poise. Taken together, our results indicate that NTRC is indispensable in ensuring the full range of dynamic responses of photosynthesis to optimize photosynthesis and maintain growth in fluctu- ating light, while Trxs ml and m2 are indispensable for full activation of photosynthesis in the high-light pe- riods but negatively affect photosynthetic efficiency in the low-light periods of fluctuating light.展开更多
Ferredoxin-NADP＋ oxidoreductase （FNR）, functioning in the last step of the photosynthetic electron transfer chain, exists both as a soluble protein in the chloroplast stroma and tightly attached to chloroplast memb...Ferredoxin-NADP＋ oxidoreductase （FNR）, functioning in the last step of the photosynthetic electron transfer chain, exists both as a soluble protein in the chloroplast stroma and tightly attached to chloroplast membranes. Surface plasmon resonance assays showed that the two FNR isoforms, LFNR1 and LFNR2, are bound to the thylakoid membrane via the C-terminal domains of Tic62 and TROL proteins in a pH-dependent manner. The tic62 trol double mutants contained a reduced level of FNR, exclusively found in the soluble stroma. Although the mutant plants showed no visual phenotype or defects in the function of photosystems under any conditions studied, a low ratio of NADPH/NADP~ was detected. Since the CO2 fixation capacity did not differ between the tic62 trol plants and wild-type, it seems that the plants are able to funnel reducing power to most crucial reactions to ensure survival and fitness of the plants. However, the activity of malate dehydrogenase was down-regulated in the mutant plants. Apparently, the plastid metabolism is able to cope with substantial changes in directing the electrons from the light reactions to stromal metabolism and thus only few differences are visible in steady-state metabolite pool sizes of the tic62 trol plants.展开更多
The translocon on the outer membrane of mitochondria （TOM） facilitates the import of nuclear-encoded proteins. The principal machinery of mitochondrial protein transport seems conserved in eukaryotes; however, diver...The translocon on the outer membrane of mitochondria （TOM） facilitates the import of nuclear-encoded proteins. The principal machinery of mitochondrial protein transport seems conserved in eukaryotes; however, divergence in the composition and structure of TOM components has been observed between mammals, yeast, and plants. TOM9, the plant homolog of yeast Tom22, is significantly smaller due to a truncation in the cytosolic receptor domain, and its precise function is not understood. Here we provide evidence showing that TOM9.2 from Arabidopsis thaliana is involved in the formation of mature TOM com- plex, most likely by influencing the assembly of the pore-forming subunit TOM40. Dexamethasone- induced RNAi gene silencing of TOM9.2 results in a severe reduction in the mature TOM complex, and the assembly of newly imported TOM40 into the complex is impaired. Nevertheless, mutant plants are fully viable and no obvious downstream effects of the loss of TOM complex, i.e., on mitochondrial import ca- pacity, were observed. Furthermore, we found that TOM9.2 can bind calmodulin （CAM） in vitro and that CaM impairs the assembly of TOM complex in the isolated wild-type mitochondria, suggesting a regula- tory role of TOM9.2 and a possible integration of TOM assembly into the cellular calcium signaling network.展开更多
文摘Sunlight represents the energy source for photosynthesis and plant growth. When growing in the field, plant photosynthesis has to manage strong fluctuations in light intensities. Regulation based on the thio- redoxin （Trx） system is believed to ensure light-responsive control of photosynthetic reactions in the chlo- roplast. However, direct evidence for a role of this system in regulating dynamic acclimation of photosyn- thesis in fluctuating conditions is largely lacking. In this report we show that the ferredoxin-dependent Trxs ml and m2 as well as the NADPH-dependent NTRC are both indispensable for photosynthetic acclimation in fluctuating light intensities. Arabidopsis mutants with combined deficiency in Trxs ml and m2 show wild- type growth and photosynthesis under constant light condition, while photosynthetic parameters are strongly modified in rapidly alternating high and low light. Two independent trxmlm2 mutants show lower photosynthetic efficiency in high light, but surprisingly significantly higher photosynthetic efficiency in low light. Our data suggest that a main target of Trx ml and m2 is the NADP-malate dehydrogenase involved in export of excess reductive power from the chloroplast. The decreased photosynthetic efficiency in the high-light peaks may thus be explained by a reduced capacity of the trxm lm2 mutants in the rapid light acti-vation of this enzyme. In the ntrc mutant, dynamic responses of non-photochemical quenching of excita- tion energy and plastoquinone reduction state both were strongly attenuated in fluctuating light intensities, leading to a massive decrease in PSII quantum efficiency and a specific decrease in plant growth under these conditions. This is likely due to the decreased ability of the ntrc mutant to control the stromal NADP（H） redox poise. Taken together, our results indicate that NTRC is indispensable in ensuring the full range of dynamic responses of photosynthesis to optimize photosynthesis and maintain growth in fluctu- ating light, while Trxs ml and m2 are indispensable for full activation of photosynthesis in the high-light pe- riods but negatively affect photosynthetic efficiency in the low-light periods of fluctuating light.
文摘Ferredoxin-NADP＋ oxidoreductase （FNR）, functioning in the last step of the photosynthetic electron transfer chain, exists both as a soluble protein in the chloroplast stroma and tightly attached to chloroplast membranes. Surface plasmon resonance assays showed that the two FNR isoforms, LFNR1 and LFNR2, are bound to the thylakoid membrane via the C-terminal domains of Tic62 and TROL proteins in a pH-dependent manner. The tic62 trol double mutants contained a reduced level of FNR, exclusively found in the soluble stroma. Although the mutant plants showed no visual phenotype or defects in the function of photosystems under any conditions studied, a low ratio of NADPH/NADP~ was detected. Since the CO2 fixation capacity did not differ between the tic62 trol plants and wild-type, it seems that the plants are able to funnel reducing power to most crucial reactions to ensure survival and fitness of the plants. However, the activity of malate dehydrogenase was down-regulated in the mutant plants. Apparently, the plastid metabolism is able to cope with substantial changes in directing the electrons from the light reactions to stromal metabolism and thus only few differences are visible in steady-state metabolite pool sizes of the tic62 trol plants.
文摘The translocon on the outer membrane of mitochondria （TOM） facilitates the import of nuclear-encoded proteins. The principal machinery of mitochondrial protein transport seems conserved in eukaryotes; however, divergence in the composition and structure of TOM components has been observed between mammals, yeast, and plants. TOM9, the plant homolog of yeast Tom22, is significantly smaller due to a truncation in the cytosolic receptor domain, and its precise function is not understood. Here we provide evidence showing that TOM9.2 from Arabidopsis thaliana is involved in the formation of mature TOM com- plex, most likely by influencing the assembly of the pore-forming subunit TOM40. Dexamethasone- induced RNAi gene silencing of TOM9.2 results in a severe reduction in the mature TOM complex, and the assembly of newly imported TOM40 into the complex is impaired. Nevertheless, mutant plants are fully viable and no obvious downstream effects of the loss of TOM complex, i.e., on mitochondrial import ca- pacity, were observed. Furthermore, we found that TOM9.2 can bind calmodulin （CAM） in vitro and that CaM impairs the assembly of TOM complex in the isolated wild-type mitochondria, suggesting a regula- tory role of TOM9.2 and a possible integration of TOM assembly into the cellular calcium signaling network.
