Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, re- sulting in the formation of a trans-thylakoid proton gradient （△pH） and membrane potential （△ψ）. Ion trans...Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, re- sulting in the formation of a trans-thylakoid proton gradient （△pH） and membrane potential （△ψ）. Ion trans-porters and channels localized to the thylakoid membrane regulate the contribution of each component to the proton motive force （pmf）. Although both △pH and △ψ contribute to ATP synthesis as pmf, only ~pH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen by inducing thermal dissipation of excessive absorbed light energy from photosystem II antennae and slowing down of the electron transport through the cytochrome bsf complex. To optimize the tradeoff between efficient light energy utilization and protection of both photosystems against photodamage, plants have to regulate the pmf amplitude and its components, △pH and △ψ. Cyclic electron transport around photosystem I （PSI） is a major regulator of the pmf amplitude by generating pmf independently of the net production of NADPH by linear electron transport. Chloroplast ATP synthase relaxes pmf for ATP synthesis, and its activity should be finely tuned for maintaining the size of the pmf during steady-state photosynthesis. Pseudo-cyclic electron transport mediated by flavodiiron protein （FIv） forms a large electron sink, which is essential for PSI photoprotection in fluctuating light in cyanobacteria. FIv is conserved from cyanobacteria to gymno- sperms but not in angiosperms. The Arabidopsis proton gradient regulation 50（pgr5） mutant is defective in the main pathway of PSI cyclic electron transport. By introducing Physcomitrella patens genes encoding Flvs, the function of PSI cyclic electron transport was substituted by that of FIv-dependent pseudo-cyclic electron transport. In transgenic plants, the size of the pmf was complemented to the wild-type level but the contribution of △pH to the total pmf was lower than that in the wild type. In the pgr5 mutant, the size of the pmf was drastically lowered by the absence of PSI cyclic electron transport. In the mutant, △pH occupied the majority ofpmf, suggesting the presence of a mechanism for the homeostasis of luminal pH in the light. To avoid damage to photosynthetic electron transport by periods of excess solar energy, plants employ an intricate regulatory network involving alternative electron transport pathways, ion transporters/channels, and pH-dependent mechanisms for downregulating photosynthetic electron transport.展开更多
文摘Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, re- sulting in the formation of a trans-thylakoid proton gradient （△pH） and membrane potential （△ψ）. Ion trans-porters and channels localized to the thylakoid membrane regulate the contribution of each component to the proton motive force （pmf）. Although both △pH and △ψ contribute to ATP synthesis as pmf, only ~pH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen by inducing thermal dissipation of excessive absorbed light energy from photosystem II antennae and slowing down of the electron transport through the cytochrome bsf complex. To optimize the tradeoff between efficient light energy utilization and protection of both photosystems against photodamage, plants have to regulate the pmf amplitude and its components, △pH and △ψ. Cyclic electron transport around photosystem I （PSI） is a major regulator of the pmf amplitude by generating pmf independently of the net production of NADPH by linear electron transport. Chloroplast ATP synthase relaxes pmf for ATP synthesis, and its activity should be finely tuned for maintaining the size of the pmf during steady-state photosynthesis. Pseudo-cyclic electron transport mediated by flavodiiron protein （FIv） forms a large electron sink, which is essential for PSI photoprotection in fluctuating light in cyanobacteria. FIv is conserved from cyanobacteria to gymno- sperms but not in angiosperms. The Arabidopsis proton gradient regulation 50（pgr5） mutant is defective in the main pathway of PSI cyclic electron transport. By introducing Physcomitrella patens genes encoding Flvs, the function of PSI cyclic electron transport was substituted by that of FIv-dependent pseudo-cyclic electron transport. In transgenic plants, the size of the pmf was complemented to the wild-type level but the contribution of △pH to the total pmf was lower than that in the wild type. In the pgr5 mutant, the size of the pmf was drastically lowered by the absence of PSI cyclic electron transport. In the mutant, △pH occupied the majority ofpmf, suggesting the presence of a mechanism for the homeostasis of luminal pH in the light. To avoid damage to photosynthetic electron transport by periods of excess solar energy, plants employ an intricate regulatory network involving alternative electron transport pathways, ion transporters/channels, and pH-dependent mechanisms for downregulating photosynthetic electron transport.
