title: Elucidation of Photoprotective Mechanisms of PSI Against Fluctuating Light photoinhibition.
abstract: It has been claimed that the cyclic electron flow around PSI (CEF-PSI) plays an important role in protection of PSI against fluctuating light photoinhibition. However, the photoprotective mechanism of PSI is not fully elucidated. Here, we examined the mechanism, using two CEF-PSI mutants of Arabidopsis thaliana, and antimycin A, an inhibitor of the PGR5 (proton gradient regulation 5)-mediated CEF-PSI. Dark-adapted leaves in these plants were illuminated in fluctuating light alternating between high light at 1,200 µmol m-2s-1and low light at 30 µmol m-2s-1every 2 min, and PSI and PSII parameters were simultaneously measured for 160 min with 830 nm absorption and Chl fluorescence, respectively. When CEF-PSI, especially PGR5-mediated CEF-PSI, did not operate, the acceptor-side limitation of PSI, Y(NA), increased stepwise, leading to marked PSI photoinhibition. The deficiency of CFE-PSI decreased not only the electron transport rate through PSI but also the donor-side limitation of PSI, Y(ND), in high light phases. These results showed that the large Y(ND), observed only when CEF-PSI operated, contributed to suppression of PSI photoinhibition. Taken together with our previous report that high Y(NA) was alleviated by the enhancement of CEF-PSI, a model for the protective mechanisms of PSI is proposed. In this model, both alleviation of Y(NA) and acceleration of Y(ND) are indispensable, and for realization of such a situation, regulation of the electron flows, especially the PGR5-mediated CEF-PSI, plays a key role. It is important for effective protection to regulate the balance of Y(ND) and Y(NA) through CEF-PSI.
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, -3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan konom07@bs.s.u-tokyo.ac.jp.
authors.forename: Masaru
authors.initials: M
authors.lastname: Kono
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, -3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Terashima
mesh_terms: D017360:Arabidopsis / Q000502:physiology* / Q000528:radiation effects*; D029681:Arabidopsis Proteins / Q000378:metabolism; D004579:Electron Transport / Q000528:radiation effects; D008027:Light; D008954:Models, Biological; D009245:NADH Dehydrogenase / Q000378:metabolism; D055668:Photochemical Processes / Q000528:radiation effects*; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D009245:NADH Dehydrogenase
keywords: Arabidopsis thaliana; Cyclic electron flow around PSI; Donor- and acceptor-side limitations of PSI; Fluctuating light; PSI photoinhibition; Transthylakoid ΔpH