The Role Of PSⅡ Protein Phosphorylation In Photoprotection Of Plants

Photosynthesis, which is the only way to convert the sunlight energy to thechemical energy to maintain all living organisms, is the most important chemicalreaction on earth. Even though sunlight is the ultimate energy source for thephotosynthesis, it can also harm plants. The efficiency of photosynthesis can besignificantly reduced in plants when exposed to high light. This phenomenon ofdecline in photosynthetic efficiency is referred as photoinhibition. Particularly underenvironmental stress, such as low temperature and water deficiency, too much lightcan cause the photodamage to the photosynthetic apparatus and the primary targetfor the damage is photosystem II. However, plants have evolved an array ofprotective mechanisms against photodamage. Our experimental results indicate thatPSII protein phosphorylation plays an important role in protecting PSII fromphotodamage.1. D1 protein phosphorylation/dephosphorylation alone has noeffect on the electron transport activity of photosystem II in soybeanleaves In order to explore the relationship between the phosphorylation of D1 proteinand the function of PSII, the changes in chlorophyll fluorescence parameters, PSIIelectron transport activity and D1 protein amount were observed afterdephosphorylation of phosphorylated D1 proteins caused by FSBA(5'-p-fluorosulfonylbenzoyl adenosine, an inhibitor of protein kinase) treatment. VI光系统 II 蛋白磷酸化在植物光破坏防御中的作用Also, phosphorylated and non-phosphorylated D1 proteins (D1* and D1,respectively) were analyzed by sodium dodecyl sulfate-polyacrylamide gelelectrophoresis (SDS-PAGE) and Western Blotting. The following results wereobtained. (1) A high level of phosphorylated D1 protein (D1*) was observed indark-adapted soybean leaves and D1* could be completely dephosphorylated byFSBA (1 mmol/L) treatment for 2 h. (2) Dephosphorylation of D1* resulted inneither substantial net loss of D1 proteins nor the significant changes in value ofchlorophyll fluorescence parameters. (3) The electron transport activity of PS II(H2O → 1,4-BQ) was not changed significantly after D1* was completelydephosphorylated. Based on these results, it is concluded that D1 proteinphosphorylation/dephosphorylation alone has no significant effect on the function ofPS II reaction center in soybean leaves.2. Different mechanisms for Photosystem II reversible inactivationin pumpkin and soybean leaves at saturating light A comparative investigation on the response to saturating light was madebetween pumpkin and soybean leaves. Some similar responses were observed in theboth species. (1) After saturating illumination for 3 h, the original fluorescence Foincreased while the PSII photochemical efficiency Fv/Fm declined significantly, butthese parameters could largely recover to the levels of dark-adapted leaves after 3 hof subsequent dark recovery. (2) No net loss of the D1 proteins occurred after thesaturating illumination. However, soybean and pumpkin leaves also had somedifferent responses to the saturating illumination. (1) Low temperature fluorescenceparameters, F685 and F685/F735 decreased significantly in soybean but not inpumpkin leaves. (2) Part of LHCII dissociated from PSII complexes in soybeanleaves but not in pumpkin leaves, as shown by the results of sucrose density gradientcentrifugation and the SDS-PAGE. (3) The light-saturated PSII electron transportactivity declined significantly in pumpkin thylakoids but not in soybean thylakoids.In addition, a high level of phosphorylated D1 proteins was found in dark-adaptedsoybean leaves but not in dark-adapted pumpkin leaves. These results may implythat at excessive light soybean and pumpkin have the same protective strategyagainst photodamage, reversible inactivation of PSII, but two different mechanisms,namely the reversible inactivation is related to the dissociation of LHCII in soybeanbut not in pumpkin leaves.