Arabidopsis plants lacking PsbS protein possess photoprotective energy dissipation

Abstract

It is commonly accepted that the photosystem II subunit S protein, PsbS, is required for the dissipation of excess light energy in a process termed 'non-photochemical quenching' (NPQ). This process prevents photo-oxidative damage of photosystem II (PSII) thus avoiding photoinhibition which can decrease plant fitness and productivity. In this study Arabidopsis plants lacking PsbS (the npq4 mutant) were found to possess a competent mechanism of excess energy dissipation that protects against photoinhibitory damage. The process works on a slower timescale, taking about 1 h to reach the same level of NPQ achieved in the wild type in just a few minutes. The NPQ in npq4 was found to display very similar characteristics to the fast NPQ in the wild type. Firstly, it prevented the irreversible light-induced closure of PSII reaction centres. Secondly, it was uncoupler-sensitive, and thus triggered by the ΔpH across the thylakoid membrane. Thirdly, it was accompanied by significant quenching of the fluorescence under conditions when all PSII reaction centres were open (F o state). Fourthly, it was accompanied by NPQ-related absorption changes (ΔA535). Finally, it was modulated by the presence of the xanthophyll cycle carotenoid zeaxanthin. The existence of a mechanism of photoprotective energy dissipation in plants lacking PsbS suggests that this protein plays the role of a kinetic modulator of the energy dissipation process in the PSII light-harvesting antenna, allowing plants to rapidly track fluctuations of light intensity in the environment, and is not the primary cause of NPQ or a direct carrier of the pigment acting as the non-photochemical quencher. © 2009 Blackwell Publishing Ltd.