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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 DeltapH 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 (DeltaA535). 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.
Johnson et al. (Fri,) studied this question.