The role of the PsbS protein in the protection of photosystems I and II against high light in Arabidopsis thaliana

The PsbS protein is recognised in higher plants as an important component in dissipating excess light energy via its regulation of non-photochemical quenching. We investigated photosynthetic responses in the arabidopsis npq4 mutant, which lacks PsbS, and in a mutant over-expressing PsbS (oePsbS). Growth under low light led to npq4 and wild-type plants being visibly indistinguishable, but induced a phenotype in oePsbS plants, which were smaller and had shorter flowering spikes. Here we report that chloroplasts from npq4 generated more singlet oxygen (O-1(2)) than those from oePsbS. This accompanied a higher extent of photosystern II photoinhibition of leaves from npq4 plants. In contrast, oePsbS was more damaged by high light than npq4 and the wild-type at the level of photosystem I. The plastoquinone pool, as measured by thermoluminescence, was more oxidised in the oePsbS than in npq4, whilst the amount of photo-oxidisable P-700, as probed with actinic light or saturating flashes, was higher in oePsbS compared to wild-type and npq4. Taken together, this indicates that the level of PsbS has a regulatory role in cyclic electron flow. Overall, we show that under high light oePsbS plants were more protected from O-1(2) at the level of photosystem whereas lack of cyclic electron flow rendered them susceptible to damage at photosystem I. Cyclic electron flow is concluded to be essential for protecting photosystem I from high light stress. (C) 2012 Elsevier B.V. All rights reserved.