Synergistic action of light and salt stress to impair photosystem II by inhibition of the expression of psbA genes

Abstract

The effects of light stress and salt stress, alone and in combination, on the activity of photosystem II (PSII) were examined in the cyanobacterium Synechocystis sp. PCC 6803. Neither strong light at 500 µE m-2 s-1 nor exposure of cells to 1.0 M NaCl in darkness impaired the activity of PSII but together they rapidly inactivated PSII. The activity of PSII that had been lost during incubation under very strong light was restored when cells were transferred to normal light at 70 µE m-2 s-1. However, the presence of 1.0 M NaCl prevented such recovery, as did lincomycin, an inhibitor of protein synthesis. These results suggested that salt stress might inhibit the repair of PSII from light-induced damage. Western blotting analysis demonstrated that NaCl inhibited the synthesis de novo of the precursor to the D1 protein, which is a component of the photochemical reaction center of PSII, while Northern blotting analysis revealed that NaCl inhibited the accumulation of transcripts of psbA genes which encode the precursor to the D1 protein. These observations suggest that salt stress might inhibit the repair of photodamaged PSII by inhibiting the transcription and translation of psbA genes.