Dissociation of the PSI-N subunit from photosystem I restults in decreased electron transport

Abstract

The photosystem I subunit F (PSI-F) is a transmembrane protein with a large lumenal domain. Regions of PSI-F are highly conserved between species, but eukaryotic PSI-F has an N-terminal extension. The role of PSI-F was investigated in Arabidopsis plants transformed with an antisense construct of psaF cDNA (Haldrup et al. 2000). The plants were very small and struggled to survive. Plants with reduced amounts of PSI-F were chronically photoinhibited in both photosystem I and photosystem II. The compensatory responses of the plant to the perturbation of the photosynthetic apparatus was investigated. We looked for changes in peptide formation, fluorescence measurements, analysed the xanthophyll cycle, and measured state transitions. We found that PSI-F deficient plants exhibit high non-photochemical quenching as a result of high energy-dependent quenching. This is related to an increased formation of zeaxanthin. The plants respond to the stressful situation by down-regulating the levels of zeaxanthin epoxidase (40% of wild-type) and up-regulating violaxanthin de-epoxidase (250% of wild-type). The plants have 30% higher levels of VAZ (expressed per chlorophyll a), compared to wild-type plants under normal growth conditions. The epoxylation index ((1/2 A + V)/VAZ)) was 10% lower in plants without PSI-F compared to wild-type. This lowered epoxylation index was maintained even after 16 h of recovery in darkness. The amount of PsbS involved in non-photochemical quenching and NDH involved in cyclic electron transport are both up-regulated. These findings show that the plants deficient in PSI-F, which are stressed at normal growth conditions, respond similarly to wild-type plants under high light conditions.