A novel chloroplast protein is required for optimal PSII function during acclimation to high light

Abstract

Chlorophyll fluorescence has been used to monitor photosynthesis in mutagenised Arabidopsis seedlings subjected to a change in their growth conditions, leading to the identification of a mutant which is defective in photosynthetic acclimation. Following a transfer from low to high growth light, wild type plants rapidly adjust the composition of the thylakoid membrane (within 2-3 days), with large increases in photosystem II (PS II) content together with proteolytic degradation of excess light-harvesting complexes (LHCII); this response is much reduced in the mutant. As well as exhibiting this acclimation defect, exposure of the mutant to an increase in growth light results in sustained reductions in the photochemical efficiency of PS II and in the quantum yield of photosynthesis, with severe consequences for the mutant¿s subsequent growth rate compared to the wild type. The basis for the mutation carried by these plants is a T-DNA insertion into a gene designated APE1. Cloning of the APE1 gene has shown that it encodes a protein which has no previously identified function, but which is heavily conserved throughout oxygenic photosynthetic organisms. Analysis of the predicted aminoacid sequence has identified potential chloroplast and thylakoid targeting signals and two putative trans-membrane helices, strongly suggesting that APE1 is a thylakoid protein. Current studies on the role of APE1 in the response to high light include the use of an antibody raised against a synthetic oligopeptide to confirm the sub-cellular location of APE1 protein, and Western blot, northern blot and RT-PCR expression analysis.