Characterization of photosynthesis in Arabidopsis ER-to-plastid lipid trafficking mutants

Vascular plants use two pathways to synthesize galactolipids, the predominant lipid species in chloroplasts-a prokaryotic pathway that resides entirely in the chloroplast, and a eukaryotic pathway that involves assembly in the endoplasmic reticulum. Mutants deficient in the endoplasmic reticulum pathway, trigalactosyldiacylglycerol (tgd1-1 and tgd2-1) mutants, had been previously identified with reduced contents of monogalactosyldiacylglycerol and digalactosyldiacylglycerol, and altered lipid molecular species composition. Here, we report that the altered lipid composition affected photosynthesis in lipid trafficking mutants. It was found that proton motive force as measured by electrochromic shift was reduced by similar to 40 % in both tgd mutants. This effect was accompanied by an increase in thylakoid conductance attributable to ATPase activity and so the rate of ATP synthesis was nearly unchanged. Thylakoid conductance to ions also increased in tgd mutants. However, gross carbon assimilation in tgd mutants as measured by gas exchange was only marginally affected. Rubisco activity, electron transport rate, and photosystem I and II oxidation status were not altered. Despite the large differences in proton motive force, responses to heat and high light stress were similar between tgd mutants and the wild type.