Digalactosyldiacylglycerol synthesis in chloroplasts of the Arabidopsis dgd1 mutant.
Galactolipid biosynthesis in plants is highly complex. It involves multiple pathways giving rise to different molecular species. To assess the contribution of different routes of galactolipid synthesis and the role of molecular species for growth and photosynthesis, we initiated a genetic approach of analyzing double mutants of the digalactosyldiacylglycerol (DGDG) synthase mutant dgd1 with the acyltransferase mutant, act1, and the two desaturase mutants, fad2 and fad3. The double mutants showed different degrees of growth retardation: act1,dgd1 was most severely affected and growth of fad2,dgd1 was slightly reduced, whereas fad3,dgd1 plants were very similar to dgd1. In act1,dgd1, lipid and chlorophyll content were reduced and photosynthetic capacity was affected. Molecular analysis of galactolipid content, fatty acid composition, and positional distribution suggested that the growth deficiency is not caused by changes in galactolipid composition per se. Chloroplasts of dgd1 were capable of synthesizing monogalactosyldiacylglycerol, DGDG, and tri- and tetragalactosyldiacylglycerol. Therefore, the reduced growth of act1,dgd1 and fad2,dgd1 cannot be explained by the absence of DGDG synthase activity from chloroplasts. Molecular analysis of DGDG accumulating in the mutants during phosphate deprivation suggested that similarly to the residual DGDG of dgd1, this additional lipid is synthesized in association with chloroplast membranes through a pathway independent of the mutations, act1, dgd1, fad2, and fad3. Our data imply that the severe growth defect of act1,dgd1 is caused by a reduced metabolic flux of chloroplast lipid synthesis through the eukaryotic and prokaryotic pathway as well as by the reduction of photosynthetic capacity caused by the destabilization of photosynthetic complexes.