Phosphatidylglycerol biosynthesis in chloroplasts of Arabidopsis mutants deficient in acyl-ACP glycerol-3- phosphate acyltransferase.

The biosynthesis of phosphatidylglycerol represents a central pathway in lipid metabolism in all organisms. The enzyme catalyzing the first reaction of the pathway in the plastid, glycerol-3-phosphate acyl-acyl carrier protein acyltransferase, is thought to be encoded in Arabidopsis by the ATS1 locus. A number of genetic mutants deficient in this activity have been described. However, the corresponding mutant alleles have not yet been analyzed at the molecular level and a causal relationship between the mutant phenotypes and a deficiency at the ATS1 locus has not been established. The presence in all known ats1 mutants of near wild-type amounts of phosphatidylglycerol raised the question of whether an alternative pathway of phosphatidylglycerol assembly in the plastid exists. However, detailed analysis of several independent ats1 mutant alleles revealed that all are leaky. Reduction by RNAi of ats1-1 RNA levels in the ats1-1 mutant background led to a more severe growth phenotype (small green plants and reduced seed set), but did not decrease the relative amount of phosphatidylglycerol. In contrast, when the amount of ATS2 mRNA encoding the plastidic lysophosphatidic acid acyltransferase catalyzing the second reaction of the pathway was reduced by RNAi in the ats1-1 mutant background, phosphatidylglycerol amounts decreased, leading to a growth phenotype (small pale-yellow plants) that is reminiscent of the pgp1-1 mutant deficient in a late step of plastidic phosphatidylglycerol biosynthesis. These observations indicate coordinated regulation of plastid lipid metabolism and plant development.