PHOSPHATE AVAILABILITY AFFECTS THE THYLAKOID LIPID-COMPOSITION AND THE EXPRESSION OF SQD1, A GENE REQUIRED FOR SULFOLIPID BIOSYNTHESIS IN ARABIDOPSIS-THALIANA

Photosynthetic membranes of higher plants contain specific nonphosphor ous lipids like the sulfolipid sulfoquinovosyl diacylglycerol in addit ion to the ubiquitous phospholipid phosphatidylglycerol. In bacteria, an environmental factor that drastically affects thylakoid lipid compo sition appears to be the availability of phosphate, Accordingly, we di scovered an increase in the relative amount of sulfolipid and a concom itant decrease in phosphatidylglycerol in Arabidopsis thaliana grown o n medium with reduced amounts of phosphate, as well as in the pho1 mut ant of A. thaliana deficient in phosphate transport, To investigate th e molecular basis of the observed change in lipid composition, we isol ated a cDNA of A. thaliana, designated SQD1, that encodes a protein in volved in sulfolipid biosynthesis as suggested by three lines of evide nce, First, the cDNA shows high sequence similarity to bacterial sqdB genes known to be essential for sulfolipid biosynthesis; second, the S QD1 gene product is imported into chloroplasts where sulfolipid biosyn thesis takes place; and third, transgenic plants expressing SQD1 in an tisense orientation show a reduction in sulfolipid content. In the pho 1 mutant as well as in wild-type plants grown under reduced phosphate availability, increased amounts of SQD1 mRNA and SQD1 protein are dete cted, suggesting that the increase in sulfolipid content under phospha te limitation is the result of an increased expression of at least one gene required for sulfolipid biosynthesis in A. thaliana. It is sugge sted that a certain amount of anionic thylakoid lipid is maintained by substituting sulfolipid for phosphatidylglycerol under reduced phosph ate availability.