Phosphate availability affects the thylakoid lipid composition and the expression of SQD1 , a gene required for sulfolipid biosynthesis in Arabidopsis thaliana

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Abstract
Photosynthetic membranes of higher plants contain specific nonphosphorous lipids like the sulfolipid sulfoquinovosyl diacylglycerol in addition to the ubiquitous phospholipid phosphatidylglycerol. In bacteria, an environmental factor that drastically affects thylakoid lipid composition appears to be the availability of phosphate. Accordingly, we discovered an increase in the relative amount of sulfolipid and a concomitant decrease in phosphatidylglycerol in Arabidopsis thaliana grown on medium with reduced amounts of phosphate, as well as in the pho1 mutant of A. thaliana deficient in phosphate transport. To investigate the molecular basis of the observed change in lipid composition, we isolated a cDNA of A. thaliana , designated SQD1 , that encodes a protein involved in sulfolipid biosynthesis as suggested by three lines of evidence. First, the cDNA shows high sequence similarity to bacterial sqdB genes known to be essential for sulfolipid biosynthesis; second, the SQD1 gene product is imported into chloroplasts where sulfolipid biosynthesis takes place; and third, transgenic plants expressing SQD1 in antisense orientation show a reduction in sulfolipid content. In the pho1 mutant as well as in wild-type plants grown under reduced phosphate availability, increased amounts of SQD1 mRNA and SQD1 protein are detected, suggesting that the increase in sulfolipid content under phosphate limitation is the result of an increased expression of at least one gene required for sulfolipid biosynthesis in A. thaliana . It is suggested that a certain amount of anionic thylakoid lipid is maintained by substituting sulfolipid for phosphatidylglycerol under reduced phosphate availability.