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

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.

Figure 1

Changes in relative amounts of complex lipids of wild-type A. thaliana in response to changing phosphate concentration. The plants were germinated and grown for 15 days on agar-solidified mineral medium as described under methods. Values represent the means ± SD of at least three independent measurements. Error bars are indicated, if they are not smaller than the symbol size. DGDG, digalactosyl diacylglycerol; MGDG, monogalactosyl diacylglycerol; PC, phosphatidylcholine; PE, phosphatidyl ethanolamine; PG, phosphatidylglycerol; SQDG, sulfoquinovosyl diacylglycerol.

Figure 2

Comparison of the amino acid sequences predicted from the sqdB gene of the cyanobacterium Synechococcus sp. PCC7942 (Syne) and the SQD1 cDNA of A. thaliana (Arab). Identical amino acid residues are marked by black boxes.

Figure 3

Uptake and processing of a carboxy-terminally truncated SQD1 preprotein of A. thaliana by isolated pea chloroplasts. Lane 1, translation product; lane 2, translation product incubated in the presence of pea chloroplasts; lane 3, translation product incubated in the presence of pea chloroplasts and thermolysin. Proteins were labeled with [³⁵S]methionine and visualized by autoradiography after SDS/PAGE. The approximate molecular mass (kDa) of the proteins is indicated.

Figure 4

Expression of the SQD1 gene in leaves of A. thaliana. (A) Wild type, the strongest SQD1 antisense line AS1, and the pho1 mutant grown for 14 days in soil. (B) Wild type germinated and raised for 7 days on agar-solidified Murashige–Skoog medium and subsequently grown for 7 days on mineral medium containing a concentration of P_i (mM) as indicated. (Upper) SQD1 mRNA (SQD1) or the presumed antisense RNA (asSQD1) detected by hybridization with the SQD1 cDNA (the approximate sizes are indicated). (Middle) SQD1 protein (SQD1) detected by immunoblotting (the apparent molecular mass is indicated). (Lower) Central part of iodine-stained lipid chromatograms (DGDG, digalactosyl diacylglycerol; PE, phosphatidylethanolamine; SQDG, sulfoquinovosyl diacylglycerol). RNA, protein, and lipids were isolated from identical batches of plants. Equal amounts of total RNA, protein, or lipids were loaded based on visual examination of stained gels or TLC plates.