Two Acyltransferases Contribute Differently to Linolenic Acid Levels in Seed Oil.

ABSTRACT

Acyltransferases are key contributors to triacylglycerol (TAG) synthesis and, thus, are of great importance for seed oil quality. The effects of increased or decreased expression of ACYL-COENZYME ADIACYLGLYCEROL ACYLTRANSFERASE1 (DGAT1) or PHOSPHOLIPIDDIACYLGLYCEROL ACYLTRANSFERASE (PDAT) on seed lipid composition were assessed in several Camelina sativa lines. Furthermore, in vitro assays of acyltransferases in microsomal fractions prepared from developing seeds of some of these lines were performed. Decreased expression of DGAT1 led to an increased percentage of 183n-3 without any change in total lipid content of the seed. The tri-183 TAG increase occurred predominantly in the cotyledon, as determined with matrix-assisted laser desorption/ionization-mass spectrometry, whereas species with two 183n-3 acyl groups were elevated in both cotyledon and embryonal axis. PDAT overexpression led to a relative increase of 182n-6 at the expense of 183n-3, also without affecting the total lipid content. Differential distributions of TAG species also were observed in different parts of the seed. The microsomal assays revealed that C.sativa seeds have very high activity of diacylglycerol-phosphatidylcholine interconversion. The combination of analytical and biochemical data suggests that the higher 182n-6 content in the seed oil of the PDAT overexpressors is due to the channeling of fatty acids from phosphatidylcholine into TAG before being desaturated to 183n-3, caused by the high activity of PDAT in general and by PDAT specificity for 182n-6. The higher levels of 183n-3 in DGAT1-silencing lines are likely due to the compensatory activity of a TAG-synthesizing enzyme with specificity for this acyl group and more desaturation of acyl groups occurring on phosphatidylcholine.