Genomic Analysis Reveals Molecular Mechanisms Underlying Enhanced Alpha-Linolenic Acid Accumulation in Developing Flax Seed

Flax (Linum usitatissimum L.), which produces oil containing a high amount of α-linolenic acid (ALA; 18:3^cisΔ9,12,15), is considered one of the most important plant-based sources of ALA. The goal of our study was to achieve a better understanding of the molecular mechanisms accounting for the high accumulation of ALA in flax oil. Using the flax genome database, we first identified and functional characterized enzymes involved in the final step of triacylglycerol (TAG) biosynthesis, including seven acyl CoA:diacylglycerol acyltransferases (DGATs) and six phospholipid:diacylglycerol acyltransferases (PDATs). Our data provided several lines of evidence in support of flax containing unique PDATs, which can efficiently transfer ALA to TAG. We further performed a comprehensive genome-wide analysis of the PDAT gene family across green plants and revealed that the functional divergence of PDAT paralogs is mainly caused by different selection pressures during evolution. Furthermore, our study revealed a second mechanism for enriching the ALA content of TAG, which involves coupling of the DGAT-catalyzed reaction for TAG production to the reverse reaction catalyzed by acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT). Our in vivo and in vitro data support the hypothesis that the phosphatidylcholine (PC) de-acylation reaction catalyzed by the LPCAT reverse action can transfer ALAs produced on PC directly into the acyl-CoA pool and make them available for the DGAT-catalyzed reaction for incorporation into TAG. The knowledge obtained from this work will benefit the future development of novel biotechnological strategies to produce value-added seed oils in flax and other oilseed crops.