Expression of FAD and SAD Genes in Developing Seeds of Flax Varieties under Different Growth Conditions

Flax seed is one of the richest plant sources of linolenic acid (LIN) and also contains unsaturated linoleic acid (LIO) and oleic acid (OLE). Stearoyl-ACP desaturases (SAD) and fatty acid desaturases (FAD) play a key role in the synthesis of flax fatty acids (FA). We sequenced flax seed transcriptomes at the 3, 7, 14, 21, and 28 day after flowering (DAF) for ten flax varieties with different oil FA compositions grown under three temperature/watering conditions. Expression levels of 25 genes of SAD, FAD2, and FAD3 families were evaluated. FAD3b, FAD3a, FAD2b-2, SAD3-1, SAD2-1, SAD2-2, SAD3-2, FAD2a-1, and FAD2a-2 had the highest expression levels, which changed significantly during seed development. These genes probably play a key role in the FA synthesis in flax seeds. High temperature and insufficient watering shifted the maximum expression levels of the FAD and SAD genes to earlier development stages, while the opposite trend was observed for low temperature and excessive watering. Differences in FAD and SAD expression profiles under different growth conditions could contribute to the FA composition of linseed oil. Stop codons in the FAD3a gene, resulting in reduced LIN content, decreased the level of the FAD3a transcript. Some difference in reaching the maximum expression level during seed development was observed between 1) SAD3-1, SAD3-2, and FAD2b-2 and 2) SAD2-1, SAD2-2, FAD2a-1, FAD2a-2, FAD3a, and FAD3b. These groups possibly contribute somewhat differently to the synthesis of FA at different development stages.