Key FAD2 , FAD3 , and SAD Genes Involved in the Fatty Acid Synthesis in Flax Identified Based on Genomic and Transcriptomic Data.

FAD (fatty acid desaturase) and SAD (stearoyl-ACP desaturase) genes play key roles in the synthesis of fatty acids (FA) and determination of oil composition in flax ( Linum usitatissimum L.). We searched for FAD and SAD genes in the most widely used flax genome of the variety CDC Bethune and three available long-read assembled flax genomes-YY5, 3896, and Atlant. We identified fifteen FAD2 , six FAD3 , and four SAD genes. Of all the identified genes, 24 were present in duplicated pairs. In most cases, two genes from a pair differed by a significant number of gene-specific SNPs (single nucleotide polymorphisms) or even InDels (insertions/deletions), except for FAD2a-1 and FAD2a-2 , where only seven SNPs distinguished these genes. Errors were detected in the FAD2a-1 , FAD2a-2 , FAD3c-1 , and FAD3d-2 sequences in the CDC Bethune genome assembly but not in the long-read genome assemblies. Expression analysis of the available transcriptomic data for different flax organs/tissues revealed that FAD2a-1 , FAD2a-2 , FAD3a , FAD3b , SAD3-1 , and SAD3-2 were specifically expressed in embryos/seeds/capsules and could play a crucial role in the synthesis of FA in flax seeds. In contrast, FAD2b-1 , FAD2b-2 , SAD2-1 , and SAD2-2 were highly expressed in all analyzed organs/tissues and could be involved in FA synthesis in whole flax plants. FAD2c-2 , FAD2d-1 , FAD3c-1 , FAD3c-2 , FAD3d-1 , FAD3d-2 , SAD3-1 , and SAD3-2 showed differential expression under stress conditions- Fusarium oxysporum infection and drought. The obtained results are essential for research on molecular mechanisms of fatty acid synthesis, FAD and SAD editing, and marker-assisted and genomic selection for breeding flax varieties with a determined fatty acid composition of oil.