Understanding The Correlation Between FAD2 Gene Family And The Divergence Of Vegetable Oil Content In Plants

Vegetable oil mainly composes of unsaturated fatty acids including oleic acid,linoleic acid and linolenic acid.Seed oil content and fatty acid composition are varied greatly in plants.Fatty acid desaturase 2(FAD2)catalyzes oleic acid into linoleic acid and linolenic acid transformations,which is essential to the profile of FAs in seeds.To analyze the role of FAD2 s in seed fatty acid synthesis and oil content,we explore the FAD2 gene family about molecular evolution,biocatalysis,directed evolution and transcriptome sequencing.The results are as follows:Firstly,the molecular evolution of the FAD2 gene family showed that most of the FAD2 genes formed monophyletic clades.However,the FAD2 genes in some eudicots diverged into constitutive and seed-specific expression clades.The genes structures of FAD2 s showed that their protein structures were relatively conservative,while intron-exon structures were diverged during the evolution.In the algae,FAD2 genes contain seven to nine introns.In basal land plants,no introns in the FAD2 genes were detected.However,in the ancient of seed plants,the introns emerged again with the numbers ranging from one to four.In addition,the promoters of FAD2 s were also diverged with different motifs.Secondly,the seed-specific or high-expressed FAD2 genes in soybean,perilla,rice and spruce,were overexpressed in Saccharomyces cerevisiae.Through the whole cell catalysis,the catalytic activity of FAD2 enzyme was detected and analyzed.The result revealed that their catalytic activity was strongly correlated with the total oil content of seeds.The result also showed that FAD2 could affect seed oil content in plants.Thirdly,the 143 site of GmaFAD2-1 in the histidine motifs was diverged into I and Y in the seed-specific and constitutive expression clades of Fabaceae,respectively.Furthermore,the site-directed mutation demonstrated that the mutant between I and Y could increase and decrease the catalytic activity of GmaFAD2,respectively.However,the phenomenon was not found in perilla.So,the 143 site might be a key site for the activity of FAD2 in differentiated eudicots,especially in legumes.Finally,transcriptome analysis of different soybeans indicated that the expression of the seed-specific FAD2 genes was extremely high in fatty acid synthesis and metabolism.The expression levels of FAD2 in high oil accession were significantly higher than low oil one,which further indicated the importance of FAD2 s in the synthesis of seed oil.Therefore,the FAD2 gene family has diverged during evolution.Further,our results indicated that the catalytic activity and expression of FAD2 co-contributed to the difference of total oil content in plants.The results provided an important theoretical basis for the improvement of oil crops.