ACYL-ACYL CARRIER PROTEIN DESATURASE2 and 3 Are Responsible for Making Omega-7 Fatty Acids in the Arabidopsis Aleurone

Omega-7 monounsaturated fatty acids (v-7s) are specifically enriched in the aleurone of Arabidopsis (Arabidopsis thaliana) seeds. We found significant natural variation in seed v-7 content and used a Multiparent Advanced Generation Inter-Cross population to fine-map a major quantitative trait loci to a region containing ACYL-ACYL CARRIER PROTEIN DESATURASE1 (AAD1) and AAD3. We found that AAD3 expression is localized to the aleurone where mutants show an approximately 50% reduction in v-7 content. By contrast, AAD1 is localized to the embryo where mutants show a small reduction in v-9 content. Enzymatic analysis has previously shown that AAD family members possess both stearoyl-and palmitoyl-ACP D9 desaturase activity, including the predominant isoform SUPPRESSOR OF SALICYLIC ACID INSENSITIVE2. However, aad3 ssi2 aleurone contained the same amount of v-7s as aad3. Within the AAD family, AAD3 shares the highest degree of sequence similarity with AAD2 and AAD4. Mutant analysis showed that AAD2 also contributes to v-7 production in the aleurone, and aad3 aad2 exhibits an approximately 85% reduction in v-7s. Mutant analysis also showed that FATTY ACID ELONGASE1 is required for the production of very long chain v-7s in the aleurone. Together, these data provide genetic evidence that the v-7 pathway proceeds via D9 desaturation of palmitoyl-ACP followed by elongation of the product. Interestingly, significant variation was also identified in the v-7 content of Brassica napus aleurone, with the highest level detected being approximately 47% of total fatty acids.