Differential Activation of Partially Redundant Δ9 Stearoyl-ACP Desaturase Genes is Critical for Omega-9 Monounsaturated Fatty Acid Biosynthesis During Seed Development in Arabidopsis

The spatiotemporal pattern of deposition, final amount, and relative abundance of oleic acid (cis-ω-9 C18:1) and its derivatives in the different lipid fractions of the seed denote an abundant synthesis of omega-9 monoenes in this organ. Accordingly, four Δ9 stearoyl-ACP desaturase (SAD)-coding genes (FAB2, AAD5, AAD1, and AAD6) are transcriptionally induced in seeds. The three most highly expressed ones are directly activated by the WRINKLED1 transcription factor. The thorough characterization of a collection of 30 simple, double, triple, and quadruple mutants affected in SAD-coding genes has revealed the functions of these desaturases all along seed development. First, production of oleic acid by FAB2 and AAD5 appears to be critical at the onset of embryo morphogenesis. Double homozygous plants from crossing fab2 and aad5 could never be obtained, and further investigations revealed that the double mutation results in the arrest of embryo development before the globular stage. During later stages, these two SADs, together with AAD1, participate in the elaboration of the embryonic cuticle, a barrier essential for embryo-endosperm separation during the phase of invasive embryo growth through the endosperm. This study finally demonstrates that the four desaturases redundantly contribute to storage lipid production during the maturation phase.