Spatial and temporal regulation of three different microsomal oleate desaturase genes (FAD2) from normal-type and high-oleic varieties of sunflower (Helianthus annuus L.)

In addition to the normal-type sunflower (Helianthus annuus L.) where linoleic acid is the major seed fatty acid, a dominant negative high-oleic mutant with oleic acid as the predominant fatty acid was previously obtained. We report the isolation and characterization of three different cDNA sequences, designated Ha89FAD2-1, Ha89FAD2-2, and Ha89FAD2-3, encoding sunflower microsomal oleate desaturases (FAD2), using a PCR strategy. All three deduced amino acid sequences showed significant homology to the known plant FAD2 sequences. Genomic Southern blot analysis revealed that at least one copy of each of these genes is present in the sunflower genome, except for the FAD2-1 gene from the high-oleic mutant, which might be duplicated. The FAD2-2 and FAD2-3 genes were weakly expressed in all tissues studied from both varieties. In contrast, the FAD2-1 gene was expressed strongly and exclusively in developing embryos of normal-type sunflower, whereas its expression in high-oleic developing embryos was drastically reduced. Functional expression of the corresponding cDNAs in yeast confirmed that they encode microsomal oleate desaturases. Furthermore, the FAD2-1 gene from the high-oleic variety also expresses a fully active enzyme. These results suggest that the high-oleic mutation in sunflower interferes with the regulation of the transcription of the seed-specific FAD2 gene.