Identification and Molecular Characterization of Homeologous Δ9-Stearoyl Acyl Carrier Protein Desaturase 3 Genes from the Allotetraploid Peanut (Arachis hypogaea)

The stearoyl–acyl carrier protein (ACP) desaturase (SAD) is a nuclear-encoded, plastid-localized soluble desaturase that catalyzes the conversion of stearoyl-ACP to oleoyl-ACP and plays a key role in the determination of the properties of the majority of cellular glycerolipids. Sad genes from a variety of plant species have been cloned and characterized. However, in peanut (Arachis hypogaea), an important edible and oilseed crop, these genes have not yet been characterized. By searching peanut expressed sequence tag (EST) and parallel sequencing (454) libraries, we have identified three members of the ahSad gene family. Among them, only one gene, ahSad3, was exclusively expressed during seed development and in a manner fully corresponding to oil accumulation. Both ahSad3 homeologous genes (ahSad3A and ahSad3B) were recovered from the allotetraploid peanut, and their mRNA expression levels were characterized. The open reading frames for ahSad3A and ahSad3B are 98% identical and consist of 1,158 bp, encoding a 386-full-amino-acid protein, with one intron in the coding sequence. Comparisons of the sequences of these two homeologous genes revealed seven single-nucleotide polymorphisms and one triplet insertion in the coding region. Southern blot analysis indicated that there are only two copies of the ahSad3 gene in the peanut genome. Homeolog-specific gene expression analysis showed that both ahSad3 homeologs are expressed in developing seeds, but gene expression is significantly biased toward the B genome. Our results point to ahSad3 as a possible target gene for manipulation of fatty acid saturation in A. hypogaea.