Abstract
Plant oils rich in oleic acid (OA) are attracting considerable attention for their high nutritional value and significant industrial potential. Stearoyl-acyl carrier protein desaturases (SADs) are a class of soluble desaturases that play a key role in OA accumulation in plants. In this study, the first genome-wide characterization of the SAD gene family was conducted in tigernut (Cyperus esculentus L. var. sativus Baeck., Cyperaceae), an oil-rich tuber plant typical for its high OA content. Six SAD genes identified from the tigernut genome are comparative to seven reported in two model plants Arabidopsis thaliana and Oryza sativa, but relatively more than four were found in most Cyperaceae species examined in this study. A comparison of 161 SAD genes from 29 representative plant species reveals the monogenic origin and lineage-specific family evolution in Poales. C. esculentus SAD genes (CeSADs) were shown to constitute two evolutionary groups (i.e., FAB2 and AAD) and four out of 12 orthogroups identified in this study, i.e., FAB2a, FAB2b, FAB2c, and AAD1. Whereas FAB2a and AAD1 are widely distributed, FAB2b and FAB2c are specific to Cyperaceae, which may arise from FAB2a via tandem and dispersed duplications, respectively. Though FAB2d and AAD2 are also broadly present in monocots, they are more likely to be lost in the Cyperaceae ancestor sometime after the split with its close family, Juncaceae. In tigernut, FAB2a appears to have undergone species-specific expansion via tandem duplication. Frequent structural variation and apparent expression divergence were also observed. Though FAB2a and AAD1 usually feature two and one intron, respectively, gain of certain introns was observed in CeSAD genes, all of which have three introns. Despite recent expansion of the FAB2 group, CeFAB2-1 has evolved into the dominant member that was highly and constitutively expressed in all tested organs. Moreover, CeFAB2-1, CeAAD1, as well as CeFAB2-5 have evolved to be predominantly expressed in tubers and thus contribute to high OA accumulation. These findings highlight lineage-specific evolution of the SAD family and putative roles of CeSAD genes in tuber oil accumulation, which facilitate further functional analysis and genetic improvement in tigernut and other species.