Superoxide dismutase multigene family in longan somatic embryos: a comparison of CuZn-SOD, Fe-SOD, and Mn-SOD gene structure, splicing, phylogeny, and expression

Superoxide dismutases (SODs), which are encoded by a multigene family, are important antioxidant enzymes guarding against superoxide toxicity. Because no systematic characterization of this gene family has been conducted in plant embryos, the function of SODs in longan embryos is not completely clear. In this study, we isolated 20 full-length cDNAs encoding cytoplasmic CSD1a and DlCSD1b, chloroplastic DlCSD2a and DlFSD1a, plastidic DlFSD1b, and mitochondrial DlMSD from longan embryogenic callus using reverse transcription-PCR and rapid amplification of cDNA ends. Each gene possessed multiple polyadenylation sites. The DlCSD1a, DlCSD1b, DlCSD2a, DlFSD1a, DlFSD1b, and DlMSD genes contained eight, six, seven, six, seven, and five introns, respectively, of greatly varying length. Seven variants with different splicing modes, demonstrating functional diversity during longan somatic embryogenesis (SE), were also cloned. Each SOD gene type had multiple transcription start sites (TSSs). The choice of TSS affected the length of the 5'-untranslated region but did not contribute to protein diversity. DlCSD1a, DlCSD2a, DlFSD1a, and DlMSD promoters were isolated and found to contain many cis-acting elements responsive to stimuli such as light, gibberellins, auxin, methyl jasmonate, dehydration, coldness, and wounding. Comprehensive expression profile analysis indicated that different SOD types exhibit different spatial and temporal expression modes and play a key role in longan SE, especially during the middle and later developmental stages. This study represents the first comprehensive analysis of the SOD gene family in plant SE, encompassing full-length cDNA cloning, gene structures, alternatively spliced variants, phylogeny, TSS, promoter isolation, and expression patterns. The comparisons in this study provide insights into diverse functions of SOD isoforms during longan somatic embryo formation.