A sweet potato cinnamate 4-hydroxylase gene, IbC4H, increases phenolics content and enhances drought tolerance in tobacco

Sweet potato [Ipomoea batatas (L.) Lam.] is a vegetable crop with significant economic value that is utilized both as a staple food and as an industrial raw material. Sweet potato possesses a variety of healthful beneficial secondary metabolites, which also contribute towards its adaptation to the environment and capacity to withstand stresses. Cinnamate 4-hydroxylase constitutes a primary enzyme in the phenylpropanoid pathway that is involved in the biosynthesis of numerous secondary metabolites. Elucidating the molecular and expressional characteristics of C4H is essential for understanding the functional genetic diversity of sweet potato germplasms. Based on the transcriptome analysis of five sweet potato genotypes with different anthocyanin contents, a cinnamate 4-hydroxylase (C4H) gene, termed IbC4H, was isolated from a purple sweet potato cv. Xuzishu 3. The cDNA sequence of IbC4H contained a 1518-bp ORF encoding a 505 amino acid protein with a molecular weight of 58.15 kDa. Genomic DNA analysis revealed that the IbC4H gene possessed three exons and two introns. The IbC4H protein shared 86.34-97.82% sequence identity with that reported for other plants. A significant positive correlation between the expression of IbC4H and phenolic acid content suggested that IbC4H might be integral to the regulation of phenolic biosynthesis in sweet potato. Furthermore, IbC4H was significantly upregulated in the young leaves and roots of cv. Xuzishu 3 and was strongly induced when subjected to drought stress. Drought stress tolerance in transgenic tobacco plants was significantly improved by the heterologous expression of the transgene IbC4H, which was simultaneously associated with the increased expression of stress-related genes, higher superoxide dismutase (SOD) activity, and proline (Pro) content, along with lower malondialdehyde (MDA) production. Collectively, these results highlight the important role of IbC4H in the phenylpropanoid pathway and support the protective function of polyphenols in the improved scavenging of reactive oxygen species of plants under stress.