Molecular Characterization and Expression of the Cinnamyl Alcohol Dehydrogenase Gene Family in Sweet Potato (Ipomoea batatas) under Environmental Stresses

Cinnamyl alcohol dehydrogenase (CAD) catalyzes the final step of monolignol biosynthesis and is a key enzyme for lignification. Most of the CAD genes in angiosperms belong to a multigene family. As in other plants, 13 cDNAs encoding CAD (IbCADs) were isolated from the expressed sequence tag (EST) library of sweet potato (Ipomoea batatas). A phylogenetic analysis revealed that IbCADs belonged to the nondevelopmental CAD proteins, which are not conserved as bona fide CADs. The IbCAD proteins were classified into four groups (Groups I-IV) according to their amino acid similarity. In all groups, IbCADs contain the Zn-binding domains, suggesting that they belong to a family of zinc-dependent alcohol dehydrogenases. Each group showed sequence divergence in the residues related in substrate specificity. Different structural characteristics of IbCADs were supported by their different transcriptional expression patterns. The IbCAD genes in four groups were highly induced in response to environmental stresses (cold, H2O2, and wounding), but each group showed different patterns and levels of expression. The differential response of the IbCAD gene family under different stresses might indicate a more complex nature of the CAD gene expression in sweet potato. These diverse expression patterns represented an important function of each IbCAD gene in tolerance to environmental stresses. To the best of our knowledge, this is the first study to characterize genes of the CAD family genes in a root crop. These results could be useful in understanding the physiological roles and characteristics of the IbCAD multigene family.