Molecular characterization of an apple cDNA encoding cinnamyl alcohol dehydrogenase

The study of lignin, a major component of secondary cell wall, has been partly focused on its removal from the woody part in the kraft pulping industry. Cinnamyl alcohol dehydrogenase (CAD; EC1.1.195) catalyzes the synthesis of cinnamyl alcohols from corresponding cinnamaldehydes. A cDNA clone, MdCAD1, encoding putative CAD from apples (Malus domestica Borkh. cv Fuji) was characterized in this study. The clone contains an open reading frame of 325 amino acid residues, which shows a greater than 80% identity with Eucalyptus CAD1. MdCAD1 mRNA was detectable in vegetative tissues and was strongly expressed in the fruit. The expression pattern of MdCAD1 mRNA in the fruit peel after light exposure was also examined. The mRNA was rapidly increased until 1 day after light exposure and remained stable thereafter, suggesting that MdCAD1 is light inducible. The inducibility of the MdCAD1 gene was examined using several environmental stresses. Mechanical wounding of leaves increased the MdCAD1 mRNA level and the induction was further increased by salicylic acid. Southern blot hybridization showed that there is either one or a few copies of CAD genes in apples. To our knowledge, it is believed that MdCAD1 is the first CAD clone expressed predominantly in fruit.