Involved in the latter stages of lignin biosynthesis. Catalyzes one of the last steps of monolignol biosynthesis, the conversion of cinnamoyl-CoAs into their corresponding cinnamaldehydes. (E)-cinnamaldehyde + CoA + NADP(+) = (E)-cinnamoyl-CoA + H(+) + NADPH Aromatic compound metabolism; phenylpropanoid biosynthesis. A number of isoforms are produced. According to EST sequences. Expressed in leaves, stems and flowers. Dwarf phenotype, delayed senescence, collapsed xylem and significant reduction of lignin content in ecotype Columbia (PubMed:19674336). Retarded growth, impaired upright growth, altered leaf morphology, dark green leaves, collapsed xylem and strong decrease in lignin content in ecotype Landsberg erecta (PubMed:11389761). Belongs to the NAD(P)-dependent epimerase/dehydratase family. Dihydroflavonol-4-reductase subfamily.