Downregulation of caffeic acid 3-O-methyltransferase and caffeoyl CoA 3-O-methyltransferase in transgenic alfalfa: Impacts on lignin structure and implications for the biosynthesis of G and S lignin

Transgenic alfalfa plants were generated harboring caffeic acid 3-O-methylt ransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) cDNA seque nces under control of the bean phenylalanine ammonia-lyase PAL2 promoter. S trong downregulation of COMT resulted in decreased lignin content, a reduct ion in total guaiacyl (G) lignin units, a near total loss of syringyl (S) u nits in monomeric and dimeric lignin degradation products, and appearance o f low levels of 5-hydroxy guaiacyl units and a novel dimer. No soluble mono lignol precursors accumulated. In contrast, strong downregulation of CCOMT led to reduced lignin levels, a reduction in G units without reduction in S units, and increases in beta -5 linked dimers of G units. Accumulation of soluble caffeic acid P-D-glucoside occurred only in CCOMT downregulated pla nts. The results suggest that CCOMT does not significantly contribute to th e 3-O-methylation step in S lignin biosynthesis in alfalfa and that there i s redundancy with respect to the 3-O-methylation reaction of G lignin biosy nthesis. COMT is unlikely to catalyze the in vivo methylation of caffeic ac id during lignin biosynthesis.