-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 3O -methyltransferase (COMT) and caffeoyl CoA 3O -methyltransferase (CCOMT) cDNA sequences under control of the bean phenylalanine ammonia-lyase PAL2 promoter. Strong downregulation of COMT resulted in decreased lignin content, a reduction in total guaiacyl (G) lignin units, a near total loss of syringyl (S) units in monomeric and dimeric lignin degradation products, and appearance of low levels of 5-hydroxy guaiacyl units and a novel dimer. No soluble monolignol 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 b -5 linked dimers of G units. Accumulation of soluble caffeic acid b D -glucoside occurred only in CCOMT downregulated plants. The results suggest that CCOMT does not significantly contribute to the 3O -methylation step in S lignin biosynthesis in alfalfa and that there is redundancy with respect to the 3O -methylation reaction of G lignin biosynthesis. COMT is unlikely to catalyze the in vivo methylation of caffeic acid during lignin biosynthesis.