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-methyltransferase (COMT) and caffeoyl CoA 3-O-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 beta-5 linked dimers of G units. Accumulation of soluble caffeic acid beta-d-glucoside occurred only in CCOMT downregulated plants. The results suggest that CCOMT does not significantly contribute to the 3-O-methylation step in S lignin biosynthesis in alfalfa and that there is redundancy with respect to the 3-O-methylation reaction of G lignin biosynthesis. COMT is unlikely to catalyze the in vivo methylation of caffeic acid during lignin biosynthesis.
Guo
D
D
Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401, USA.
Chen
F
F
Inoue
K
K
Blount
J W
JW
Dixon
R A
RA
D001483:Base Sequence; D017931:DNA Primers; D007887:Fabaceae / Q000235:genetics; D008031:Lignin / Q000096:biosynthesis*; D000455:Medicago sativa / Q000201:enzymology* / Q000378:metabolism; D008780:Methyltransferases / Q000235:genetics*; D030821:Plants, Genetically Modified / Q000201:enzymology* / Q000378:metabolism; D011401:Promoter Regions, Genetic
D017931:DNA Primers; D008031:Lignin; D008780:Methyltransferases; C059825:caffeoyl-CoA O-methyltransferase; C019608:caffeate O-methyltransferase