A novel lignin in poplar trees with a reduced caffeic acid/5‐hydroxyferulic acid O‐methyltransferase activity

Abstract

Lignin is a polymeric constituent of the cell wall that needs to be removed during the paper making process. Bispecific caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) catalyses the O-methylation of caffeic acid and 5-hydroxyferulic acid to ferulic acid and sinapic acid, respectively. These compounds are intermediates in the biosynthesis of the lignin precursors. Therefore, COMTs are potential target enzymes for reducing the amount, or modifying the composition, of lignin in plants. Different antisense and sense constructs have been expressed of a gene encoding a COMT from poplar (Populus trichocarpa X II: deltoides) in a I! tremula X P. alba clone under the control of the cauliflower mosaic virus 35S promoter. From all analysed transformants, four lines transformed with an antisense construct had a reduced COMT activity. Two showed a 50% reduction of COMT activity, which altered only slightly the monomeric composition. In the two other transform ants, the COMT activity was reduced by 95%. In the latter case, the syringyl/guaiacyl ratio (S/G) was reduced by sixfold (due to a decrease of S and an increase of G), as analysed by thioacidolysis. A new component of lignin, the 5-hydroxy-guaiacyl residue, was detected among the thioacidolysis products. Moreover, in contrast to the white/yellow colour of wild-type wood, the xylem of the transgenic lines with a 95% reduction of COMT activity was pale rose. A similar phenotype was observed in brown-midrib mutants of maize and sorghum, known for their altered lignification. Although the lignin composition was consistently modified, the lignin content of the transgenic poplars was similar to that of the controls

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

“…Interestingly, complete loss of CCOMT protein in the CCOMT antisense line ACC305 was accompanied by an increase in COMT protein level, as documented in Figure 5C, and in COMT enzymatic activity (see below). The above results indicate that expression of OMT sequences from the bean PAL2 promoter results in downregulation of COMT and CCOMT that is similar to or greater than that obtained in previous studies in tobacco and poplar (Ni et al, 1994;Atanassova et al, 1995;Van Doorsselaere et al, 1995;Zhong et al, 1998), in contrast with our previous results using 35S promoter-driven constructs in alfalfa.…”

“…Most studies on genetic modification of lignin biosynthesis in transgenic plants have utilized the cauliflower mosaic virus 35S promoter to drive expression of sense or antisense lignification-associated genes (Halpin et al, 1994;Ni et al, 1994;Atanassova et al, 1995;Doorsselaere et al, 1995;Piquemal et al, 1998;Zhong et al, 1998;Baucher et al, 1999). However, antisense downregulation of COMT and CCOMT in transgenic alfalfa is inefficient and relatively weak using 35S promoter constructs (V.J.H.…”

“…There have been several reports on the effects of downregulation of COMT activity on lignin content and composition in transgenic plants (Ni et al, 1994;Atanassova et al, 1995;Van Doorsselaere et al, 1995;Zhong et al, 1998). The results of these studies have been somewhat contradictory, possibly due to unspecified differences in tissue maturity, use of homologous versus heterologous transgenes, and use of different methods for lignin analysis.…”

Lignin and Lignan Biosynthesis: Distinctions and Reconciliations

“…Instead, the molecular approach has primarily been devoted to attempts at determining the effects of differentially expressing genes encoding putative regulatory enzymes in the lignin biosynthetic pathway (Figure 3) (56)(57)(58)(59)(60)(61)(62). As can be seen, there are essentially only four types of transformations beyond cinnamic acid, namely aromatic hydroxylations (63)(64)(65), O-methylations (66-68), CoA ligations and (consecutive) NADPH dependent reductions (69).…”

4-Coumarate:Coenzyme A Ligase in Hybrid Poplar1

“…The lignin in poplar secondary xylem (wood) is composed primarily of guaiacyl units, presumed to be derived from coniferyl alcohol and feruloyl-CoA, and syringyl units, presumed to be derived from syringyl alcohol and sinapoyl-CoA. It has been possible to alter the composition of this lignin by antisense suppression of caffeate 3-O-methyltransferase and CAD activity (van Doorsselaere et al, 1995a;Baucher et al, 1996) in transgenic poplar trees.…”