A NOVEL LIGNIN IN POPLAR TREES WITH A REDUCED CAFFEIC ACID 5-HYDROXYFERULIC ACID O-METHYLTRANSFERASE ACTIVITY

Lignin is a polymeric constituent of the cell wall that needs to be re moved during the paper making process. Bispecific caffeic acid/5-hydro xyferulic 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 biosynthe sis of the lignin precursors. Therefore, COMTs are potential target en zymes for reducing the amount, or modifying the composition, of lignin in plants. Different antisense and sense constructs have been express ed of a gene encoding a COMT from poplar (Populus trichocarpa X II: de ltoides) in a I! tremula X P. alba clone under the control of the caul iflower mosaic virus 35S promoter. From all analysed transformants, fo ur lines transformed with an antisense construct had a reduced COMT ac tivity. Two showed a 50% reduction of COMT activity, which altered onl y slightly the monomeric composition. In the two other transform ants, the COMT activity was reduced by 95%. In the latter case, the syringy l/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 thio acidolysis products. Moreover, in contrast to the white/yellow colour of wild-type wood, the xylem of the transgenic lines with a 95% reduct ion of COMT activity was pale rose. A similar phenotype was observed i n brown-midrib mutants of maize and sorghum, known for their altered l ignification. Although the lignin composition was consistently modifie d, the lignin content of the transgenic poplars was similar to that of the controls.