Repression of O-methyltransferase genes in transgenic tobacco affects lignin synthesis and plant growth

Among the different enzymatic steps leading to lignin biosynthesis: two met hylation reactions introduce the methyl groups borne by guaiacyl (G) and sy ringyl (S) units. Tobacco possesses a complex system of methylation compris ing three classes of CCoAOMTs (caffeoyl-CoA-O-methyltransferases) and two c lasses of COMTs (caffeic acid OMTs). Antisense plants transformed with the CCoAOMT sequence alone or fused to COMT I sequence have been produced and c ompared to ASCOMT I plants in order to study the specific role of each OMT isoform in lignin biosynthesis, plant development and resistance to pathoge ns. Tobacco plants strongly inhibited in OMT activities have been selected and analyzed. Whereas antisense COMT I plants exhibited no visual phenotype , CCoAOMT repression was shown to strongly affect the development of both s ingle and double transformants: a reduction of plant growth and the alterat ion of flower development were observed in the most inhibited plants. Ligni n analysis performed by Klason and thioacidolysis methods, showed a decreas e in the lignin quantity and changes in the lignin structure of ASCCoAOMT a nd ASCCoAOMT/ASCOMT I transgenics but not in ASCOMT I plants. Inhibition of COMT I in single as well as in double transformed tobacco was demonstrated to decrease S unit synthesis and to provoke the accumulation of 5-hydroxyg uaiacyl lignin units. ASCCoAOMT/ASCOMT I tobacco was affected in lignin amo unt and composition, thus demonstrating additive effects of inhibition of b oth enzymes. The changes of lignin profiles and the phenotypical and molecu lar alterations observed in the different transgenic lines were particularl y prominent at the later stages of plant development. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.