IMPACT OF DIFFERENT LEVELS OF CINNAMYL ALCOHOL-DEHYDROGENASE DOWN-REGULATION ON LIGNINS OF TRANSGENIC TOBACCO PLANTS

The effects of cinnamyl alcohol dehydrogenase (CAD, EC.1.1.1.195) down -regulation on lignin profiles of plants were analysed in four selecte d transgenic lines of tobacco (Nicotiana tabacum L. cv. Samsun) exhibi ting different levels of CAD activity (8-56% of the control). A signif icant decrease in thioacidolysis yields (i.e. yield of beta-O-4 linked monomers) and in the ratio of syringyl to guaiacyl monomers (S/G) was observed for three transgenic lines and the most drastic reduction (u p to 50%) was correlated with the lowest level of CBD activity. Higher lignin extractability by mild alkali treatment was confirmed, and, in addition to a tenfold increase in C-6-C-1 aldehydes, coniferyl aldehy de was detected by high-performance liquid chromatography in the alkal i extracts from the xylem of transgenic plants. In-situ polymerisation of cinnamyl aldehydes in stem sections of untransformed tobacco gave a xylem cell wall coloration strikingly similar to the reddish-brown c oloration of the xylem of antisense CAD-down-regulated plants. Overall , these data provide new arguments for the involvement of polymerised cinnamyl aldehydes in the formation of the red-coloured xylem of CAD-d ownregulated plants.