Lignification in transgenic poplars with extremely reduced caffeic acid O-methyltransferase activity

Transgenic poplars (Populus tremula X Populus alba) were obtained by introd uction of a sense homologous transgene encoding caffeic acid O-methyltransf erase (COMT) under the control either of the cauliflower mosaic virus doubl e 355 promoter or of the eucalyptus cinnamyl alcohol dehydrogenase promoter . Although these constructs conferred a moderate overexpression of COMT in some lines, a transgenic line with the double 355 promoter was found where COMT activity in woody tissues was close to zero due to a gene-silencing ph enomenon. For the first time in COMT down-regulated trees, this alteration substantially reduced lignin level in 6-month-old trees (17% decrease). Lig nin structure was found to be strongly altered, with a two times higher con tent in condensed bonds, an almost complete lack of syringyl units, and the incorporation of 5-hydroxyguaiacyl units to the most remarkable Extent rep orted so far. Consistent with the higher cellulose content and with the hig her condensation degree of the lignin, the impact of the transformation on the kraft-pulping performances of the poplar trees positively affected the pulp yield (10% relative increase), but made lignins less amenable to indus trial degradations.