Structural alterations of lignins in transgenic poplars with depressed cinnamyl alcohol dehydrogenase or caffeic acid O-methyltransferase activity have an opposite impact on the efficiency of industrial kraft pulping

We evaluated lignin profiles and pulping performances of 2-year-old transge nic poplar (Populus tremula x Populus alba) lines severely altered in the e xpression of caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) or cinnamyl alcohol dehydrogenase (CAD). Transgenic poplars with CAD or COM T antisense constructs showed growth similar to control trees. CAD downregu lated poplars displayed a red coloration mainly in the outer xylem. A 90% l ower COMT activity did not change lignin content but dramatically increased the frequency of guaiacyl units and resistant biphenyl linkages in lignin. This alteration severely lowered the efficiency of kraft pulping. The Klas on lignin level of CAD-transformed poplars was slightly lower than that of the control. Whereas CAD down-regulation did not change the frequency of la bile ether bonds or guaiacyl units in lignin, it increased the proportion o f syringaldehyde and diarylpropane structures and, more importantly with re gard to kraft pulping, of free phenolic groups in lignin. In the most depre ssed line, ASCAD21, a substantially higher content in free phenolic units f acilitated lignin solubilization and fragmentation during kraft pulping. Th ese results point the way to genetic modification of lignin structure to im prove wood quality for the pulp industry.