RED XYLEM AND HIGHER LIGNIN EXTRACTABILITY BY DOWN-REGULATING A CINNAMYL ALCOHOL-DEHYDROGENASE IN POPLAR

Cinnamyl alcohol dehydrogenase (CAD) catalyzes the last step in the bi osynthesis of the lignin precursors, the monolignols. We have down-reg ulated CAD in transgenic poplar (Populus tremula x Populus alba) by bo th antisense and co-suppression strategies. Several antisense and sens e CAD transgenic poplars had an approximately 70% reduced CAD activity that was associated with a red coloration of the xylem tissue. Neithe r the lignin amount nor the lignin monomeric composition (syringyl/gua iacyl) were significantly modified. However, phloroglucinol-HCl staini ng was different in the down-regulated CAD plants, suggesting changes in the number of aldehyde units in the lignin. Furthermore, the reacti vity of the cell wall toward alkali treatment was altered: a lower amo unt of lignin was found in the insoluble, saponified residue and more lignin could be precipitated from the soluble alkali fraction. Moreove r, large amounts of phenolic compounds, vanillin and especially syring aldehyde, were detected in the soluble alkali fraction of the CAD down -regulated poplars. Alkaline pulping experiments on 3-month-old trees showed a reduction of the kappa number without affecting the degree of cellulose degradation. These results indicate that reducing the CAD a ctivity in trees might be a valuable strategy to optimize certain proc esses of the wood industry, especially those of the pulp and paper ind ustry.