Higher extractability of lignin in poplar (Populus tremula x P-alba) by reducing cinnamyl alcohol dehydrogenase activity

Cinnamyl alcohol dehydrogenase (CAD) catalyses the last step in the biosynthesis of the lignin precursors, which is the conversion of cinnamaldehydes into cinnamyl alcohols. A poplar (P. trichocarpa x P. deltoides) CAD cDNA was introduced in sense and in antisense orientation into P. tremula x P. alba via an Agrobacterium-mediated transformation. The CAD activity was reduced by 70% in the xylem tissue of three antisense and two co-suppressed lines. A red coloration of the xylem was detected in all poplar lines with a residual CAD activity of less than 60%. The lignin content and the lignin composition (syringyl/guaiacyl) were similar in co-suppressed, antisense CAD and control lines. However, chemical properties of the cell wall were different. First, a phloroglucinol staining showed a different coloration in the xylem of the down-regulated CAD poplars, being red-brown as compared to the typical pink staining of the control xylem. Second, after NaOH treatment, lignin could be precipitated by acidification of the alkali-soluble fraction of the cell wall of the red poplars but not of that of the white poplars. A spectral characterisation of the alkali fraction from red poplars showed an increased level of vanillin and syringaldehyde. Third, pulping experiments showed less residual lignin in the pulp of CAD down-regulated poplars.