Effect of down-regulation of cinnamyl alcohol dehydrogenase on cell wall composition and on degradability of tobacco stems

The effect of down-regulation of tobacco cinnamyl alcohol dehydrogenase (CAD) on cell wall composition and degradability has been assessed. CAD activity was only 20, 16, 14 and 7%, relative to the controls, in four populations of plants (designated 40-1, 40-2, 48 and 50, respectively) transformed with CAD antisense mRNA. Cell wall residues of stem samples were analysed for polysaccharide composition, gravimetric and acetyl bromide lignins and lignin nitrobenzene oxidation products. In situ disappearance and cellulase solubility of both initial dry matter and CWR were determined. The populations of plants with depressed CAD activity showed no change in lignin content but some consistent changes in cell wall composition and digestibility were identified. The syringyl content of lignins decreased and the syringaldehyde to vanillin ratio (S/V) was consequently reduced. Dry matter degradability, as measured by both methods, was significantly improved in all CAD-depressed samples except for population 40-1, which was the least CAD-depressed. Increased in situ disappearance of cell wall (ISCWD) was found in all plants exhibiting more than 80% CAD down-regulation and was maximal (7 percentage units) in population 50 which had the greatest CAD depression. The rates of ISCWD increased slightly in some populations (40-2 and 50). The relationship between S/V and ISCWD was significant (R = -0.68) only in the samples from a selected population of mature, most depleted plants. Other modifications may therefore also contribute to the improvement in degradability. However the changes in lignin composition that were observed in CAD-depressed tobacco are largely similar to those seen in some maize and sorghum mutants with altered lignification and improved digestibility. These data therefore suggest that depressing CAD activity may be an effective method for improving digestibility in forage crops.