Impact of lignin composition on cell-wall degradability in an Arabidopsis mutant

An Arabidopsis mutant that does not deposit syringyl-type lignin was used to test the hypothesis that lignin composition impacts cell-wall degradability. Two lines of the ferulate-5-hydroxylase-deficient fah1 mutant and the wild-type control Line were grown in the greenhouse. In Experiment 1, the plants were harvested at the mature seed stage. For Experiment 2, plants were harvested 5, 6, 7 and 8 weeks after sowing. In both experiments stems were collected and analysed for cell-wall concentration and composition, and in vitro degradability of cell-wall polysaccharide components by rumen micro-organisms. The absence of syringyl-type lignin was confirmed for the mutant lines by nitrobenzene oxidation and pyrolysis-GC-MS. Lignin concentration was the same for all three Arabidopsis Lines, at all stages of maturity. The Arabidopsis stems were similar to forage legumes in that the potentially degradable cell-wall fraction was very quickly degraded. Cell-wall polysaccharide degradability did not differ among the Arabidopsis lines in the first experiment after 24-h fermentations, but the cell-wall polysaccharides of the fah1-2 mutant line were less degradable after 96-h than either the wild-type or the fah1-5 mutant. In contrast, in Experiment 2 no differences among lines were found for cell-wall polysaccharide degradability after either 24- or 96-h fermentations; however, signficantly higher levels of ester-bound ferulic acid were found in the walls of the fah1 mutant lines. As expected, increasing stem maturity was correlated with reduced degradation of cell-wall polysaccharides. These experiments indicate that either lignin composition, as measured by syringyl-to-guaiacyl ratio, does not alter cell-wall degradability in Arabidopsis, or that the fah1 mutation has other effects on the cell walls of these mutants such that the impact of the change in syringyl-to-guaiacyl 1 ratio is masked. (C) 1999 Society of Chemical Industry.