Effect of phenolic structures on the degradability of cell walls isolated from newly extended apical internode of tall fescue (Festuca arundinacea Schreb.).

Apical internodes of tall fescue (Festuca arundinacea Schreb. var. Clarine) harvested at flowering were sectioned into 5 or 10 equal parts to study in situ degradability and cell wall composition, respectively. The basal (youngest) section had the greatest primary wall content. Cell walls in the upper (older) sections had the highest xylose/arabinose ratio and lignin content and a lignin rich in syringyl units, all typical of extensive secondary wall development. Almost all of the p-coumaric (p-CA) and about half of the ferulic acid (FA) were released by 1 M NaOH and presumed to be ester-linked. The total FA content was approximately double that of p-CA in all sections other than the youngest with a distribution similar to that of total p-CA. However, the ratio of esterified to ether and ether plus ester linked (Et & Et+Es) FA differed with age. Whereas the esterified form remained essentially constant ( approximately 4.5 g/kg of cell wall), Et & Et+Es ferulate increased with increasing age of the tissue and was significantly related to lignin deposition (r = 0.79, P < 0.01). The extent of cell wall degradation after 48 h of incubation in the rumen was inversely related to maturity, falling from 835 g/kg of dry matter in the youngest section to 396 g/kg in the oldest. Both the rate and extent of cell wall degradation were significantly negatively related to the ratio of xylose to arabinose, lignin content, proportion of syringyl units present in lignin, and concentration of Et & Et+Es FA present. A positive relationship between Et & Et+Es FA was also found, with the rate (P < 0.01) being better correlated than the extent (P < 0.05) of cell wall degradation. Application of the newly extended internode model to fescue produced results consistent with the view that both the lignin content and the extent to which lignin was covalently bound to the other wall polymers crucially influenced the rate and extent of degradation.

Transformation of (sup14)C-Lignin-Labeled Cell Walls of Wheat by Syntrophococcus sucromutans, Eubacterium oxidoreducens, and Neocallimastix frontalis.

Wheat cell walls, saponified or not, labeled with [U-(sup14)C]phenylalanine or [O-methyl-(sup14)C]sinapate were fermented by Neocallimastix frontalis or Syntrophococcus sucromutans plus Eubacterium oxidoreducens or a mixed culture. Phenolics were less solubilized but more transformed by bacteria than by the fungus, and mineralization was slight. S. sucromutans O-demethylated [O-methyl-(sup14)C]syringyl lignins, yielding labeled acetate.

Down-regulation of cinnamyl alcohol dehydrogenase in transgenic alfalfa (Medicago sativa L.) and the effect on lignin composition and digestibility.

To improve the digestibility of the forage crop alfalfa (Medicago sativa L.), cinnamyl alcohol dehydrogenase (CAD), which catalyses the last step in the biosynthesis of the lignin monomers, was down-regulated by using an antisense approach. A subset of six transgenic lines with reduced CAD activity and control lines were analysed when grown in the greenhouse and in the field. The down-regulation of the CAD enzyme was associated with a red coloration of the stem. The lignin quantity remained unchanged, but the lignin composition, as determined by thioacidolysis, was altered. The highest reduction of CAD activity was associated with a lower syringyl/guaiacyl (S/G) ratio and a lower S+G yield, mainly because of a decreased amount of S units. An increase in in situ disappearance of dry matter and of cell wall residue was detected in one of the transgenic lines grown in the greenhouse, and for two of the lines grown in the field the rate of disappearance of dry matter slightly improved. Furthermore, these two lines had a higher solubility in alkali as shown by the lower yield of saponified residue. This study opens perspectives for improving forage crop digestibility by the modulation of enzymes involved in lignin biosynthesis.