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 t he biosynthesis of the lignin monomers, was down-regulated by using an anti sense approach. A subset of six transgenic lines with reduced CAD activity and control lines were analysed when grown in the greenhouse and in the fie ld. The down-regulation of the CAD enzyme was associated with a red colorat ion of the stem. The lignin quantity remained unchanged, but the lignin com position, as determined by thioacidolysis, was altered. The highest reducti on of CAD activity was associated with a lower syringyl/guaicayl (S/G) rati o and a lower S+G yield, mainly because of a decreased amount of S units. A n 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 fo r two of the lines grown in the field the rate of disappearance of dry matt er slightly improved. Furthermore, these two lines had a higher solubility in alkali as shown by the lower yield of saponified residue. This study ope ns perspectives for improving forage crop digestibility by the modulation o f enzymes involved in lignin biosynthesis.