Genetic Engineering of Pasture Legume and Grasses for Reduced Lignin Content and Increased Digestibility

Plants of tropical pastures have a higher lignin content than those of temperate pastures, which limits the digestibility and nutritive value of these forages. Ribozyme and antisense RNA technology is being investigated as a method of reducing lignin levels and increasing digestibility in the tropical forage Stylosanthes spp, and in the model system tobacco. Ribozymes to the tobacco anionic peroxidase mRNA have been designed and shown to be capable of cleaving this mRNA in vitro. Constructs for the expression of antisense and ribozyme RNA to reduce peroxidase gene expression have been prepared, Transgenic tobacco plants containing these constructs have been produced and are being assessed for peroxidase activity. Several cDNA clones for the lignification enzymes phenylalanine ammonia­lyase (PAL), caffeic acid O-methyltransferase (COMT) and peroxidase (PX) have been isolated from Stylosanthes humilis. Antisense constructs to PAL have been prepared in plant expression vectors and transformation experiments are underway.