Genetic modification of fibre digestion

Plant biomass is an extensive renewable resource which already plays a central role in herbivore nutrition, but has the further potential to serve as a raw material for industrial processes leading to the production of bulk organics, fine chemicals and biofuel.Microbial enzymes which reside in the gut of both ruminant and non-ruminant herbivores and degrade plant structural polysaccharides, represent the means by which most of the energy and carbon contained in plant biomass may be released.Such enzymes have been the subject of much biochemical research, but since the advent of recombinant DNA (rDNA) technology during the late 1970s, various research groups have, in addition, focused on molecular aspects of these enzymes, and in particular on the introduction of cellulose-degrading capacity into non-cellulolytic organisms by techniques known broadly as genetic engineering.The major components of plant cell walls are hemicellulose, cellulose and lignin (Demeyer, 1981).Cellulose, the linear P l + 4 homopolymer of glucose forms tightly packed parallel molecules linked together by hydrogen bonds.The resulting fibrils are embedded in a ligno-hemicellulose complex.The most abundant hemicelluloses are branched chain P l -+ 4 xylans and glucomannan which are covalently bound to lignin, an heterogeneous phenolic polymer (Whistler & Richards, 1970).Pectin, which is an a1 + 4 polymer of galacturonic acid, is a minor component of grass cell walls.To realize the full potential of plant biomass in animal nutrition requires an efficient system for the complete enzymic hydrolysis of plant structural polysaccharides into their constituent monomers.The weighty problem of improving the effectiveness of naturallyoccurring cellulolytic micro-organisms or introducing plant-cell-wall hydrolysing capacity into non-cellulolytic organisms, has two main aspects.First, the analysis of enzymes which hydrolyse plant cell walls and the genes which encode them.This will facilitate the future construction of novel genes, encoding multi-functional hybrid enzymes with enhanced activity against cellulose and hemicellulose.Second, the development of host-vector systems which will enable the genes developed in the first phase to be efficiently expressed in organisms that would then be used to elicit improved fibre utilization by both ruminants and non-ruminants.