Functional Genomics of Plant Cell Wall Biosynthetic Enzymes: Type-II Membrane-Bound Glycosyltransferases

Functional analyses of plant cell wall biosynthetic enzymes date back to the isolation of plasma-membrane and/or Golgi-enriched microsomal membranes, which were subjected to biochemical analysis of enzymes of interest. These analyses were usually performed with a radiolabelled UDP-sugar substrate and a suitable acceptor molecule. However, preparations of both substrates and/or candidate oligosaccharide acceptors generally yielded low levels of enzyme activity and minute amounts of product. With the advent of recombinant DNA technology and implementation of heterologous expression systems to improve the amount of available protein, new opportunities arose through which to obtain more meaningful biochemical data. Although, heterologous expression of DNA fragments proved useful for the characterization of soluble enzymes, the procedure was not so successful in the generation of membrane-bound enzymes that were correctly folded and therefore active. The completion of the rice and Arabidopsis genome sequences, together with the deposition of numerous plant ESTs into public databases opened the way for the development of other techniques. Analyses of T-DNA insertion lines for glycosyltransferase genes with reference to cell wall polysaccharide linkage types and wall composition, especially in Arabidopsis, were amongst the successful methods used in addressing the function of candidate glycosyltransferases. More recently quantitative genetics and the identification of Quantitative Trait Loci (QTL) have started to shed some light on our understanding of the genes and enzymes involved in wall biogenesis, deposition and re-modelling.