Update on Mechanisms of Plant Cell Wall Biosynthesis Update on Mechanisms of Plant Cell Wall Biosynthesis: How Plants Make Cellulose and Other (1/4)-b-D-Glycans

The discovery of a gene that encodes a cotton (Gossypium hirsutum) cellulose synthase (Pear et al., 1996) revolutionized and invigorated the plant cell wall community to find the genes that encode the machinery of cell wall polysaccharide synthesis. The landscape was framed by the completion of the genome sequence of Arabidopsis (Arabidopsis thaliana; Arabidopsis Genome Initiative, 2000), which gave a complete gene inventory for a model plant species, but one with many genes yet to be annotated for function. An estimated 10% of the plant genome, about 2,500 genes, is devoted to construction, dynamic architecture, sensing functions, and metabolism of the plant cell wall. Based largely on prior discoveries of function in prokaryotic organisms, most of the tentatively annotated genes are organized into gene families for substrate generation, glycosyl transfer, targeting and trafficking, cell wall rearrangement, and modification by hydrolases, esterases, and lyases (Yong et al., 2005; Penning et al., 2009). However, the biochemical activities of most enzymes involved in glycosyl transfer within these families remain to be verified, and an additional 40% of the genome encodes genes whose functions are not known. As many of these proteins contain secretory signal peptides (Arabidopsis Genome Initiative, 2000), it is reasonable to infer that some have roles in cell wall construction. The Arabidopsis cellulose synthase/cellulose synthaselike (CesA/Csl) gene superfamily, which includes 10CesA genes and 29 Csl genes in six distinct groups, was one of the first large families to be described (Richmond and Somerville, 2000), and comparative analyses of a reference dicot, Arabidopsis, with a reference grass, rice (Oryza sativa), revealed substantive differences in family structures, adding two groups not seen in the dicot genome (Hazen et al., 2002). Extension of these annotations to compare all cell wall-related gene families of the grasses with those of the dicots reveals some correlation of family structure with the differences between plants with type I walls and those of the grasses with type II walls (Fig. 1A; Penning et al., 2009). For CesA genes and certain Csl genes, establishment of specific function for the synthases they encode comes from the analysis of mutants lacking a particular function and, in some specific examples, by heterologous expression. However, genetic approaches alone do not inform us about the biological mechanism of synthesis. The knowledge gained from molecular genetic approaches now needs to be augmented by biochemical and cell biological approaches to achieve a greater understanding of proteins and their interactions within a synthase complex, their organization at membranes, and their dynamics. This Update focuses on the biochemical mechanisms of the synthesis of a single type of linkage, the (1/4)-b-D linkage, in which one sugar is inverted nearly 180 with respect to each neighboring sugar in the chain. This linkage presents a unique steric problem for processive catalysis that all living organisms have solved but we are still struggling to understand. This Update reviews our present state of knowledge of the biochemical mechanisms of polysaccharide synthesis, including some classic discoveries, and presents an alternative hypothesis on the biochemical mechanisms and organization of complexes involved in synthase reactions that yield (1/4)-b-D linkages.