Glycosyltransferases in Plant Cell Wall Synthesis

Abstract

Abstract Through their influence on plant cell wall polysaccharide structure, plant cell wall glycosyltransferases influence growth, development, cell division and environmental responses as well as plant‐derived food products, biofuels, textiles, paper and timber. Plant cell wall synthesizing enzymes can be processive (glycan synthases) or nonprocessive, and are integral membrane proteins with single or multiple transmembrane domains. Plant cell wall glycosyltransferases can be challenging to study biochemically, as they tend to be labile, present in multimeric complexes and encoded by large gene families whose members may have overlapping function. Candidate proteins may be identified by homology, but their function must be confirmed with biochemical evidence. Strategies for confirmation include expression in heterologous systems followed by assays of enzymatic activity or detection of carbohydrate products. Loss of function or gain‐of‐function techniques may be useful. Despite challenges, numerous plant cell wall glycosyltransferases have been identified; hundreds more, however, await further characterization. Key concepts Plant cell walls are critical for both biological processes and plant‐derived products, and are formed largely of polysaccharides synthesized by glycosyltransferases. Plant cell wall glycosyltransferases use sugar nucleotides (NDP‐sugars) as donors, and transfer the sugar residue to acceptors to form glycosidic bonds. These enzymes may transfer a single sugar molecule in nonprocessive fashion, or may transfer molecules iteratively to elongate a polymer. Processive glycosyltransferases are termed synthases whereas nonprocessive enzymes are simply termed glycosyltransferases. Cellulose synthesis and callose synthesis occurs at the plasma membrane. All other plant cell wall polymers are synthesized within the Golgi and secreted to the cell wall in vesicles. Nonprocessive glycosyltransferases tend to have a single transmembrane domain, whereas processive glycan synthases tend to have multiple transmembrane domains. There are exceptions to this rule, however. Glycosyltransferases and glycan synthases tend to be highly specific for each donor and acceptor, and are specialized for formation of one type of carbohydrate linkage. Numerous strategies exist for biochemical characterization of glycosyltransferases and glycan synthases, including heterologous expression and use of loss of function and gain‐of‐function genetic methods. Plant cell wall glycosyltransferases and glycan synthases tend to be encoded by genes that are members of large multigene families. Plant cell wall glycosyltransferases and glycan synthases often function in multimeric complexes. Numerous enzymes involved in synthesis of plant cell wall biosynthesis have been identified. These include cellulose synthase, callose synthase, xyloglucan biosynthetic enzymes, pectin biosynthetic enzymes, mixed‐linkage glycan synthase and enzymes synthesizing mannans. However, many hundreds of enzymes remain to be identified at the biochemical level.