Biosynthesis of plant cell wall polysaccharides — a complex process
Introduction
Plant cell walls are composed mainly of polysaccharides. Both primary and secondary cell walls contain cellulose and hemicelluloses; primary walls also contain pectins as well as many enzymes and structural proteins, whereas secondary walls contain little protein or pectin, but normally contain lignin [1]. Among the polysaccharides, cellulose is the most important and normally the most abundant wall component. Cellulose microfibrils are embedded in a matrix that contains other polysaccharides, glycoproteins and proteins. Significant progress has been made in defining the structures of the various polysaccharides that make up plant cell walls [2]. However, large gaps exist in our understanding of the biosynthesis of wall polysaccharides. Although cellulose is made at the plasma membrane and deposited directly into the wall [3], most other matrix components are made in the Golgi and delivered to the wall in secretory vesicles [1]. The processes and players involved in assembling functional cell walls from components synthesized at different cellular locations are largely unknown.
One reason for the lack of information regarding the biosynthesis of cell wall polysaccharides is the difficulty of using traditional biochemical techniques to study the process. In recent years, the application of genetic and genomic strategies has led to significant progress in identifying the genes and proteins required for cell wall biosynthesis. Thus, at least for some polysaccharides, we now have a list of proteins involved in their biosynthesis; more work is needed to determine whether the lists are complete. As described in this review, however, many important questions regarding the biochemistry, cell biology, and regulation of polysaccharide biosynthesis remain unanswered.
Section snippets
Cellulose
Cellulose is ubiquitous among plants in which it constitutes the major polysaccharide of cell walls; it is also considered to be the most abundant biopolymer on Earth [4]. Cellulose is at the core of plant cell walls, where it serves as a scaffold for the binding of other wall components. Many excellent reviews provide background and additional details on cellulose structure and biosynthesis [3, 4, 5, 6, 7, 8, 9], so we focus primarily on recent advances in understanding cellulose biosynthesis.
Hemicelluloses
Hemicellulosic polysaccharides are complex molecules that associate with cellulose microfibrils, providing a cross-linked matrix. Hemicelluloses can be divided into four main classes: xyloglucans (XyG), which contain a heavily substituted β-1,4-glucan backbone; (gluco)mannans, containing a variably substituted backbone that includes β-1,4-linked mannose (glucose and mannose) residues; glucuronoarabinoxylans (GAX), containing a substituted β-1,4-linked xylan backbone; and mixed-linkage glucans
Pectins
Pectins are complex polysaccharides comprised of domains of homogalacturonan (HG), rhamnogalacturonan I (RG-I), and relatively minor amounts of rhamnogalacturonan II (RG-II) (see [60, 61, 62] for reviews of pectin structure and function). Although pectin biosynthesis is predicted to require at least 53 different GTs [61], none of these biosynthetic enzymes have been purified using traditional biochemical purification techniques. To circumvent the difficulties associated with the purification of
Concluding remarks
Significant progress has been achieved over the past few years in identifying genes encoding proteins that are required for plant cell wall polysaccharide biosynthesis. For instance, we now know that various members of the CSL gene family encode enzymes that synthesize the backbones of hemicellulosic polysaccharides. Despite this progress, many important questions remain with respect to the biochemistry and the cell biology of polysaccharide biosynthesis, as well as wall deposition and
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors would like to thank Ms Karen Bird for providing editorial assistance on the manuscript. Research in KK's laboratory is supported in part by a grant from the Plant Genome Research Program at the National Science Foundation (NSF) and in part by a grant from the Energy Biosciences Program at the US Department of Energy.
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