Monitoring Polysaccharide Dynamics in the Plant Cell Wall.

All plant cells are surrounded by complex walls that play a role in the growth and differentiation of tissues. Walls provide mechanical integrity and structure to each cell and represent an interface with neighboring cells and the environment (Somerville et al., 2004). Cell walls are composed primarily of multiple polysaccharides that can be grouped into three major classes: cellulose, pectins, and hemicelluloses. While cellulose fibrils are synthesized by the plant cells directly at the plasma membrane (PM), the matrix polysaccharides are produced in the Golgi apparatus by membrane-bound enzymes from multiple glycosyltransferase families (Oikawa et al., 2013). After secretion to the wall via exocytosis, the structures of the noncellulosic polysaccharides are modified by various apoplastic enzymes. In addition to polysaccharides, most plant cell walls contain small amounts of structural proteins such as extensins and arabinogalactan proteins.Cell walls are dynamic entities, rather than rigid and recalcitrant shells, that can be remodeled during plant development and in response to abiotic and biotic stresses. Cell expansion requires the deposition of additional material in the surrounding primary walls as well as the reorganization and loosening of existing polymers to allow for wall relaxation and controlled expansion (Cosgrove, 2005). The latest model of the primary wall structure proposes that cellulose-cellulose junctions only occur at a limited number of biomechanical hotspots, where protein catalysts must act selectively to initiate wall loosening (Cosgrove, 2018). In tissues undergoing growth, the recycling of polysaccharides via a suite of enzymes can contribute to the construction of elongating walls (Barnes and Anderson, 2018). Once elongation ceases, some cells deposit thick secondary walls that incorporate additional polysaccharides. Many secondary walls are impregnated with the polyphenol lignin and thereby become relatively fixed structures that exclude water and resist hydrolysis