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These substituents influence solubility, viscosity, and interactions with other polysaccharides and proteins within the cell wall.
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In contrast to cellulose, the pectic and non-cellulosic polysaccharides can be further distinguished by sugar substitutions and side chains that are attached to the polysaccharide backbone during biosynthesis ( Scheller and Ulvskov, 2010). Other abundant non-cellulosic polysaccharides include xyloglucan, β-1,3:1,4-glucan, xylan, mannan, and callose, which fulfill various roles in mechanical support, reserve storage and development. Pectins, which are arguably the most complex and heterogeneous of the cell wall polysaccharides, exist predominantly in the primary cell wall and have roles in expansion, strength, porosity, adhesion, and intercellular signaling.
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In brief, cellulose is a water insoluble carbohydrate found in both primary and secondary cell walls whose fibrous structure enables the maintenance of structural integrity. The major components (>90%) are polysaccharides, the structure, and biosynthesis of which have been extensively reviewed in recent times ( Atmodjo et al., 2013 Pauly et al., 2013 Rennie and Scheller, 2014 Kumar et al., 2016). Typical components of the cell wall include cellulose, non-cellulosic, and pectic polysaccharides, proteins, phenolic compounds, and water. The secondary wall is seen as a crucial adaptation that allows terrestrial plants to withstand and facilitate upright growth. The thicker and more durable secondary wall lies between the primary wall and plasma membrane, and is deposited at a later stage when the cell has stopped growing and dividing. A dynamic primary wall is established in young cells during division and acts to provide flexibility and basic structural support, protecting the cell, and mediating cell-cell interactions. In general, two wall types surrounding plant cells are often referred to as the primary wall and secondary wall. Consistent with a role in many processes, plant cell wall structure is incredibly varied, not only between plant species but also between tissue types. The cell wall hosts a wide range receptors, pores and channels that regulate molecular movement and responses to local and long-range elicitors including hormones, sugars, proteins, and RNAs. In addition to maintaining structural integrity by resisting internal hydrostatic pressures, the cell wall provides flexibility to support cell division, a biochemical scaffold that enables differentiation, and a pathological and environmental barrier that defends against stress ( Scheller and Ulvskov, 2010 Hamann, 2012 Tucker and Koltunow, 2014). The plant cell wall is a complex structure that fulfills a diverse array of functions throughout the plant lifecycle. We propose that the emerging picture of cell wall remodeling during stress is one that utilizes a common toolkit of cell wall-related genes, multiple modifications to cell wall structure, and a defined set of stress-responsive transcription factors that regulate them. Despite this, comparisons between publically available datasets indicate that in many instances cell wall-related genes respond similarly to different pathogens and abiotic stresses, even across the monocot-dicot boundary. These studies highlight that the molecular signatures of cell wall modification are often complex and dynamic, with multiple genes appearing to respond to a given stimulus. In this review we summarize recent genetic and transcriptomic data from the literature supporting a role for specific cell wall-related genes in stress responses, in both dicot and monocot systems. Genes encoding enzymes capable of synthesizing or hydrolyzing components of the plant cell wall show differential expression when subjected to different stresses, suggesting they may facilitate stress tolerance through changes in cell wall composition. The cell wall must also retain some flexibility, such that when subjected to developmental, biotic, or abiotic stimuli it can be rapidly remodeled in response. It provides a structural framework to support plant growth and acts as the first line of defense when the plant encounters pathogens. The plant cell wall has a diversity of functions. 2Australian Research Council Centre of Excellence in Plant Cell Walls and School of Agriculture, Food and Wine, Waite Research Institute, The University of Adelaide, Glen Osmond, SA, Australia.1Cell and Molecular Sciences, The James Hutton Institute, Dundee, UK.Tucker 2† Jamil Chowdhury 2 Neil Shirley 2 Alan Little 2†