Advances in Cell Wall Matrix Research with a Focus on Mixed-Linkage Glucan

Mixed beta(1,3;1,4)-linkage glucan (MLG) is commonly found in the monocot lineage, at particularly high levels in the Poaceae family, but also in the evolutionally distant genus, Equisetum. MLG has several properties that make it unique from other plant cell wall polysaccharides. It consists of beta 1,4-linked polymers of glucose interspersed with beta 1,3-linkages, but the presence of beta 1,3-linkages provides quite different physical properties compared to its closest form of the cell wall component, cellulose. The mechanisms of MLG biosyn- thesis have been investigated to understand whether single or multiple enzymes are required to build mixed linkages in the glucan chain. Currently, MLG synthesis by a single enzyme is supported by mutagenesis analyses of cellulose synthase- like F6, the major MLG synthase, but further investigation is needed to gather mechanistic insights. Because of transient accumulation of MLG in elongating cells and vegetative tis- sues, several hypotheses have been proposed to explain the role of MLG in the plant cell wall. Studies have been carried out to identify gene expression regulators during development and light cycles as well as enzymes involved in MLG organization in the cell wall. A role of MLG as a storage molecule in grains is evident, but the role of MLG in vegetative tissues is still not well understood. Characterization of a cell wall component is difficult due to the complex het- erogeneity of the plant cell wall However, as detailed in this review, recent exciting research has made significant impacts in the understanding of MLG biology in plants.