Mixed linkage (1→3),(1→4)-β-D-glucans of grasses

The mixed-linkage (1-->3),(1-->4)-beta-D-glucans are unique to the Poales, the taxonomic order that includes the cereal grasses. (1-->3), (1-->4)-beta-Glucans are the principal molecules associated with cellulose microfibrils during cell growth, and they are enzymatically hydrolyzed to a large extent once growth has ceased. They appear again during the developmental of the endosperm cell wall and maternal tissues surrounding them. The roles of (1-->3),(1-->4)-beta-glucans in cell wall architecture and in cell growth are beginning to be understood. From biochemical experiments with active synthases in isolated Golgi mem-branes, the biochemical features and topology of synthesis are found to more closely parallel those of cellulose than those of all other non-cellulosic P-linked polysaccharides. The genes that encode part of the (1-->3),(1-->4)-beta-glucan synthases are likely to be among those of the CESA/CSL gene superfamily, but a distinct glycosyl transferase also appears to be integral in the synthetic machinery. Several genes involved in the hydrolysis of (1-->3),(1-->4)-beta-glucan have been cloned and sequenced, and the pattern of expression is starting to unveil their function in mobilization of beta-glucan reserve material and in cell growth.