Walls of the grasses contain mixed-linkage (1,3;1,4)-p-D-glucans (MLGs) which are not present in walls of dicotyledons or most other monocotyledonous plants (147). Some alga and liverworts may also have MLGs (148). The (1,3;1,4)- (3 — D-glucans have an unusual struc­ture consisting of an unbranched, unsubstituted glucan chain with two linkages arranged in a non-repeating, but non-random, fashion. The glucan chains consist of primarily cel — lotriosyland cellotetraosyl units separated by single (1 ^ 3)-p — linkages (149). MLGs can be synthesized in vitro from Golgi membrane fractions with UDP-Glc as a substrate (150,151). The amount of UDP-Glc used in the assay alters the ratio of cellotriosyl and cellotetraosyl units, indicating that this is not a fixed property of the biosynthetic enzyme (152).

Recently, following prescient speculation as to possible structural similarity between cel­lulose synthase and the MLG synthase (153), it was shown that expression of a rice CslF gene in Arabidopsis led to the accumulation of (1,3;1,4)-p-D-glucan biosynthesis in Arabidopsis (154). Thus, it appears that CSLF encodes the mixed glucan synthase. The generally low levels of (1,3;1,4)-p-D-glucan in walls of the transformed Arabidopsis plants is consistent with the concept that limiting levels of other components might be required for high-level synthesis of the polysaccharide or its transfer to the cell wall. Similarly, the preferential de­position of the (1,3;1,4)-p-D-glucan in the epidermal layers of the transgenic Arabidopsis lines, despite the fact that transgene expression was driven by the constitutive 35 S promoter, could indicate that the epidermal cells contain ancillary factors that are not abundant in other cells of the leaf. The identification of the gene opens up new approaches to understand­ing the fascinating process by which an enzyme catalyzes substantially different transferase reactions (151, 153).

The role of the MLGs is not clear. The MLGs are synthesized in relatively large amounts during growth and may coat cellulose microfibrils during the synthesis and expansion phase, but they are degraded when elongation ceases (155, 156).