B bGlucosidases

The functional role of ^-glucosidases in cereals is more difficult to define. In barley, the genes encoding the enzyme are expressed only in the maturing endosperm of the grain (26). The enzyme itself is also detected in the late stages of grain development and in ungermi-nated grain; activity does not increase after germination (26, 45). Possible functions of the ^-glucosidases in developing and germinated grain have been reviewed by Leah et al. (26), and some of these are related to cell wall metabolism. The ^-glucosidases are expressed at a time when (1 ! 3, 1 ! 4)-^-glucan is being deposited in the walls of starchy endosperm cells (26), and might be involved in trimming or turnover of wall (1 ! 3, 1 ! 4)-^-glucans during their synthesis.

Finally, the ^-glucosidases could be responsible for salvaging glucose units from the (1 ! 3, 1 ! 4)-^-oli-goglucosides released from wall (1 ! 3, 1 ! 4)-^-glu-cans by endohydrolases. The ^-glucosidases are able to hydrolyze not only the more abundant tri- and tetra-saccharides released by the endohydrolases, but also the longer-chain "cellulosic" ^-oligoglucosides that have single (1 ! 3)-^-glucosyl residues at their reducing termini (Figs. 1, 3) (27). The detection of the glucosidases in germinated grain is consistent with such a role in the complete depolymerization of wall (1 ! 3, 1 ! 4)-y8-glucans to glucose, and, based on current evidence, we believe that this might be a likely function of the barley ^-glucosidases.

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