Polysaccharide hydrolases in germinated barley and their role in the depolymerization of plant and fungal cell walls

Abstract

Cell wall degradation is an important event during endosperm mobilization in the germinated barley grain. A battery of polysaccharide and oligosaccharide hydrolases is required for the complete depolymerization of the arabinoxylans and (1→3,1→4)- β-glucans which comprise in excess of 90% by weight of these walls. The (1→3,1→4)- β-glucan endohydrolases release oligosaccharides from their substrate and are probably of central importance for the initial solubilization of the (1→3,1→4)- β-glucans, but β-glucan exohydrolases and β-glucosidases may be important additional enzymes for the conversion of released oligosaccharides to glucose. The latter enzymes have recently been purified from germinated barley and characterized. There is an increasing body of evidence to support the notion that the (1→3,1→4)- β-glucan endohydrolases from germinated barley evolved from the pathogenesis-related (1→3)- β-glucanases which are widely distributed in plants and which hydrolyse polysaccharides that are abundant in fungal cell walls. Arabinoxylan depolymerization is also mediated by a family of enzymes, but these are less well characterized. (1→4)- β-Xylan endohydrolases have been purified and the corresponding cDNAs and genes isolated. While the presence of (1→4)- β-xylan exohydrolases and α- l-arabinofuranosidases has been reported many times, the enzymes have not yet been studied in detail. Here, recent advances in the enzymology and physiology of cell wall degradation in the germinated barley grain are briefly reviewed.