Apoplastic Xylanases and (1→3,1→4)-β- Glucanases in Cell Wall Metabolism in Barley

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Cell walls of the endosperm and young vegetative tissues of barley (Hordeum vulgare L.) contain relatively high levels of arabinoxylans and (1⟶3,1⟶4)-β-glucans. Here, we briefly review the characteristics of enzymes that mediate arabinoxylan and (1⟶3,1⟶4)-β-glucan hydrolysis, their structures and mechanisms of action, their genetics and the regulation of the corresponding genes. In germinated grain, wall components are depolymerised by the combined action of endo- and exohydrolases, which are usually secreted into the apoplastic space by cells of the aleurone or the scutellar epithelium. Thus, multiple enzyme systems are required to completely hydrolyse both the arabinoxylans and the (1⟶3,1⟶4)-β-glucans. Degradation of the walls of the starchy endosperm by the action of these enzymes ensures that other hydrolytic enzymes secreted from the aleurone or scutellum can reach starch, protein and other polymeric substrates that are stored within starchy endosperm cells. In young vegetative tissues, levels of (1⟶3,1⟶4)-β-glucans appear to decrease progressively during growth, and endo- and exohydrolases capable of hydrolysing this polysaccharide or oligomeric products derived from it have been detected in young leaves and roots, and in elongating coleoptiles. In contrast, there is little information on the activity of endo- or exohydrolases that could potentially hydrolyse arabinoxylans in elongating coleoptiles or other vegetative tissues.
帝塚山大学の論文

Apoplastic Xylanases and (1→3,1→4)-β- Glucanases in Cell Wall Metabolism in Barley

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