Abstract
Among land-plant hemicelluloses, xyloglucan is ubiquitous, whereas mixed-linkage (1→3),(1→4)-β-D-glucan (MLG) is confined to the Poales (e.g. cereals) and Equisetales (horsetails). The enzyme MLG:xyloglucan endotransglucosylase (MXE) grafts MLG to xyloglucan. In Equisetum, MXE often exceeds extractable xyloglucan endotransglucosylase (XET) activity; curiously, cereals lack extractable MXE. We investigated whether barley possesses inextractable MXE. Grafting of endogenous MLG or xyloglucan onto exogenous [(3)H]xyloglucan oligosaccharides in vivo indicated MXE and XET action, respectively. Extractable MXE and XET activities were assayed in vitro. MXE and XET actions were both detectable in living Equisetum fluviatile shoots, the MXE : XET ratio increasing with age. However, only XET action was observed in barley coleoptiles, leaves and roots (which all contained MLG) and in E. fluviatile intercalary meristems and callus (which lacked MLG). In E. fluviatile, extractable MXE activity was high in mature shoots, but extremely low in callus and young shoots; in E. arvense strobili, it was undetectable. Barley possesses neither extractable nor inextractable MXE, despite containing both of its substrates and high XET activity. As the Poales are xyloglucan-poor, the role of their abundant endotransglucosylases remains enigmatic. The distribution of MXE action and activity within Equisetum suggests a strengthening role in ageing tissues.
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