Mixed‐linkage glucan:xyloglucan endotransglucosylase ( MXE ) re‐models hemicelluloses in E quisetum shoots but not in barley shoots or E quisetum callus

Summary 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 E quisetales (horsetails). The enzyme MLG :xyloglucan endotransglucosylase ( MXE ) grafts MLG to xyloglucan. In E quisetum , 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 E quisetum 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 P oales are xyloglucan‐poor, the role of their abundant endotransglucosylases remains enigmatic. The distribution of MXE action and activity within E quisetum suggests a strengthening role in ageing tissues.