Root hair initiation is coupled to a highly localized increase of xyloglucan endotransglycosylase action in arabidopsis roots

Root hairs are formed by two separate processes: initiation and subsequent tip growth. Root hair initiation is always accompanied by a highly localize d increase in xyloglucan endotransglycosylase (XET) action at the site of f uture bulge formation, where the trichoblast locally loosens its cell wall. This suggests an important role of XET in the first stages of root hair in itiation. The tip of growing root hairs is not marked by localized high XET action. Experiments in which root hair initiation was modulated and observ ations on root hair mutants support this view. The ethylene precursor 1-ami nocyclopropane-1-carboxylic acid shifts both root hair initiation and the l ocal increase in XET action toward the root tip. On the other hand, roots t reated with the ethylene inhibitor aminoethoxyvinyl-glycine, as well as roo ts of mutants affected in root hair initiation (rhl1, rhd6-1, and axr2-1) r evealed no localized increases of XET action at all and consequently did no t initiate root hairs. Disruption of actin and microtubules did not prevent the localized increase in XET action. Also, the temporal and spatial patte rn of action as the specific pH dependence suggest that different isoforms of XET act in different processes of root development.