Abstract

Plants are sessile and therefore their development needs to be environmentally responsive. One such plastic developmental process is shoot branching, where axillary meristems are produced which may remain dormant or active to produce a branch. The phytohormone auxin has been proposed to mediate this aspect of development by its transport. One hypothesis is that axillary meristem activity depends on the canalization of auxin transport away from the axillary meristem and out into the main stem. Auxin transport canalization involves a feedback mechanism whereby an initial flux of auxin from a source to a sink upregulates the production and polarization of its own efflux transport proteins (eg. PINs), establishing files of cells actively transporting auxin from the source to the sink. A computational model based on this mechanism can reproduce a range of branching phenomena. In this model, diverse effects of a second hormone, strigolactone (SL), can be reproduced if SL acts to promote PIN removal from the plasma membrane. This role of strigolactone was supported by experimental results in which strigolactone treatment triggered PIN1 depletion from the basal plasma membrane in xylem parenchyma cells. This removal is independent of protein synthesis but dependent on clathrin-mediated endocytosis.