The plant
signalling molecule auxin provides positional information in a variety of developmental processes by
means of its differential distribution (gradients) within plant tissues. Thus, cellular
auxin levels often determine the developmental output of
auxin signalling. Conceptually,
transmembrane transport and
metabolic processes regulate the steady-state levels of
auxin in any given cell. In particular, PIN auxin-efflux-carrier-mediated, directional transport between cells is crucial for generating
auxin gradients. Here we show that
Arabidopsis thaliana PIN5, an atypical member of the PIN
gene family, encodes a functional
auxin transporter that is required for auxin-mediated development. PIN5 does not have a direct role in cell-to-cell transport but regulates
intracellular auxin homeostasis and
metabolism. PIN5 localizes, unlike other characterized
plasma membrane PIN proteins, to
endoplasmic reticulum (ER), presumably mediating
auxin flow from the
cytosol to the lumen of the ER. The ER localization of other PIN5-like transporters (including the moss PIN) indicates that the diversification of PIN protein functions in mediating
auxin homeostasis at the ER, and cell-to-cell
auxin transport at the
plasma membrane, represent an ancient event during the evolution of land plants.
