Polar auxin transport. New support for an old model

INSIGHT In 1880, Charles Darwin noted that " some influence, " later shown to be in-dole-3-acetic acid (IAA), moves from the tip of an oat coleoptile to the region below the tip, where it controls elongation (Darwin, 1880). Darwin's statement was probably the first description of polar auxin transport, a phenomenon that has received considerable attention in the ensuing decades for two main reasons. First, polar auxin transport is ubiquitous among higher plant species, and second , the effects of inhibiting polar auxin transport with chemical inhibitors such as naphthylphthalamic acid (NPA) are profound (Lomax et al., 1995). For example , NPA acts to disrupt tropic responses , reduce apical dominance, inhibit floral bud formation, and inhibit lateral root formation. These effects strongly suggest that auxin transport has a central role in auxin-regulated growth processes. We now know that auxin moves basi-petally at a velocity of between 5 and 20 mm per hr in the shoots and coleop-tiles of a wide range of plant species (Lomax et al., 1995). We also know that acropetal transport is minimal. What is less clear is how this directionality to auxin transport is achieved. One model, termed the chemios-motic hypothesis, has informed our thinking on auxin transport for the past 25 years. As illustrated in Figure 1, this model, which was independently proposed by Rubery and Sheldrake (1974) and Raven (1975), posits that polar auxin transport occurs through the action of cellular auxin influx and efflux carriers located in the plasma membrane of transporting cells. To explain the strong polarity of transport, the model proposes that the efflux carrier is asymmetrically localized to the basal side of cells. In support of this aspect of the model, Jacobs and Gilbert (1983) used an immunological approach to demonstrate basal localization of a pu-tative efflux carrier. In 1996, Bennett et al. reported the molecular characterization of a candidate auxin influx carrier, a protein called AUX1. The auxin resistant1 (aux1) mutants of Arabidopsis are resistant to IAA and have agravitropic roots, suggesting a role in some aspect of auxin physiology (Estelle, 1996). AUX1 is a presump-tive membrane protein similar to amino acid permeases, suggesting that it may function to transport auxin across a membrane (Bennett et al., 1996). Because plant amino acid permeases function as proton-driven symporters, and auxin influx is also thought to occur by a proton cotransport mechanism, Bennett et al. (1996) suggested that AUX1 may …