AUX1 regulates root gravitropism in Arabidopsis by facilitating auxin uptake within root apical tissues

Plants employ a specialized transport system composed of separate influx an d efflux carriers to mobilize the plant hormone auxin between its site(s) o f synthesis and action. Mutations within the permease-like AUX1 protein sig nificantly reduce the rate of carrier-mediated auxin uptake within Arabidop sis roots, conferring an agravitropic phenotype, We are able to bypass the defect within auxin uptake and restore the gravitropic root phenotype of au x1 by growing mutant seedlings in the presence of the membrane-permeable sy nthetic awin, l-naphthaleneacetic acid. We illustrate that AUX1 expression overlaps that previously described for the auxin efflux carrier, AtPIN2, us ing transgenic lines expressing an AUX1 promoter::uidA (GUS) gene. Finally, we demonstrate that AUX1 regulates gravitropic curvature by acting in unis on with the auxin efflux carrier to co-ordinate the localized redistributio n of auxin within the Arabidopsis root apex. Our results provide the first example of a developmental role for the auxin influx carrier within higher plants and supply new insight into the molecular basis of gravitropic signa lling.