Long-standing models propose that plant growth responses to light or gravity are mediated by asymmetric distribution of the phytohormone auxin 1 -3 . Physiological studies implicated a specific transport system that relocates auxin laterally, thereby effecting differential growth 4 ; however, neither the molecular components of this system nor the cellular mechanism of auxin redistribution on light or gravity perception have been identified. Here, we show that auxin accumulates asymmetrically during differential growth in an efflux-dependent manner. Mutations in the Arabidopsis gene PIN3, a regulator of auxin efflux, alter differential growth. PIN3 is expressed in gravity-sensing tissues, with PIN3 protein accumulating predominantly at the lateral cell surface. PIN3 localizes to the plasma membrane and to vesicles that cycle in an actin-dependent manner. In the root columella, PIN3 is positioned symmetrically at the plasma membrane but rapidly relocalizes laterally on gravity stimulation. Our data indicate that PIN3 is a component of the lateral auxin transport system regulating tropic growth. In addition, actin-dependent relocalization of PIN3 in response to gravity provides a mechanism for redirecting auxin flux to trigger asymmetric growth.Plants orientate their growth with respect to the direction of light (phototropism) or gravity (gravitropism)1 . As early as 1926 a widely accepted model for plant tropisms, the Cholodny -Went hypothesis, was presented 2 . It proposes differential distribution of the plant hormone auxin in lateral direction on gravity or light stimulation. Subsequently, different auxin levels elicit differential growth rates, which ultimately lead to bending of the shoot or root 3 . Visualization of asymmetrically distributed auxin response in gravistimulated tobacco stems 5 and Arabidopsis roots 6 experimentally supported this hypothesis. Polar auxin transport represent a plausible means of lateral auxin distribution, as its chemical inhibition affects differential growth responses such as tropisms and apical hook formation 7,8 . Physiologically characterized components of polar auxin transport are cellular efflux carriers, whose polar localization within cells is thought to determine the direction of auxin flux 9 . The recently identified PIN genes of Arabidopsis appear to encode essential components of these carriers 7 . A role of PIN2 in regulation of basipetal auxin transport and gravitropism in root 6,10,11 as well as a role of PIN1 in basipetal auxin transport in the stem have been reported 12 ; however, so far the molecular basis of shoot tropic responses remains elusive. Lateral auxin transport with a specific, laterally localized auxin efflux carrier was proposed 4 to explain the exchange of auxin between vasculature, where the main basipetal auxin stream occurs 13 , and peripheral tissues controlling elongation 14 . Nevertheless the lack of any molecular data supporting this concept still leaves the existence of such a system in question.We analysed the relationship between auxi...

Intercellular flow of the phytohormone auxin underpins multiple developmental processes in plants. Plant-specific pin-formed (PIN) proteins and several phosphoglycoprotein (PGP) transporters are crucial factors in auxin transport-related development, yet the molecular function of PINs remains unknown. Here, we show that PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors. Conditional gain-of-function alleles and quantitative measurements of auxin accumulation in Arabidopsis and tobacco cultured cells revealed that the action of PINs in auxin efflux is distinct from PGP, rate-limiting, specific to auxins, and sensitive to auxin transport inhibitors. This suggests a direct involvement of PINs in catalyzing cellular auxin efflux.