Coordinated Polar Localization

The plant hormone auxin is transported in a polar manner along the shoot-root axis, which requires efßux carriers such as PIN1. Asymmetric localization of PIN1 develops from a random distribution in Arabidopsis early embryogenesis. Coordinated polar localization of PIN1 is defective in gnom embryos. GNOM is a membrane-associated guanine-nucleotide exchange factor on ADP-ribosyla-tion factor G protein (ARF GEF). Thus, GNOM-dependent vesicle trafÞcking may establish cell polarity, resulting in polar auxin transport. Polar auxin transport has been implicated in a variety of developmental processes, includ-ing phototropism, gravitropism, apical domi-nance, root formation, and vascular tissue patterning (1, 2). These vectorial processes require coordinated polar organization of cells that is reflected in the localization of putative auxin efflux carrier, PIN1, at th