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he plant hormone auxin affects many important aspects of plant growth and development. For example, aux-in influences growth of plants relative to gravity (gravitropism) and light (phototro-pism), placement of leaf primordia, and the establishment of stem cell niches (1–4). These processes all depend on dif-ferences in the local concentrations of auxin. Such differential auxin concentra-tions are established through the directed (polar) transport of auxin from sites of biosynthesis (leaves) to sites of action in the shoot and root. In turn, polar auxin transport depends on the asymmetric lo-calization in plant cells of proteins called PINFORMED (PIN) auxin transport facil-itators (5). The location of PIN proteins, and hence the direction of polar auxin trans-port, varies depending on the type of tis-sue. For instance, in central portions of the root, PIN proteins are localized in basal areas of cells and auxin flow is directed downward. In contrast, in emerging leaf and floral primordia, PIN proteins are lo-calized apically and auxin flow is directed upward (4–6) (see the figure). Because the localization of PIN proteins has such an important influence on polar auxin trans-port, plant biologists have sought to un-derstand what determines the placement of PIN proteins in plant cells. On page 862 of this issue, Friml et al. (7) provide evidence that a major determinant of PIN protein lo-calization in the model plant Arabidopsis is the serine-threonine kinase PINOID (PID). High levels of PID activity lead to the apical localization of PIN, whereas low levels lead to the basal localization of PIN. Arabidopsis mutants that carry a defec-tive PINFORMED1 (pin1) gene make bar-ren “pin-like ” inflorescences that largely lack floral primordia (see the figure). Polar auxin transport is reduced in such pin1 mu-tants, and inhibitors of polar auxin trans-port induce the development of pin-like in-florescences in wild-type plants (8, 9). Application of auxin to these barren inflo-rescences rescues their ability to make pri