Auxins, a family of plant hormones, are powerful regulators of plant development and growth that need to be actively transported into and out of cells. Here, Jiří Friml and colleagues report that the two auxin export systems, the phosphoglycoprotein (PGP) and the PIN protein systems,although independent, cooperate during Arabidopsis developmental patterning, and they propose a new model in which PGPs function partly by rationing the auxin that is available for PINs to directionally transport (see ). In the Arabidopsis embryo, PINs localise to polarised patches at the plasma membrane. The authors now show that PGPs are dispersed throughout the plasma membrane of embryonic tissue. Their mutant analysis reveals, among other findings, that the two systems function synergistically in the spatial patterning of the auxin response in embryos and roots. It is here, the authors propose, that PGPs regulate auxin flow both by directly interacting with PINs and by generally exporting auxin from cells, thus limiting the auxin that is available for directional transport.
