Figure 8.

Auxin Signaling and GNOM-dependent PIN1 localization.

(A) Auxin accumulates in certain cells through coordinated transport mediated by the PIN proteins and this accumulation leads to the activation of the ARFs. At low auxin concentration, ARFs are maintained in a complex with the Aux/IAA proteins that act as transcriptional repressors. At high auxin concentrations, auxin bound to a TIR1 related F-Box protein within a SCF complex stabilizes the interaction between the complex and its target, the Aux/IAA protein. The SCF complex catalyzes the ubiquitination (U) of the Aux/IAA target and marks it for degradation by the proteasome, releasing ARF activity. This, in turn, may affect PIN genes expression and auxin transport properties of the respective cells, leading to potentially complex mutual influences between auxin distribution and transport patterns.

(B) Polar targeting of PIN1: The original transport of PIN1 from the ER/Golgi (on the right side of the figure) is non-polar. However, an ARF GEF-dependent transcytosis mechanism then targets the PIN1 protein to the apical or basal side of the cell. The ARF GEF GNOM is involved in the basal targeting of PIN1. Image reproduced from Kleine-Vehn and Friml (2008) with permission from Annual Reviews.

PIN1 is represented in blue, ARF in green and ARF GEF in yellow.