The widely accepted chemiosmotic model on PAT conveys the idea that auxin influx and efflux carriers reside in the plasma membrane where they exhibit transport functions. However, physiological studies already suggested that a fraction of the auxin efflux complexes have a short half-life in the plasma membrane and cycle rapidly through an unidentified intracellular compartment (Morris, 2000). These findings were unexpected and difficult to reconcile with classical models. After isolation of PAT components and with the availability of tools to visualize them, the subcellular dynamics of these proteins was addressed directly using cell biological approaches.
The fungal toxin BFA is in both animal and plant systems a well-characterized compound that interferes with subcellular vesicle trafficking. The molecular targets of
Figure 1.4 Subcellular movement of PIN proteins. Schematic model to explain internalization of PIN proteins upon BFA treatment. BFA blocks GNOM ARF-GEF responsible for activation of endosomal ARF GTPases, which mediate recycling of PIN1 to the plasma membrane. Ongoing endocytosis is BFA insensitive. AEIs such as TIBA interfere with both steps of PIN cycling.
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