GNOM alone

Polarized vesicular trafficking is essential for a plethora of fundamental biological functions. In mammals, this includes processes as diverse as the maintenance of epithelial cell polarity, downregulation of cell-surface receptors and progress through key stages of development. In plants, by contrast, we know almost nothing about the significance of polarized transport. Now, a study by Geldner et al. (Cell 112, 219–230 (2003)) provides evidence of a direct link between transport of auxin, an essential regulator of plant growth, and polarized membrane transport.

There have been some recent hints to suggest that polarized transport is essential for fundamental processes in plants. The first came from studies of auxin, which is actively transported throughout the plant via specific transporters that are organized in a polar distribution. One candidate transporter for auxin, PIN1, is continuously recycled through the endomembrane system, suggesting a potential requirement for polarized vesicle transport. The second hint came from analysis of the Arabidopsis thaliana protein, GNOM, a guanine nucleotide-exchange factor for ARF GTPases — essential regulators of vesicle trafficking in many organisms. Mutations in GNOM caused defects that were highly reminiscent of the effects of blocking auxin transport. At a sub-cellular level, PIN1 was mis-localized in gnom mutants. Furthermore, treatment with Brefeldin A (BFA), which is known to reversibly block vesicular trafficking steps by inhibiting the activity of specific ARF-GEFs, also disrupted auxin transport and resulted in the mis-localization of PIN1 to internal membranes. Despite this circumstantial evidence, however, no direct molecular link between the movement of auxin and polarized trafficking has been demonstrated.

Now, Geldner et al. provide direct evidence of such a link. Their first interesting observation was that GNOM is localized to endosomes. This was unexpected, as related GEFs from yeast and animals have only been associated with secretory transport routes. In addition, they found that the endosomal morphology of gnom mutants was significantly disrupted, suggesting an essential role of GNOM in the maintenance of endosomal integrity and function. Next, in an elegant series of experiments, the authors engineered a mutant form of GNOM that is fully functional, but resistant to the effects of BFA. This allowed them to specifically dissect out the function of GNOM from other BFA-sensitive trafficking steps. Crucially, when plants expressing the mutant protein were then treated with BFA, PIN1, but not other plasma membrane proteins, was correctly localized to the cell surface. This provides convincing evidence that GNOM directly regulates the continual polarized recycling of PIN1 and suggests that ARF-GEFs regulate specific membrane trafficking pathways in plants. In addition, Geldner et al. show that the block in auxin transport induced by BFA is restored in cells expressing BFA-resistant GNOM. The next step is to identify the particular ARF on which GNOM exerts its effects. Nevertheless, this study directly links a component of membrane traffic to the transport of auxin, highlighting the importance of polarized trafficking in fundamental aspects of plant biology.

Figure 1: GNOM redistributes to an endosomal compartment after BFA treatment.

GNOM (red) and γ-COP (green) in Arabidopsis root cells after treatment with 50 μM BFA for 1 h. The nucleus is stained with DAPI (blue). Figure adapted from Geldner et al. © (2003) with permission from Elsevier Science.