Gravity-induced PIN transcytosis for polarization of auxin fluxes in gravity-sensing root cells.

Auxin is an essential plant-specific regulator of patterning processes that also controls directional growth of roots and shoots. In response to gravity stimulation, the PIN3 auxin transporter polarizes to the bottom side of gravity-sensing root cells, presumably redirecting the auxin flux toward the lower side of the root and triggering gravitropic bending. By combining live-cell imaging techniques with pharmacological and genetic approaches, we demonstrate that PIN3 polarization does not require secretion of de novo synthesized proteins or protein degradation, but instead involves rapid, transient stimulation of PIN endocytosis, presumably via a clathrin-dependent pathway. Moreover, gravity-induced PIN3 polarization requires the activity of the guanine nucleotide exchange factors for ARF GTPases (ARF-GEF) GNOM-dependent polar-targeting pathways and might involve endosome-based PIN3 translocation from one cell side to another. Our data suggest that gravity perception acts at several instances of PIN3 trafficking, ultimately leading to the polarization of PIN3, which presumably aligns auxin fluxes with gravity vector and mediates downstream root gravitropic response.