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

Abstract

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 clathrindependent 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.polarity | exocytosis | gravitropism | plant cell biology

Inactivation of Plasma Membrane–Localized CDPK-RELATED KINASE5 Decelerates PIN2 Exocytosis and Root Gravitropic Response inArabidopsis

“…Although the level of PIN7-GFP protein was somewhat lower compared with wild-type in crk5-1 roots, its basal localization in stele cells and apolar pattern in the columella did not differ in vertically grown and gravistimulated wild-type and crk5-1 roots (see Supplemental Figure 7C online). Using color-coded heat maps for PIN3-GFP localization in columella cells according to Kleine-Vehn et al (2010), we found that PIN3-GFP showed apolar PM localization in 90% of vertically positioned roots of both wild-type and crk5-1 plants. After 30 min of gravistimulation by 90°reorientation, translocation of PIN3-GFP to basal PM was observed only in 10% of both wild-type and crk5-1 roots, in which PIN3-GFP showed previous partial polarization also in the absence of gravistimulus (see Supplemental Figure 8A online) as reported by Kleine-Vehn et al (2010).…”

“…Using color-coded heat maps for PIN3-GFP localization in columella cells according to Kleine-Vehn et al (2010), we found that PIN3-GFP showed apolar PM localization in 90% of vertically positioned roots of both wild-type and crk5-1 plants. After 30 min of gravistimulation by 90°reorientation, translocation of PIN3-GFP to basal PM was observed only in 10% of both wild-type and crk5-1 roots, in which PIN3-GFP showed previous partial polarization also in the absence of gravistimulus (see Supplemental Figure 8A online) as reported by Kleine-Vehn et al (2010). A subsequent time course indicated that PIN3-GFP was nonpolarized and partially polarized in 90% versus 10% of vertically grown roots equally in both the wild type and crk5-1 mutant, and this ratio did not change during subsequent gravistimulation for 30, 60, or 120 min (see Supplemental Figure 8B online).…”

Shoot and root branch growth angle control—the wonderfulness of lateralness

“…In the case of gravitropism, this asymmetry is generated by changes in the subcellular distribution of PIN auxin efflux carriers within the gravity-sensing cells, or statocytes, of the shoot and root (Figure 1) [34,35]. A predominantly downward polarity of PIN-mediated auxin flow out of these cells appears to be linked, in ways that are still entirely unknown, to the sedimentation of starch-rich amyloplasts within each statocyte.…”

Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism

“…Specialized starch-filled organelles termed statoliths move within 5 min of a gravity stimulus in gravity-sensing columella cells (26,27). It has been proposed that statolith sedimentation onto the lower side of these cells triggers the formation of the lateral auxin gradient (28,29). To establish a link between statolith sedimentation and auxin redistribution, we monitored gravity-induced changes in DII-VENUS in the starchless pgm-1 (30) mutant that exhibits impaired gravitropic responses.…”