Gravitropism ofArabidopsis thalianaRoots Requires the Polarization of PIN2 toward the Root Tip in Meristematic Cortical Cells  

Abstract

In the root, the transport of auxin from the tip to the elongation zone, referred to here as shootward, governs gravitropic bending. Shootward polar auxin transport, and hence gravitropism, depends on the polar deployment of the PIN-FORMED auxin efflux carrier PIN2. In Arabidopsis thaliana, PIN2 has the expected shootward localization in epidermis and lateral root cap; however, this carrier is localized toward the root tip (rootward) in cortical cells of the meristem, a deployment whose function is enigmatic. We use pharmacological and genetic tools to cause a shootward relocation of PIN2 in meristematic cortical cells without detectably altering PIN2 polarization in other cell types or PIN1 polarization. This relocation of cortical PIN2 was negatively regulated by the membrane trafficking factor GNOM and by the regulatory A1 subunit of type 2-A protein phosphatase (PP2AA1) but did not require the PINOID protein kinase. When GNOM was inhibited, PINOID abundance increased and PP2AA1 was partially immobilized, indicating both proteins are subject to GNOM-dependent regulation. Shootward PIN2 specifically in the cortex was accompanied by enhanced shootward polar auxin transport and by diminished gravitropism. These results demonstrate that auxin flow in the root cortex is important for optimal gravitropic response.

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

“…PIN2 shows either apolar or apical localization in the neighboring cortical cells, consistent with a possible defect of backward auxin recycling from the cortex to the root tip. The effect of crk5-1 mutation thus appears to be very similar to the consequence of treating roots with low concentrations of BFA, which stimulates basal-to-apical relocation of PIN2 in the cortex inhibiting root gravitropic response (Rahman et al, 2010). In fact, we observed that the crk5-1 mutation not only reduces the amount of PM-associated PIN2 in the epidermis of transition zone, but also markedly accelerates brefeldin-sensitive internalization of PIN2 in epidermal cells.…”

“…Upwards of the crk5-1 transition zone, PIN2 was detected, if at all, as weak internalized signal in the epidermis, and its localization was shifted to apical in the cortex. Since such apical localization of PIN2 in the cortex induced by low concentration BFA treatment was previously demonstrated to abolish root gravitropic responses (Rahman et al, 2010), these data suggested that aberrant polar localization of PIN2 in both epidermis and cortex was likely responsible for the delayed gravitropic response of crk5-1 mutant roots.…”

“…Brefeldin inhibition of GNOM-dependent basal targeting stimulates PIN accumulation in so-called brefeldin compartments and leads ultimately to their relocalization to apical membranes . Without changing apical localization of PIN2 in the epidermis, low-level BFA treatment stimulates basal to apical PIN2 transcytosis in the cortex, which abolishes root gravitropic response, demonstrating the importance of cortical auxin recycling to the root tip (Rahman et al, 2010). Basal-toapical switching of PINs is stimulated by the PINOID (PID), WAG1, and WAG2 AGC kinases, which phosphorylate TPRxS motifs in the hydrophilic loops of PINs (Michniewicz et al, 2007;Dhonukshe et al, 2010).…”

Cellular Auxin Homeostasis under High Temperature Is Regulated through a SORTING NEXIN1–Dependent Endosomal Trafficking Pathway

“…Newly synthesized nonpolar PIN1 and PIN2 in the cortex relocalize themselves toward the rootward domain of the plasma membrane through ADP ribosylation factor GTP-exchanging factors, such as GNOM, and the phosphorylation status of the proteins, which is regulated by the counterbalancing activities of PINOID Kinase and protein phosphatase 2A (Geldner et al, 2001;Michniewicz et al, 2007;Sukumar et al, 2009;Rahman et al, 2010). However, polarization of PIN2 in epidermal and lateral root cap cells is independent of this pathway (Rahman et al, 2010). Recycling and intracellular trafficking of PIN2 require the retromer proteins SNX1 and VACUOLAR PROTEIN SORTING29 (VPS29) (Jaillais et al, 2006(Jaillais et al, , 2007.…”

Multiscale modelling of auxin transport in the plant-root elongation zone

“…This is of particular interest as a recent experimental study (Rahman et al 2010) (focussing on the meristem) suggests that changing the direction of the cortical cells' efflux carriers significantly alters the auxin distribution and gravitropic response. It is straightforward to see how changing the location of these efflux carriers affects the analytical results: in the single-layer analysis, the effective velocity in the cortex becomes v cor = B 3 P P I N /2, and we accordingly use this formula for v cor in place of (3.26e) when evaluating the dynamics of the three-layer model, (3.32) with (3.14d-j).…”