Vectorial Information for Arabidopsis Planar Polarity Is Mediated by Combined AUX1, EIN2, and GNOM Activity

Abstract

Cell polarity is commonly coordinated within the plane of a single tissue layer (planar polarity), and hair positioning has been exploited as a simple marker for planar polarization of animal epithelia . The root epidermis of the plant Arabidopsis similarly reveals planar polarity of hair localization close to root tip-oriented (basal) ends of hair-forming cells . Hair position is directed toward a concentration maximum of the hormone auxin in the root tip , but mechanisms driving this plant-specific planar polarity remain elusive. Here, we report that combinatorial action of the auxin influx carrier AUX1, ETHYLENE-INSENSITIVE2 (EIN2) , and GNOM genes mediates the vector for coordinate hair positioning. In aux1;ein2;gnom eb triple mutant roots, hairs display axial (apical or basal) instead of coordinate polar (basal) position, and recruitment of Rho-of-Plant (ROP) GTPases to the hair initiation site reveals the same polar-to-axial switch. The auxin concentration gradient is virtually abolished in aux1;ein2;gnom eb roots, where locally applied auxin can coordinate hair positioning. Moreover, auxin overproduction in sectors of wild-type roots enhances planar ROP and hair polarity over long and short distances. Hence, auxin may provide vectorial information for planar polarity that requires combinatorial AUX1, EIN2, and GNOM activity upstream of ROP positioning.

Root Hairs

“…The eto1 plants produce more ethylene than wild type and root hairs form closer to the apical end of the cell . Genes affecting auxin transport, such as AUXIN1 (AUX1: At2g38120), which encodes an auxin influx carrier, and GNOM (GN: At1g13980), which affects PIN auxin efflux carriers, also alter the locations of root hairs on cells, with root hairs forming towards the end of the cell where the auxin concentration is likely to be highest (Fischer et al, 2006). This suggests that auxin gradients govern where on each cell a root hair forms.…”

AUXIN BINDING PROTEIN1: The Outsider

“…Other ROP-dependent processes, such as polar root hair positioning, might also require local auxin gradients (Fischer et al, 2006;Ikeda et al, 2009). However, the underlying mechanism is unknown and it would be interesting to see whether ABP1 regulates the polarized activity of ROPs during root hair positioning.…”

Hormonal Regulation of Lateral Root Development in Arabidopsis Modulated byMIZ1and Requirement of GNOM Activity forMIZ1Function

“…GNOM regulates polar targeting and constitutive endosomal recycling of PIN auxin efflux carriers (Steinmann et al, 1999;Geldner et al, 2003). Polar PIN localization and auxin gradients could not be established in either partial or complete loss-offunction gnom mutants and, thus, failed to complete normal embryogenesis or development of root architectures (Steinmann et al, 1999;Geldner et al, 2004;Fischer et al, 2006;Kleine-Vehn et al, 2008). However, polar localization of PIN proteins was not affected by overexpression of MIZ1 or the miz2 mutation (Fig.…”