Vectorial information for Arabidopsis planar polarity is mediated by combined AUX1, EIN2, and GNOM activity

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 [1]. 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 [2-4]. Hair position is directed toward a concentration maximum of the hormone auxin in the root tip [4, 5], but mechanisms driving this plant-specific planar polarity remain elusive. Here, we report that combinatorial action of the auxin influx carrier AUX1 [6, 7], ETHYLENE-INSENSITIVE2 (EIN2) [8], and GNOM [9] 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 [10, 11] 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.