AtRAC7, a small Rac-like GTPase, is involved in auxin signaling

RACs are small molecular switches that belong to the Ras superfamily of small GTPases. Arabidopsis contains a family of 11 RAC-like GTPases (AtRACs) that appear to be master regulators of signal transduction in plants. Previous work has shown that AtRACs are involved in diverse cellular processes, such as cell morphogenesis, plant defence, and stress responses. AtRAC GTPases have also been suggested to take part in responses to abscisic acid and auxin. The plant hormone auxin is a central regulator of cell identity, cell division and cell expansion. Polar transport of auxin, generated through the action of auxin efflux facilitators (PINs), produces auxin gradients that are necessary for processes such as embryogenesis, organogenesis, vascular tissue differentiation and root meristem maintenance. AtRAC7 is expressed in the embryo, root and shoot meristems and leaf provascular tissue. Several lines of evidence point towards a role for AtRAC7 in auxin signaling. The AtRAC7 promoter contains putative binding sites for auxin response factors (ARFs). AtRAC7 expression is strongly reduced in the auxin mutants axr2 and axr3. A transposon insertion mutant of AtRAC7 shows defects in leaf vasculature and root growth; microarray studies of the atrac7 mutant indicate phosphate responses. The results presented will be discussed in view of a possible role for AtRAC7 in polar auxin transport and/or auxin responses.