RAC/ROP GTPases and Auxin Signaling

Abstract

Auxin functions as a key morphogen in regulating plant growth and development. Studies on auxin-regulated gene expression and on the mechanism of polar auxin transport and its asymmetric distribution within tissues have provided the basis for realizing the molecular mechanisms underlying auxin function. In eukaryotes, members of the Ras and Rho subfamilies of the Ras superfamily of small GTPases function as molecular switches in many signaling cascades that regulate growth and development. Plants do not have Ras proteins, but they contain Rho-like small G proteins called RACs or ROPs that, like fungal and metazoan Rhos, are regulators of cell polarity and may also undertake some Ras functions. Here, we discuss the advances made over the last decade that implicate RAC/ROPs as mediators for auxin-regulated gene expression, rapid cell surface-located auxin signaling, and directional auxin transport. We also describe experimental data indicating that auxin-RAC/ROP crosstalk may form regulatory feedback loops and theoretical modeling that attempts to connect local auxin gradients with RAC/ROP regulation of cell polarity. We hope that by discussing these experimental and modeling studies, this perspective will stimulate efforts to further refine our understanding of auxin signaling via the RAC/ ROP molecular switch.

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana

“…A few plasma membrane integral or associated polar localised proteins have been described in plants. They play a role in the establishment of cell polarity and/or are involved in cell-cell communication by transporting or generating signalling molecules (Fu and Yang, 2001;Swarup et al, 2001;Motose et al, 2004;Vieten et al, 2005;Dong et al, 2009;Humphries et al, 2011;Wu et al, 2011). The OPS protein does not have any known functional domains that could give evidence of its role.…”

AUXIN BINDING PROTEIN1: The Outsider

“…In these pavement cells, auxin seemingly instructs interdigitated lobing and indentation by ROP2-and ROP6-dependent formation and rearrangement of the actin and microtubule cytoskeleton, respectively (Yang, 2008;Xu et al, 2010). ROP2/ROP6 are, as all RAC/ROP GTPases, plant-specific (Rhorelated) molecular switches, of which the activity is regulated via shuttling between GDP-(inactive) and GTP-bound (active) states (Wu et al, 2011). Auxin rapidly activates ROP2 and ROP6 within minutes and requires an ABP1-dependent perception mechanism (Xu et al, 2010).…”

Uniform auxin triggers the Rho GTPase-dependent formation of interdigitation patterns in pavement cells

“…43 These observations raise an interesting possibility that ABP1 might also mediate auxin signaling through other ROPs in ER. 44 Importantly the work by Xu and colleagues also reveal a ROP GTPasedependent connection between PIN polarization and ABP1 in these cells. 1 The regulation of PIN distribution by the cell surface ABP1 is also demonstrated in another recent report.…”