Cell polarity: ROPing the ends together.

Cell polarity plays an important role in plant development, but the mechanisms that first establish polarity cues remain obscure. By contrast, a flurry of information has recently emerged on the elaboration of cell shape from such unknown initial cell-polarity cues. Recent studies suggest that Rho-related GTPases in plants (ROPs), and their effector targets among the ROP-interactive CRIB motif-containing proteins (RICs), mediate two antagonistic pathways that have opposing action on cell polarization. ROP proteins appear to interact directly with upstream regulators of the ARP2/3 complex, which are conserved modulators of the actin cytoskeleton. ROP function is dependent on the class 1 ADP-ribosylation factors (ARFs), which are core components of the vesicle transport machinery that are also involved in the polar localization of PIN-FORMED (PIN) family auxin efflux facilitators.