Arabidopsis thaliana Rop GTPases are localized to tips of root hairs and control polar growth

Plants contain a novel unique subfamily of Rho GTPases, vital components of cellular signalling networks. Here we report a general role for some membe rs of this family in polarized plant growth processes. We show that Arabido psis AtRop4 and AtRop6 encode functional GTPases with similar intrinsic GTP hydrolysis rates. We localized AtRop proteins in root meristem cells to th e cross-wail and cell plate membranes. Polar localization of AtRops in tric hoblasts specifies the growth sites for emerging root hairs. These sites we re visible before budding and elongation of the Arabidopsis root hair when AtRops accumulated at their tips. Expression of constitutively active AtRop 4 and AtRop6 mutant proteins in root hairs of transgenic Arabidopsis plants abolished polarized growth and delocalized the tip-focused Ca2+ gradient. Polar localization of AtRops was inhibited by brefeldin A, but not by other drugs such as latrunculin B, cytochalasin D or caffeine. Our results demon strate a general function of AtRop GTPases in tip growth and in polar diffu se growth.