A Tip-Localized RhoGAP Controls Cell Polarity by Globally Inhibiting Rho GTPase at the Cell Apex

Abstract

Summary Background Highly elongated eukaryotic cells (e.g., neuronal axons, fungal hyphae, and pollen tubes) are generated through continuous apically restricted growth (tip growth), which universally requires tip-localized Rho GTPases. We used the oscillating pollen tube as a model system to determine the function and regulation of Rho GTPases in tip growth. Our previous work showed that the spatiotemporal dynamics of the apical cap of the activated ROP1 Rho GTPase is critical for tip growth in pollen tubes. However, the underlying mechanism for the generation and maintenance of this dynamic apical cap is poorly understood. Results A screen for mutations that enhance ROP1 overexpression-induced depolarization of pollen tube growth identified REN1 (ROP1 enhancer 1) in Arabidopsis, whose null mutations turn elongated pollen tubes into bulbous cells. REN1 encodes a novel Rho GTPase-activating protein (RhoGAP) required for restricting the ROP1 activity to the pollen tube tip. REN1 was localized to exocytic vesicles accumulated in the pollen tube apex as well as to the apical plasma membrane at the site of ROP1 activation. The apical localization of REN1 and its function in controlling growth polarity was compromised by disrupting ROP1-dependent F-actin and vesicular trafficking, which indicates that REN1 targeting and function is regulated by ROP1 downstream signaling. Conclusions Our findings suggest that the REN1 RhoGAP controls a negative feedback-based global inhibition of ROP1. This function provides a critical self-organizing mechanism, by which ROP signaling is spatially limited to the growth site and temporally oscillates during continuous tip growth. Similar spatiotemporal control of Rho GTPase signaling may also play an important role in cell polarity control in other systems, including tip growth in fungi and cell movement in animals.

The Small GTPase ROP10 of Medicago truncatula Is Required for Both Tip Growth of Root Hairs and Nod Factor-Induced Root Hair Deformation

“…Various regulators that modulate the shuttling between inactive and active forms of ROP GTPases have been identified, including guanine nucleotide exchange factors (RopGEFs) that stimulate GDP-to-GTP exchange to activate ROP GTPases, ROP GTPase-activating proteins (RopGAPs) that accelerate GTP hydrolysis, and guanine nucleotide dissociation inhibitors (RhoGDIs) that inhibit GDP release, thus shifting ROP GTPases toward the inactive state. Some of these regulators have been shown to modulate ROP GTPase activity in pollen tubes and root hairs (Carol et al, 2005;Gu et al, 2006;Zhang and McCormick, 2007;Hwang et al, 2008Hwang et al, , 2010Riely et al, 2011).…”

Calcium participates in feedback regulation of the oscillating ROP1 Rho GTPase in pollen tubes

“…As a negative regulator of ROPs, RhoGAP is critical for polarized pollen tube growth (28). The simulation of our models predicted that ROP1 activity oscillation is highly sensitive to increasing amounts of RhoGAP (Fig.…”

“…1B). In Arabidopsis pollen tubes, apically localized REN1 RhoGAP participates in the negative feedback regulation of ROP1 and may be subject to regulation by calcium (28,29).…”

“…Pollen grains of Arabidopsis flowers were germinated in solid germination medium for 4 h at room temperature and then immunostained with AntiRop1Ps polyclonal antibody and FITC-conjugated goat anti-rabbit IgG antibody (Sigma), as described previously (28). Mounted pollen tubes from the WS wild-type background and aca9 -2 mutant background were observed under a Leica SP2 confocal microscope.…”

ROPGAPs of Arabidopsis limit susceptibility to powdery mildew

“…16,17 Another type of ROPGAP, a pleckstrin-homology domain-containing ROPGAP called REN1 also functions in regulation of pollen tube polarity in Arabidopsis. 18 Depolarization of normally tip-growing root hairs also takes place when a constitutively activated (CA) version of barley HvRACB is stably expressed in barley. 12 The same CA HvRACB plants are super-susceptible to powdery mildew.…”