Cell Polarity

Cone of Inactivation

Science Signaling  06 Jan 2009:
Vol. 2, Issue 52, pp. ec4
DOI: 10.1126/scisignal.252ec4

Pollen tubes must grow in a polarized manner such that elongation occurs only from the extreme apical area. Furthermore, this apical growth zone must be continuously regenerated as the pollen tube lengthens. ROP1 is a pollen-specific, plasma membrane–associated Rho family guanine triphosphatase (GTPase) that controls F-actin dynamics at the apex of pollen tubes from Arabidopsis and other plant species and whose oscillatory activity must be tightly controlled to ensure pollen tube polarized growth. Hwang et al. identified a gene whose loss accentuated the phenotype caused by overexpression of ROP1, which causes Arabidopsis pollen tubes to grow laterally (so that they are short and wide) instead of longitudinally. ROP enhancer 1 (REN1) encoded a Rho GTPase-activating protein (RhoGAP) present only in mature pollen that promoted ROP1 GTPase activity in vitro. In tobacco flower pollen tubes, the growth phenotype caused by overexpression of green fluorescent protein (GFP)–tagged ROP1 was partially rescued by REN1 coexpression but was slightly enhanced by coexpression of a REN1 mutant that lacked GAP activity. In addition, ROP1 localization and activity [as measured by the distribution of one of its effectors, RIC4 (Rop-interactive Cdc42-Rac-interactive binding motif-containing protein 4)] was no longer limited to the apical tip in tobacco flower pollen tubes lacking REN1 compared with wild-type pollen tubes. In Arabidopsis pollen tubes, endogenous REN1 or GFP-tagged REN1 was localized to exocytic vesicles throughout the apical tip. Pharmacological disruption of the accumulation of exocytic vesicles to the apical tip or of exocytotic vesicle formation caused REN1 distribution to become more uniform and altered the morphology of Arabidopsis pollen tubes. Like ROP1 activity, REN1 localization to the apical tip of Arabidopsis pollen tubes oscillated, peaking after ROP1 activity peaks and before ROP1 deactivation. The authors propose that ROP1 activity at the apical tip induces F-actin assembly and recruits vesicles bearing REN1 to the plasma membrane of the pollen tube apex, which in turn limits ROP1 activation and ensures polarized growth. The authors suggest that these mechanisms could also be present in other cells that exhibit elongated growth, such as fungal hyphae and animal neuronal axons.

J.-U. Hwang, V. Vernoud, A. Szumlanski, E. Nielsen, Z. Yang, A tip-localized RhoGAP controls cell polarity by globally inhibiting Rho GTPase at the cell apex. Curr. Biol. 18, 1907–1916 (2008). [PubMed]