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. Author manuscript; available in PMC: 2012 Oct 1.
Published in final edited form as: Semin Cell Dev Biol. 2011 Jun 25;22(8):816–824. doi: 10.1016/j.semcdb.2011.06.004

Figure 3. A model for the generation and maintenance of the apical cap of active ROP1 in growing pollen tubes.

Figure 3

The localized ROP1 activity in the center of tube apical PM is amplified through a positive-feedback loop of ROP1 activation, such as recruitment of RhoGEF or other upstream ROP activator, which induces a rapid increase of local ROP1 activity and then its lateral propagation through the apex, generating the active ROP1 cap. The ROP1-mediated tip F-actin dynamic might also contribute to the rapid lateral propagation of ROP1 activity by facilitating the diffusion of ROP1 and its regulators in the PM through polarized exocytosis. RhoGAP and RhoGDI globally inhibit ROP1 in the apex, preventing excess lateral propagation and finally terminating one cycle of ROP1 activity increase. ROP1 activity starts to increase again, probably via positive feedback from the remnant of the previous active ROP1 cap. A tightly balanced interaction of ROP1 activation and inactivation might continuously generate the dynamic apical ROP1 activity for the continuous tip growth. When the balance is broken by loss of critical RhoGDI or RhoGAP activity (RhoGDI2a and REN1 RhoGAP in Arabidopsis pollen tube), ROP1 becomes activated, resulting in the depolarization of apical ROP1 cap and pollen-tube tip growth.