A model for the coordination of vesicle transport and exocytosis by ROP1-dependent actin dynamics during oscillatory pollen tip growth. (A) Pollen tip growth is controlled by locally distributed ROP1 activity through its effect on actin assembly and disassembly at the tip of pollen tubes. ROP1 is activated at the apical region of the PM, and active ROP1 subsequently activates two distinct pathways. The RIC4 pathway promotes F-actin assembly required for the accumulation of exocytic vesicles to the tip but inhibits vesicle exocytosis to the apical PM (orange). Meanwhile, ROP1 also activates the RIC3–calcium pathway. As RIC3-dependent Ca²⁺ levels rise, the tip F-actin is depolymerized, and vesicles can then tether on and fuse with the PM. This event is accompanied by reduced vesicle accumulation in the tip (blue). The RIC3 and RIC4 pathways counteract to control F-actin dynamics and consequently coordinate vesicle targeting and exocytosis. (B) A temporal relationship among ROP1, F-actin, and Ca²⁺ oscillation based on this work and previous studies (Messerli et al., 2000; Hwang et al., 2005). An approximate temporal relationship among ROP1 activity, tip F-actin accumulation, tip expansion (growth) rate, and tip Ca²⁺ concentration is illustrated using different color-coded curves, whereas the peak points of vesicle accumulation and anticipated exocytosis are indicated by arrows. Tip-localized active ROP1 activates both RIC4 and RIC3, respectively, leading to rapid actin assembly and a delayed Ca²⁺ gradient formation and actin disassembly. We propose that when F-actin is assembled, vesicles are targeted to the tip, but do not dock and fuse to the PM until the Ca²⁺ level is high to disassemble cortical F-actin. The exact timing of vesicle exocytosis has yet to be determined, but is expected to be in phase with Ca²⁺, which slightly lags behind tip expansion (Messerli et al., 2000). If so, the PM at the tip may be stretched during the tip expansion, and the compensatory exocytosis should immediately follow the tip expansion, which could explain why Ca²⁺ slightly lags behind the tip expansion.