Figure 2

ROP-mediated signaling controls the actin dynamics necessary for pollen tube growth. (a) Pollen tubes elongate through vesicle secretion at the tip of the tube. The delivery of vesicles (blue circles) requires cytoplasmic streaming (arrows) generated by stable actin bundles (1) and the movement of these vesicles through a meshwork of highly dynamic actin filaments (2) to an actin-free zone at the tube tip (3). Precise organization of actin is required for tube growth, as is a calcium gradient that increases towards the tip (b). (c) As shown by Zhenbiao Yang, ROPs control actin organization in the pollen tube. An unknown signal activates multiple ROPs at the tip, forming the active, GTP-bound state. ROPs activate various RICs, which indirectly control actin organization either by promoting the calcium gradient which stimulates the effects of profilin on actin, or by promoting actin polymerization, possibly via the ARP2/3 complex.