A model for the role of AGPs in pollen tube growth. (A) Accepted model for pollen tube growth [70, 79], illustrating vesicle exocytosis (1), smooth endocytosis (2), clathrin vesicle endocytosis (3), the pathway into degradation (4) and the recycling pathway (5). The apical dome (red), the exocytosis region (green), the multivesicular body (purple) and the vacuole (blue) are shown. (B) Proposed model for AGP signaling role in the pollen tube apical dome. AGPs act as receptors for extracellular signals and interact with transmembrane proteins, possibly receptor kinases or C2 domain-containing proteins. These interactions lead to opening of calcium channels, triggering various intracellular events. During pollen tube growth, AGPs are recycled by endocytosis, and either reused or sent for degradation, through multivesicular bodies. (C) Proposed model for the sub-apical zone. The hardening of cell wall pectins resulting from complexing with calcium hinders AGP contact with putative signal molecules, rendering AGPs inactive.