MICROTUBULES AND ACTIN FILAMENTS PARTECIPATE IN DIFFERENT STEPS OF ENDOCYTOSIS IN POLLEN TUBE OF NICOTIANA TABACUM (L.)

The pollen tube is a highly polarized cell that plays a fundamental role in the fertilization process of higher plants. Pollen tube growth depends on transport and accumulation of secretory vesicles (SVs) in the apical region where they fuse in a small area of the plasma membrane (PM) supplying material for the construction of new cell wall and providing, at the same time, new surface of PM. The incorporation of new PM area largely exceeds the quantity of membrane surface required for tube elongation, so the excess of PM must be removed through an active internalization process, known as endocytosis (Steer and Steer 1989; Derksen et al. 1995, Hepler et al. 2001). In vivo time-lapse experiments using FM4-64 combined with quantitative analysis allowed the observation that different endocytic pathways occurs in distinct zones of the tube, and that endocytic vesicles might substantially contribute to V-shaped vesicle accumulation in addition to SVs (Moscatelli et al. 2007; Bove et al. 2008; Zonia and Munnick 2008;). Even if membranous compartments and the cytoskeleton inside the pollen tube have been often analyzed as independent units, the evidence demonstrates that they can interact through complex molecular interactions that mutually influence their organization, suggesting a sort of “molecular-continuum” between the extracellular environment and the pollen tube cytoplasm. In this study we disturb the system of MTs using low concentration of Nocodazole, to verify its effect on endocytosis and endosome trafficking. Our results demonstrated that MTs are able to bind vacuoles and pre vacuolar compartmens and that the perturbation of dynamic MTs retards the progression of endocytic vesicles from the tip regions to the vacuoles, suggesting a role of MTs in one degradative pathway. However, MTs do not seem involved in plasma membrane internalization. In addition, MTs appeared to be involved in maintaining the differential pattern of exocytosis, occurring in different zones of the apex in cooperation with AFs.