Different endocytic pathways require the coordinated action of actin filaments and microtubules in tobacco pollen tube

Pollen tubes are highly polarized cells in which both cytoplasmic constituents and proteins/lipids are distributed differentially in the cytoplasm and on the plasma membrane (PM). The equilibrium between exo- and endocytosis, defining PM composition, allows pollen tubes to successfully interact with molecules of the female tissues during guided migration through the style. It was shown that incorporation of new segments of PM exceeds the quantity of membrane required for the extension, so the excess of PM must be removed by endocytosis. Recent studies showed internalization of subapical PM domains that were mainly recycled to exocytosis through the Golgi apparatus and a second mainly degradative pathway. PM internalization and movements of the endomembrane system during the pollen tube growth depends on the concerted action and integrity of the cytoskeleton. The use of low concentration of Latrunculin B showed that both actin-dependent and actin-indepedent endocytosis work in tobacco pollen tube, internalizing tracts of PM destined to recycling and to degradation and evidenced the presence of distinct degradation pathways. Intriguingly, although most studies concentrate on exocytosis and distension in the tip region, this paper demonstrate that an actin-dependent, still uncharacterized secretory activity occurs in the flanks of tobacco pollen tubes. On the other hand, perturbation of microtubules by using Nocodazole showed that they are not involved in PM internalization. Colocalization experiments using dyes for Golgi, acidic compartments and FM4-64 in vivo assays suggested that Microtubule play a role in the transport of endosomes internalized in the tip region. These data have been also confirmed by ultrastructural observations of negatively charged naogold, whose transport to vacuoles appeared to be retarded in the presence of nocodazole.