Apical vesicles: Social networking at the pollen tube tip

Pollen tube growth is asymmetric and highly focused at the tip. It is regarded as an ideal model cell system in plants to study polar cell growth and cell polarity. The rapid pollen tube growth is empowered by exocytosis of large amounts of secretion vesicles that contain proteins, lipids and cell wall materials to the apex. Meanwhile, active retrograde endocytosis recycles excessively secreted vesicles for multiple rounds of exocytosis. Meanwhile, it also mediates uptake of extracellular molecules for signal transduction or vacuolar degradation. The dome morphogenesis and fast expansion of the pollen tube tip demand dynamic coordination and balance of antagonistic exocytosis and endocytosis. Nevertheless, the origins, molecular identities and their underlying molecular machineries of the apical vesicles still remain largely uncovered. In this review, we briefly summarize recent progresses in our understanding of the origins and identities of apical vesicles in the pollen tube tip. Furthermore, we focus on discussing elusive aspects underlying the apical vesicles to dominate pollen tube tip growth and highlight a few research challenges that need to be addressed by newly emerging high-resolution bio-imaging and biochemical analysis approaches.