Exocytosis and Endocytosis: Yin-Yang Crosstalk for Sculpting a Dynamic Growing Pollen Tube Tip

Abstract

The growing pollen tube has become one of the most fascinating model cell systems for investigations into cell polarity and polar cell growth in plants. Rapidly growing pollen tubes achieve tip-focused cell expansion by vigorous anterograde exocytosis, through which various newly synthesized macromolecules are directionally transported and deposited at the cell apex. Meanwhile, active retrograde endocytosis counter balances the exocytosis at the tip which is believed to recycle the excessive exocytic components for multiple rounds of secretion. Therefore, apical exocytosis and endocytosis are the frontline cellular processes which drive the polar growth of pollen tubes, although they represent opposite vesicular trafficking events with distinct underpinning mechanisms. Nevertheless, the molecular basis governing the spatiotemporal crosstalk and counterbalance of exocytosis and endocytosis during pollen tube polarization and growth remains elusive. Here we discuss recent insight into exocytosis and endocytosis in sculpturing high rates of polarized pollen tube growth. In addition, we especially introduce the novel integration of mathematical modeling in uncovering the mysteries of cell polarity and polar cell growth.

Autophagy and its mediated mitochondrial quality control maintain pollen tube growth and male fertility in Arabidopsis

“…However, what is the biological role of autophagy during this process? We know that large numbers of mitochondria are required for rapidly growing pollen tubes to support the energy demands of metabolism, cell growth and transport related functions [ 2 , 5–7 ]. The autophagy of mitochondria as a general means of cellular quality control is also a well-studied phenomenon.…”

“…Pollen tubes are one of the fastest growing cell types in nature. To support the rapid and sustainable pollen tube growth, high levels of energy production is required to support the active metabolism and vigorous intracellular transport [ 2 , 4 , 5 ]. The number of mitochondria in pollen is about 20-fold greater compared to vegetative cells in maize [ 66 ].…”

“…Additionally, quick degradation and recycling of damaged cellular components (e.g. mitochondria and proteins) seems to be a more efficient way to meet the requirement of fast pollen tube growth than de novo synthesis [ 5–7 ]. Vacuole-mediated degradation is one of the major degradation pathways in plant cells [ 8–10 ].…”

Membrane trafficking functions of the ANTH/ENTH/VHS domain‐containing proteins in plants

“…The polar growth of the pollen tube is achieved via complex mechanisms, which involve the tight coordination between endocytosis and exocytosis, plant-specific Rho GTPase ROP1, the actin cytoskeleton, exocyst complex, phospholipids, SNARE proteins, cell wall biochemical mechanics, Ca 2+ , pH and reactive oxygen species (ROS) (Fig. B2) [58]. The apical region of the pollen tube is divided into the apex, subapical and shank regions.…”

Going with the membrane flow: the impact of polarized secretion on bulk plasma membrane flows

“…cerevisiae , endocytosis forms a ring around the secretion site at the time of bud emergence [ 7 , 8 ]; in filamentous fungi, the zone of endocytosis is largely distinct from the secretion zone, forming a collar around the exocytic apical zone (reviewed in [ 9 ]). Similarly, exo‐ and endocytosis sites overlap at the apex of pollen tubes [ 10 ], but these display a clear sub‐apical region where endocytosis takes place [ 11 , 12 ], suggesting an organization very similar to that observed in filamentous fungi. In this configuration, computational simulations showed that membrane flows would occur even without net membrane growth [ 1 ].…”