Exocytosis and endocytosis: coordinating and fine-tuning the polar tip growth domain in pollen tubes

Abstract

Pollen tubes rapidly elongate, penetrate, and navigate through multiple female tissues to reach ovules for sperm delivery by utilizing a specialized form of polar growth known as tip growth. This process requires a battery of cellular activities differentially occurring at the apical growing region of the plasma membrane (PM), such as the differential cellular signaling involving calcium (Ca2+), phospholipids, and ROP-type Rho GTPases, fluctuation of ions and pH, exocytosis and endocytosis, and cell wall construction and remodeling. There is an emerging understanding of how at least some of these activities are coordinated and/or interconnected. The apical active ROP modulates exocytosis to the cell apex for PM and cell wall expansion differentially occurring at the tip. The differentiation of the cell wall involves at least the preferential distribution of deformable pectin polymers to the apex and non-deformable pectin polymers to the shank of pollen tubes, facilitating the apical cell expansion driven by high internal turgor pressure. Recent studies have generated inroads into how the ROP GTPase-based intracellular signaling is coordinated spatiotemporally with the external wall mechanics to maintain the tubular cell shape and how the apical cell wall mechanics are regulated to allow rapid tip growth while maintaining the cell wall integrity under the turgor pressure. Evidence suggests that exocytosis and endocytosis play crucial but distinct roles in this spatiotemporal coordination. In this review, we summarize recent advances in the regulation and coordination of the differential pectin distribution and the apical domain of active ROP by exocytosis and endocytosis in pollen tubes.

Keywords: Cell wall; ROP GTPase; endocytosis; exocytosis; pollen tube; tip growth.

Fig. 1.

Proposed sites of exocytosis and endocytosis on the PM of pollen tubes. (A) PM of pollen tubes can be divided into three regions, the apical, shoulder, and shank region, based on the distribution of distinct molecules along the cell surface. An apical cap of active ROP1 defines the growth domain of pollen tubes. (B) The bulk of exocytosis occurs in the apical region, while certain subpopulations of exocytic vesicles may also be secreted to the shoulder region. Evidence also hints at the likely existence of an exocytic site in the shank region. Further identification of the cargos delivered to the shoulder and the shank regions is required to confirm the exocytosis events in the above two sites. (C) Clathrin-mediated endocytosis (CME) and clathrin-independent endocytosis (CIE) have been proposed to occur in both the apical region and the shank region of pollen tubes; however, only vesicle internalization by the CME has been experimentally observed to occur in the apical, shoulder, and shank region.

Fig. 2.

Role of exocytosis and endocytosis in the coordination and fine-tuning of the active ROP1 domain and the cell wall dynamics. (A) Reciprocal regulation of polar exocytosis and ROP signaling coordinated the active ROP1 domain and cell wall deposition. ROP GTPase organizes and coordinates polar secretion by regulating the dynamics of the actin filaments and recruiting the exocytosis machinery through a battery of downstream effectors. Exocytosis regulates the pollen tube growth and guidance through multiple pathways. (i) Exocytosis delivers building blocks of the cell wall, including pectin, PME, and PMEI to the apical growth domain, where the PME activity is inhibited by the PMEI to maintain the soft methylesterified status of pectin (sPectin). In the shoulder region, where the activity of PME is released due to internalization of the PMEI, pectin is demethylesterified by PME and cross-linked via Ca²⁺ bridges to form a hard network of stiff pectin polymers (hPectin). (ii) Exocytosis mediates positive feedback regulation on the ROP signaling by polar secretion of ligands and their receptors (PRK2) to the growth domain in an autocrine manner. (iii) Exocytosis delivers REN1 RopGAP to the tip of pollen tubes for global inhibition of active ROP1. (iv) Exocytosis participates in maintenance of the cell wall integrity (CWI) by secreting signal peptides (RALF4/19) and CWI sensor LRX family proteins; the secreted RALF4/19 peptides are also sensed by the PM-localized ANX1/2 and BUPS1/2 protein complexes. (v) Preferential delivery of the receptor-like kinase PRK6, the receptor for the guidance peptides AtLUREs, to the side of the PM that facing the AtLUREs is likely to be mediated by polar exocytosis. (B) Endocytosis shapes growth domains in pollen tubes using at least two mechanisms: it releases the activity of AtPPME1 by selective internalization of its inhibitor AtPMEI2 in the shoulder region and establishes a border between the two pectin zones; and it internalizes active ROP1 by REN4-mediated endocytosis in the apical and the shoulder region on the PM to maintain an optimal level of active ROP1 in the extreme apex and control the robustness of growth directionality in pollen tubes.