Plasma membrane and cytoplasmic compartmentalization: A dynamic structural framework required for pollen tube tip growth.

This article focuses on the dynamic structural framework of plasma membrane (PM) and cytoplasmic compartmentalization required for pollen tube tip growth in Nicotiana tabacum (tobacco). Using quantitative fluorescence microscopy and lipid/protein markers, the study maps distinct PM domains enriched in signaling proteins (notably RAC/ROP GTPases and their regulators), lipids (such as phosphatidylinositol 4,5-bisphosphate [PI4,5P2], phosphatidic acid [PA], and phosphatidylserine [PS]), and regions of high membrane order, alongside cytoplasmic structures involved in apical membrane trafficking. The findings reveal a spatially organized apical secretory domain overlapping with active RAC/ROP and NtGEF12a (a guanine nucleotide exchange factor) domains, flanked by a lateral endocytic domain enriched in lipid-modifying enzymes and signaling lipids, separated by a diffusion barrier associated with a cortical F-actin fringe and a subapical trans-Golgi network compartment. Elevated cytoplasmic Ca2+ concentrations overlap with the vesicle accumulation region (VAR) beneath the apical PM, suggesting a role in priming vesicle fusion rather than directly triggering secretion. The study integrates these spatial domains into a model highlighting coordinated membrane trafficking, lipid metabolism, and cytoskeletal interactions essential for polarized pollen tube growth, providing a foundation for future research on molecular mechanisms underlying plant cell morphogenesis.