ROP GTPase-dependent polarity establishment during tip growth in plants

Polar cell growth in plants requires a cell peripheral region that undergoes membrane extension and cell wall remodeling. Since the 1990s, RHO-RELATED GTPASES FROM PLANTS (ROPs) have been identified as master regulators that determine the site of cell growth. ROPs function to regulate actin and microtubule cytoskeletons, calcium gradients, and exocytosis, thus directing the delivery of materials for membrane and cell wall extension. In recent years, our understanding of the regulatory mechanisms underlying polar localization and the activation of ROPs has greatly advanced. Evidence points to the crucial roles of membrane lipids, receptor-like kinases, and cell wall components. In this review, we provide updates on the mechanisms underlying polarity control in tip-growing cells, with a focus on ROP effectors and membrane-associated signals. By integrating knowledge from pollen tubes, root hairs, and findings in bryophyte protonema cells and rhizoids, we hope to offer important insights into a common conceptual framework on polarity establishment governed by intercellular and extracellular signals.