Molecular Mechanisms Regulating Root Hair Tip Growth: A Comparison with Pollen Tubes

The developmental program of roots is constantly modified according to environmental signals and often includes an elevation in the density of root hairs, which increases the root’s absorptive surface in an attempt to meet the ion and water demands of the plant. Root hairs emerge from certain epidermal cells and this depends on a complex genetic cascade. Once this has determined root hair cell fate, local wall loosening and turgor pressure initiate a bulge in the cell wall. The transition from root hair initiation to actual tip growth begins with the accumulation of secretory vesicles at the apical part of the bulge. A complex interplay between ion oscillations, cytoskeleton architecture, vesicle trafficking, cell wall metabolism and hormonal and environmental signals allows the root hair to maintain growth at the tip. This review summarizes the current knowledge on the core components regulating root hair tip growth, critically identifies challenges for future research and points to commonalities and differences with the current knowledge on pollen tube tip growth.