Oscillations in extracellular pH and reactive oxygen species modulate tip growth of Arabidopsis root hairs

Root hairs show highly localized cell expansion focused to their growing tips. This growth pattern is accomplished through restriction of secretion to the elongating apex and modulation of cell wall properties, with the wall just behind the tip becoming rigidified to resist the lateral expansive forces of turgor. In this report we show that root hairs exhibit oscillating growth that is associated with oscillating increases in extracellular pH and reactive oxygen species (ROS), which lag growth by ≈7 s. Consistent with a role for these changes in growth control, artificially increasing extracellular pH arrested root hair elongation, whereas decreasing pH elicited bursting at the tip. Similarly, application of exogenous ROS arrested elongation, whereas scavenging of ROS led to root hair bursting. Roots hairs of the root hair-defective rhd2-1 mutant, which lack a functional version of the NADPH oxidase ATRBOH C, burst at the transition to tip growth. This phenotype could be rescued by elevating the pH of the growth medium to ≥6.0. Such rescued root hairs showed reduced cytoplasmic ROS levels and a lack of the oscillatory production of ROS at the tip. However, they exhibited apparently normal tip growth, including generation of the tip-focused Ca²⁺ gradient thought to drive apical growth, indicating that ATRBOH C is not absolutely required to sustain tip growth. These observations indicate that root hair elongation is coupled to spatially distinct regulation of extracellular pH and ROS production that likely affect wall properties associated with the polarized expansion of the cell.