Polar growth in root hairs and pollen tubes is an excellent model for investigating plant cell size regulation. While linear plant growth is historically explained by the acid growth theory, which considers that auxin triggers apoplastic acidification by activating plasma membrane P-type H + -ATPases (AHAs) along with cell wall relaxation over long periods, the apoplastic pH ( apo pH) regulatory mechanisms are unknown for polar growth. Polar growth is a fast process mediated by rapid oscillations that repeat every ∼20–40 s. In this review, we explore a reactive oxygen species (ROS)-dependent mechanism that could generate oscillating apo pH gradients in a coordinated manner with growth and Ca 2+ oscillations. We propose possible mechanisms by which apo pH oscillations are coordinated with polar growth together with ROS and Ca 2+ waves.
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