How Does pH Fit in with Oscillating Polar Growth?

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 similar to 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 Ca2+ oscillations. We propose possible mechanisms by which (apo)pH oscillations are coordinated with polar growth together with ROS and Ca2+ waves.