How Does pH Fit in with Oscillating Polar Growth?

• Published in: Trends in plant science Vol. 23; no. 6; pp. 479 - 489
• Main Authors: Mangano, Silvina, Martínez Pacheco, Javier, Marino-Buslje, Cristina, Estevez, José M.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-06-2018; Elsevier BV
• Subjects: Acidification; apoplastic pH; Arabidopsis - physiology; Arabidopsis thaliana; Ca2; Calcium - metabolism; Calcium ions; Calcium signalling; Cell Differentiation - physiology; Cell size; Cells; Historical account; Hormones; Hydrogen-Ion Concentration; Membranes; Oscillations; P-type H+-ATPase; pH effects; Plant growth; Plant Roots - growth & development; Plant Roots - physiology; Plasma; polar cell expansion; Pollen; Pollen Tube - growth & development; Pollen Tube - physiology; Pollen tubes; Reactive oxygen species; Reactive Oxygen Species - metabolism; Regulatory mechanisms (biology); Root hairs; Tubes; Arabidopsis thaliana; polar cell expansion; apoplastic pH; Ca2; P-type H+-ATPase; reactive oxygen species; Ca(2+); P-type H(+)-ATPase
• ISSN: 1360-1385; 1878-4372
• DOI: 10.1016/j.tplants.2018.02.008

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 (apopH) regulatory mechanisms are unknown for polar growth. Polar growth is a fast process mediated by rapid oscillations that repeat every ∼20–40s. In this review, we explore a reactive oxygen species (ROS)-dependent mechanism that could generate oscillating apopH gradients in a coordinated manner with growth and Ca2+ oscillations. We propose possible mechanisms by which apopH oscillations are coordinated with polar growth together with ROS and Ca2+ waves.
Recent reports provide new molecular evidence in support of the acid growth theory, which proposes that auxin triggers linear plant cell growth via the TIR1/AFB-Aux/IAA pathway and apopH acidification (e.g., in hypocotyls and root cells).
apopH in linear growing cells is regulated by AHAs via the autoinhibitory C-terminal domain, phosphorylation, and protein interactors.
Root hairs and pollen tubes undergo polar growth in an oscillatory manner as rapidly as several hundred microns over a few hours. This oscillatory polar growth is regulated by Ca2+, ROS, and pH gradients. A complete understanding of how oscillating apopH is regulated remains elusive.
apopH oscillations are coupled Ca2+ and ROS waves that coordinate growth by unknown mechanisms.