Hydrogen, a deceptively simple element, plays crucial roles in regulating life on Earth. The concentration of hydrogen ions (H+) determines the pH of biological systems and dictates virtually all biochemical processes. The pH modulates the structure, physicochemical properties and function of most macromolecules. The plant cell surface is characterized by tremendous variations in apoplastic pH, serving as informative signals shaping plant development and its interaction with the environment. Here, we discuss the principles underlying cell surface H+ homeostasis, the molecular tipping points that regulate fast, controlled and informative changes in apoplastic pH, as well as open questions regarding the regulation of plasma membrane H+-ATPases.

Plants develop characteristic shoot architectures by extending branches at specific angles. Primary shoots bend in response to gravity and then adjust the orientation through an organ-straightening process to achieve a mechanically favorable shape. However, how plants integrate branch structure with the shoot architecture remains uncertain. Here, we examined the lateral branch morphology of Arabidopsis thaliana mutants for myosin XI motor proteins through a combination of three-dimensional reconstruction and temporal imaging. The wild type and myosin xif mutant formed S-shaped branches and gradually adjusted the branch angle upwards. The myosin xik mutant exhibited straighter and drooping branches and maintained branch angles. The myosin xif xik double mutant formed branches with irregular directional changes with fluctuating angles. These results suggest that MYOSIN XIk and XIf are required for the establishment of branch morphology through upward bending, stabilizing growth direction, and maintaining curvature.