Ca2+ Regulates Reactive Oxygen Species Production and pH during Mechanosensing in Arabidopsis Roots

Mechanical stimulation of plants triggers a cytoplasmic Ca2+ increase that is thought to link the touch stimulus to appropriate growth responses. We found that in roots of Arabidopsis thaliana, external and endogenously generated mechanical forces consistently trigger rapid and transient increases in cytosolic Ca2+ and that the signatures of these Ca2+ transients are stimulus specific. Mechanical stimulation likewise elicited an apoplastic alkalinization and cytoplasmic acidification as well as apoplastic reactive oxygen species (ROS) production. These responses showed the same kinetics as mechanically induced Ca2+ transients and could be elicited in the absence of a mechanical stimulus by artificially increasing Ca2+ concentrations. Both pH changes and ROS production were inhibited by pretreatment with a Ca2+ channel blocker, which also inhibited mechanically induced elevations in cytosolic Ca2+. In trichoblasts of the Arabidopsis root hair defective2 mutant, which lacks a functional NADPH oxidase RBOH C, touch stimulation still triggered pH changes but not the local increase in ROS production seen in wild-type plants. Thus, mechanical stimulation likely elicits Ca2+-dependent activation of RBOH C, resulting in ROS production to the cell wall. This ROS production appears to be coordinated with intra-and extracellular pH changes through the same mechanically induced cytosolic Ca2+ transient.