Calcium-triggered apoplastic ROS bursts balance gravity and mechanical signals to navigate soil

Reactive oxygen species (ROS) have been implicated repeatedly in multiple signaling processes in plants but the underlying mechanisms and roles remain enigmatic. Here, we developed live imaging of apoplastic ROS at the root surface. Different signals, including auxin, extracellular ATP and RALF1 peptide, all induce cytosolic calcium transients and apoplastic ROS bursts. Genetic and optogenetic manipulations identified calcium transients as necessary and sufficient for ROS bursts via activation of NADPH oxidases RBOHC and RBOHF. Apoplastic ROS bursts are not required but rather limit the gravity-induced root bending. Root bending is sensed by stretch-activated calcium channel MCA1 leading to NADPH oxidase activation at the stretched side. The resulting ROS production stiffens cell wall for better soil penetration. Apoplastic ROS thus provides a means to balance tissue flexibility and stiffness to efficiently navigate soil.