HPCA1 is required for systemic ROS and calcium cell-to-cell signaling and plant acclimation to stress
As multicellular organisms, plants constantly balance and coordinate many metabolic, physiological, and molecular responses between different cell types and tissues. This process is essential for plant development, growth, and response to different environmental cues. Because plants lack a nervous system, they transmit different signals over long distances via cell-to-cell signaling. Recent studies revealed that reactive oxygen species (ROS), produced by respiratory burst oxidase homologs (RBOHs) at the apoplast play a key role in cell-to-cell signaling. A state of enhanced ROS production by one cell is thereby sensed by a neighboring cell, causing it to produce ROS, creating a continuous chain of cell-to-cell ROS accumulation termed the ROS wave. This process was found to mediate systemic signals throughout the plant and is required for plant acclimation to different stresses. Although RBOHs were found to produce ROS essential for this process, the identity of the receptor(s) perceiving the apoplastic ROS signal is currently unknow. Here we reveal that the leucine-rich-repeat receptor-like kinase HPCA1 (H2O2-induced Ca2+ increases 1) acts as a central ROS receptor required for the propagation of cell-to-cell ROS signals, systemic signaling in response to different biotic and abiotic stresses, and plant acclimation to stress. We further report that HPCA1 is required for systemic calcium signals, but not systemic membrane depolarization responses, and identify key calcium-dependent signal transduction proteins involved in this process. Our findings reveal that HPCA1 plays a key role in mediating and coordinating systemic cell-to-cell ROS and calcium signals that are required for plant acclimation to stress.