Hydrogen peroxide sensor HPCA1 is an LRR receptor kinase in Arabidopsis.

Hydrogen peroxide (HO) is a major reactive oxygen species in unicellular and multicellular organisms, and is produced extracellularly in response to external stresses and internal cues. HO enters cells through aquaporin membrane proteins and covalently modifies cytoplasmic proteins to regulate signalling and cellular processes. However, whether sensors for HO also exist on the cell surface remains unknown. In plant cells, HO triggers an influx of Ca ions, which is thought to be involved in HO sensing and signalling. Here, by using forward genetic screens based on Ca imaging, we isolated hydrogen-peroxide-induced Ca increases (hpca) mutants in Arabidopsis, and identified HPCA1 as a leucine-rich-repeat receptor kinase belonging to a previously uncharacterized subfamily that features two extra pairs of cysteine residues in the extracellular domain. HPCA1 is localized to the plasma membrane and is activated by HO via covalent modification of extracellular cysteine residues, which leads to autophosphorylation of HPCA1. HPCA1 mediates HO-induced activation of Ca channels in guard cells and is required for stomatal closure. Our findings help to identify how the perception of extracellular HO is integrated with responses to various external stresses and internal cues in plants, and have implications for the design of crops with enhanced fitness.