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Hydrogen peroxide (H
2
O
2
) is a major reactive oxygen species in unicellular and multicellular organisms, and is produced extracellularly in response to external stresses and internal cues
1
–
4
. H
2
O
2
enters cells through aquaporin membrane proteins and covalently modifies cytoplasmic proteins to regulate signalling and cellular processes. However, whether sensors for H
2
O
2
also exist on the cell surface remains unknown. In plant cells, H
2
O
2
triggers an influx of Ca
2+
ions, which is thought to be involved in H
2
O
2
sensing and signalling. Here, by using forward genetic screens based on Ca
2+
imaging, we isolated
hydrogen-peroxide-induced Ca
2+
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 H
2
O
2
via covalent modification of extracellular cysteine residues, which leads to autophosphorylation of HPCA1. HPCA1 mediates H
2
O
2
-induced activation of Ca
2+
channels in guard cells and is required for stomatal closure. Our findings help to identify how the perception of extracellular H
2
O
2
is integrated with responses to various external stresses and internal cues in plants, and have implications for the design of crops with enhanced fitness.
HPCA1, a member of a previously uncharacterized subfamily of leucine-rich-repeat receptor-like kinases, is the hydrogen-peroxide sensor at the plasma membrane in
Arabidopsis
.