Liina Jakobson, Lauri Vaahtera, Kadri Tõldsepp, Maris Nuhkat, Cun Wang, Yuh-Shuh Wang, Hanna Hõrak, Ervin Valk, Priit Pechter, Yana Sindarovska, Jing Tang, Chuanlei Xiao, Yang Xu, Ulvi Gerst Talas, Alfonso T. García-Sosa, Saijaliisa Kangasjärvi, Uko Maran, Maido Remm, M. Rob G. Roelfsema, Honghong Hu, Jaakko Kangasjärvi, Mart Loog, Julian I. Schroeder, Hannes Kollist, Mikael Brosché

• Plant gas exchange is regulated by guard cells that form stomatal pores. Stomatal adjustments are crucial for plant survival; they regulate uptake of CO\(_{2}\) for photosynthesis, loss of water, and entrance of air pollutants such as ozone. We mapped ozone hypersensitivity, more open stomata, and stomatal CO\(_{2}\)-insensitivity phenotypes of the Arabidopsis thaliana accession Cvi-0 to a single amino acid substitution in MITOGEN-ACTIVATED PROTEIN (MAP) KINASE 12 (MPK12). In parallel, we showed that stomatal CO\(_{2}\)-insensitivity phenotypesPlant gas exchange is regulated by guard cells that form stomatal pores. Stomatal adjustments are crucial for plant survival; they regulate uptake of CO\(_{2}\) for photosynthesis, loss of water, and entrance of air pollutants such as ozone. We mapped ozone hypersensitivity, more open stomata, and stomatal CO\(_{2}\)-insensitivity phenotypes of the Arabidopsis thaliana accession Cvi-0 to a single amino acid substitution in MITOGEN-ACTIVATED PROTEIN (MAP) KINASE 12 (MPK12). In parallel, we showed that stomatal CO\(_{2}\)-insensitivity phenotypes of a mutant cis (CO\(_{2}\)-insensitive) were caused by a deletion of MPK12. Lack of MPK12 impaired bicarbonate-induced activation of S-type anion channels. We demonstrated that MPK12 interacted with the protein kinase HIGH LEAF TEMPERATURE 1 (HT1)—a central node in guard cell CO\(_{2}\) signaling—and that MPK12 functions as an inhibitor of HT1. These data provide a new function for plant MPKs as protein kinase inhibitors and suggest a mechanism through which guard cell CO\(_{2}\) signaling controls plant water management.…