Stomatal CO2/bicarbonate sensor consists of two interacting protein kinases, Raf-like HT1 and non-kinase-activity activity requiring MPK12/MPK4

The continuing rise in the atmospheric carbon dioxide (CO2) concentration causes stomatal closing, thus criti-cally affecting transpirational water loss, photosynthesis, and plant growth. However, the primary CO2 sensor remains unknown. Here, we show that elevated CO2 triggers interaction of the MAP kinases MPK4/MPK12 with the HT1 protein kinase, thus inhibiting HT1 kinase activity. At low CO2, HT1 phosphorylates and activates the downstream negatively regulating CBC1 kinase. Physiologically relevant HT1-mediated phosphorylation sites in CBC1 are identified. In a genetic screen, we identify dominant active HT1 mutants that cause insensitivity to elevated CO2. Dominant HT1 mutants abrogate the CO2/bicarbonate-induced MPK4/12-HT1 interaction and HT1 inhibition, which may be explained by a structural AlphaFold2-and Gaussian-accelerated dynamics -gener-ated model. Unexpectedly, MAP kinase activity is not required for CO2 sensor function and CO2-triggered HT1 inhibition and stomatal closing. The presented findings reveal that MPK4/12 and HT1 together constitute the long-sought primary stomatal CO2/bicarbonate sensor upstream of the CBC1 kinase in plants.

Automated summary

The rise in atmospheric CO2 concentration affects stomatal closing in plants, impacting transpirational water loss, photosynthesis, and growth. This study identifies MPK4/12 and HT1 as the primary CO2 sensors in plants, located upstream of the CBC1 kinase. These findings are significant for understanding the plant response to CO2.