Mitogen-activated protein kinases are carbon dioxide receptors in plants.

The amount of CO2 in the atmosphere is increasing continuously in the industrial era, posing a threat to the ecological balance on Earth. There are two ways to reduce elevated CO2 concentrations ([CO2]high): reducing human emissions or increasing their absorption by oceans and plants. However, in response to [CO2]high, plants diminish gas exchange and CO2 uptake by closing stomata. Surprisingly, we do not know how plants sense CO2 in their environment, and the basic mechanisms of the plant response to [CO2]high are very poorly understood. Here, we show that mitogen-activated protein kinases (MAPKs) are plant CO2 receptors. We demonstrate that MPK4, a prominent MAPK that is known to be involved in the stomatal response to [CO2]high1-3, is capable of binding CO2 and is directly activated by a very low increase in [CO2] in vivo and in vitro. Unlike MPK4 activation by infections4, stress and hormones within known MAPK signalling cascades, [CO2]high-induced MPK4 activation is independent of the upstream regulators MKK1 and MKK2. Moreover, once activated, MPK4 is prone to inactivation by bicarbonate. The identification of stress-responsive MPK4 as a CO2 receptor sheds new light on the integration of various environmental signals in guard cells, setting up MPK4 as the main hub regulating CO2 availability for photosynthesis. This result could help to find new ways to increase CO2 uptake by plants.