Mitogen-activated protein kinases are carbon dioxide receptors in plants

Abstract The amount of CO 2 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 CO 2 concentrations ([CO 2 ] high ): reducing human emissions or increasing their absorption by oceans and plants. However, in response to [CO 2 ] high , plants diminish gas exchange and CO 2 uptake by closing stomata. Surprisingly, we do not know how plants sense CO 2 in their environment, and the basic mechanisms of the plant response to [CO 2 ] high are very poorly understood. Here, we show that mitogen-activated protein kinases (MAPKs) are plant CO 2 receptors. We demonstrate that MPK4, a prominent MAPK that is known to be involved in the stomatal response to [CO 2 ] high 1–3 , is capable of binding CO 2 and is directly activated by a very low increase in [CO 2 ] in vivo and in vitro . Unlike MPK4 activation by infections 4 , stress and hormones within known MAPK signalling cascades, [CO 2 ] 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 CO 2 receptor sheds new light on the integration of various environmental signals in guard cells, setting up MPK4 as the main hub regulating CO 2 availability for photosynthesis. This result could help to find new ways to increase CO 2 uptake by plants.