Exploring the Role of the HT1-MPK12 Module in Guard Cell CO2 Sensing

Abstract

To optimise the balance between CO₂ uptake for photosynthesis and loss of water by transpiration, plant stomatal pores regulate their aperture in response to changes in CO₂ concentration. The HT1–MPK12/4 complexes have been implicated as key CO₂ sensors in stomatal guard cells, but the molecular basis of MPK12 and MPK4 function in CO₂ signalling, as well as the exact CO₂-sensing mechanism, remain unclear. MPK12 is specific to the guard cells, whereas MPK4 plays diverse roles in many cell types. Interestingly, MPK11, as a close homolog of MPK4 and MPK12, is not involved in stomatal regulation by CO₂. This thesis aimed to determine which regions of MPK12 are required for its CO₂-specific function and whether the HT1–MPK12 module can directly sense CO₂ via lysine carbamylation. We constructed MPK12/MPK11 chimeras and MPK12 truncation mutants and tested their ability to bind HT1 and restore normal CO₂-induced stomatal responses in plant lines in which native MPK12 was deleted. We also identified potential CO₂-binding lysines on MPK12 and HT1 and evaluated their functional importance in planta. Chimeric and truncation analyses revealed key features of MPK12 domains: the C-terminal lobe is essential for interaction with HT1, while the N-lobe is required for a wild-type CO₂ response. Several amino acids in the N-lobes of MPK12 and MPK4 were proposed to contribute to CO₂ signalling and HT1 inhibition, warranting further investigation. Preventing potential carbamylation on selected lysine residues in MPK12 and HT1 did not abolish stomatal CO₂ responses. These findings suggest that non covalent HCO₃⁻ binding, rather than lysine carbamylation, serves as the primary CO₂-sensing mechanism within the HT1–MPK12/4 module.