The Transcription Factor ZmMYB56 Regulates High CO-Induced Stomatal Closure in Maize Seedlings by Regulating ZmHLT1 Expression.

Stomata are channels through which plants exchange HO and CO with the external environment. The regulation of stomatal movement has significant impacts on plant growth, development and stress adaptation. Here, we present a maize R2R MYB transcription factor, ZmMYB56, which regulates high CO-induced stomatal closure in maize seedlings. ZmMYB56 is highly expressed in stomatal guard cells and is negatively regulated by CO and HCO . Loss of ZmMYB56 function leads to insensitivity to high CO. As a transcription factor, ZmMYB56 binds to a cis-acting element in the ZmHLT1 promoter and regulates its expression. ZmHLT1 plays a key role in CO- induced maize stomatal movement, and its absence causes a severe weakening of maize's response to ambient CO. ZmHLT1 expression is negatively regulated by bicarbonate, which does not occur in Zmmyb56 mutants, highlighting the significance of this regulatory relationship in plant responses to CO and HCO . Taken together, these results show that ZmMYB56-regulated ZmHLT1 expression is important for high CO-induced stomatal closure in maize. Our findings provide insight into genetic pathways that could be manipulated to improve maize growth and stress tolerance, especially under increasing atmospheric CO concentrations.