The Transcription Factor ZmMYB56 Regulates High CO 2 -Induced Stomatal Closure in Maize Seedlings by Regulating ZmHLT1 Expression.

Stomata are channels through which plants exchange H 2 O and CO 2 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 3 - MYB transcription factor, ZmMYB56, which regulates high CO 2 -induced stomatal closure in maize seedlings. ZmMYB56 is highly expressed in stomatal guard cells and is negatively regulated by CO 2 and HCO 3 - . Loss of ZmMYB56 function leads to insensitivity to high CO 2 . 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 2 - induced maize stomatal movement, and its absence causes a severe weakening of maize's response to ambient CO 2 . 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 2 and HCO 3 - . Taken together, these results show that ZmMYB56-regulated ZmHLT1 expression is important for high CO 2 -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 2 concentrations.