ZmMYB56 Regulates Stomatal Closure and Drought Tolerance in Maize Seedlings by Regulating ZmTOM7 Expression at the Transcriptional Level

The growth and yield of important crops such as maize are seriously threatened by drought stress. Closing stomata, reducing water dissipation and improving water use efficiency are important aspects of plant response to drought. In the present study, the MYB-like transcription factor ZmMYB56, specifically expressed in maize guard cells, was identified to play important roles in regulating stomatal closure and drought tolerance. Mutations in ZmMYB56 caused elevated stomatal conductance, rapid water loss in isolated leaves and severe drought sensitivity in plants. ZmMYB56 has transcriptional activation activity and is specifically expressed in stomatal guard cells. As a R2R3 transcription factor, ZmMYB56 can bind the cis-acting element on the ZmTOM7 promoter sequence thereby activating its expression. Correspondingly, the ZmTOM7 transcript level is severely downregulated in Zmmyb56 seedlings. Transgenic Arabidopsis plants overexpressing ZmTOM7 exhibits lower stomatal conductance and elevated drought tolerance, whereas the ZmTOM7 mutation is associated with higher stomatal conductance and severe drought sensitivity in maize seedlings. Based on these findings, we conclude that ZmTOM7 acts as one of the key target genes of ZmMYB56 and is involved in ZmMYB56-regulated stomatal closure and drought stress tolerance in maize. Our findings concerning the functional mechanisms of maize ZmMYB56 transcription factors in stomatal closure and drought stress provide a potential genetic resource for improving drought resistance of maize.