The Transcriptional Coactivator CmMBF1c is Required for Waterlogging Tolerance in Chrysanthemum Morifolium

Waterlogging is one of the most serious abiotic stressors affecting Chrysanthemum morifolium during its lifespan. However, the molecular mechanisms underlying the waterlogging tolerance of chrysanthemum remain unclear. In this study, we discovered that the transcriptional coactivator MULTIPROTEIN BRIDGING FACTOR 1c (CmMBF1c) was significantly induced by waterlogging stress in chrysanthemums. Promoter sequence analysis and transient dual-luciferase assay using chrysanthemum protoplasts showed that the waterlogging-tolerant cultivar 'Nannongxuefeng' carried more response elements involved in waterlogging and hypoxia stress compared with the waterlogging-sensitive cultivar 'Qinglu', conferring on 'Nannongxuefeng' a stronger hypoxia responsive activity and higher CmMBF1c expression under waterlogging conditions. Subcellular localization and transcriptional activity assays showed that CmMBF1c protein was localized to the nucleus and had no transcriptional activation activity. Overexpression of CmMBF1c in 'Qinglu' enhanced its waterlogging tolerance by promoting its reactive oxygen species (ROS) scavenging ability and maintaining low ROS levels. However, RNAi-mediated knockdown of CmMBF1c in cultivar 'Nannongxuefeng' resulted in the opposite tendency. Yeast two-hybrid screening and tobacco bimolecular fluorescence complementation assays revealed that CmHRE2, a pivotal regulator of hypoxia response, could interact with CmMBF1c. In summary, this study demonstrates that CmMBF1c improves chrysanthemum waterlogging tolerance by regulating its ROS signaling pathway and interacting with CmHRE2. These findings together offer, to our knowledge, new mechanistic insights into chrysanthemum waterlogging tolerance and provide a rational foundation for future research on the genetic improvement of horticultural crops for waterlogging stress tolerance.