ZmMKK1 phosphorylates ZmCAT2 to regulate H₂O₂ homeostasis and improve heat stress tolerance in maize

High-temperature stress severely impairs plant growth, compromising crop yield and quality. The mitogen-activated protein kinase (MAPK) cascade (MAPKKK-MAPKK-MAPK) is a highly conserved signaling module involved in plant responses to diverse biotic and abiotic stresses. As the central component, MAPKK plays a critical role in signal transduction. However, its role in heat stress and underlying mechanisms remains poorly understood. This study establishes ZmMKK1 as a crucial regulator of thermotolerance in maize. ZmMKK1 knockout mutants exhibited reduced catalase activity, accumulated higher levels of H₂O₂, and displayed hypersensitivity to heat stress. Conversely, ZmMKK1 overexpression significantly enhanced thermotolerance, confirming its positive function in heat defense. Further analysis showed that ZmMKK1 physically interacts with catalase ZmCAT2 and phosphorylates it at Thr352 and Thr421. The phosphomimetic variant ZmCAT2^T352,421D displayed enhanced catalase activity in vitro and in vivo, and rescued the thermosensitive phenotype of zmmkk1 mutants. These results indicate that ZmMKK1-mediated thermotolerance depends on ZmCAT2 phosphorylation. Our findings reveal that ZmMKK1 activates ZmCAT2 via phosphorylation independently of the canonical MAPK cascade, facilitating reactive oxygen species scavenging and thermotolerance. Together, these results define a non-canonical mechanism whereby a MAPKK directly phosphorylates a non-MAPK substrate to confer thermotolerance, offering a new strategy for improving crop stress resistance.