Cold acclimation is the major process that prepares plants for freezing tolerance. In addition to extensive transcription regulation by coldinducible master transcription factors, oxidative stress signaling has been postulated to play a role in freezing tolerance. Activation of oxidative signaling through the expression of an active mitogenactivated protein kinase kinase kinase provided benefits in transgenic tobacco at freezing temperature bypassing cold acclimation. Because involvement of the mitogen-activated protein kinase cascade in oxidative stress signaling is evolutionarily conserved in eukaryotes from yeast to mammals, we tested the effect of expressing a heterologous tobacco mitogen-activated protein kinase kinase kinase (Nicotiana PK1), which can mimic H 2O2 signaling, in a major cereal crop. We demonstrate that low-level but constitutive expression of the Nicotiana PK1 gene enhances freezing tolerance in transgenic maize plants that are normally frost sensitive. Our results suggest that a new molecular approach can be designed to genetically enhance freezing tolerance in important crops.Nicotiana protein kinase ͉ stress tolerance ͉ Zea mays

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