The mitogen-activated protein kinase cascade MKK1-MPK4 mediates salt signaling in rice

Mitogen-activated protein kinase (MAPK) pathways have been implicated in signal transduction of both biotic and abiotic stresses in plants. In this study, we found that the transcript of a rice (Oryza sativa) MAPKK (OsMKK1) was markedly increased by salt stress. By examining the survival rate and Na(+) content in shoot, we found that OsMKK1-knockout (osmkk1) mutant was more sensitive to salt stress than the wild type. OsMKK1 activity in the wild-type seedlings and protoplasts was increased by salt stress. Yeast two-hybrid and in vitro and in vivo kinase assays revealed that OsMKK1 targeted OsMPK4. OsMPK4 activity was increased by salt, which was impaired in osmkk1 plants. In contrast, overexpression of OsMKK1 increased OsMPK4 activity in protoplasts. By comparing the transcription factors levels between WT and osmkk1 mutant, OsMKK1 was necessary for salt-induced increase in OsDREB2B and OsMYBS3. Taken together, the data suggest that OsMKK1 and OsMPK4 constitute a signaling pathway that regulates salt stress tolerance in rice.