OMTK1, a novel MAPKKK, channels oxidative stress signaling through direct MAPK interaction

In common with other eukaryotes, plants utilize mitogen-activated protein kinase (MAPK) cascades to mediate responses to a wide variety of stimuli. In contrast to other eukaryotes, plants have an unusually large number of MAPK components, such as more than 20 MAPKs, 10 MAPK kinases (MAPKKs), and 60 MAPKK kinases (MAPKKKs) in Arabidopsis (MAPK Group (2002) Trends Plant Sci. 7, 301-308). Presently it is mostly unknown how MAPK signaling specificity is generated in plants. Here we have isolated OMTK1 (oxidative stress-activated MAP triple-kinase 1), a novel MAPKKK from alfalfa (Medicago sativa). In plant protoplasts, OMTK1-showed basal kinase activity and was found to induce cell death. Among a panel of hormones and stresses tested, only H₂O ₂ was found to activate OMTK1. Out of four MAPKs, OMTK1 specifically activated MMK3 resulting in an increased cell death rate. Pull-down analysis between recombinant proteins indicated that OMTK1 directly interacts with MMK3 and that OMTK1 and MMK3 are part of a protein complex in vivo. These results indicate that OMTK1 plays a MAPK scaffolding role and functions in activation of H₂O₂-induced cell death in plants.