STRESS SIGNALING IN PLANTS - A MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY IS ACTIVATED BY COLD AND DROUGHT

Yeast and animals use mitogen-activated protein (MAP) kinase cascades to mediate stress and extracellular signals, We have tested whether MA P kinases are involved in mediating environmental stress responses in plants, Using specific peptide antibodies that were raised against dif ferent alfalfa MAP kinases, we found exclusive activation of p44(MMK4) kinase in drought- and cold-treated plants. p44(MMK4) kinase was tran siently activated by these treatments and was correlated with a shift in the electrophoretic mobility of the p44(MMK4) protein, Although tra nscript levels of the MMK4 gene accumulated after drought and cold tre atment, no changes in p44(MMK4) steady state protein levels were obser ved, indicating a posttranslational activation mechanism, Extreme temp eratures, drought, and salt stress are considered to be different form s of osmotic stress, However, high salt concentrations or heat shock d id not induce activation of p44(MMK4), indicating the existence of dis tinct mechanisms to mediate different stresses in alfalfa, Stress adap tation in plants is mediated by abscisic acid (ABA)-dependent and ABA- independent processes, Although ABA rapidly induced the transcription of an ABA-inducible marker gene, MMK4 transcript levels did not increa se and p44(MMK4) kinase was not activated, These data indicate that th e MMK4 kinase pathway mediates drought and cold signaling independentl y of ABA.