1 Plant Pathology, Department of Plant Biology, Faculty of Life Sciences, Københavns Universitet2 Faculty of Science Administration, Centre m.v., Faculty of Science, Københavns Universitet3 Freshwater Biology, Department of Biology, Faculty of Science, Københavns Universitet4 Functional Genomics, Department of Biology, Faculty of Science, Københavns Universitet5 unknown6 Copenhagen University7 Section for Transport Biology, Department of Plant and Environmental Sciences, Faculty of Science, Københavns Universitet8 University of California San Francisco9 Max-Planck Institute for Plant Breeding Research10 Freshwater Biology, Department of Biology, Faculty of Science, Københavns Universitet11 Functional Genomics, Department of Biology, Faculty of Science, Københavns Universitet12 Section for Transport Biology, Department of Plant and Environmental Sciences, Faculty of Science, Københavns Universitet
Arabidopsis MPK4 has been implicated in plant defense regulation because mpk4 knockout plants exhibit constitutive activation of salicylic acid (SA)-dependent defenses, but fail to induce jasmonic acid (JA) defense marker genes in response to JA. We show here that mpk4 mutants are also defective in defense gene induction in response to ethylene (ET), and that they are more susceptible than wild-type (WT) to Alternaria brassicicola that induces the ET/JA defense pathway(s). Both SA-repressing and ET/JA-(co)activating functions depend on MPK4 kinase activity and involve the defense regulators EDS1 and PAD4, as mutations in these genes suppress de-repression of the SA pathway and suppress the block of the ET/JA pathway in mpk4. EDS1/PAD4 thus affect SA-ET/JA signal antagonism as activators of SA but as repressors of ET/JA defenses, and MPK4 negatively regulates both of these functions. We also show that the MPK4-EDS1/PAD4 branch of ET defense signaling is independent of the ERF1 transcription factor, and use comparative microarray analysis of ctr1, ctr1/mpk4, mpk4 and WT to show that MPK4 is required for induction of a small subset of ET-regulated genes. The regulation of some, but not all, of these genes involves EDS1 and PAD4.