SlEpk1 appears to act upstream of MAPK signaling. Leaves of N. benthamiana plants silenced for Ec1, SlEpk1, MAPKKKα, MEK2 or SlPrf were syringe-infiltrated with Agrobacterium carrying transgenes for MAPKKKα, MAPKKKα KD-, MEK2DD or MEK2 WT. PCD percentage was calculated as described in the Materials and methods using six plants per construct. Asterisks indicate significant differences compared with Ec1-silenced plants using Fisher’s exact test (P <0.05). PCD, programmed cell death; MEK2DD, constitutive-active MAP kinase kinase 2; MEK2 WT, wild type MEK2; MAPKKKα KD-, MAPKKKα with an inactive kinase domain.