Tomato MAPKKKε is a positive regulator of cell-death signaling networks associated with plant immunity

Mitogen-activated protein (MAP) kinase cascades are fundamental components of the signaling pathways associated with plant immunity. Despite the large number of MAP kinase kinase kinases (MAPKKK) encoded in the plant genome, only very few of them have an assigned function. Here, we identified MAPKKK gene of tomato (Solanum lycopersicum), SIMAPKKK epsilon, which is required for hypersensitive response cell death and disease resistance against Gram-negative bacterial pathogens. Silencing of SIMAPKKK epsilon compromised tomato resistance to Xanthomonas campestris and Pseudomonas syringae strains, resulting in the appearance of disease symptoms and enhanced bacterial growth. In addition, silencing of NbMAPKKK epsilon in Nicotiana benthamiana plants significantly inhibited the cell death triggered by expression of different R gene/effector gene pairs. Conversely, overexpression of either the full-length SIMAPKKK epsilon gene or its kinase domain in N. benthamiana leaves caused pathogen-independent activation of cell death that required an intact kinase catalytic domain. Moreover, by suppressing the expression of various MAPKK and MAPK genes and overexpressing the SIMAPKKK epsilon kinase domain, we identified a signaling cascade acting downstream of SIMAPKKK epsilon that includes MEK2, WIPK and SIPK. Additional epistasis experiments revealed that SIPKK functions as a negative regulator of SIMAPKKK epsilon-mediated cell death. Our results provide evidence that SIMAPKKK epsilon is a signaling molecule that positively regulates cell death networks associated with plant immunity.