MAPKKKα is a positive regulator of cell death associated with both plant immunity and disease

Many plant pathogens cause disease symptoms that manifest over days as regions of localized cell death. Localized cell death (the hypersensitive response; HR) also occurs in disease‐resistant plants, but this response appears within hours of attempted infection and may restrict further pathogen growth. We identified a MAP kinase kinase kinase gene ( MAPKKK α) that is required for the HR and resistance against Pseudomonas syringae . Significantly, we found that MAPKKK α also regulates cell death in susceptible leaves undergoing P. syringae infection. Overexpression of MAPKKK α in leaves activated MAPKs and caused pathogen‐independent cell death. By overexpressing MAPKKK α in leaves and suppressing expression of various MAPKK and MAPK genes by virus‐induced gene silencing, we identified two distinct MAPK cascades that act downstream of MAPKKK α. These results demonstrate that signal transduction pathways associated with both plant immunity and disease susceptibility share a common molecular switch.