The dark side of the salad: Salmonella typhimurium overcomes the innate immune response of Arabidopsis thaliana and shows an endopathogenic lifestyle.

Salmonella enterica serovar typhimurium contaminated vegetables and fruits are considerable sources of human infections. Bacteria present in raw plant-derived nutrients cause salmonellosis, the world wide most spread food poisoning. This facultative endopathogen enters and replicates in host cells and actively suppresses host immune responses. Although Salmonella survives on plants, the underlying bacterial infection mechanisms are only poorly understood. In this report we investigated the possibility to use Arabidopsis thaliana as a genetically tractable host system to study Salmonella-plant interactions. Using green fluorescent protein (GFP) marked bacteria, we show here that Salmonella can infect various Arabidopsis tissues and proliferate in intracellular cellular compartments. Salmonella infection of Arabidopsis cells can occur via intact shoot or root tissues resulting in wilting, chlorosis and eventually death of the infected organs. Arabidopsis reacts to Salmonella by inducing the activation of mitogen-activated protein kinase (MAPK) cascades and enhanced expression of pathogenesis related (PR) genes. The induction of defense responses fails in plants that are compromised in ethylene or jasmonic acid signaling or in the MKK3-MPK6 MAPK pathway. These findings demonstrate that Arabidopsis represents a true host system for Salmonella, offering unique possibilities to study the interaction of this human pathogen with plants at the molecular level for developing novel drug targets and addressing current safety issues in human nutrition.