Salmonella colonization activates the plant immune system and benefits from association with plant pathogenic bacteria.

Despite increasing incidences of human salmonellosis caused by consumption of contaminated vegetables, relatively little is known about how the plant immune system responds to and may inhibit Salmonella colonization. Here we show that Salmonella Typhimurium activates the plant immune system primarily due to its recognition of the flg22 region in Salmonella flagellin. Several previously identified plant genes that play a role in immunity were found to affect the host response to Salmonella. The Salmonella flg22 (Seflg22) peptide induced the immune response in leaves which effectively restricted the growth of Salmonella as well as the plant pathogenic bacterium, Pseudomonas syringae pv. tomato. Induction of immune responses by Seflg22 was dependent on the plant FLS2 receptor. Salmonella multiplied poorly on plant tissues similar to other bacteria which are non-pathogenic to plants. However, Salmonella populations increased significantly when co-inoculated with P. syringae pv. tomato but not when co-inoculated with a type III secretion system mutant of this pathogen. Our results suggest that Salmonella benefits from the immune-suppressing effects of plant pathogenic bacteria, and this growth enhancement may increase the risk of salmonellosis.