Human pathogenic bacterium Shigella infects Arabidopsis plants using type-III effectors that suppress conserved MAP kinase signaling

Abstract

Originality-significance statementIncreased incidence of food-borne disease outbreaks caused by fresh produce contaminated with Escherichia coli O157:H7 and Salmonella spp. has prompted researchers to examine the interaction between these bacteria and various plant species under different environmental conditions. Although studies show that human enteropathogenic bacteria survive both on the surface of and inside plants, little is known about the molecular mechanism underlying plant invasion and colonization. Here, we examined the interaction between the human pathogenic bacterium Shigella and the model plant Arabidopsis. We found that four Shigella spp. strains proliferated successfully in Arabidopsis, causing symptom-like lesions in tissues. Using mutants lacking T3S effectors (i.e., noninvasive human strains), we demonstrated that effectors regulating pathogenesis of shigellosis in humans also play a central role in bacteria-plant interactions. To the best of our knowledge, this is the first study to examine Shigella-mediated virulence and host immune suppression in a plant host at a molecular level.\n\nSummaryAlthough there is debate about whether human intestinal pathogenic bacteria are also plant pathogens, it is clear that these bacteria use plants as an alternative host. Shigella, which infects primates, is reportedly transmitted by fresh vegetables; however, its molecular interactions with plants have not been extensively studied. Here, we show that four Shigella strains, S. boydii (S. b), S. sonnei (S. s), S. flexneri 2a (S. f 2a), and S. flexneri 5a (S. f 5a), proliferated at different levels in Arabidopsis thaliana. Microscopic studies revealed that these bacteria were present inside leaves and damaged plant cells. GFP-labeled S. b, S. f 2a, and S. f 5a entered plants via guard cells, and S. f 2a infiltrated root tissues and colonized roots. Using mutants lacking type III secretion systems (T3SS), we found that T3SS of Shigella that regulate the pathogenesis of shigellosis in humans also play a central role in attachment and multiplication in Arabidopsis. Furthermore, the immunosuppressive activity of two T3S effectors, OspF and OspG, were needed for the proliferation of Shigella in Arabidopsis. These findings demonstrate that Shigella-mediated virulence determinants are expressed, and pathogenic symptoms are observed, in model plants.