Mixed Infection in Tomato Plants by Dominant Bacterial Spot Pathogen and the Co-occurring Bacterial Species

Abstract

Pathogen dynamics can be altered by co-occurring pathogenic or opportunistic entities inhabiting the same host species. Mixed infection or co-infection caused by multiple pathogenic strains are common in nature. However, traditional plant pathogen studies have always focused on the binary interaction between single host and single pathogen. In this study, I have looked beyond the binary interaction between single host and single pathogen and observed the effect of co-occurring bacterial species on the pathogenesis of dominant pathogen. Xanthomonas arboricola and various Pseudomonas species co-occur with the bacterial leaf spot pathogen Xanthomonas perforans. Altered disease severity was observed when tomato plants were co-inoculated with these three bacterial species. In-planta population of each of these bacterial species revealed that the dominant pathogen X. perforans has the highest population when it was present alone, but its population decreases in presence of the co-occurring bacterial species. Presence of X. perforans always induced the in-planta population of X. arboricola and the Pseudomonas species. PAMP triggered immunity (PTI) assay revealed the inability of the Pseudomonas sp. to suppress PTI. Genome analysis confirmed the lack of effector molecules in the Pseudomonas species. The principal cell wall degrading enzymes were not detected in the Pseudomonas species. Similar nutritional profile of the three bacterial species indicated niche overlap and potential competition for resources which explained the decrease in X. perforans population. In mixed infection the Pseudomonas species and the X. arboricola were observed to colonize the apoplast in the later stage of the infection after X. perforans already colonized it. Our study revealed that mixed infection with the weak taxa changed the population dynamics of the dominant taxa whereas the weak bacterial species could exploit the dominant pathogen to colonize the host plant.