Roles of different forms of lipopolysaccharides in Ralstonia solanacearum pathogenesis

Lipopolysaccharides (LPS) are critical components for the fitness of most gram-negative bacteria. Ralstonia solanacearum causes a deadly wilting disease in many crops; however, the pathogenic roles of different forms of LPS and their pathways of biogenesis remain unknown. By screening for phage-resistant mutants of R. solanacearum Pss4, whose genome sequence is unavailable, mutants with various types of structural defects in LPS were isolated. Pathogenesis assays of the mutants revealed that production of rough LPS (R-LPS), which does not contain O-polysaccharides, was sufficient to cause necrosis on Nicotiana benthamiana and induce the hypersensitive response on N. tabacum. However, biosynthesis of smooth LPS (S-LPS), which contains O-polysaccharides, was required for bacterial proliferation at infection sites on N. benthamiana leaves and for proliferation and causing wilt on tomato. Complementation tests confirmed the involvement of the previously unidentified cluster RSc2201 to RSc2204 in the formation of R. solanacearum S-LPS. With these data and the availability of the annotated genomic sequence of strain GMI1000, certain loci involved in key steps of R. solanacearum LPS biosynthesis were identified. The strategy of this work could be useful for similar studies in other bacteria without available genome sequences.