Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2010
Publication Date: 1/1/2011
Citation: Mavrodi, D.V., Joe, A., Mavrodi, O., Hassan, K.A., Weller, D.M., Paulsen, I.T., Loper, J.E., Alfano, J.R., Thomashow, L.S. 2011. Structural and functional analysis of the type III secretion system from Pseudomonas fluorescens Q8r1-96. Journal of Bacteriology. 193(1):177-189. Interpretive Summary: Pseudomonas fluorescens Q8r1-96 is a beneficial strain typical of wheat root-associated bacteria that are responsible for the natural control the cereal disease known as take-all. Strain Q8r1-96 produces the natural antibiotic 2,4-diacetylphloroglucinol and is an excellent colonizer of the roots of cereal crops. In a search for genes that contribute to the beneficial properties of strain Q8r1-96, we analyzed its DNA and identified a type III secretion system (T3SS) gene cluster similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. The T3SS is a syringe-like mechanism used by plant- and animal-associated bacteria to deliver protein molecules called effectors into the cells of their hosts. We also identified similar T3SS genes in 29 of 30 strains closely related to Q8r1-96, indicating that T3SS are widely distributed among root-associated bacteria. The Q8r1-96 genome contained three effector genes named ropAA, ropM, and ropB. These effector genes encoded proteins that were secreted in culture media and were recognized and injected into plant cells by the T3SS of both P. syringae and Q8r1-96. The Q8r1-96 T3SS genes were expressed by bacteria on roots, but mutants lacking a functional T3SS were not altered in their behavior on roots. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing two plant immune responses but this suppression was not associated with the production of disease symptoms. The results of this study show that the T3SS of Q8r1-96 is functional, but that it is unlikely to have a major role in interactions between this bacterial strain and its plant host.
Technical Abstract: Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a T3SS gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1-1 and hopM1, as well as a novel type III effector gene designated ropB. These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.