Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2011
Publication Date: 7/1/2011
Citation: Subramoni, S., Gonzalez, J.F., Johnson, A., Pechy-Tarr, M., Rochat, L., Paulsen, I., Loper, J.E., Keel, C., Venturi, V. 2011. Bacterial subfamily of LuxR regulators that respond to plant compounds. Applied and Environmental Microbiology. 77(13):4579-4588. Interpretive Summary: Biological control is a promising and environmentally-friendly approach for the management of plant diseases in the future. Certain strains of Pseudomonas fluorescens live on root surfaces and protect roots from infection by plant pathogens that live in the soil. The Pseudomonas bacteria do this by producing antibiotics that kill the fungal or bacterial plant pathogens. Factors that influence the production of these antibiotics also influence the success of biological control. In this study, we identified a new type of regulator that is present in P. fluorescens and influences antibiotic production. This regulator receives a chemical signal from plant roots and transmits it within the bacterial cell to control the expression of many genes including the genes for antibiotic biosynthesis. We made a derivative strain of P. fluorescens that is deficient in this regulator and showed that it no longer functions as well in suppressing plant diseases. These results are important because they have uncovered a new class of regulators in bacteria that respond to signals from plants. Also, the results highlight a new factor influencing biological control of plant disease, which opens the door to future studies aiming to improve biological control for use in agriculture.
Technical Abstract: Certain strains of Pseudomonas fluorescens inhabit the rhizosphere where they can suppress plant diseases caused by soilborne pathogens. The expression of genes coding for the biosynthesis of antibiotics is crucial to the biological control properties of these bacteria, but factors influencing the expression of these genes are poorly understood. Here we describe the identification and functional characterization of PsoR, a LuxR solo or orphan regulator of P. fluorescens. We show that PsoR protein is solubilized only in the presence of macerated rice and wheat but not of macerated cucumber, indicating the presence of a plant molecule(s) that binds to PsoR. This observation was substantiated by the regulation of the expression of a lux box-containing promoter in a PsoR-dependent manner in the presence of rice and wheat extract, but not of cucumber extract. Comparison of gene expression profiles of a P. fluorescens wild-type and a PsoR-overexpressing (PsoR+) strain revealed that several genes involved in plant colonization and inhibition of plant pathogens were differentially expressed. These include genes coding for chitinase and iron metabolism, as well as genes encoding several regulatory and structural proteins required for the biosynthesis of secondary metabolites with anti-fungal activity. In biocontrol assays, a psoR mutant was significantly reduced for its ability to protect wheat plants from root rot and damping-off caused by Pythium ultimum infection. Overall, these results indicate that PsoR senses a molecule(s) in wheat or rice extracts and modulates expression of its target genes contributing to its role in biocontrol activity. PsoR and several closely related proteins form a sub-family of LuxR family regulators present in plant-associated bacteria.