Bacteria from the citrus phylloplane can disrupt cell-cell signalling in Xanthomonas citri and reduce citrus canker disease severity
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Xanthomonas citri subsp. citri (Xcc) is the causal agent of citrus canker, a disease that affects almost all types of citrus crops. Production of particular Xcc pathogenicity factors is controlled by a gene cluster rpf, which encodes elements of a cell-cell communication system called quorum sensing (QS), mediated by molecules of the diffusible signal factor (DSF) family. Interference with cell-cell signalling, also termed quorum quenching, either by signal degradation or over-production, has been suggested as a strategy to control bacterial disease. In this study, three bacterial strains were isolated from citrus leaves that displayed the ability to disrupt QS signalling in Xcc. Pathogenicity assays in sweet orange (Citrus sinensis) showed that bacteria of the genera Pseudomonas and Bacillus also have a strong ability to reduce the severity of citrus canker disease. These effects were associated with alteration in bacterial attachment and biofilm formation, factors that are known to contribute to Xcc virulence. These quorum-quenching bacteria may represent a highly valuable tool in the process of biological control and offer an alternative to the traditional copper treatment currently used to treat citrus canker disease.