Visualizing bacterial quorum sensing and quorum quenching interaction on Medicago roots

Abstract Background Defining interactions of bacteria in the rhizosphere (area near and on the plant root) is important to understand how they affect plant health. Some rhizosphere bacteria, including plant growth promoting rhizobacteria (PGPR) engage in the intraspecies communication known as quorum sensing (QS). Many use the extracellular autoinducer signal molecule N-acyl homoserine lactone (AHL) and other rhizobacteria, including PGPR, can interfere or disrupt QS through quorum quenching (QQ). Traditional AHL biosensor assays used for screening and identifying QS and QQ bacteria interactions fail to account for the role of the plant root. Methods Medicago spp. seedlings germinated on Lullien’s agar are transferred to soft-agar plates containing the broad-range AHL biosensor Agrobacterium tumefaciens KYC55 and X-gal substrate. Cultures of QS and QQ bacteria as well as pure AHLs and QQ protein are applied to the plant roots and incubated for 3 days. Results We show that this expanded use of an AHL biosensor successfully allowed for visualization of QS/QQ interactions localized at the plant root. KYC55 detected pure AHLs as well as AHLs from live bacteria cultures grown directly on the media. We also showed clear detection of QQ interactions occurring in the presence of the plant root. Conclusions Our novel tri-trophic system using an AHL biosensor is useful to study QS interspecies interactions in the rhizosphere.