Interaction between 2,4-Diacetylphloroglucinol- and Hydrogen Cyanide-Producing Pseudomonas brassicacearum LBUM300 and Clavibacter michiganensis subsp michiganensis in the Tomato Rhizosphere

We have previously demonstrated that inoculation of tomato plants with 2,4-diacetylphloroglucinol (DAPG)-and hydrogen cyanide (HCN)-producing Pseudomonas brassicacearum LBUM300 could significantly reduce bacterial canker symptoms caused by Clavibacter michiganensis subsp. michiganensis. In this study, in order to better characterize the population dynamics of LBUM300 in the rhizosphere of tomato plants, we characterized the role played by DAPG and HCN production by LBUM300 on rhizosphere colonization of healthy and C. michiganensis subsp. michiganensis-infected tomato plants. The impact of C. michiganensis subsp. michiganensis presence on the expression of DAPG and HCN biosynthetic genes in the rhizosphere was also examined. In planta assays were performed using combinations of C. michiganensis subsp. michiganensis and wild-type LBUM300 or DAPG (LBUM300.phlD) or HCN (LBUM300.hcnC) isogenic mutant strains. Populations of LBUM300 and phlD and hcnC gene expression levels were quantified in rhizosphere soil at several time points up to 264 h postinoculation using culture-independent quantitative PCR (qPCR) and reverse transcriptase quantitative PCR (RT-qPCR) TaqMan assays, respectively. The presence of C. michiganensis subsp. michiganensis significantly increased rhizospheric populations of LBUM300. In C. michiganensis subsp. michiganensis-infected tomato rhizospheres, the populations of wild-type LBUM300 and strain LBUM300.hcnC, both producing DAPG, were significantly higher than the population of strain LBUM300.phlD. A significant upregulation of phlD expression was observed in the presence of C. michiganensis subsp. michiganensis, while hcnC expression was only slightly increased in the mutant strain LBUM300.phlD when C. michiganensis subsp. michiganensis was present. Additionally, biofilm production was found to be significantly reduced in strain LBUM300.phlD compared to the wild-type and LBUM300.hcnC strains. IMPORTANCE The results of this study suggest that C. michiganensis subsp. michiganensis infection of tomato plants contributes to increasing rhizospheric populations of LBUM300, a biocontrol agent, as well as the overexpression of the DAPG biosynthetic operon in this bacterium. The increasing rhizospheric populations of LBUM300 represent one of the key factors in controlling C. michiganensis subsp. michiganensis in tomato plants, as DAPG-producing bacteria have shown the ability to decrease bacterial canker symptoms in tomato plants.