R-tailocin Production and Biological Function in the Plant Growth Promoting Rhizobacteria, Pseudomonas chlororaphis
Abstract
R-tailocins are bacteriocins resembling contractile bacteriophage tails. Recent studies identified R-tailocin gene clusters within several plant growth promoting (PGPR) Pseudomonas genomes, but the ecological significance of these R-tailocins remains unknown. In this study, we analyzed sequenced PGPR Pseudomonas genomes and found that the R-tailocin gene cluster within most P. chlororaphis and some P. fluorescens strains contain two R-tailocin genetic modules. Phylogenetic analysis revealed that these genetic modules may have different ancestral origins. Beyond genomic analysis, we demonstrate that the PGPR, P. chlororaphis 30-84, produces two distinct R-tailocin particles, each with a discrete killing spectrum. Further study found that P. chlororaphis 30-84 not only produces two distinct tailocin particles, but that one of them is produced with two different types of tail fibers that widen its killing spectrum. The killing spectrum of tailocin 1 is limited to Pseudomonas whereas tailocin 2 targets both Pseudomonas and Xanthomonas strains. The spectra of pseudomonads killed by the two R-tailocins differed, although a few Pseudomonas species were either killed by or insusceptible to both. Tailocin release was disrupted by deletion of the holin gene within the tailocin gene cluster, demonstrating that the lysis cassette is required for the release of both R-tailocins. The loss of functional tailocin production reduced the ability of P. chlororaphis 30-84 to compete with an R-tailocin-sensitive strain within biofilms and rhizosphere communities. The breadth of influence of the R-tailocins on native wheat rhizobacteria was examined by screening a collection of wheat rhizobacteria (484 strains) for susceptibility to either tailocin. Nearly 12% of the collection was targeted by one of the tailocins, the majority being susceptible to tailocin 2. Loss of production of one or both tailocins resulted in decreased P. chlororaphis 30- 84 persistence within the wheat rhizosphere when in competition with the native microflora, but not bulk soil. PGPR need to survive and persist within complex and dynamic rhizosphere microbial communities. This study demonstrates that P. chlororaphis possess a unique R-tailocin gene cluster and that R-tailocins serve as a competitive mechanism that enables P. chlororaphis persistence within the rhizopshere microbial community. R-tailocin production should be considered when vetting PGPR for inoculants.
Citation
Dorosky, Robert Justin (2018). R-tailocin Production and Biological Function in the Plant Growth Promoting Rhizobacteria, Pseudomonas chlororaphis. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /173497.