The biocontrol strain Pseudomonas putida IsoF, which was isolated from a tomato rhizosphere, is a known N-acyl-homoserine lactone (AHL) producer with only one LuxI/LuxR-like quorum-sensing (QS) system. The production and degradation of AHLs were analysed in different growth phases of the bacterium. Using the analytical tools of ultra performance liquid chromatography and high resolution MS, it was possible to determine not only the various AHLs synthesized over time but also their degradation products. 3-oxo-decanoyl-homoserine lactone was found to be the dominant AHL, which reached its maximum in the early logarithmic growth phase. Although the pH of the medium was neutral, the AHLs were degraded thereafter rapidly to the corresponding homoserines and other metabolites. The proposed lactonase gene of P. putida IsoF could not be identified, because it is apparently quite different from hitherto described lactonases. The analytical data were used to calculate the rates and thresholds of AHL production by mathematical modelling, allowing quantitative predictions and a further understanding of the QS-based regulations in this bacterium. This study, combining microbiological, chemical and mathematical approaches, suggests that AHL degradation is an integral part of the whole autoinducer circuit of P. putida IsoF.

Over the last few decades, the ability of rhizosphere bacteria to promote plant growth has been considered to be of scientific, ecological, and economic interest. The properties and mechanisms of interaction of these root-colonizing bacteria have been extensively investigated, and plant protection agents that are based on these bacterial strains have been developed for agricultural applications. In the present study, the root colonization of barley by Pseudomonas sp. DSMZ 13134, that is contained in the commercially available plant protection agent Proradix, was examined using the fluorescence in situ hybridization method with oligonucleotide probes and specific gfp-tagging of the inoculant strain in combination with confocal laser scanning microscopy. In the first phase of root colonization, the inoculant strain competed successfully with seed and soil-borne bacteria (including Pseudomonads) for the colonization of the rhizoplane. Pseudomonas sp. DSMZ 13134 could be detected in all parts of the roots, although it did not belong to the dominant members of the root-associated bacterial community. Gfp-tagged cells were localized particularly in the root hair zone, and high cell densities were apparent on the root hair surface. To investigate the impact of the application of Proradix on the structure of the dominant root-associated bacterial community of barley, T-RFLP analyses were performed. Only a transient community effect was found until 3 weeks post-application.

In this interdisciplinary approach, the dynamics of production and degradation of the quorum sensing signal 3-oxo-decanoylhomoserine lactone were studied for continuous cultures of Pseudomonas putida IsoF. The signal concentrations were quantified over time by use of monoclonal antibodies and ELISA. The results were verified by use of ultra-high-performance liquid chromatography. By use of a mathematical model we derived quantitative values for non-induced and induced signal production rate per cell. It is worthy of note that we found rather constant values for different rates of dilution in the chemostat, and the values seemed close to those reported for batch cultures. Thus, the quorum-sensing system in P. putida IsoF is remarkably stable under different environmental conditions. In all chemostat experiments, the signal concentration decreased strongly after a peak, because emerging lactonase activity led to a lower concentration under steady-state conditions. This lactonase activity probably is quorum sensing-regulated. The potential ecological implication of such unique regulation is discussed.

Beneficial rhizobacteria strains are of substantial interest as biological plant protection agents in agriculture. Bacteria of the genus Pseudomonas have been studied for many years for their role in plant growth and biocontrol. In this study, we analyzed the influence of the commercially available agent Proradix † , which contains the strain Pseudomonas sp. DSMZ 13134, on barley. In controlled infection experiments, we showed that Pseudomonas sp. DSMZ 13134 induces resistance to the barley leaf pathogen Rhynchosporium secalis and inhibits the growth of the barley root pathogen Gaeumannomyces graminis. In greenhouse experiments, Pseudomonas sp. DSMZ 13134-treated plants showed enhanced growth and yield under nutrient deprivation. In field trials, an increase of yield and straw weight was observed. While the quality of grains, as determined by starch and protein content, was not affected, the yield increased by up to 20%. Our results demonstrate the value of Pseudomonas sp. DSMZ 13134 in agronomical applications for barley.

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