Interrelations between Azospirillum and Rhizobium nitrogen-fixers and arbuscular mycorrhizal fungi in the rhizosphere of alfalfa in sterile, AMF-free or normal soil conditions

Co-inoculations of the alfalfa (Medicago sativa L.) plants with the associative- and/or the obligate nitrogen-fixing bacteria (Azospirillum brasilense, S; Rhizobium meliloti, R) and/or the vesicular arbuscular mycorrhiza fungus (Glomus fasciculatum, M) were evaluated in a pot experiment under controlled conditions. The effect of these beneficial microbes, as single- (M, R, S), dual- (MR, MS) or multilevel (MRS) inoculation-treatments were assessed in a calcareous loamy chernozem soil, originating from a grass-type natural ecosystem. A range of substrates were used to separate the influence of the indigenous microbes: C, untreated original soil (i.e. including all of the usual microflora); G, gamma-sterilised soil (no competitive microbes); GB, sterile soil (re-suspension of a mycorrhiza-free soil extract). The weight of the host, nodule-number, macro- and microelement contents and the colonisation by the inoculated bacterial and fungal microsymbionts were recorded. In the gamma-sterilised substrate all of the mono- (M), dual- (MR, MS) or multilevel (MRS) co-inoculations with the selected, Glomus fasciculatum M 107 strain were effective in improving plant growth, nutrient-uptake and abundance of the microsymbionts in the rhizosphere of alfalfa. In contrast a competition from the indigenous microflora in the non-sterilised soil, greatly reduced the functioning of the applied mycorrhizal inoculum. Although the associative Azospirillum bacteria (MS) slightly reduced effects relative to single mycorrhizal inoculation (M), the multilevel treatments, with both of the diazotrophs (MRS), showed a further enhancement (a synergistic effect) for almost all of the tested parameters and substrates. The functional compatibility of the obligate- and associative diazotrophs in the mycorhizosphere are discussed

Domain shuffling in a sensor protein contributed to the evolution of insect pathogenicity in plant-beneficial Pseudomonas protegens.

Pseudomonas protegens is a biocontrol rhizobacterium with a plant-beneficial and an insect pathogenic lifestyle, but it is not understood how the organism switches between the two states. Here, we focus on understanding the function and possible evolution of a molecular sensor that enables P. protegens to detect the insect environment and produce a potent insecticidal toxin specifically during insect infection but not on roots. By using quantitative single cell microscopy and mutant analysis, we provide evidence that the sensor histidine kinase FitF is a key regulator of insecticidal toxin production. Our experimental data and bioinformatic analyses indicate that FitF shares a sensing domain with DctB, a histidine kinase regulating carbon uptake in Proteobacteria. This suggested that FitF has acquired its specificity through domain shuffling from a common ancestor. We constructed a chimeric DctB-FitF protein and showed that it is indeed functional in regulating toxin expression in P. protegens. The shuffling event and subsequent adaptive modifications of the recruited sensor domain were critical for the microorganism to express its potent insect toxin in the observed host-specific manner. Inhibition of the FitF sensor during root colonization could explain the mechanism by which P. protegens differentiates between the plant and insect host. Our study establishes FitF of P. protegens as a prime model for molecular evolution of sensor proteins and bacterial pathogenicity