Enhanced tomato plant growth in soil under reduced P supply through microbial inoculants and microbiome shifts.

Soil microbial communities interact with roots, affecting plant growth and nutrient acquisition. In the present study, we aimed to decipher the effects of the inoculants Trichoderma harzianum T-22, Pseudomonas sp. DSMZ 13134, Bacillus amyloliquefaciens FZB42 or Pseudomonas sp. RU47 on the rhizosphere microbial community and their beneficial effects on tomato plants grown in moderately low phosphorous soil under greenhouse conditions. We analyzed the plant mass, inoculant colony forming units and rhizosphere communities on 15, 22, 29 and 43 days after sowing. Selective plating showed that the bacterial inoculants had a good rhizocompetence and accelerated shoot and root growth and nutrient accumulation. 16S rRNA gene fingerprints indicated changes in the rhizosphere bacterial community composition. Amplicon sequencing revealed that rhizosphere bacterial communities from plants treated with bacterial inoculants were more similar to each other and distinct from those of the control and the Trichoderma inoculated plants at harvest time, and numerous dynamic taxa were identified. In conclusion, likely both, inoculants and the rhizosphere microbiome shifts, stimulated early plant growth mainly by improved spatial acquisition of available nutrients via root growth promotion. At harvest, all tomato plants were P-deficient, suggesting a limited contribution of inoculants and the microbiome shifts to the solubilization of sparingly soluble soil P.

Analysis of the genome sequence of plant beneficial strain Pseudomonas sp. RU47.

Pseudomonas sp. RU47 (RU47) is a rhizosphere-competent strain showing plant growth-promoting and biocontrol activities. In this study, the genome sequence of strain RU47 was obtained and phylogenetic and comparative genome analyses were performed. Multilocus sequence analysis (MLSA) coupled with the calculation of average nucleotide identity (ANI) and in silico DNA-DNA hybridization (DDH) values suggested that strain RU47 belongs to the P. koreensis group of the P. fluorescens complex, but cannot be assigned to any known Pseudomonas species. Multiple genes and operons encoding functions that likely contribute to its previously reported high rhizosphere competence, biocontrol of Rhizoctonia solani and plant growth promotion in soils with reduced fertilization were identified. Putative genes and gene clusters for the production of hydrogen cyanide, cyclic lipopeptides, bacteriocins, siderophores, indole-3-acetic acid, spermidine, alkaline protease A, chitinase and ß-1,3-glucanase were identified, as well as loci associated with solubilization of inorganic phosphate. Several of these functions were also confirmed by in vitro testing. Distribution of putative genes and gene clusters that may contribute to the plant growth-promoting and biocontrol activities was largely similar among strains belonging to P. koreensis group, as revealed by comparative genome analysis. Data presented in this study further support the potential of RU47 for its application in agriculture and may be a valuable resource for further studies.

Sewage sludge amendment and inoculation with plant-parasitic nematodes do not facilitate the internalization of Salmonella Typhimurium LT2 in lettuce plants.

Contamination of fruits and vegetables with Salmonella is a serious threat to human health. In order to prevent possible contaminations of fresh produce it is necessary to identify the contributing ecological factors. In this study we investigated whether the addition of sewage sludge or the presence of plant-parasitic nematodes foster the internalization of Salmonella enterica serovar Typhimurium LT2 into lettuce plants, posing a potential threat for human health. Greenhouse experiments were conducted to investigate whether the amendment of sewage sludge to soil or the presence of plant-parasitic nematodes Meloidogyne hapla or Pratylenchus crenatus promote the internalization of S. Typhimurium LT2 from soil into the edible part of lettuce plants. Unexpectedly, numbers of cultivable S. Typhimurium LT2 decreased faster in soil with sewage sludge than in control soil but not in root samples. Denaturing gradient gel electrophoresis analysis revealed shifts of the soil bacterial communities in response to sewage sludge amendment and time. Infection and proliferation of nematodes inside plant roots were observed but did not influence the number of cultivable S. Typhimurium LT2 in the root samples or in soil. S. Typhimurium LT2 was not detected in the leaf samples 21 and 49 days after inoculation. The results indicate that addition of sewage sludge, M. hapla or P. crenatus to soil inoculated with S. Typhimurium LT2 did not result in an improved survival in soil or internalization of lettuce plants.