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High-throughput sequencing, culture-dependent workflows, and microbiome transfer experiments reveal whether potassium phosphite (KP), an environmentally acceptable agricultural chemical, could specifically enrich the antagonistic bacterial community that inhibited the growth of the pathogen Ralstonia solanacearum. The application of KP enriched the potential antagonistic bacteria Paenibacillus and Streptomyces in soil, but depleted most dominant genera belonging to gram negative bacteria, such as Pseudomonas, Massilia, and Flavobacterium on day 7. Moreover, the KP-modulated soil microbiome suppressed R. solanacearum growth in soil. The predicted functions related to the synthesis of antagonistic substances, such as streptomycin, and the predicted functions related to tellurite resistance and nickel transport system were significantly enriched, but the synthesis of lipopolysaccharide (distinct component lipopolysaccharide in gram negative bacteria) were significantly depleted in the KP-treated soils. In addition, the copy numbers of specific sequences for Streptomyces coelicoflavus and Paenibacillus favisporus were significantly increased in the soil amended with KP, inhibited the growth of R. solanacearum, and had a higher tolerance of KP than R. solanacearum. Our study linked the application of fertilizers to the enrichment of antagonistic bacteria, which could support future work that aims to precisely regulate the soil microbiome to protect the host from infection by soil-borne pathogens. [ABSTRACT FROM AUTHOR]

Chemotaxis towards root exudates and subsequent biofilm formation are very important for root colonization and for providing the beneficial functions of plant growth-promoting rhizobacteria (PGPRs). In this study, in comparison with other root-secreted compounds, d-galactose in the root exudates of cucumber was found to be a strong chemoattractant at the concentration of 1 μM for Bacillus velezensis SQR9. Chemotaxis assays with methyl-accepting chemotaxis proteins (MCPs) deletion strains demonstrated that McpA was solely responsible for chemotaxis towards d-galactose. Interestingly, d-galactose significantly enhanced the biofilm formation of SQR9 in an McpA-dependent manner. Further experiment showed that d-galactose also enhanced root colonization by SQR9. In addition, the secretion of d-galactose by cucumber roots could be induced by inoculation with SQR9, indicating that d-galactose may be an important signal in the interaction between plant and SQR9. These findings suggested that the root-secreted d-galactose was a signal, the secretion of which was induced by the beneficial bacteria, and which in turn induced colonization of the bacteria. [ABSTRACT FROM AUTHOR]

Aim Trichoderma guizhouense: NJAU 4742 (Tgui) can serve as a promising strain for the development of novel biofertilizers and biofungicides. Plants primed with Tgui via inoculation were investigated to clarify the underlying mechanisms that promote root growth and development and activate the plant innate immune response. Methods: The relative expression of defence-related genes and of genes involved in the auxin signalling pathway in Zea mays and Arabidopsis thaliana was quantified. Scanning electron microscopy (SEM) was performed to visualize the colonization of Tgui in maize roots, and a proteomic approach was used to identify Tgui-derived elicitors. Results: The establishment of Tgui in the rhizosphere of maize leads to the stimulation of the auxin synthesis pathway in maize and subsequently leads to increased plant growth. And the extracellular proteins of Tgui induced systemic resistance (ISR) of maize plants to Fusarium verticillioides (Fv) (Hypocreales, Ascomycota); the ISR of maize plants may be linked to the accumulation of reactive oxygen species (ROS) and increased deposition of callose in maize tissue. Conclusions: Activation of the maize immune response was triggered by the mixture of extracellular proteins secreted by Tgui into the rhizosphere. Our study thereby contributes to a better understanding of the interaction between T. guizhouense and plant roots. [ABSTRACT FROM AUTHOR]

Aside from applications in the production of commercial enzymes and metabolites, Bacillus amyloliquefaciens is also an important group of plant growth-promoting rhizobacteria that supports plant growth and suppresses phytopathogens. A host-genotype-independent counter-selectable marker would enable rapid genetic manipulation and metabolic engineering, accelerating the study of B. amyloliquefaciens and its development as both a microbial cell factory and plant growth-promoting rhizobacteria. Here, a host-genotype-independent counter-selectable marker pheS was constructed through a point mutation of the gene pheS, which encodes the α-subunit of phenylalanyl-tRNA synthetase in Bacillus subtilis strain 168. In the presence of 5 mM p-chloro-phenylalanine, 100 % of B. amyloliquefaciens strain SQR9 cells carrying pheS were killed, whereas the wild-type strain SQR9 showed resistance to p-chloro-phenylalanine. A simple pheS and overlap-PCR-based strategy was developed to create the marker-free deletion of the amyE gene as well as a 37-kb bmy cluster in B. amyloliquefaciens SQR9. The effectiveness of pheS as a counter-selectable marker in B. amyloliquefaciens was further confirmed through the deletion of amyE genes in strains B. amyloliquefaciens FZB42 and NJN-6. In addition, the potential use of pheS in other Bacillus species was preliminarily assessed. The expression of PheS in B. subtilis strain 168 and B. cereus strain ATCC 14579 caused pronounced sensitivity of both hosts to p-chloro-phenylalanine, indicating that pheS could be used as a counter-selectable marker (CSM) in these strains. [ABSTRACT FROM AUTHOR]