Beijerinckia fluminensis BFC-33, a novel multi-stress-tolerant soil bacterium: Deciphering the stress amelioration, phytopathogenic inhibition and growth promotion in Triticum aestivum (L.)

Environmental challenges like drought, salinity, heavy metals and pesticides directly/indirectly influence the environment and decreased the agricultural output. During its long developmental stages, cereal crops including wheat is exposed to a variety of abiotic/biotic stressors. Certain beneficial soil bacteria that can ameliorate environmental stresses can be exploited as crop growth promoters/enhancers under adverse situations. In this study, Beijerinckia fluminensis BFC-33 (accession no. MT672580) isolated from potato rhizosphere tolerated variousabiotic (drought, salinity, temperature, heavy metals, and fungicides) stresses. Strain BFC-33 demonstrated multifarious plant-growth-promoting (PGP) characteristics, such as the production of indole-3-acetic acid, P-solubilization, ACC deaminase, ammonia, siderophore, HCN, EPS, and extracellular enzymes. The antagonistic potential of BFC-33 against major fungal pathogens was ranked: Alternaria alternata (79.2%)>Rhizoctonia solani (69%)>Fusarium oxysporum (23.5%)>Ustilaginoidea virens (17%). Furthermore, bacterization of wheat seeds witha multi-stress-tolerant strain revealed B. fluminensis as a plant growth enhancer and biocontrol agent. For instance, increase in root length (cm) in BFC-33 inoculated wheat exposed to abiotic and biotic stresses at the seedling stage was ranked: B. fluminensis (24.2)>B. fluminensis + 100μgTBZLmL^-1 (21.3) = B. fluminensis + 2%PEG (21.3)>B. fluminensis + 100 mM NaCl (19.7)>B. fluminensis + 100μgPbmL^-1 (19) = B. fluminensis 100μgMNZBmL^-1 (19)>B. fluminensis + A. alternata (17.4)>B. fluminensis + 100μgCdmL^-1 (17)>B. fluminensis + F. oxysporum (13.4). In addition, increase in carotenoid accumulation (mg g^-1FW) in the foliage of BFC-33 inoculated wheat exposed to fungal infection was ranked: BFC-33 (3.88)>BFC-33+ A. alternata (3.0)>BFC-33+ R. solani (2.78)>BFC-33+ F. oxysporum (2.44). Moreover, BFC-33 inoculation significantly (p ≤ 0.05) reduced stress-induced stressor molecules (proline and TBARS) and electrolyte leakage. Furthermore, B. fluminensis BFC-33 potentially enhanced the defense responses in wheat seedlings by increasing phenylalanine ammonia lyase (PAL), β-1,3 glucanase, and polyphenol oxidase (PPO), which play a significant role in protecting plants from phytopathogens. Even so, by successfully establishing a product with the requisite effects under field settings, selecting multi-stress-tolerant and antagonistic plant growth promoting rhizobacteria (PGPRs) would be helpful to end-users. Future use of native multi-stress-tolerant bacteria as biocontrol agents in conjunction with existing drought, salinity, heavy metal, and pesticide tolerance might contribute to global food security.

Diversity and geographic distribution of soil streptomycetes with antagonistic potential against actinomycetoma-causing Streptomyces sudanensis in Sudan and South Sudan

BACKGROUND

Production of antibiotics to inhibit competitors affects soil microbial community composition and contributes to disease suppression. In this work, we characterized whether Streptomyces bacteria, prolific antibiotics producers, inhibit a soil borne human pathogenic microorganism, Streptomyces sudanensis. S. sudanensis represents the major causal agent of actinomycetoma - a largely under-studied and dreadful subcutaneous disease of humans in the tropics and subtropics. The objective of this study was to evaluate the in vitro S. sudanensis inhibitory potential of soil streptomycetes isolated from different sites in Sudan, including areas with frequent (mycetoma belt) and rare actinomycetoma cases of illness.

RESULTS

Using selective media, 173 Streptomyces isolates were recovered from 17 sites representing three ecoregions and different vegetation and ecological subdivisions in Sudan. In total, 115 strains of the 173 (66.5%) displayed antagonism against S. sudanensis with different levels of inhibition. Strains isolated from the South Saharan steppe and woodlands ecoregion (Northern Sudan) exhibited higher inhibitory potential than those strains isolated from the East Sudanian savanna ecoregion located in the south and southeastern Sudan, or the strains isolated from the Sahelian Acacia savanna ecoregion located in central and western Sudan. According to 16S rRNA gene sequence analysis, isolates were predominantly related to Streptomyces werraensis, S. enissocaesilis, S. griseostramineus and S. prasinosporus. Three clusters of isolates were related to strains that have previously been isolated from human and animal actinomycetoma cases: SD524 (Streptomyces sp. subclade 6), SD528 (Streptomyces griseostramineus) and SD552 (Streptomyces werraensis).

CONCLUSION

The in vitro inhibitory potential against S. sudanensis was proven for more than half of the soil streptomycetes isolates in this study and this potential may contribute to suppressing the abundance and virulence of S. sudanensis. The streptomycetes isolated from the mycetoma free South Saharan steppe ecoregion show the highest average inhibitory potential. Further analyses suggest that mainly soil properties and rainfall modulate the structure and function of Streptomyces species, including their antagonistic activity against S. sudanensis.