A Rhizobacterium, Streptomyces albulus Z1-04-02, Displays Antifungal Activity against Sclerotium Rot in Mungbean

In silico and in vitro analyses reveal the potential use of Streptomyces parvulus VNUA74 as bioagent for sustainable banana production

Actinobacteria are well-known producers of diverse secondary metabolites by the presence of biosynthetic gene clusters (BGCs). Biological control of banana pathogens using antagonistic actinomycetes is recently considered a promising strategy. Therefore, this study aimed to assess the plant growth-promoting activities and the antagonistic potential of the newly identified Streptomyces sp. VNUA74 strain that isolated from banana rhizosphere in Hung Yen province, Vietnam. The morphological, biochemical and physiological characteristics together with the whole genome and 16S rRNA based taxonomic analyses confirmed that VNUA74 strain belongs to Streptomyces parvulus. In silico genome mining revealed that S. parvulus VNUA74 contains rich source of potential BGCs for secondary metabolites involved in antagonistic activities. Notably, eleven BGCs showed 100% similarity in gene contents with the known clusters possessing antibacterial and antifungal activities such as actimomycin D, germicidin, istamycins, albaflavenone, and cyclic Lanthipeptide SapB. The functional genome analysis also revealed genes participated in plant growth-promoting. Furthermore, in vitro biochemical assays indicated that S. parvulus VNUA74 exhibited strong antagonistic activities against a range of important phytopathogens on banana, including Fusarium oxysporum f. sp. cubense Tropical race 4, F. solani, F. oxysporum, Colletotrichum gloeosporioides, Corynespora cassiicola, Xanthomonas axonopodis, Ralstonia solanacearum and Clavibacter michiganensis. Finally, the VNUA74 strain showed notable enhancements of all examined growth traits of banana plantlets in the pot experiment. In summary, the results showed that the S. parvulus VNUA74 strain possesses multiple characteristics of being the effective biocontrol and biofertilizer agents for the sustainable production of banana and other agricultural crops. In further, the genomic approaches will provide an opportunity to discover novel bioactive compounds as well as manipulating novel gene clusters from S. parvulus VNUA74 strain.

Taxonomic identification and antagonistic activity of Streptomyces luomodiensis sp. nov. against phytopathogenic fungi

Banana wilt caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is a devastating fungal disease. Biocontrol strategies hold immense potential for inhibiting the spread of Foc TR4. Here, 30 actinobacteria were isolated from soils and screened for their antagonistic activity against Foc TR4. Strain SCA4-21T was selected due to its strongest antagonistic activity against Foc TR4. Strain SCA4-21T also exhibited strong antagonistic activity against the other eight phytopathogenic fungi. The strain was identified as the genus Streptomyces according to its physiological, biochemical, and phenotypic characteristics. The phylogenetic trees of 16S rRNA sequences demonstrated that strain SCA4-21T formed a subclade with S. iranensis HM 35T and/or S. rapamycinicus NRRL B-5491T with low bootstrap values. Considering that 16S rRNAs did not provide sufficient resolution for species-level identification, the whole genome of strain SCA4-21T was sequenced. Multilocus sequence analysis (MLSA) based on five housekeeping gene alleles (atpD, gyrB, recA, rpoB, and trpB) revealed that strain SCA4-21T clustered into S. hygroscopicus subsp. hygroscopicus NBRC 13472T with 100% of bootstrap value. The analysis of the genome-based phylogeny also approved the results. Average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) were 91.26 and 44.30%, respectively, with values below the respective species level threshold of 95 and 70%. Hence, strain SCA 4-21T represented a novel species within the genus Streptomyces, named Streptomyces luomodiensis sp. nov. The type strain is SCA4-21T (=GDMCC4.340T = JCM36555T). By the CAZymes analysis, 348 carbohydrate-active enzymes (CAZymes) were detected, including 15 chitinases and eight β-1,3-glucanases. The fermentation broth of strain SCA4-21T, exhibiting strong antagonistic activity against Foc TR4, demonstrated high activities of chitinase and β-1,3-glucanase, which might be involved in antifungal activity. Our results showed an innovative potential biocontrol agent for managing plant fungal diseases, specifically banana fusarium wilt.

Exogenous application of antagonistic Streptomyces sp. SND-2 triggers defense response in Vigna radiata (L.) R. Wilczek (mung bean) against anthracnose infection

Anthracnose is a devastating disease caused by the fungus Colletotrichum lindemuthianum (CL) in Vigna radiata (L.) R. Wilczek (mung bean). In the present study, an eco-friendly approach to control anthracnose infection, growth promotion and enhancement of defense response in mung bean plants using endophytic actinomycetes was performed. Among the 24 actinomycetes isolates from the Cleome rutidosperma plant, the isolate SND-2 exhibited a broad spectrum of antagonistic activity with 63.27% of inhibition against CL in the dual culture method. Further, the isolate SND-2 was identified as Streptomyces sp. strain SND-2 (SND-2) through the 16S rRNA gene sequence. In-vitro screening of plant growth trials confirmed that SND-2 has the potential to produce indole acetic acid, hydrogen cyanide, ammonia, phosphate solubilization, and siderophore. The in-vivo biocontrol study was performed with exogenous application of wettable talcum-based formulation of SND-2 strain to mitigate CL infection in mung bean seedlings. The results displayed maximum seed germination, vigor index, increased growth parameters, and lowest disease severity (43.63 ± 0.73) in formulation treated and pathogen challenged mung bean plants. Further, the application of SND-2 formulation with pathogen witnessed increased cellular defense through the maximum accumulation of lignin, hydrogen peroxide and phenol deposition in mung bean leaves compared with control treatments. Biochemical defense response exhibited upregulation of antioxidant enzymes such as phenylalanine ammonia-lyase, β-1,-3-Glucanase, and peroxidase enzymes activities with increased phenolic (3.64 ± 0.11 mg/g fresh weight) and flavonoid (1.14 ± 0.05 mg/g fresh weight) contents in comparison with other treatments at 0, 4, 12, 24, 36, and 72 h post pathogen inoculation. This study demonstrated that formulation of Streptomyces sp. strain SND-2 is a potential source as a suppressive agent and plant growth promoter in mung bean plants upon C. lindemuthianum infestation and witnesses the elevation in cellular and biochemical defense against anthracnose disease.