Antifungal activity against pathogens of cereals and characterization of antibiotics of Streptomyces sp. strain K-541 isolated from extreme ecosystems in Kazakhstan

The general requirement for biologicals is that they must be insensitive to climate change and soil conditions, including soil physicochemical composition, fertility levels, and pH values. Actinomycetes isolated from extreme habitats are able to produce biologically active substances not only under neutral conditions but also in saline, alkaline and acidic environments, which determines their importance in the biopreparations, being developed for plant protection. This study is the first to report the Streptomyces sp. strain K-541 antibiosis against the causative agents of several cereal fungal infections under various environmental conditions and the identification of the antibiotic produced. Streptomyces sp. strain K-541 isolated from extreme ecosystems of Kazakhstan was cultured under neutral (pH 7.0) and alternative growth conditions at 25.0 g/l NaCl (pH 7.2) or 2.5 g/l Na2CO3 (pH 8.0). Antifungal activity was determined in agar block diffusion experiments and under paired co-incubation with phytopathogenic fungi Fusarium solani (Mart.) Sacc., F. oxysporum Schlecht., F. heterosporum Nees, F. sporotrichiella Sherb., Piricularia oryzae Cavara, Alternaria triticina Prasada & Prabhu, A. alternate (Fr.) Keissl., Bipolaris sorokiniana (Sacc.) Shoemaker, and Aspergillus niger van Tieghem. For antibiotic A-541 production, the strain was cultured on an orbital shaker (180-200 rpm) for 120 hours at 28 °С. The antibiotic was extracted with organic solvents and analyzed using thin layer chromatography and spectrophotometry. The studies have shown high antifungal activity of K-541 against all the phytopathogens examined. After 72 hour incubation at 25 °С the growth inhibition zones were 20-56 mm in diameter depending on growth conditions which simulated different ecological niches. In co-culturing the strain K-541 and the phytopathogenic fungi, the fungal colonies decreased 1.8-2.7 times in diameter indicating the possibility of K-541 introduction into soil biocenoses for biocontrol of cereal fungal pathogens. High inhibition of growth was also observed under saline (2 % NaCl) and alkaline (0.2 % Na2CO3) conditions. The antibiotic produced by strain K-541 was classified as a member of polyene group, a subgroup of the hexaenes. So strain K-541 is recognized as promising for the development of a new biopreparation with fungicidal activity against causal agents of cereal fungal infections under different environmental conditions.