Abstract
Plant diseases caused by pathogenic fungi are a severe problem for agriculture, and some organic fertilizers have shown a suppressive capacity due to the antagonistic action of microbial communities inhabiting those substrates. The purpose of this work was to isolate and identify actinomycetes from soil, compost and vermicompost able to antagonize phytopathogens. In total, out of 352 actinomycetes successfully recovered on Pochon medium and of which were selected representative 22 mycelium forming isolates, which were subjected to morphological and genotypic characterization. Genetic characterization based on 16S rDNA fragment sequencing revealed that, all the actinomycete isolates belong to the genus Streptomyces. The antifungal activity of isolates was tested against eight phytopathogenic fungi, and they were most activated against Rhizoctonia solani, Alternaria tenuissima, Aspergillus niger and Penicillium expansum. Isolates 51VK13 (Streptomyces sampsonii) and 12VK13 (Streptomyces flavovariabilis) isolated from soil amended with vermicompost showed the greatest antagonistic activity. Their bioactive potential was also confirmed by presence of genes for nonribosomal peptide synthetase (NRPS) and polyketide synthase types I and II (PKS-I and PKS-II). It can, therefore, be concluded that soil amended with organic fertilizers such as compost, and in particular vermicompost, provides a lower incidence of phytopathogens, especially fungi. Isolated actinomycetes have also shown high potential for the production of bioactive compounds useful in the control of plant diseases.
Highlights
Actinomycetes present in soil, play major roles in the cycling of organic matter; they inhibit the growth of several plant pathogens in the rhizosphere and decompose complex mixtures of polymers in dead plant, animal and fungal material, resulting in the production of many extracellular enzymes which are conducive to crop production (Bhatti et al, 2017; Charousová et al, 2016a; Charousová et al, 2017)
Actinomycetes were isolated from soil (Haplic Fluvisol) located near Nitra (Slovakia – latitude 48° 31' 35.97'' N, longitude 18° 11' 46.82'' E), one compost, one vermicompost and soil amended with compost or vermicompost
The lowest number of actinomycetes was recorded in the soil without the addition of composts; the addition of both composts increased the number of actinomycetes compared to that in untreated soil, especially in the case of vermicompost application, by up to 12 times
Summary
Actinomycetes present in soil, play major roles in the cycling of organic matter; they inhibit the growth of several plant pathogens in the rhizosphere and decompose complex mixtures of polymers in dead plant, animal and fungal material, resulting in the production of many extracellular enzymes which are conducive to crop production (Bhatti et al, 2017; Charousová et al, 2016a; Charousová et al, 2017). NRPS and PKS participate in the production of secondary metabolites like antibiotics, siderophores, pigments, antihelmintics, antifungals, antitumor agents and immunosuppressives (Ayuso-Sacido and Genilloud, 2005; Gartemann et al, 2003). Both PKS-I and NRPS are multifunctional polypeptides encoded by a variable number of modules with multiple enzymatic activities. Each PKS-I module encodes at least three domains corresponding to a ketosynthase (KS), acyltransferase (AT), and acyl carrier protein (ACP) involved in the selection and condensation of the correct extender unit They can include additional enoylreductase, dehydratase, and ketoreductase activities involved in the reduction of the β-keto group formed in the condensation (Ayuso-Sacido and Genilloud, 2005). Antifungal activity and overall bioactive potential of actinomycetes isolates were evaluated by in vitro antagonism assay, and by PCR amplification of specific sequences of genes encoding NRPS and PKS type I and II (PKS-I, PKS-II)
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