Antiviral activities of three Streptomyces spp. against Zucchini yellow mosaic virus (ZYMV) infecting squash (Cucurbita pepo L.) plants

Background Zucchini yellow mosaic virus (ZYMV) is the major devastating disease worldwide, which leads to substantial economic losses (up to 100%) to yield and fruits quality produced of squash plants. Application of agro-pesticides is efficient and incompatible with organic agriculture and reportedly has harmful effects on human health and ecosystem. Nowadays, Streptomyces spp., a rich source of potential bioactive secondary metabolites, is extensively used to manage various biotic stresses for sustainable agriculture and considered to be eco-friendly.Results An isolate of ZYMV was isolated from squash plants and identified based on biological and molecular characterization using RT-PCR for several genes, i.e., coat protein gene (CP), DAG, P1 and P3 coding regions in the virus RNA, and then, nucleotide sequences were compared to other isolates submitted in GenBank having accession numbers, i.e., OM925548.1, OM925549.1, OM925550.1 and OM925551.1, respectively. Phylogenetic trees of CP, DAG, P1 and P3 sequences compared to other ZYMV nucleotide sequences presented in the GenBank. In order to determine new efficient substances elicitors derived from Streptomyces spp. to control ZYMV, greenhouse trials were designed with seven treatments including culture broth of three Streptomyces spp. (S. sampsonii, S. rochei and S. griseus) individually or in combinations. Early application of Streptomyces spp. revealed potent antiviral activity against ZYMV infection, inhibited virus replication and promoted plant growth as well as induced systemic resistance. Moreover, physiological stress markers as indicators for systemic acquired resistance were distinguished via significantly enhanced proline, phenols and defense-related enzymes, i.e., catalase, superoxide dismutase and glutathione peroxidase by culture broth treatments, despite the presence of infection. Real-time qPCR assay was a more reliable and accurate detection for quantification ZYMV than conventional PCR. The results revealed that the three Streptomyces spp. novel biocontrol agents produced Behenic alcohol (Docosanol) which provided clues to be potential antiviral mechanisms capable to down-regulate P1 gene expression responsible for virus replication and movement from cell to cell to induce systemic infection as well as safe eco-friendly candidates for the controlling approaches against plant viral pathogens.Conclusion Results suggest that the three Streptomyces spp. provided clues as a novel biocontrol agent having potential antiviral with protective activity and eco-friendly alternative pesticides for managing plant viruses.