Abstract
Wheat is the main cereal crop consumed in Europe. Its contamination with fungal pathogens is one of the main threats, leading to massive yield losses and the potential occurrence of mycotoxins. Puccinia triticina causes leaf rust, Magnaporthe oryzae causes wheat blast, Septoria nodorum blotch and Septoria tritici blotch are caused by Parastagonospora nodorum and Zymoseptoria tritici, respectively, Fusarium head blight is caused by Fusarium graminearum and Fusarium verticillioides emerges as a wheat pathogen. Aspergillus flavus and Penicillium verrucosum, develop mostly during grain storage. Decrease on chemical fungicides motivated the search for alternatives for fungal management. Streptomyces genus are promising as biocontrol agents (BCAs) due to their wide production of bioactive secondary metabolites and enzymes, but also due to their capacity to detoxify mycotoxins. In this study, the impact of Streptomyces isolates on fungal growth and mycotoxin accumulation was evaluated. Maximal decreases of fungal growth varied from 66% (F. graminearum) down to 10% of the control (P. nodorum). Mycotoxin accumulation was decreased from 35% of the control (DON), to a complete elimination (AFB1), while some isolates caused an overaccumulation. Bacterial Cell Free Extracts (CFEs) were in general less efficient at decreasing fungal growth, and DON, AFB1 and OTA accumulation. Sometimes causing a strong overproduction (at least 8 times more fumonisins). This highlights the importance of assessing the impact on mycotoxins when evaluating BCAs. OTA was strongly degraded by bacterial isolates mainly in liquid medium (down to 38% of the control in average), while AFB1 was degraded in solid and liquid medium and by bacterial CFEs (33%, 43% and 69% of the control on average, respectively). Moreover, the evaluation of AFB1 residual genotoxicity indicated an effective detoxification down to 27% of the control. Clustering analysis and Pearson correlations allowed to classify the bacterial isolates according to their performances and to evoke possible antagonistic mechanisms, such as the production of antifungal compounds, mycotoxin degrading enzymes and potential inhibitors of mycotoxin biosynthesis. Bacteria colonized wheat leaves during in planta assays and remained viable after 1 week. However, only S. griseoviridis from Mycostop®, protected against the tested fungi. This work presents an integrated approach for the screening of potential BCAs, focusing on the global analysis of the results to elucidate antagonistic mechanisms and to identify the most promising isolates.