Abstract
The bacterium, Streptomyces yanglinensis 3-10, shows promise in the control of many phytopathogenic fungi. In this study, S. yanglinensis and its antifungal substances, culture filtrate (CF3-10) and crude extracts (CE3-10), were evaluated for their activity in reducing growth and aflatoxin AFB1 production by Aspergillus flavus, both in vitro and in vivo on peanut kernels. The results showed that in dual culture conditions, S. yanglinensis reduced the mycelial growth of A. flavus about 41% as compared to control. The mycelial growth of A. flavus was completely inhibited on potato dextrose agar amended with CF3-10 at 3% (v/v) or CE3-10 at 2.5 μg/ml. In liquid culture experiments, growth inhibition ranged from 32.3 to 91.9% with reduction in AFB1 production ranging from 46.4 to 93.4% using different concentrations of CF3-10 or CE3-10. For in vivo assays, CF3-10 at 0.133 ml/g (v/w) or CE3-10 at 13.3 μg/g (w/w) reduced the postharvest decay of peanut kernels by inhibiting visible growth of A. flavus leading to an 89.4 or 88.1% reduction in AFB1 detected, respectively. Compared with the controls, CF3-10 and CE3-10 in A. flavus shake culture significantly reduced expression levels of two AFB1 biosynthesis genes, aflR and aflS. Furthermore, electron microscopy observation showed that CF3-10 (2%, v/v) caused hyphae growth to be abnormal and shriveled, cell organelles to degenerate and collapse, large vacuoles to appear. These results suggest that S. yanglinensis 3-10 has potential as an alternative to chemical fungicides in protecting peanut kernels and other agricultural commodities against postharvest decay from A. flavus.
Highlights
Aspergillus flavus is an important pathogenic fungus affecting peanuts during storage (Amaike and Keller, 2011)
In the tests on potato dextrose agar (PDA) plates, different concentrations of CF3−10 or CE3−10 incorporated in the PDA exhibited differential inhibitory effects on A. flavus
The efficacy (YCF) of inhibition of mycelial growth of A. flavus by the cultural filtrates of S. yanglinensis was positively related to the concentration of the cultural filtrates (XCF) incorporated in PDA
Summary
Aspergillus flavus is an important pathogenic fungus affecting peanuts during storage (Amaike and Keller, 2011). The pathogen can produce aflatoxins, a group of fungal secondary metabolites, which are the most toxic carcinogens among known mycotoxins (Calvo et al, 2002; Klich, 2007; Krishnamurthy et al, 2008). These metabolites are considered potent hepatocarcinogens in animals and may be involved in primary liver cancer and other cancers in kidney, lung, and colon tissues in humans (Bullerman, 1976; Amaike and Keller, 2011). The gene cluster for aflatoxin biosynthesis is 70 kb in length, containing 25 genes (Yu et al, 2004).
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