Streptomyces sp. ASBV‐1 reduces aflatoxin accumulation by Aspergillus parasiticus in peanut grains

Aims: To evaluate the ability of Streptomyces sp. (strain ASBV‐1) to restrict aflatoxin accumulation in peanut grains. Methods and Results: In the control of many phytopathogenic fungi the Streptomyces sp. ASBV‐1 strain showed promise. An inhibitory test using this strain and A. parasiticus was conducted in peanut grains to evaluate the effects of this interaction on spore viability and aflatoxin accumulation. In some treatments the Streptomyces sp ASBV‐1 strain reduced the viability of A. parasiticus spores by c. 85%, and inhibited aflatoxin accumulation in peanut grains. The values of these reductions ranged from 63 to 98% and from 67% to 96% for aflatoxins B₁ and G₁, respectively. Conclusions: It was demonstrated that Streptomyces sp. ASBV‐1 is able to colonize peanut grains and thus inhibit the spore viability of A. parasiticus, as well as reducing aflatoxin production. Significance and Impact of the Study: The positive finding for aflatoxin accumulation reduction in peanut grains seems promising and suggests a wider use of this actinobacteria in biological control programmes.     

The RNA interference—virus interplay: tools of nature for gene modulation,morphogenesis, evolution and a possible mean for aflatoxin control

This article points out, that viruses, in an interplay with RNA interference and as vehicles for intergenic and interspecies gene transfer, may work as agents for intracellular gene modulation, for steering of individual morphogenesis and as a driving force of evolution in the toolbox of nature. This is illustrated in particular in the light of a fungal double-stranded RNA virus that may be employed as a suitable agent for a biological control of aflatoxins, the most carcinogenic natural substances occurring in food and feedstuff.     

Effect of nontoxigenic Aspergillus flavus and A. parasiticus on aflatoxin contamination of wounded peanut seeds inoculated with agricultural soil containing natural fungal populations

Peanuts and other seed and grain crops are commonly contaminated with carcinogenic aflatoxins, secondary metabolites produced by Aspergillus flavus and A. parasiticus. Aflatoxin contamination of peanuts in the field can be reduced by 77–98% with biological control through the application of nontoxigenic strains of these species, which competitively exclude native aflatoxin-producing strains from developing peanuts. In this study, viable peanut seeds were artificially wounded and inoculated with field soil containing natural fungal populations that were supplemented with conidia of nontoxigenic A. flavus NRRL 21882 (niaD nitrate-nonutilizing mutant) and A. parasiticus NRRL 21369 (conidial color mutant). Increasing soil densities of applied nontoxigenic strains generally resulted in an increase in the incidence of seed colonization by applied nontoxigenic strains, a decrease in seed colonization by native A. flavus and A. parasiticus, and a decrease in aflatoxin concentration in seeds. Reduction of aflatoxins in peanut seeds depended on both the density and the aflatoxin-producing potential of native populations and on the fungal strain used for biological control. Wild-type strain A. flavus NRRL 21882 and its niaD mutant were equally effective in reducing aflatoxins in peanuts, indicating that nitrate-nonutilizing mutants, which are easily monitored in the field, can be used for evaluating the efficacy of biocontrol strains.     

Quantification of conidial density of Aspergillus flavus and A. parasiticus in soil from almond orchards using real-time PCR

Aims:  To design the Aspergillus flavus and Aspergillus parasiticus -specific primers and a real-time PCR assay for quantification of the conidial density in soil.
Methods and Results:  Aspergillus flavus and A. parasiticus -specific DNA primers were designed based on internal transcribed spacer sequences to distinguish these two species and from other Aspergillus and other fungal species. A method of pathogen DNA extraction directly from soil samples was developed. Using the designed primers, a real-time PCR assay was developed to quantitatively determine the conidial density of each A. flavus and A. parasiticus in soil, after generating corresponding standard curves. Known conidial densities of each A. flavus or A. parasiticus in soil significantly correlated with those tested with the real-time PCR.
Conclusions:  This study demonstrated the applicability of the real-time PCR assay in studies of quantifying A. flavus and A. parasiticus in soil as inoculum sources.
Significance and Impact of the Study:  The A. flacus and A. parasitic -specific primers can be widely used in aflatoxin research. The real-time PCR assay developed in this study provides a potential approach to quantify the plant pathogen density from not only soil but also other sources in relation to aflatoxin contamination from environment, food and feed commodities.     

Biosynthesis of dothistromin

Dothistromin is a mycotoxin that is remarkably similar in structure to versicolorin B, a precursor of both aflatoxin and sterigmatocystin. Dothistromin-producing fungi also produce related compounds, including some aflatoxin precursors as well as alternative forms of dothistromin. Dothistromin is synthesized by pathogenic species of Dothistroma in the red bands of pine needles associated with needle blight, but is also made in culture where it is strongly secreted into the surrounding medium. Orthologs of aflatoxin and sterigmatocystin biosynthetic genes have been found that are required for the biosynthesis of dothistromin, along with others that are speculated to be involved in the same pathway on the basis of their sequence similarity to aflatoxin genes. An epoxide hydrolase gene that has no homolog in the aflatoxin or sterigmatocystin gene clusters is also clustered with the dothistromin genes, and all these genes appear to be located on a minichromosome in Dothistroma septosporum. The dothistromin genes are expressed at an early stage of growth, suggesting a role in the first stages of plant invasion by the fungus. Future studies are expected to reveal more about the role of dothistromin in needle blight and about the genomic organization and expression of dothistromin genes: these studies will provide for interesting comparisons with these aspects of aflatoxin and sterigmatocystin biosynthesis.     

黄曲霉毒素生物合成的遗传调控研究进展

黄曲霉毒素是一类具有较强毒性和致癌力的次级代谢产物,在小麦、水稻、玉米和花生等多种粮食、油料、饲料和食品中检出率均比较高。因此,黄曲霉毒素不仅给人和动物的健康造成极其严重的威胁,而且也给食品和饲料等行业造成了巨大的经济损失。黄曲霉毒素主要由黄曲霉和寄生曲霉产生。自上个世纪60年代首次发现黄曲霉毒素以来,研究者在黄曲霉毒素合成途径、降解、合成机制和致病机理等方面做了大量研究。本文主要综述近年来国内外以黄曲霉为对象的黄曲霉毒素合成的遗传调控机制研究进展。从转录调控、蛋白翻译后修饰、信号转导途径、参与生长发育和形态建成的蛋白和其他酶等方面对黄曲霉毒素合成机制展开综述,为今后进一步深入系统研究黄曲霉毒素合成机制奠定基础,同时为制定防治黄曲霉及其毒素的策略提供理论基础。     

Soil fungi of some low-altitude desert cotton fields and ability of their extracts to inhibit Aspergillus flavus

Soil is presumed to be a major source of inoculum for Aspergillus flavus which contaminates cottonseed and produces the potent carcinogen, aflatoxin. Little is known about the mycoflora of the low desert soils of cotton fields where aflatoxin is a chronic problem. In this study, soils from cotton fields in southwestern Arizona and southeastern California were assayed for filamentous fungi. Forty-two taxa, predominantly in the genera Aspergillus, Penicillium and Fusarium, were isolated. To determine whether or not compounds produced by these fungi could be potential inhibitors of A. flavus, extracts of strains of each taxon were tested for their ability to inhibit growth of A. flavus. Twelve taxa produced compounds inhibitory to A. flavus, including several strains of Fusarium solani, Penicillium vinaceum and Aspergillus auricomus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Aflatoxin formation and varietal difference of cow pea (Vigna unguiculata (L.) Walp.) and garden pea (Pisum sativum L.) cultivars

Different cultivars of cow pea and garden pea seeds were surveyed for susceptibility or resistance towards the toxigenic and aflatoxin-producing mould (Aspergillus flavus IMI 102135). The results show that aflatoxin production varied among the different cultivars of both cow pea and garden pea. Morphological and histological characters of the different cultivars tested did not show any relation between colour, shape and size of seeds and the amount of aflatoxin produced. The chemical analysis of the different constituents obtained from both seed coats and seed kernels with susceptible, partially resistant and resistant cow pea and garden pea cultivars revealed that the resistant cultivars of cow pea (namely: Balady cultivar) and garden pea (namely: Melting Sugar cultivar) contained lower levels of sodium and higher levels of phosphate and potassium.