Abstract
Drought stress in the field has been shown to exacerbate aflatoxin contamination of maize and peanut. Drought and heat stress also produce reactive oxygen species (ROS) in plant tissues. Given the potential correlation between ROS and exacerbated aflatoxin production under drought and heat stress, the objectives of this study were to examine the effects of hydrogen peroxide (H2O2)-induced oxidative stress on the growth of different toxigenic (+) and atoxigenic (−) isolates of Aspergillus flavus and to test whether aflatoxin production affects the H2O2 concentrations that the isolates could survive. Ten isolates were tested: NRRL3357 (+), A9 (+), AF13 (+), Tox4 (+), A1 (−), K49 (−), K54A (−), AF36 (−), and Aflaguard (−); and one A. parasiticus isolate, NRRL2999 (+). These isolates were cultured under a H2O2 gradient ranging from 0 to 50 mM in two different media, aflatoxin-conducive yeast extract-sucrose (YES) and non-conducive yeast extract-peptone (YEP). Fungal growth was inhibited at a high H2O2 concentration, but specific isolates grew well at different H2O2 concentrations. Generally the toxigenic isolates tolerated higher concentrations than did atoxigenic isolates. Increasing H2O2 concentrations in the media resulted in elevated aflatoxin production in toxigenic isolates. In YEP media, the higher concentration of peptone (15%) partially inactivated the H2O2 in the media. In the 1% peptone media, YEP did not affect the H2O2 concentrations that the isolates could survive in comparison with YES media, without aflatoxin production. It is interesting to note that the commercial biocontrol isolates, AF36 (−), and Aflaguard (−), survived at higher levels of stress than other atoxigenic isolates, suggesting that this testing method could potentially be of use in the selection of biocontrol isolates. Further studies will be needed to investigate the mechanisms behind the variability among isolates with regard to their degree of oxidative stress tolerance and the role of aflatoxin production.
Highlights
The contamination of agricultural products with aflatoxins produced by Aspergillus flavus poses a serious threat to human health and food security, in developing countries [1,2]
This opens the possibility that reactive oxygen species (ROS) may function in the host-pathogen interaction between Aspergillus spp. and their host plants as a form of “cross-kingdom communication” [16,19]. This seems plausible given the observed correlation between drought stress, which results in the accumulation of ROS in host plant tissues, and the exacerbation of aflatoxin contamination in oilseed crops such as maize and peanut [6,7,21,22]. Given this correlation among drought and heat stresses, ROS and aflatoxin production, it is possible that the ability to produce aflatoxin may influence the growth of Aspergillus spp. isolates when exposed to drought stress-derived ROS such as hydrogen peroxide (H2O2) the objectives of this study were twofold: to examine the effects of H2O2-induced oxidative stress on the growth of different isolates of toxigenic (+) and atoxigenic (−) isolates of Aspergillus flavus, and to test whether aflatoxin production affects the concentrations of H2O2 in which the isolates could grow and survive
In order to determine the effects of oxidative stress encountered during drought on toxigenic and atoxigenic isolates, we simulated this stress in vitro using H2O2 supplemented aflatoxin production conducive media
Summary
The contamination of agricultural products with aflatoxins produced by Aspergillus flavus poses a serious threat to human health and food security, in developing countries [1,2]. Research into aflatoxin contamination prevention began in the 1960s following the outbreak of what was termed turkey X disease which resulted in the deaths of over 100,000 turkey poults due to aflatoxin contaminated feed [3] These efforts were intensified in the area of pre-harvest host resistance following outbreaks in US maize in the 1970s, and their importance has been further underscored by the 2004 Kenya outbreak which resulted in 125 human deaths due to direct aflatoxicosis from consumption of contaminated maize [4,5]. Host drought tolerance and reduced aflatoxin accumulation are correlated [6,7] Given this observation, it is important to better understand the mechanisms at play in this relationship and how they may relate to both environmental stress tolerance and to the regulation of aflatoxin production in the pathogen
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