Effects of Hydrogen Peroxide on Different Toxigenic and Atoxigenic Isolates of Aspergillus flavus

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.

Oxidative stress and carbon metabolism influence Aspergillus flavus transcriptome composition and secondary metabolite production

“…Previously, we observed that the aflatoxin production capabilities of A. flavus isolates were correlated with the levels of H 2 O 2 stress individual isolates could tolerate or survive32. Here, we found a related trend with isolates with higher stress tolerance and aflatoxin production exhibiting fewer significant DEGs than atoxigenic, less tolerant isolates in both YES and YEP (Table 1).…”

“…Among the atoxigenic isolates, those which are used as biological controls, Aflaguard and AF36, were previously found to tolerate higher levels of oxidative stress than non-adapted isolates including K54A32. In the present study, these biological control isolates exhibited fewer DEGs in comparison to K54A (Table 1).…”

“…In the present study, these biological control isolates exhibited fewer DEGs in comparison to K54A (Table 1). In addition, the atoxigenic isolates, which generally tolerated lower levels of oxidative stress than the toxigenic isolates in the previous study32, tended to exhibit greater numbers of significant DEGs than the toxigenic isolates, particularly at higher levels of stress (Table 1). …”

“…On examining the level of oxidative tolerance of these isolates, our previous studies found that isolates with greater levels of aflatoxin production tend to tolerate greater levels of oxidative stress32. In order to examine the transcriptional responses of the toxigenic isolates to oxidative stress we identified the significantly differentially expressed genes in the isolates in YES medium in response to increasing levels of oxidative stress.…”

“…The close association of a simple sugar metabolic gene cluster with the aflatoxin biosynthetic gene cluster has also been demonstrated in A. parasiticus , though a similar association was not observed in all Aspergillus spp 1630. In A. flavus isolates, supplementing culture media with alternative carbon sources such as peptone results in reduced or inhibited aflatoxin production3132. The modulation of aflatoxin production through cultural methods including carbon source variation and ROS amendment has shown that isolates that produce aflatoxin tend to exhibit greater tolerance to oxidative stress, and that the reduction of aflatoxin production can adversely affect said tolerance1828.…”