Impacts of Climate Change Interacting Abiotic Factors on Growth, aflD and aflR Gene Expression and Aflatoxin B ₁ Production by Aspergillus flavus Strains In Vitro and on Pistachio Nuts

Abstract

Pistachio nuts are an important economic tree nut crop which is used directly or processed for many food-related activities. They can become colonized by mycotoxigenic spoilage fungi, especially Aspergillus flavus, mainly resulting in contamination with aflatoxins (AFs), especially aflatoxin B ₁ (AFB ₁). The prevailing climate in which these crops are grown changes as temperature and atmospheric CO ₂ levels increase, and episodes of extreme wet/dry cycles occur due to human industrial activity. The objectives of this study were to evaluate the effect of interacting Climate Change (CC)-related abiotic factors of temperature (35 vs. 37 °C), CO ₂ (400 vs. 1000 ppm), and water stress (0.98–0.93 water activity, a _w) on (a) growth (b) aflD and aflR biosynthetic gene expression and (c) AFB ₁ production by two strains A. flavus (AB3, AB10) in vitro on milled pistachio-based media and when colonizing layers of shelled raw pistachio nuts. The A. flavus strains were resilient in terms of growth on pistachio-based media and the colonisation of pistachio nuts with no significant difference when exposed to the interacting three-way climate-related abiotic factors. However, in vitro studies showed that AFB ₁ production was significantly stimulated ( p < 0.05), especially when exposed to 1000 ppm CO ₂ at 0.98–0.95 a _w and 35 °C, and sometimes in the 37 °C treatment group at 0.98 a _w. The relative expression of the structural aflD gene involved in AFB ₁ biosynthesis was decreased or only slightly increased, relative to the control conditions at elevated CO, regardless of the a _w level examined. For the regulatory aflR gene expression, there was a significant ( p < 0.05) increase in 1000 ppm CO ₂ and 37 °C for both strains, especially at 0.95 a _w. The in situ colonization of pistachio nuts resulted in a significant ( p < 0.05) stimulation of AFB ₁ production at 35 °C and 1000 ppm CO ₂ for both strains, especially at 0.98 a _w. At 37 °C, AFB ₁ production was either decreased, in strain AB3, or remained similar, as in strain AB10, when exposed to 1000 ppm CO ₂. This suggests that CC factors may have a differential effect, depending on the interacting conditions of temperature, exposure to CO ₂ and the level of water stress on AFB ₁ production.