Effect of temperature and water activity on gene expression and aflatoxin biosynthesis in Aspergillus flavus on almond medium

Almonds are among the commodities at risk of aflatoxin contamination by Aspergillus flavus. Temperature and water activity are the two key determinants in pre and post-harvest environments influencing both the rate of fungal spoilage and aflatoxin production. Varying the combination of these parameters can completely inhibit or fully activate the biosynthesis of aflatoxin, so it is fundamental to know which combinations can control or be conducive to aflatoxin contamination. Little information is available about the influence of these parameters on aflatoxin production on almonds. The objective of this study was to determine the influence of different combinations of temperature (20 degrees C, 28 degrees C, and 37 degrees C) and water activity (0.90, 0.93, 0.96, 0.99 a(w)) on growth, aflatoxin B-1 (AFB(1)) production and expression of the two regulatory genes, aflR and aflS, and two structural genes, aflD and aflO, of the aflatoxin biosynthetic cluster in A. flavus grown on an almond medium solidified with agar. Maximum accumulation of fungal biomass and AFB(1) production was obtained at 28 degrees C and 0.96 a(w); no fungal growth and AFB(1) production were observed at 20 degrees C at the driest tested conditions (0.90 and 0.93 a(w)). At 20 and 37 degrees C AFB(1) production was 70-90% lower or completely suppressed, depending on a(w). Reverse transcriptase quantitative PCR showed that the two regulatory genes (aflR and aflS) were highly expressed at maximum (28 degrees C) and minimum (20 degrees C and 37 degrees C) AFB(1) production. Conversely the two structural genes (aflD and aflO) were highly expressed only at maximum AFB(1) production (28 degrees C and 0.96-0.99 a(w)). It seems that temperature acts as a key factor influencing aflatoxin production which is strictly correlated to the induction of expression of structural biosynthesis genes (aflD and aflO), but not to that of aflatoxin regulatory genes (aflR and aflS), whose functional products are most likely subordinated to other regulatory processes acting at post-translational level. The results of this study are useful to select conditions that could be used in the almond processing chain to suppress aflatoxin production in this important product. (C) 2015 Elsevier B.V. All rights reserved.