Interaction of water activity and temperature on the growth, gene expression and aflatoxin production by Aspergillus flavus on paddy and polished rice

Water activity (a(w)) and temperature are two pivotal environmental factors affecting Aspergillus flavus growth and aflatoxin production. Here, we found that AFB(1) production on polished rice can occur over a wider range of temperature x a(w) levels than that on paddies. For fungal growth on polished rice, the optimum conditions were a(w) 0.92-0.96 and 28-37 degrees C. The maximum amounts of AFB(1) on polished rice was observed at 33 degrees C and a(w) 0.96. Compared to 33 degrees C, all tested genes of A. flavus on polished rice were significantly up-regulated at 25 degrees C under a(w) 0.96. The late structural genes of pathway were significantly down-regulated at 37 degrees C under a(w) 0.96, although aflR and aflS and most of early structural genes were up-regulated. Compared to a(w) 0.96, most of pathway genes were significantly down-regulated at a(w) 0.90 and 0.99 under 33 degrees C, although two regulatory genes were up-regulated at a(w) 0.90.