Role of Oxidative Stress in Sclerotial Differentiation and Aflatoxin B1 Biosynthesis in Aspergillus flavus

Abstract

cWe show here that oxidative stress is involved in both sclerotial differentiation (SD) and aflatoxin B1 biosynthesis in Aspergillus flavus. Specifically, we observed that (i) oxidative stress regulates SD, as implied by its inhibition by antioxidant modulators of reactive oxygen species and thiol redox state, and that (ii) aflatoxin B1 biosynthesis and SD are comodulated by oxidative stress. However, aflatoxin B1 biosynthesis is inhibited by lower stress levels compared to SD, as shown by comparison to undifferentiated A. flavus. These same oxidative stress levels also characterize a mutant A. flavus strain, lacking the global regulatory gene veA. This mutant is unable to produce sclerotia and aflatoxin B1. (iii) Further, we show that hydrogen peroxide is the main modulator of A. flavus SD, as shown by its inhibition by both an irreversible inhibitor of catalase activity and a mimetic of superoxide dismutase activity. On the other hand, aflatoxin B1 biosynthesis is controlled by a wider array of oxidative stress factors, such as lipid hydroperoxide, superoxide, and hydroxyl and thiyl radicals. Humans and animals are exposed to carcinogenic aflatoxins through contaminated food and feed, air, and drinking water (1, 2). Aspergillus flavus is the primary cause of aflatoxin-contaminated crops. A. flavus is a heterothallic fungus, and laboratory crosses produce ascospore-bearing ascocarps embedded within sclerotia. In the field, sclerotia are dispersed during crop harvest and require an additional incubation period on the soil for sexual reproduction (3). Despite the significant contribution of A. flavus to crop aflatoxin contamination, it is not yet known what the role of oxidative stress is for its sclerotial differentiation (SD) and aflatoxin B1 biosynthesis. Deciphering this relationship could contribute to the development of nontoxic antifungal means via the coinhibition of A. flavus SD and aflatoxin B1 biosynthesis.Several toxigenic and phytopathogenic fungi spread and survive in nature through the formation of conidiophores and resistant sclerotia. It has been known that oxidative stress regulates the sclerotial differentiation of filamentous phytopathogenic fungi such as Rhizoctonia solani, Sclerotium rolfsii, Sclerotinia sclerotiorum, and Sclerotinia minor (4, 5). Moreover, it has been established that the regulation of morphogenesis in aspergilli and other fungi is genetically linked to secondary metabolism (6-9). In A. flavus, both SD and aflatoxin biosynthesis are governed by the regulatory protein VeA (10). Deletion of the veA gene in this fungus results in the inhibition of sclerotia formation and aflatoxin biosynthesis (10). However, it is not known whether SD in A. flavus is regulated by oxidative stress and whether the deletion of veA could alter its oxidative stress levels.Previous reports have linked aflatoxin biosynthesis with oxidative stress in A. flavus and A. parasiticus both at the metabolic and transcriptional levels. Specifically, aflatoxin biosynthesis in both species is activated by high ...

Effects of Growth Parameters on the Analysis of Aspergillus flavus Volatile Metabolites

“…Relative humidity, temperature, substrate (growth medium), number of fungal spores inoculated and oxidation stress are the main factors influencing fungal growth, metabolism and MVOCs' production in a laboratory setting [20][21][22]. For example, López-Malo et al [23] studied the effect of incubation temperature (10-30˝C), pH (3.0-4.0) and vanillin concentration (350-1200 ppm) on the growth of A. flavus.…”

Further characterization of the role of the mitochondrial high-mobility group box protein in the intracellular redox environment of Aspergillus nidulans

“…An intriguing relation between oxidative stress, secondary metabolite production and sexual development has been unfolding during the last few years in Aspergillus, which was initiated by the work of Reverberi et al (2007) on Aspergillus parasiticus and Aspergillus ochraceus, and further studied by many others (Baidya et al, 2014;Grintzalis et al, 2014;Reverberi et al, 2012;Wartenberg et al, 2012;Yin et al, 2013). In A. nidulans, the stress response proteins NapA and DlpA have been shown to play a role in developmental processes and sterigmatocystin production (Wartenberg et al, 2012;Yin et al, 2013).…”

Aflatoxins: biosynthesis, prevention and eradication

“…AFB 1 biosynthesis is controlled by a wider array of oxidative stress factors, such as lipid hydroperoxide, superoxide, and hydroxyl and thiol radicals (Grintzalis et al, 2014). Grintzalis et al 2014 observed that (i) oxidative stress regulates fungal sexual development, as implied by its inhibition by ROS and thiol redox state, and that (ii) the AFB 1 biosynthesis and sexual development are coregulated by the oxidative stress. Numerous studies have shown that the AF production can be exacerbated by ROS and their reactive products including oxylipins and peroxidized lipids (Fabbri et al, 1983;Gao and Kolomiets, 2009).…”

“…AF production apparently could direct the ROS excess accumulated during cell ageing, or as a response to external insults, into extending the fungal lifespan (Hong et al, 2013). AFB 1 biosynthesis is controlled by a wider array of oxidative stress factors, such as lipid hydroperoxide, superoxide, and hydroxyl and thiol radicals (Grintzalis et al, 2014). Grintzalis et al 2014 observed that (i) oxidative stress regulates fungal sexual development, as implied by its inhibition by ROS and thiol redox state, and that (ii) the AFB 1 biosynthesis and sexual development are coregulated by the oxidative stress.…”

“…has been extensively studied, and has been shown to be connected to reactive oxygen species (ROS) formation and oxidative stress responses (Zaccaria et al, 2015). The AF biosynthesis is not only regulated by intracellular ROS of different origins (Reverberi et al, 2006;Grintzalis et al, 2014), but the AF biosynthetic pathway itself may serve as a source of ROS due to involvement of P450 enzymes (Roze et al, Tab. 1.…”