Loss of msnA, a Putative Stress Regulatory Gene, in Aspergillus parasiticus and Aspergillus flavus Increased Production of Conidia, Aflatoxins and Kojic Acid

Abstract

Production of the harmful carcinogenic aflatoxins by Aspergillus parasiticus and Aspergillus flavus has been postulated to be a mechanism to relieve oxidative stress. The msnA gene of A. parasiticus and A. flavus is the ortholog of Saccharomyces cerevisiae MSN2 that is associated with multi-stress response. Compared to wild type strains, the msnA deletion (∆msnA) strains of A. parasiticus and A. flavus exhibited retarded colony growth with increased conidiation. The ∆msnA strains also produced slightly higher amounts of aflatoxins and elevated amounts of kojic acid on mixed cereal medium. Microarray assays showed that expression of genes encoding oxidative stress defense enzymes, i.e., superoxide dismutase, catalase, and cytochrome c peroxidase in A. parasiticus ∆msnA, and the catalase A gene in A. flavus ∆msnA, was up-regulated. Both A. parasiticus and A. flavus ∆msnA strains produced higher levels of reactive oxygen species (ROS), and ROS production of A. flavus msnA addback strains was decreased to levels comparable to that of the wild type A. flavus. The msnA gene appears to be required for the maintenance of the normal oxidative state. The impairment of msnA resulted in the aforementioned changes, which might be used to combat the increased oxidative stress in the cells.

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

“…At the transcriptional level, the activation of the gene cluster encoding the proteins for aflatoxin biosynthesis requires AflR expression. Moreover, transcription factors that control the expression of genes involved in the oxidative response (e.g., AtfB, AP1, MsnA, and Srr) also contribute to the modulation of the genes influencing aflatoxin biosynthesis (20)(21)(22).…”

“…At the transcriptional level, the activation of the gene cluster encoding the proteins for aflatoxin biosynthesis requires AflR expression. Moreover, transcription factors that control the expression of genes involved in the oxidative response (e.g., AtfB, AP1, MsnA, and Srr) also contribute to the modulation of the genes influencing aflatoxin biosynthesis (20)(21)(22).In this context, the main objective of the present study is to elucidate (i) whether oxidative stress regulates sclerotial differentiation in A. flavus, (ii) whether SD and aflatoxin B1 biosynthesis are comodulated by oxidative stress, through comparative analysis with a ⌬veA mutant strain that does not produce sclerotia and aflatoxin, and (iii) whether aflatoxin B1 biosynthesis and SD are controlled by different parameters of oxidative stress. …”

A transcription factor, MrMsn2, in the dimorphic fungus Metarhizium rileyi is essential for dimorphism transition, aggravated pigmentation, conidiation and microsclerotia formation

“…Moreover, similar to A. parasiticus and A. flavus (Chang et al ., 2011), deletion of MrMsn2 inhibited growth but increased production of conidia (Fig. 1).…”

“…MrMsn2 belongs to a group of proteins containing C 2 H 2 ‐like Zn finger domains that are important in development, secondary metabolism and stress responses (Chang et al ., 2011; Liu et al ., 2013; Zhang et al ., 2014; Tian et al ., 2017). In this study, ▵MrMsn2 mutants negatively controlled the yeast‐to‐hypha transition (Figs 1 and 2).…”

“…They are similar to ScMsn2/4 of Saccharomyces cerevisiae (Schmitt and Mcentee, 1996) and have been characterized in Aspergillus parasiticus , A. flavus , Beauveria bassiana , Magnaporthe oryzae , Metarhizium robertsii and Verticillium dahliae (Chang et al ., 2011; Liu et al ., 2013; Zhang et al ., 2014; Tian et al ., 2017). Under abiotic and biotic stresses, Msn2/4 is phosphorylated for translocation from the cytoplasm to the nucleus, where it drives the transcription of stress‐induced genes (Hansen et al ., 2015; Yi and Huh, 2015; Li et al ., 2017).…”