Aflatoxin Biosynthesis and Sclerotial Development in Aspergillus flavus and Aspergillus parasiticus

Chang, Perng‐Kuang

doi:10.1007/978-3-642-00725-5_6

Cited by 25 publications

(32 citation statements)

References 89 publications

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“…This earlier initiation of conidiation probably diverts the metabolic flux toward conidial formation, resulting in no sclerotia being produced, in contrast to the small amounts of sclerotia produced by the latter type of fluG complemented strains and the wild-type strain (Fig. 1B) (9). Conidiation and sclerotial production probably share common regulatory factors that enable the shift toward a developmental process depen- dent on either physiological or environmental conditions.…”

Section: Discussionmentioning

confidence: 97%

Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

Chang

Scharfenstein

Mack

et al. 2012

Appl Environ Microbiol

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The fluG gene is a member of a family of genes required for conidiation and sterigmatocystin production in Aspergillus nidulans. We examined the role of the Aspergillus flavus fluG orthologue in asexual development and aflatoxin biosynthesis. Deletion of fluG in A. flavus yielded strains with an approximately 3-fold reduction in conidiation but a 30-fold increase in sclerotial formation when grown on potato dextrose agar in the dark. The concurrent developmental changes suggest that A. flavus FluG exerts opposite effects on a mutual signaling pathway for both processes. The altered conidial development was in part attributable to delayed expression of brlA, a gene controlling conidiophore formation. Unlike the loss of sterigmatocystin production by A. nidulans fluG deletion strains, aflatoxin biosynthesis was not affected by the fluG deletion in A. flavus. In A. nidulans, FluG was recently found to be involved in the formation of dehydroaustinol, a component of a diffusible signal of conidiation. Coculturing experiments did not show a similar diffusible meroterpenoid secondary metabolite produced by A. flavus. These results suggest that the function of fluG and the signaling pathways related to conidiation are different in the two related aspergilli.A spergillus nidulans has been the model system for studying asexual conidiation at the molecular level. The genes brlA (bristle) (1), abaA (abacus) (39), and wetA (wet-white conidia) (29) are expressed sequentially and form the central developmental pathway of conidiation (17). BrlA is a transcription factor that mediates the budding growth of conidiophores. Mutations in brlA prevent vesicle formation, yielding stalks that grow indeterminately and that are unable to bear conidia (1). BrlA activates the expression of abaA, a gene that controls phialide differentiation (39). AbaA further mediates transcriptional activation of wetA (31) and also expression of brlA (3, 4). Efforts to elucidate early events leading to activation of conidiation in A. nidulans have identified six genes, fluG (fluffy) (2) and flbA to flbE (fluffy with low brlA expression), that are required for normal activation of brlA (42). Mutations of these genes result in proliferation of undifferentiated vegetative hyphae that produce fluffy cotton-like colonies. A. nidulans fluG is not transcribed in dormant conidia and is significantly downregulated during early vegetative growth; its expression before and after induction of conidiation does not change significantly (2, 7). A positive role of fluG in brlA activation has been suggested in A. nidulans, although the genetic relationship of brlA and fluG is not well defined. A. nidulans fluG mutants do not produce the secondary metabolite sterigmatocystin (20), the penultimate precursor of the carcinogenic aflatoxins. Because contact of the wild-type A. nidulans strain with the fluG null mutant remediates phenotypic defects (26), it was postulated that FluG synthesizes a diffusible factor that initiates conidiation. The effector molecules have recently ...

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Section: Discussionmentioning

confidence: 97%

Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

Chang

Scharfenstein

Mack

et al. 2012

Appl Environ Microbiol

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“…The isolates were incubated on potato dextrose agar (PDA; Difco) for 7 days at 30°C, before use. Twenty-five milliliters of potato dextrose broth (PDB; Difco) (which is aflatoxin conducive) in 50-ml Erlenmeyer flasks was inoculated with the WT, ⌴, and CM strains, using 0.2 ml of conidial suspension (ϳ10 6 conidia) for each flask; incubation was performed at 30°C for different time periods (10,12,14,18,21,24,30,36,42,48,60,72,96,120,144, and 168 h). In other experiments, 50 ml of Czapek Dox (CD) broth (Difco) (which is low-conducive for aflatoxins) was inoculated with the WT and M strains; after 4 days of incubation at 30°C, 1 mM cumene hydroperoxide (CH), 0.5 mM menadione (Men), and 1 mM and 10 mM hydrogen peroxide (H 2 O 2 ) were added to test the strain's sensitivity to oxidant stressors.…”

Section: Methodsmentioning

confidence: 99%

“…In A. nidulans, ROS can also steer the production of mitospores and meiospores in the regulation of the asexual and sexual phases in development (25,63). In Aspergillus parasiticus, ROS can control sclerotium formation (14), improving the resistance to adverse environmental conditions. In A. flavus and A. nidulans, the biosynthesis of mycotoxins is closely related to different stages of fungal develop-ment, such as conidiogenesis and sclerotium formation (51,56) in the idiophase, during which an increase in ROS occurs.…”

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confidence: 99%

Modulation of Antioxidant Defense in Aspergillus parasiticus Is Involved in Aflatoxin Biosynthesis: a Role for the Ap yapA Gene

et al. 2008

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Oxidative stress is recognized as a trigger of different metabolic events in all organisms. Various factors correlated with oxidation, such as the ␤-oxidation of fatty acids and their enzymatic or nonenzymatic byproducts (e.g., precocious sexual inducer factors and lipoperoxides) have been shown to be involved in aflatoxin formation. In the present study, we found that increased levels of reactive oxygen species (ROS) were correlated with increased levels of aflatoxin biosynthesis in Aspergillus parasiticus. To better understand the role of ROS formation in toxin production, we generated a mutant (⌬ApyapA) having the ApyapA gene deleted, given that ApyapA orthologs have been shown to be part of the antioxidant response in other fungi. Compared to the wild type, the mutant showed an increased susceptibility to extracellular oxidants, as well as precocious ROS formation and aflatoxin biosynthesis. Genetic complementation of the ⌬ApyapA mutant restored the timing and quantity of toxin biosynthesis to the levels found in the wild type. The presence of putative AP1 (ApYapA orthologue) binding sites in the promoter region of the regulatory gene aflR further supports the finding that ApYapA plays a role in the regulation of aflatoxin biosynthesis. Overall, our results show that the lack of ApyapA leads to an increase in oxidative stress, premature conidiogenesis, and aflatoxin biosynthesis.Reactive oxygen species (ROS), such as superoxide anionand lipoperoxides (LOOH), which are formed from unsaturated fatty acids and can be produced in the cell during metabolic processes, can be overproduced following the action of oxidative stressors present in the environment (32,49,57). To counteract the potentially dangerous accumulation of ROS, cells have evolved strategies (49, 61) based on enzymatic or nonenzymatic systems (28,45). The main antioxidant enzymes in cells involved in ROS removal are superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). If H 2 O 2 exceeds the cell-scavenging capacity, it can generate highly reactive HO ⅐ through a Fenton reaction, which initiates the formation of LOOH in the membrane lipids (32). When ROS accumulation occurs, the oxidant/antioxidant balance is perturbed, which can damage the cell membrane and cell metabolism (free-radical theory of aging) (26). ROS produced at certain time points during the cell's life cycle and at low physiological concentrations play a crucial role in the organism's homeostasis and cell functions. As second messengers, ROS take part in the plant's developmental processes (18,24,31) and in the defense mechanisms against pathogens and abiotic stress (5, 24, 52, 62). Similar effects have been shown in mammals, where ROS at proper levels stimulate antioxidant reactions, immune system modulation, and regulation of cell proliferation (3,4,55,59,65). One of the major objectives of studying the biology of stress is to identify the key factors that control the switch from cytoprotective responses to cell dysfunction following oxidative insult (11)....

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“…Fungal developmental processes are associated with secondary metabolism such as sporulation and sclerotial formation (Bennett et al, 1986;Calvo et al, 2002;Chang et al, 2002;Yu, 2006). Similar environmental conditions required for secondary metabolism and for sporulation are observed.…”

Section: Genetics and Biochemistry Of Aflatoxin Biosynthesismentioning

confidence: 99%

Strategies in Prevention of Preharvest Aflatoxin Contamination in Peanuts: Aflatoxin Biosynthesis, Genetics and Genomics

Guo¹,

Yu²,

Holbrook³

et al. 2009

Peanut Science

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Peanut (Arachis hypogaea L.), or groundnut, is an important crop economically and nutritionally in many tropical and subtropical areas of the world. It is also one of the most susceptible host crops to Aspergillus flavus resulting in aflatoxin contamination. The prevention or elimination of aflatoxin contamination in preharvest and postharvest crops is a serious challenge facing scientists. The recent International Conference on Groundnut Aflatoxin Management and Genomics held in Guangzhou, China, provided an international forum for discussions on the latest accomplishments, the development of strategies, and the initiation of cooperative research for the prevention of aflatoxin contamination. This review summarizes the progress in genetic and genomic research of peanuts and the toxinproducing fungus A. flavus. In particular, the pathway for production and the genetic regulation of afaltoxin, and the peanut-Aspergillus interaction are discussed. The use of a peanut-Aspergillus microarray will help scientists to study the croppathogen interaction; aids in the identification of genes involved in both fungal invasion and crop resistance, and ultimately enhance research to find solutions that prevent aflatoxin contamination in agricultural commodities.

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Aflatoxin Biosynthesis and Sclerotial Development in Aspergillus flavus and Aspergillus parasiticus

Cited by 25 publications

References 89 publications

Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

Modulation of Antioxidant Defense in Aspergillus parasiticus Is Involved in Aflatoxin Biosynthesis: a Role for the Ap yapA Gene

Strategies in Prevention of Preharvest Aflatoxin Contamination in Peanuts: Aflatoxin Biosynthesis, Genetics and Genomics

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