Abstract
Two morphologically different Aspergillus parasiticus strains, one producing aflatoxins, abundant conidia but few sclerotia (BN9) and the other producing O-methyl-sterimatocystin (OMST), copious sclerotia but a low number of conidia (RH), were used to assess the role of crzA which encodes a putative calcium-signaling pathway regulatory protein. Under standard culture conditions, BN9ΔcrzA mutants conidiated normally but decreased slightly in radial growth, regardless of illumination conditions. RHΔcrzA mutants produced only conidia under light and showed decreased conidiation and delayed sclerotial formation in the dark. Regulation of conidiation of both A. parasiticus strains by light was independent of crzA. Increased concentrations of lithium, sodium, and potassium impaired conidiation and sclerotial formation of the RHΔcrzA mutants but they did not affect conidiation of the BN9ΔcrzA mutants. Vegetative growth and asexual development of both ΔcrzA mutants were hypersensitive to increased calcium concentrations. Calcium supplementation (10 mM) resulted in 3-fold and 2-fold decreases in the relative expression of the endoplasmic reticulum calcium ATPase 2 gene in the BN9 and RH parental strains, respectively, but changes in both ΔcrzA mutants were less significant. Compared to the parental strains, the ΔcrzA mutants barely produced aflatoxins or OMST after the calcium supplementation. The relative expression levels of aflatoxin biosynthesis genes, nor1, ver1, and omtA, in both ΔcrzA mutants were decreased significantly, but the decreases in the parental strains were at much lower extents. CrzA is required for growth and development and for aflatoxin biosynthesis under calcium stress conditions.
Highlights
Several species in the genus Aspergillus, most notably A. flavus, A. parasiticus and A. nomius, produce the toxic and carcinogenic aflatoxins [1]
The results suggest that CrzA is an important determinant to calcium tolerance and has a positive role in aflatoxin biosynthesis under calcium stress
The role of A. parasiticus crzA was evaluated by a gene disruption vector constructed based on the A. flavus crzA sequence in respective A. parasiticus ∆ku70 recipient strains [36, 37]
Summary
Several species in the genus Aspergillus, most notably A. flavus, A. parasiticus and A. nomius, produce the toxic and carcinogenic aflatoxins [1]. In the U.S, A. parasiticus isolates that do not produce aflatoxins usually accumulate O-methylsterigmatocystin (OMST). It was estimated that OMST-accumulating isolates accounted for about 2.6% of an A. parasiticus population in a southwestern Georgia peanut field [3]. The synthesis of aflatoxins, a group of polyketide-derived secondary metabolites, is a complex process and is regulated at many levels [4,5,6]. Nutritional factors such as carbon and nitrogen sources [7,8,9,10,11], as well as metal ions and trace elements [12,13,14,15], affect aflatoxin production
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