Abstract
Aspergillus carbonarius, belonging to the group Nigri, is the main species responsible for contamination by ochratoxin A (OTA) in grapes and derivative products. OTA can accumulate in the mycelium and in black conidia of the fungus and released into the matrix. Here, we have deleted in A. carbonarius the alb1 orthologue gene of A. fumigatus, involved in melanin biosynthesis. Three A. carbonarius Δalb1 mutants were characterized for morphologic traits and OTA production on different media and temperatures. Δalb1 mutants showed a fawn color of conidia associated with a significant reduction of the conidiogenesis and a statistically significant increase (p ≤ 0.01) of total OTA production as compared to the wild type (WT) strain. The alb1 gene somehow affected OTA partitioning since in Δalb1 mutants OTA amount was lower in conidia and was more abundantly secreted into the medium as compared to the WT. On grape berries the Δalb1 mutants and the WT caused lesions with similar sizes but OTA amount in berry tissues was higher for the mutants. These results demonstrate that A. carbonarius conidia pigmentation is largely dependent on polyketide biosynthesis. The gene is not directly involved in virulence and its deletion affects morphological features and OTA production in the fungus.
Highlights
Aspergillus carbonarius, belonging to the section Nigri, is the main species responsible for the contamination by ochratoxin A (OTA) in grapes and derivatives [1,2,3]
A. fumigatus and the fwnA gene of A. niger involved in the first step of DHN-melanin biosynthesis. It consists of five exons separated by short (48–62 bp) introns and encodes for a type I polyketide synthase (PKS), composed by the domains ACP-transcyclase (SAT), β-ketoacyl synthase (KS), acyl transferase (AT), product template (PT), phosphopantetheine attachment site (PP) and thioesterase (TE) (Figure 1a,b)
The gene seems to be conserved in the ascomycetes (Table S1) and according to phylogenetic analysis of the AT domain, the sequence is generally clade based on sections and species within the genus Aspergillus, the highest bootstrap values include species having similar conidia pigmentation (Figure 1c)
Summary
Aspergillus carbonarius, belonging to the section Nigri (black Aspergilli), is the main species responsible for the contamination by ochratoxin A (OTA) in grapes and derivatives [1,2,3]. A common feature of all Aspergillus species belonging to section Nigri, including A. carbonarius, is the dark pigmentation of conidia, probably due to the synthesis of aspergillin resulting from the combination of the green pigment hexahydroxyl pentacyclic quinoid and the brown pigment melanin [5]. The genetic basis of conidia pigmentation has been fully elucidated in A. fumigatus, in which it is due to the synthesis of 1,8-dihydroxynaphtalene (1,8-DHN) melanin [8]. A cluster of six genes was identified to be involved in the biosynthetic pathway to produce 1,8-DHN melanin and inactivation of single genes in the cluster caused different pigmentation of conidia [9]. The cluster contains the genes alb (synonymous of pksP), ayg, arp, arp, abr, abr encoding, in the order, a polyketide synthase (PKS) involved in the production of the heptaketide naphtopyrone YWA1 [10], a putative hydrolase converting YWA1 into 1,3,6,8-tetrahydroxynaphtalene (1,3,6,8-THN) [11], an hydroxynaphthalene (HN) reductase yielding both scytalone and vermelone [8], a dehydratase converting scytalone in
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