Transcriptome analysis of non-ochratoxigenic Aspergillus carbonarius strains and interactions between some black aspergilli species

Application and antagonistic mechanisms of atoxigenic Aspergillus strains for the management of fungal plant diseases

This review covers, for the first time, all methods based on the use of Aspergillus strains as biocontrol agents for the management of plant diseases caused by fungi and oomycetes. Atoxigenic Aspergillus strains have been screened in a variety of hosts, such as peanuts, maize kernels, and legumes, during the preharvest and postharvest stages. These strains have been screened against a wide range of pathogens, such as Fusarium, Phytophthora, and Pythium species, suggesting a broad applicability spectrum. The highest efficacies were generally observed when using non-toxigenic Aspergillus strains for the management of mycotoxin-producing Aspergillus strains. The modes of action included the synthesis of antifungal metabolites, such as kojic acid and volatile organic compounds (VOCs), secretion of hydrolytic enzymes, competition for space and nutrients, and induction of disease resistance. Aspergillus strains degraded Sclerotinia sclerotiorum sclerotia, showing high control efficacy against this pathogen. Collectively, although two Aspergillus strains have been commercialized for aflatoxin degradation, a new application of Aspergillus strains is emerging and needs to be optimized.

Unlocking the Transcriptional Reprogramming Repertoire between Variety-Dependent Responses of Grapevine Berries to Infection by Aspergillus carbonarius

Aspergillus carbonarius causes severe decays on berries in vineyards and is among the main fungal species responsible for grape contamination by ochratoxin A (OTA), which is the foremost mycotoxin produced by this fungus. The main goal of this study was to investigate at the transcriptome level the comparative profiles between two table grape varieties (Victoria and Fraoula, the white and red variety, respectively) after their inoculation with a virulent OTA-producing A. carbonarius strain. The two varieties revealed quite different transcriptomic signatures and the expression profiles of the differential expressed genes (DEGs) highlighted distinct and variety-specific responses during the infection period. The significant enrichment of pathways related to the modulation of transcriptional dynamics towards the activation of defence responses, the triggering of the metabolic shunt for the biosynthesis of secondary metabolites, mainly phenylpropanoids, and the upregulation of DEGs encoding phytoalexins, transcription factors, and genes involved in plant-pathogen interaction and immune signaling transduction was revealed in an early time point in Fraoula, whereas, in Victoria, any transcriptional reprogramming was observed after a delay. However, both varieties, to some extent, also showed common expression dynamics for specific DEG families, such as those encoding for laccases and stilbene synthases. Jasmonate (JA) may play a critical modulator role in the defence machinery as various JA-biosynthetic DEGs were upregulated. Along with the broader modulation of the transcriptome that was observed in white grape, expression profiles of specific A. carbonarius genes related to pathogenesis, fungal sporulation, and conidiation highlight the higher susceptibility of Victoria. Furthermore, the A. carbonarius transcriptional patterns directly associated with the regulation of the pathogen OTA-biosynthesis gene cluster were more highly induced in Victoria than in Fraoula. The latter was less contaminated by OTA and showed substantially lower sporulation. These findings contribute to uncovering the interplay beyond this plant-microbe interaction.

Isolation and Characterization of Paenibacillus polymyxa B7 and Inhibition of Aspergillus tubingensis A1 by Its Antifungal Substances

Screening of Bacillus with antagonistic effects on paddy mold pathogens to provide strain resources for biological control of mold in Oryza sativa L. screening of Bacillus isolates antagonistic towards Aspergillus tubingensis from rhizosphere soil of healthy paddy; classification and identification of antagonistic strains by biological characteristics and 16S rDNA sequence analysis; transcriptome sequencing after RNA extraction from Bacillus-treated Aspergillus tubingensis; and extraction of inhibitory crude proteins of Bacillus by ammonium sulfate precipitation; inhibitory crude protein and Bacillus spp. were treated separately for A. tubingensis and observed by scanning electron microscopy (SEM). An antagonistic strain of Bacillus, named B7, was identified as Paenibacillus polymyxa by 16S rDNA identification and phylogenetic evolutionary tree comparison analysis. Analysis of the transcriptome results showed that genes related to secondary metabolite biosynthesis such as antifungal protein were significantly downregulated. SEM results showed that the mycelium of A. tubingensis underwent severe rupture after treatment with P. polymyxa and antifungal proteins, respectively. In addition, the sporocarp changed less after treatment with P. polymyxa, and the sporangium stalks had obvious folds. P. polymyxa B7 has a good antagonistic effect against A. tubingensis and has potential for biocontrol applications of paddy mold pathogens.

Exploring the Biocontrol Capability of Non-Mycotoxigenic Strains of Penicillium expansum

Penicillium expansum is one the major postharvest pathogens of pome fruit during postharvest handling and storage. This fungus also produces patulin, which is a highly toxic mycotoxin that can contaminate infected fruits and their derived products and whose levels are regulated in many countries. In this study, we investigated the biocontrol potential of non-mycotoxigenic strains of Penicillium expansum against a mycotoxigenic strain. We analyzed the competitive behavior of two knockout mutants that were unable to produce patulin. The first mutant (∆patK) involved the deletion of the patK gene, which is the initial gene in patulin biosynthesis. The second mutant (∆veA) involved the deletion of veA, which is a global regulator of primary and secondary metabolism. At the phenotypic level, the ∆patK mutant exhibited similar phenotypic characteristics to the wild-type strain. In contrast, the ∆veA mutant displayed altered growth characteristics compared with the wild type, including reduced conidiation and abnormal conidiophores. Neither mutant produced patulin under the tested conditions. Under various stress conditions, the ∆veA mutants exhibited reduced growth and conidiation when exposed to stressors, including cell membrane stress, oxidative stress, osmotic stress, and different pH values. However, no significant changes were observed in the ∆patK mutant. In competitive growth experiments, the presence of non-mycotoxigenic strains reduced the population of the wild-type strain during in vitro growth. Furthermore, the addition of either of the non-mycotoxigenic strains resulted in a significant decrease in patulin levels. Overall, our results suggest the potential use of non-mycotoxigenic mutants, particularly ∆patK mutants, as biocontrol agents to reduce patulin contamination in food and feed.

Ochratoxin A Defective Aspergillus carbonarius Mutants as Potential Biocontrol Agents

Aspergillus carbonarius is one of the main species responsible for wine, coffee and cocoa toxin contamination. The main mycotoxin produced by this fungus, ochratoxin A (OTA), is a secondary metabolite categorized as a possible carcinogen because of its significant nephrotoxicity and immunosuppressive effects. A polyketide synthase gene (otaA) encodes the first enzyme in the OTA biosynthetic pathway. It is known that the filamentous fungi, growth, development and production of secondary metabolites are interconnected processes governed by global regulatory factors whose encoding genes are generally located outside the gene clusters involved in the biosynthesis of each secondary metabolite, such as the veA gene, which forms part of the VELVET complex. Different fungal strains compete for nutrients and space when they infect their hosts, and safer non-mycotoxigenic strains may be able to outcompete mycotoxigenic strains during colonization. To determine the possible utility of biopesticides based on the competitive exclusion of mycotoxigenic strains by non-toxigenic ones, we used A. carbonarius ΔotaA and ΔveA knockout mutants. Our results showed that during both in vitro growth and infection of grapes, non-mycotoxigenic strains could outcompete the wild-type strain. Additionally, the introduction of the non-mycotoxigenic strain led to a drastic decrease in OTA during both in vitro growth and infection of grapes.

Functional Role of Aspergillus carbonariusAcOTAbZIP Gene, a bZIP Transcription Factor within the OTA Gene Cluster

Aspergillus carbonarius is the principal fungal species responsible for ochratoxin A (OTA) contamination of grapes and derived products in the main viticultural regions worldwide. In recent years, co-expressed genes representing a putative-OTA gene cluster were identified, and the deletion of a few of them allowed the partial elucidation of the biosynthetic pathway in the fungus. In the putative OTA-gene cluster is additionally present a bZIP transcription factor (AcOTAbZIP), and with this work, A. carbonarius ΔAcOTAbZIP strains were generated to study its functional role. According to phylogenetic analysis, the gene is conserved in the OTA-producing fungi. A Saccharomyces cerevisiae transcription factor binding motif (TFBM) homolog, associated with bZIP transcription factors was present in the A. carbonarius OTA-gene cluster no-coding regions. AcOTAbZIP deletion results in the loss of OTA and the intermediates OTB and OTβ. Additionally, in ΔAcOTAbZIP strains, a down-regulation of AcOTApks, AcOTAnrps, AcOTAp450, and AcOTAhal genes was observed compared to wild type (WT). These results provide evidence of the direct involvement of the AcOTAbZIP gene in the OTA biosynthetic pathway by regulating the involved genes. The loss of OTA biosynthesis ability does not affect fungal development as demonstrated by the comparison of ΔAcOTAbZIP strains and WT strains in terms of vegetative growth and asexual sporulation on three different media. Finally, no statistically significant differences in virulence were observed among ΔAcOTAbZIP strains and WT strains on artificially inoculated grape berries, demonstrating that OTA is not required by A. carbonarius for the pathogenicity process.

Carbon Catabolite Repression Gene AoCreA Regulates Morphological Development and Ochratoxin A Biosynthesis Responding to Carbon Sources in Aspergillus ochraceus

Carbon is one of the most important nutrients for the development and secondary metabolism in fungi. CreA is the major transcriptional factor mediating carbon catabolite repression, which is employed in the utilization of carbon sources. Aspergillus ochraceus contaminates various food and feed containing different carbon sources by producing ochratoxin A (OTA). However, little is known about the function of AoCreA in regulating the morphology and OTA production of A. ochraceus. To give an insight into the mechanism of the carbon sources regulating development of A. ochraceus and OTA production, we have identified AoCreA in A. ochraceus. The homologous recombination strategy was used to generate the AoCreA deletion mutant (ΔAoCreA). We have investigated the morphology and OTA production of the wild type (WT) and ΔAoCreA of A. ochraceus with media containing different carbon sources (glucose, fructose, maltose, D-xylose, D-mannose, acetate, D-galactose, D-mannitol and lactose). ΔAoCreA showed a significant growth and conidiation defect on all media as compared with WT. Glucose and maltose were the most inducing media for OTA production by A. ochraceus, followed by sucrose and the nutrient-rich Yeast Extract Sucrose (YES) and Potato Dextrose Agar (PDA). The deletion of AoCreA led to a drastic reduction of OTA production on all kinds of media except PDA, which was supported by the expression profile of OTA biosynthetic genes. Furthermore, infection studies of ΔAoCreA on oats and pears showed the involvement of AoCreA in the pathogenicity of A. ochraceus. Thus, these results suggest that AoCreA regulates morphological development and OTA biosynthesis in response to carbon sources in A. ochraceus.