Alternariol as virulence and colonization factor of Alternaria alternata during plant infection

Synthesis and effects of zinc-chitosan complex on morpho-growth and biochemical responses in Alternaria alternata, a cause of leaf blights and spots on plants

The increasing resistance of fungal pathogens to synthetic fungicides necessitates eco-friendly alternatives. Zinc-chitosan (Zn-Ch) complexe offer a sustainable solution due to their biocompatibility and antifungal properties. This study synthesized and characterized Zn-Ch using UV-vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and fluorescence spectral analysis, confirming strong Zn²⁺ interactions with chitosan. In vitro bioassays evaluated antifungal efficacy across different treatments: Mancozeb (chemical control), chitosan alone (Ch), non-complexed Zn (2.5 and 5.0 ppm) + Ch, and the Zn (5.0 ppm)-Ch complex. In all treatments, Mancozeb and Ch were tested at identical concentrations (0.5-2.5%) to ensure comparative analysis. Mancozeb and Ch exhibited dose-dependent inhibition, reaching 66.67% and 78.57% at 2.5%, respectively. Non-complexed Zn (2.5 and 5.0 ppm) + Ch demonstrated enhanced antifungal activity, with 5.0 ppm achieving 89.03% inhibition, while the Zn (5.0 ppm)-Ch complex exhibited the highest antifungal efficacy (99% inhibition) with severe hyphal distortion and delayed sporulation. Biochemical assays revealed an initial increase in catalase (CAT), peroxidase (POX), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activities, peaking at 1.0-1.5%, followed by significant declines at higher concentrations. The Zn (5.0 ppm)-Ch complex caused the steepest enzymatic decline at 2.0-2.5%, suggesting oxidative stress-induced fungal suppression. Collectively, these findings confirmed the potential of the Zn-Ch complex in enhancing antifungal efficacy by altering fungal morphology, suppressing growth, and activating enzymatic defense responses in plants. Zn-Ch emerged as a promising, biocompatible alternative to synthetic fungicides, warranting further in vivo validation for its application in sustainable crop protection strategies.

Metabolic Blockade-Based Genome Mining of Malbranchea circinata SDU050: Discovery of Diverse Secondary Metabolites

Malbranchea circinata SDU050, a fungus derived from deep-sea sediment, is a prolific producer of diverse secondary metabolites. Genome sequencing revealed the presence of at least 69 biosynthetic gene clusters (BGCs), including 30 encoding type I polyketide synthases (PKSs). This study reports the isolation and identification of four classes of secondary metabolites from wild-type M. circinata SDU050, alongside five additional metabolite classes, including three novel cytochalasins (7-9), obtained from a mutant strain through the metabolic blockade strategy. Furthermore, bioinformatic analysis of the BGC associated with the isocoumarin sclerin (1) enabled the deduction of its biosynthetic pathway based on gene function predictions. Bioactivity assays demonstrated that sclerin (1) and (-)-mycousnine (10) exhibited weak antibacterial activity against Gram-positive bacteria such as Staphylococcus aureus, methicillin-resistant Staphylococcus aureus (MRSA), and Bacillus subtilis. These findings underscore the chemical diversity and biosynthetic potential of M. circinata SDU050 and highlight an effective strategy for exploring marine fungal metabolites.

Haplotype diversity and phylogeny within Alternaria alternata and A. arborescens species complexes from tomatoes

Tomato (Solanum lycopersicum L.) is an economically important vegetable susceptible to various fungal diseases, including leaf spot caused by Alternaria spp. from the section Alternaria. In our study, a total of 72 tomato-associated Alternaria spp. strains from Latvia, Lithuania, Estonia, and Algeria were analysed by integrating morphological data, pathogenicity assay, multi-locus phylogeny, and haplotype assignment. Recovered Alternaria spp. strains were characterized by considerable variation in phenotypic diversity, non-pathogenicity to their host of origin and absence of the AAL-toxin biosynthesis gene (ALT1). Multi-locus phylogeny of the RNA polymerase II second largest subunit (rpb2), putative F-box-domain-containing protein (ASA-10), and putative histone-like transcription factor (ASA-19) confirmed the occurrence of both A. alternata and A. arborescens species complexes along with A. longipes and A. postmessia on symptomatic tomatoes. The discordant tree topology among single-gene phylogenies suggested the occurrence of potential recombination between phylogenetic lineages in the section Alternaria, resulting in putative alternata-arborescens and alternata-longipes hybrids. DNA polymorphism analysis of the rpb2, ASA-10, and ASA-19 loci revealed a high level of genetic diversity in the section Alternaria, and the number of single nucleotide polymorphisms (SNPs) and haplotypes varied among loci and lineages studied. A total of 16 and 6 multi-locus haplotypes were assigned in alternata and arborescens lineages, respectively. Global genetic diversity analysis of A. alternata and A. arborescens strains at the rpb2 locus confirmed that major haplotypes described from tomatoes were shared among other hosts of origin.

Research progress on inhibitors and inhibitory mechanisms of mycotoxin biosynthesis

Mycotoxins are secondary metabolites produced by fungi with harmful effects such as carcinogenicity, teratogenicity, nephrotoxicity, and hepatotoxicity. They cause widespread contamination of plant products such as crops, food, and feed, posing serious threats to the life and health of human beings and animals. It has been found that many traditionally synthesized and natural compounds are capable of inhibiting the growth of fungi and their secondary metabolite production. Natural compounds have attracted much attention due to their safety, environmental, and health friendly features. In this paper, compounds of plant origin with inhibitory effects on ochratoxins, aflatoxins, Fusarium toxins, and Alternaria toxins, including cinnamaldehyde, citral, magnolol, eugenol, pterostilbene, curcumin, and phenolic acid, are reviewed, and the inhibitory mechanisms of different compounds on the toxin production of fungi are also elucidated, with the aim of providing application references to reduce the contamination of fungal toxins, thus safeguarding the health of human beings and animals.

The combination of metabolome and transcriptome clarifies the inhibition of the Alternaria toxin accumulation by methyl ferulate

As one of the most typical pathogens in fruit postharvest diseases, Alternaria alternata (A. alternata) can produce Alternaria toxins (ATs) aggravating fruit decay and harming human health. In this study, ATs (tenuazonic acid, alternariol monomethyl ether, and alternariol) production was inhibited effectively by 200 and 8000 mg/L MF (methyl ferulate) in vitro and in vivo. 1-Octen-3-ol and 3-octanol were the potential iconic volatile organic compounds of ATs (R2 > 0.99). MF induced oxidative stress, resulting in physiological and metabolic disorders, membrane lipid oxidation and cell damage. It decreased precursors and energy supply by disturbing amino acid metabolism, ABC transporters, citrate cycle, pentose and glucuronate interconversions to regulate ATs synthesis. MF down-regulated the genes related to ATs synthesis (PksJ, AaTAS1, and OmtI), transport (AaMFS1 and MFS), and pathogenicity to affect ATs production and virulence. This study provided a theoretical basis for the control of ATs production.

Beyond morphogenesis and secondary metabolism: function of Velvet proteins and LaeA in fungal pathogenesis

Velvet proteins, as well as the epigenetic regulator LaeA, are conserved in numerous fungal species, where, in response to environmental cues, they control several crucial cellular processes, including sexual and asexual morphogenesis, secondary metabolism, response to oxidative stress, and virulence. During the last two decades, knowledge of their mechanism of action as well as understanding their functional roles, has greatly increased, particularly in Aspergillus species. Research efforts from multiple groups followed, leading to the characterization of other Velvet and LaeA homologs in species of other fungal genera, including important opportunistic plant and animal pathogens. This review focuses mainly on the current knowledge of the role of Velvet and LaeA function in fungal pathogenesis. Velvet proteins and LaeA are unique to fungi, and for this reason, additional knowledge of these critical regulatory proteins will be important in the development of targeted control strategies to decrease the detrimental impact of fungal pathogens capable of causing disease in plants and animals.

Natural resorcylic lactones derived from alternariol

In this overview, naturally occurring resorcylic lactones biosynthetically derived from alternariol and almost exclusively produced by fungi, are discussed with view on their isolation, structure, biological activities, biosynthesis, and total syntheses. This class of compounds consists until now of 127 naturally occurring compounds, with very divers structural motifs. Although only a handful of these toxins (i.e., alternariol and its 9-O-methyl ether, altenusin, dehydroaltenusin, altertenuol, and altenuene) were frequently found and isolated as fungal contaminants in food and feed and have been investigated in significant detail, further metabolites, which were much more rarely found as natural products, similarly show interesting biological activities.

Advances in CRISPR-Cas systems for fungal infections

Fungi contain a wide range of bioactive secondary metabolites (SMs) that have numerous applications in various fields, including agriculture, medicine, human health, and more. It is common for genes responsible for the production of secondary metabolites (SMs) to form biosynthetic gene clusters (BGCs). The identification and analysis of numerous unexplored gene clusters (BGCs) and their corresponding substances (SMs) has been significantly facilitated by the recent advancements in genomic and genetic technologies. Nevertheless, the exploration of secondary metabolites with commercial value is impeded by a variety of challenges. The emergence of modern CRISPR/Cas technologies has brought about a paradigm shift in fungal genetic engineering, significantly streamlining the process of discovering new bioactive compounds. This study begins with an examination of fungal biosynthetic gene clusters (BGCs) and their interconnections with the secondary metabolites (SMs) they generate. Following that, a brief summary of the conventional methods employed in fungal genetic engineering is provided. This study explores various sophisticated CRISPR/Cas-based methodologies and their utilization in examining the synthesis of secondary metabolites (SMs) in fungi. The chapter provides an in-depth analysis of the limitations and obstacles encountered in CRISPR/Cas-based systems when applied to fungal genetic engineering. It also proposes promising avenues for future research to optimize the efficiency of these systems.

Regulation of nitrogen utilization and mycotoxin biosynthesis by the GATA transcription factor AaAreA in Alternaria alternata

Alternaria alternata is a prevalent postharvest pathogen that generates diverse mycotoxins, notably alternariol (AOH) and alternariol monomethyl ether (AME), which are recurrent severe contaminants. Nitrogen sources modulate fungal growth, development, and secondary metabolism, including mycotoxin production. The GATA transcription factor AreA regulates nitrogen source utilization. However, little is known about its involvement in the regulation of nitrogen utilization in A. alternata. To examine the regulatory mechanism of AaAreA on AOH and AME biosynthesis in A. alternata, we analyzed the impact of diverse nitrogen sources on the fungal growth, conidiation and mycotoxin production. The use of a secondary nitrogen source (NaNO3) enhanced mycelial elongation and sporulation more than the use of a primary source (NH4Cl). NaNO3 favored greater mycotoxin accumulation than did NH4Cl. The regulatory roles of AaAreA were further clarified through gene knockout. The absence of AaAreA led to an overall reduction in growth in minimal media containing any nitrogen source except NH4Cl. AaAreA positively regulates mycotoxin biosynthesis when both NH4Cl and NaNO3 are used as nitrogen sources. Subcellular localization analysis revealed abundant nuclear transport when NaNO3 was the sole nitrogen source. The regulatory pathway of AaAreA was systematically revealed through comprehensive transcriptomic analyses. The deletion of AaAreA significantly impedes the transcription of mycotoxin biosynthetic genes, including aohR, pksI and omtI. The interaction between AaAreA and aohR, a pathway-specific transcription factor gene, demonstrated that AaAreA binds to the aohR promoter sequence (5'-GGCTATGGAAA-3'), activating its transcription. The expressed AohR regulates the expression of downstream synthase genes in the cluster, ultimately impacting mycotoxin production. This study provides valuable information to further understand how AreA regulates AOH and AME biosynthesis in A. alternata, thereby enabling the effective design of control measures for mycotoxin contamination.

Inhibitory Effect and Mechanism of Carvacrol against Black Mold Disease Agent Alternaria alternata in Goji Berries

Alternaria alternata, as a main decay fungus of goji berry, can produce mycotoxins such as alternariol (AOH), alternariol monomethyl ether (AME), and tenuazonic acid (TeA). Carvacrol (CVR) has exhibited a broad-spectrum antifungal activity in vitro. We assumed that CVR can also be applied to control Alternaria rot on goji berries and mycotoxins produced by the pathogens. To investigate whether CVR impacts the accumulation of mycotoxins and cell membrane damage of A. alternata, the antifungal activity of CVR on the fungal growth and mycotoxin production was evaluated in this study. The results showed that the minimum inhibitory concentration (MIC) of CVR against A. alternata was 0.12 µL/mL. Meanwhile, the destruction of plasma membrane integrity, cytoplasmic leakage, intracellular oxidative damage, and inhibitory effect in vivo were also observed in A. alternata treated with CVR. Moreover, CVR significantly reduced the accumulation of AOH, AME, and TeA. Transcriptomic profiling was performed by means of comparative RNA-Seq analysis to research the gene expression level of A. alternata, which attested to significant changes in nitrogen metabolism, carbon utilization, fatty acid oxidation, and antioxidant enzymes in CVR-treated A. alternata. This study suggests a new understanding of the molecular mechanism of response to CVR treatment in A. alternata, indicating that CVR is a novel antifungal agent with the potential to be applied to various fungi.

The HOG-pathway related AaOS1 leads to dicarboximide-resistance in field strains of Alternaria alternata and contributes, together with the Aafhk1, to mycotoxin production and virulence

BACKGROUND

Garlic leaf spot (GLS) caused by Alternaria alternata is one of the main diseases in the garlic production areas, and its management heavily relies on dicarboximide fungicides. However, the efficacy of dicarboximides against the GLS disease has decreased year on year.

RESULTS

In the present study, 10 of 148 A. alternata strains separated from Jiangsu Province were moderately resistant (MR) to a dicarboximide fungicide procymidone (ProMR). Positive cross-resistance was observed between Pro and iprodione (Ipro) or fludioxonil (Fld), but not between Pro and fluazinam or azoxystrobin. Mutations at AaOS1, but not Aafhk1, were confirmed to confer the Pro resistance by constructing replacement mutants, whereas mutations at both AaOS1 and Aafhk1 decreased the gene expression level of AapksI, as well as the ability to produce mycotoxin AOH (polyketide-derived alternariol) and virulence. Additionally, more genes (AaOS1 and Aafhk1) harboring the mutations experienced a larger biological fitness penalty.

CONCLUSION

To our knowledge, this is the first report on Pro resistance selected in garlic fields, and mutations at AaOS1 of A. alternata causing a decreased ability to produce the mycotoxin AOH. © 2024 Society of Chemical Industry.

Antibacterial insights into alternariol and its derivative alternariol monomethyl ether produced by a marine fungus

Alternaria alternata FB1 is a marine fungus identified as a candidate for plastic degradation in our previous study. This fungus has been recently shown to produce secondary metabolites with significant antimicrobial activity against various pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and the notorious aquaculture pathogen Vibrio anguillarum. The antibacterial compounds were purified and identified as alternariol (AOH) and its derivative, alternariol monomethyl ether (AME). We found that AOH and AME primarily inhibited pathogenic bacteria (MRSA or V. anguillarum) by disordering cell division and some other key physiological and biochemical processes. We further demonstrated that AOH could effectively inhibit the unwinding activity of MRSA topoisomerases, which are closely related to cell division and are the potential action target of AOH. The antibacterial activities of AOH and AME were verified by using zebrafish as the in vivo model. Notably, AOH and AME did not significantly affect the viability of normal human liver cells at concentrations that effectively inhibited MRSA or V. anguillarum. Finally, we developed the genetic operation system of A. alternata FB1 and blocked the biosynthesis of AME by knocking out omtI (encoding an O-methyl transferase), which facilitated A. alternata FB1 to only produce AOH. The development of this system in the marine fungus will accelerate the discovery of novel natural products and further bioactivity study.IMPORTANCEMore and more scientific reports indicate that alternariol (AOH) and its derivative alternariol monomethyl ether (AME) exhibit antibacterial activities. However, limited exploration of their detailed antibacterial mechanisms has been performed. In the present study, the antibacterial mechanisms of AOH and AME produced by the marine fungus Alternaria alternata FB1 were disclosed in vitro and in vivo. Given their low toxicity on the normal human liver cell line under the concentrations exhibiting significant antibacterial activity against different pathogens, AOH and AME are proposed to be good candidates for developing promising antibiotics against methicillin-resistant Staphylococcus aureus and Vibrio anguillarum. We also succeeded in blocking the biosynthesis of AME, which facilitated us to easily obtain pure AOH. Moreover, based on our previous results, A. alternata FB1 was shown to enable polyethylene degradation.

Alternaria diseases on potato and tomato

Alternaria spp. cause different diseases in potato and tomato crops. Early blight caused by Alternaria solani and brown spot caused by Alternaria alternata are most common, but the disease complex is far more diverse. We first provide an overview of the Alternaria species infecting the two host plants to alleviate some of the confusion that arises from the taxonomic rearrangements in this fungal genus. Highlighting the diversity of Alternaria fungi on both solanaceous hosts, we review studies investigating the genetic diversity and genomes, before we present recent advances from studies elucidating host-pathogen interactions and fungicide resistances.

TAXONOMY

Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes, Order Pleosporales, Family Pleosporaceae, Genus Alternaria.

BIOLOGY AND HOST RANGE

Alternaria spp. adopt diverse lifestyles. We specifically review Alternaria spp. that cause disease in the two solanaceous crops potato (Solanum tuberosum) and tomato (Solanum lycopersicum). They are necrotrophic pathogens with no known sexual stage, despite some signatures of recombination.

DISEASE SYMPTOMS

Symptoms of the early blight/brown spot disease complex include foliar lesions that first present as brown spots, depending on the species with characteristic concentric rings, which eventually lead to severe defoliation and considerable yield loss.

CONTROL

Good field hygiene can keep the disease pressure low. Some potato and tomato cultivars show differences in susceptibility, but there are no fully resistant varieties known. Therefore, the main control mechanism is treatment with fungicides.

Characterization and toxicological potential of Alternaria alternata associated with post-harvest fruit rot of Prunus avium in China

Post-harvest fruit rot caused by Alternaria species is one of the most important threats to the fruit industry. Post-harvest rot on sweet cherry (Prunus avium) fruit was observed in the fruit markets of the Haidian district of Beijing, China. The fungal isolates obtained from the infected sweet cherry fruits matched the descriptions of Alternaria alternata based on the morphology and multi-gene (ITS, endo-PG, and Alta1) sequence analysis. Pathogenicity tests indicated that ACT-3 was the most virulent isolate, exhibiting typical post-harvest fruit rot symptoms. Physiological studies revealed that the optimal conditions for the growth of ACT-3 were temperature of 28°C, water activity of 0.999, and pH of 8 with 87, 85, and 86 mm radial growth of ACT-3 on a potato dextrose agar (PDA) medium, respectively, at 12 days post-inoculation (dpi). Moreover, the fungus showed the highest growth on a Martin agar medium (MAM) modified (85 mm) and a PDA medium (84 mm) at 12 dpi. The proliferation of the fungus was visualized inside the fruit tissues by confocal and scanning electron microscope (SEM), revealing the invasion and destruction of fruit tissues. Alternaria mycotoxins, tenuazonic acid (TeA), and alternariol (AOH) were detected in five representative isolates by HPLC analysis. The highest concentrations of TeA (313 μg/mL) and AOH (8.9 μg/mL) were observed in ACT-6 and ACT-3 isolates, respectively. This study is the first to present a detailed report on the characteristics and proliferation of A. alternata associated with sweet cherry fruit rot and the detection of toxic metabolites.

Crude Lipopeptides Produced by Bacillus amyloliquefaciens Could Control the Growth of Alternaria alternata and Production of Alternaria Toxins in Processing Tomato

Alternaria spp. and its toxins are the main contaminants in processing tomato. Based on our earlier research, the current study looked into the anti-fungal capacity of crude lipopeptides from B. amyloliquefaciens XJ-BV2007 against A. alternata. We found that the crude lipopeptides significantly inhibited A. alternata growth and reduced tomato black spot disease incidence. SEM analysis found that the crude lipopeptides could change the morphology of mycelium and spores of A. alternata. Four main Alternaria toxins were detected using UPLC-MS/MS, and the findings demonstrated that the crude lipopeptides could lessen the accumulation of Alternaria toxins in vivo and in vitro. Meanwhile, under the stress of crude lipopeptides, the expression of critical biosynthetic genes responsible for TeA, AOH, and AME was substantially down-regulated. The inhibitory mechanism of the crude lipopeptides was demonstrated to be the disruption of the mycelial structure of A. alternata, as well as the integrity and permeability of the membrane of A. alternata sporocytes. Taken together, crude lipopeptides extracted from B. amyloliquefaciens XJ-BV2007 are an effective biological agent for controlling tomato black spot disease and Alternaria toxins contamination.

Complexity of fungal polyketide biosynthesis and function

Where does one draw the line between primary and secondary metabolism? The answer depends on the perspective. Microbial secondary metabolites (SMs) were at first believed not to be very important for the producers because they are dispensable for growth under laboratory conditions. However, such compounds become important in natural niches of the organisms, and some are of prime importance for humanity. Polyketides are an important group of SMs with aflatoxin as a well-known and well-characterized example. In Aspergillus spp., all 34 afl genes encoding the enzymes for aflatoxin biosynthesis are located in close vicinity on chromosome III in a so-called gene cluster. This led to the assumption that most genes required for polyketide biosynthesis are organized in gene clusters. Recent research, however, revealed an enormous complexity of the biosynthesis of different polyketides, ranging from individual polyketide synthases to a gene cluster producing several compounds, or to several clusters with additional genes scattered in the genome for the production of one compound. Research of the last decade furthermore revealed a huge potential for SM biosynthesis hidden in fungal genomes, and methods were developed to wake up such sleeping genes. The analysis of organismic interactions starts to reveal some of the ecological functions of polyketides for the producing fungi.

Conserved copper regulation of the antimicrobial isocyanide brassicicolin A in Alternaria brassicicola

Phytopathogenic Alternaria species are renown for production of toxins that contribute to virulence on host plants. Typically, these toxins belong to well-known secondary metabolite chemical classes including polyketides, non-ribosomal peptides and terpenes. However, the purported host toxin brassicicolin A produced by A. brassicicola is an isocyanide, a chemical class whose genetics and encoding gene structure is largely unknown. The chemical structure of brassicicolin A shows it to have similarity to the recently characterized fumicicolins derived from the Aspergillus fumigatus isocyanide synthase CrmA. Examination of the A. brassicicola genome identified AbcrmA, a putative homolog with 64% identity to A. fumigatus CrmA. Deletion of AbcrmA resulted in loss of production of brassicicolin A. Contrary to reports that brassicicolin A is a host-specific toxin, the ΔAbcrmA mutants were equally virulent as the wildtype on Brassica hosts. However, in line with results of A. fumigatus CrmA generated metabolites, we find that brassicicolin A increased 360-fold under copper limited conditions. Also, like A. fumigatus CrmA derived metabolites, we find brassicicolin A to be a broad-spectrum antimicrobial. We speculate that CrmA-like isocyanide synthase products provide the producing fungi a fitness advantage in copper depleted environments.

Identification and Characterization of Peptaibols as the Causing Agents of Pseudodiploöspora longispora Infecting the Edible Mushroom Morchella

Pseudodiploöspora longispora (previously known as Diploöspora longispora) is a pathogenic fungus of Morchella mushrooms. The molecular mechanism underlying the infection of P. longispora in fruiting bodies remains unknown. In this study, three known peptaibols, alamethicin F-50, polysporin B, and septocylindrin B (1-3), and a new analogue, longisporin A (4), were detected and identified in the culture of P. longispora and the fruiting bodies of M. sextelata infected by P. longispora. The primary amino sequence of longisporin A is defined as Ac-Aib1-Pro2-Aib3-Ala4-Aib5-Aib6-Gln7-Aib8-Val9-Aib10-Glu11-Leu12-Aib13-Pro14-Val15-Aib16-Aib17-Gln18-Gln19-Phaol20. The peptaibols 1-4 greatly suppressed the mycelial growth of M. sextelata. In addition, treatment with alamethicin F-50 produced damage on the cell wall and membrane of M. sextelata. Compounds 1-4 also exhibited inhibitory activities against human pathogens including Aspergillus fumigatus, Candida albicans, methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus, and plant pathogen Verticillium dahlia. Herein, peptaibols are confirmed as virulence factors involved in the invasion of P. longispora on Morchella, providing insights into the interaction between pathogenic P. longispora and mushrooms.

Fungicide-free management of Alternaria leaf blotch and fruit spot on apple indicates Alternaria spp. as secondary colonizer

The fungal genus Alternaria is a pan-global pathogen of > 100 crops, and is associated with the globally expanding Alternaria leaf blotch in apple (Malus x domestica Borkh.) which leads to severe leaf necrosis, premature defoliation, and large economic losses. Up to date, the epidemiology of many Alternaria species is still not resolved as they can be saprophytic, parasitic or shift between both lifestyles and are also classified as primary pathogen able to infect healthy tissue. We argue that Alternaria spp. does not act as primary pathogen, but only as a necrosis-dependent opportunist. We studied the infection biology of Alternaria spp. under controlled conditions and monitored disease prevalence in real orchards and validated our ideas by applying fungicide-free treatments in 3-years field experiments. Alternaria spp. isolates were not able to induce necroses in healthy tissue, but only when prior induced damages existed. Next, leaf-applied fertilizers, without fungicidal effect, reduced Alternaria-associated symptoms (- 72.7%, SE: ± 2.5%) with the same efficacy as fungicides. Finally, low leaf magnesium, sulphur, and manganese concentrations were consistently linked with Alternaria-associated leaf blotch. Fruit spot incidence correlated positively with leaf blotch, was also reduced by fertilizer treatments, and did not expand during storage unlike other fungus-mediated diseases. Our findings suggest that Alternaria spp. may be a consequence of leaf blotch rather than its primary cause, as it appears to colonize the physiologically induced leaf blotch. Taking into account existing observations that Alternaria infection is connected to weakened hosts, the distinction may appear slight, but is of great significance, as we can now (a) explain the mechanism of how different stresses result in colonization with Alternaria spp. and (b) substitute fungicides for a basic leaf fertilizer. Therefore, our findings can result in significant decreases in environmental costs due to reduced fungicide use, especially if the same mechanism applies to other crops.

Synthesis, Structure and Biological Activity of Indole-Imidazole Complexes with ZnCl2: Can Coordination Enhance the Functionality of Bioactive Ligands?

The ability of the indole-imidazole hybrid ligands to coordinate with the Zn(II) ion and the resulting structures of this new class of coordination compounds were analyzed in order to determine their structural properties and biological functionalities. For this purpose, six novel Zn(II) complexes, [Zn(InIm)₂Cl₂] (1), [Zn(InMeIm)₂Cl₂] (2), [Zn(IniPrIm)₂Cl₂] (3), [Zn(InEtMeIm)₂Cl₂] (4), [Zn(InPhIm)₂Cl₂] (5) and [Zn₂(InBzIm)₂Cl₂] (6) (where InIm is 3-((1H-imidazol-1-yl)methyl)-1H-indole), were synthesized by the reactions of ZnCl₂ and the corresponding ligand in a 1:2 molar ratio in methanol solvent at an ambient temperature. The structural and spectral characterization of these complexes was performed using NMR, FT-IR and ESI-MS spectrometry and elemental analysis, and the crystal structures of 1-5 were determined using single-crystal X-ray diffraction. Complexes 1-5 form polar supramolecular aggregates by utilizing, for this purpose, the N-H(indole)∙∙∙Cl(chloride) intermolecular hydrogen bonds. The assemblies thus formed differ depending on the distinctive molecular shape, which can be either compact or extended. All complexes were screened for their hemolytic, cytoprotective, antifungal, and antibacterial activities. The results show that the cytoprotective activity of the indole/imidazole ligand significantly increases upon its complexation with ZnCl₂ up to a value comparable with the standard antioxidant Trolox, while the response of its substituted analogues is diverse and less pronounced.

Advances and Challenges in CRISPR/Cas-Based Fungal Genome Engineering for Secondary Metabolite Production: A Review

Fungi represent an important source of bioactive secondary metabolites (SMs), which have wide applications in many fields, including medicine, agriculture, human health, and many other industries. The genes involved in SM biosynthesis are usually clustered adjacent to each other into a region known as a biosynthetic gene cluster (BGC). The recent advent of a diversity of genetic and genomic technologies has facilitated the identification of many cryptic or uncharacterized BGCs and their associated SMs. However, there are still many challenges that hamper the broader exploration of industrially important secondary metabolites. The recent advanced CRISPR/Cas system has revolutionized fungal genetic engineering and enabled the discovery of novel bioactive compounds. In this review, we firstly introduce fungal BGCs and their relationships with associated SMs, followed by a brief summary of the conventional strategies for fungal genetic engineering. Next, we introduce a range of state-of-the-art CRISPR/Cas-based tools that have been developed and review recent applications of these methods in fungi for research on the biosynthesis of SMs. Finally, the challenges and limitations of these CRISPR/Cas-based systems are discussed and directions for future research are proposed in order to expand their applications and improve efficiency for fungal genetic engineering.

Detection of microscopic filamentous fungal biofilms - Choosing the suitable methodology

Microscopic filamentous fungi are ubiquitous microorganisms that adapt very easily to a variety of environmental conditions. Due to this adaptability, they can colonize a number of various surfaces where they are able to start forming biofilms. Life in the form of biofilms provides them with many benefits (increased resistance to desiccation, UV radiation, antimicrobial compounds, and host immune response). The aim of this study is to find a reliable and reproducible methodology to determine biofilm growth of selected microscopic filamentous fungi strains. Several methods (crystal violet staining, MTT assay, XTT assay, resazurin assay) for the determination of total biofilm biomass and its metabolic activity were tested on four fungi - Alternaria alternata, Aspergillus niger, Fusarium culmorum and Fusarium graminearum, and their biofilm was also imaged by spinning disc confocal microscopy using fluorescent dyes. A reproducible biofilm quantification method is essential for the subsequent testing of the biofilm growth suppression using antifungal agents or physical methods. Crystal violet staining was found to be a suitable method for the determination of total biofilm biomass of selected strains, and the MTT assay for the determination of metabolic activity of the biofilms. Calcofluor white and Nile red fluorescent stains successfully dyed the hyphae of microscopic fungi.

Pathogenicity, Molecular Characterization, and Mycotoxigenic Potential of Alternaria spp. Agents of Black Spots on Fruit and Leaves of Pyrus communis in Italy

Brown and black spots, caused by Stemphylium and Alternaria species, are important fungal diseases affecting European pear (Pyrus communis) in orchards. Both fungal genera cause similar symptoms, which could favor misidentification, but Alternaria spp. are increasingly reported due to the changing climatic conditions. In this study, Alternaria spp. were isolated from symptomatic leaves and fruits of European pear, and their pathogenicity was evaluated on pear fruits from cultivar Abate Fétel, and molecular and chemical characterization were performed. Based on maximum likelihood phylogenetic analysis, 15 of 46 isolates were identified as A. arborescens species complex (AASC), 27 as A. alternata, and four as Alternaria sp. Both species were isolated from mature fruits and leaves. In pathogenicity assays on pear fruits, all isolates reproduced the symptoms observed in the field, by both wound inoculation and direct penetration. All but one isolate produced Alternaria toxins on European pears, including tenuazonic acid and alternariol (89.1% of the isolates), alternariol monomethyl ether (89.1%), altertoxin I (80.4%), altenuene (50.0%), and tentoxin (2.2%). These isolates also produced at least two mycotoxins, and 43.5% produced four mycotoxins, with an average total concentration of the Alternaria toxins exceeding 7.58 × 10⁶ ng/kg. Our data underline the potential risks for human health related to the high mycotoxin content found on fruits affected by black spot. This study also represents the first report of AASC as an agent of black spot on European pear in Italy.

Elicitation of Fruit Fungi Infection and Its Protective Response to Improve the Postharvest Quality of Fruits

Fruit diseases brought on by fungus infestation leads to postharvest losses of fresh fruit. Approximately 30% of harvested fruits do not reach consumers’ plates due to postharvest losses. Fungal pathogens play a substantial part in those losses, as they cause the majority of fruit rots and consumer complaints. Understanding fungal pathogenic processes and control measures is crucial for developing disease prevention and treatment strategies. In this review, we covered the presented pathogen entry, environmental conditions for pathogenesis, fruit’s response to pathogen attack, molecular mechanisms by which fungi infect fruits in the postharvest phase, production of mycotoxin, virulence factors, fungal genes involved in pathogenesis, and recent strategies for protecting fruit from fungal attack. Then, in order to investigate new avenues for ensuring fruit production, existing fungal management strategies were then assessed based on their mechanisms for altering the infection process. The goal of this review is to bridge the knowledge gap between the mechanisms of fungal disease progression and numerous disease control strategies being developed for fruit farming.

The expanding CRISPR toolbox for natural product discovery and engineering in filamentous fungi

Covering: up to May 2022Fungal genetics has transformed natural product research by enabling the elucidation of cryptic metabolites and biosynthetic steps. The enhanced capability to add, subtract, modulate, and rewrite genes via CRISPR/Cas technologies has opened up avenues for the manipulation of biosynthetic gene clusters across diverse filamentous fungi. This review discusses the innovative and diverse strategies for fungal natural product discovery and engineering made possible by CRISPR/Cas-based tools. We also provide a guide into multiple angles of CRISPR/Cas experiment design, and discuss current gaps in genetic tool development for filamentous fungi and the promising opportunities for natural product research.

Phylogeny and mycotoxin profiles of pathogenic Alternaria and Curvularia species isolated from date palm in southern Tunisia

Date palm (Phoenix dactylifera L.), is a widely cultivated crop across North Africa, with about 300 thousand tons of fruits produced per year, in Tunisia. A wide range of fungal pathogens has been associated with leaf spots of date palm, Alternaria species being the most frequently reported. Symptomatic leaves of Deglet Nour variety were randomly collected in six localities in Tunisia. We used a polyphasic approach to identify 45 Alternaria and five Curvularia strains isolated from date palm, confirming their pathogenicity. Sequencing of allergen Alt-a1, glyceraldehyde-3-phosphate dehydrogenase (gpd) and calmodulin genes allowed us to group 35 strains in Alternaria Section, and 10 strains in Ulocladioides section. Based on sequencing analyses of Internal Transcribed Spacer, gpd and elongation factor genomic regions, all Curvularia strains were identified as Curvularia spicifera. All Alternaria and Curvularia species tested on date palm plantlets proved to be pathogenic, fulfilling Koch's postulates. Although no significant differences were observed among the species, the highest mean disease severity index was observed in A. arborescens, while the lowest corresponded to C. spicifera. The capability of these strains to produce mycotoxins in vitro was evaluated. None of the A. consortialis strains produced any known Alternaria mycotoxin, whereas more than 80% of the strains included in Alternaria section Alternaria produced variable amounts of multiple mycotoxins such as alternariol, alternariol monomethyl ether, altenuene, tenuazonic acid and tentoxin. Curvularia spicifera strains produced detectable traces of fumonisins B. This work reports a first comprehensive multidisciplinary study of mycotoxigenic Alternaria species and C. spicifera associated with leaf spot disease on date palm.

Whole-Genome Sequence Analysis of an Endophytic Fungus Alternaria sp. SPS-2 and Its Biosynthetic Potential of Bioactive Secondary Metabolites

As one of the commonly isolated endophytic fungi, Alternaria has been known for the production of numerous secondary metabolites (SMs). However, its detailed genomic features and SM biosynthetic potential have not been extensively studied thus far. The present work focuses on the whole-genome sequencing and assembly of an endophytic strain Alternaria sp. SPS-2 derived from Echrysantha chrysantha Lindl. and gene annotation using various bioinformatic tools. The results of this study suggested that the genome of strain SPS-2 was 33.4 Mb in size with a GC content of 51% and an N50 scaffold of 2.6 Mb, and 9789 protein-coding genes, including 644 CAZyme-encoding genes, were discovered in strain SPS-2 through KEGG enrichment analysis. The antiSMASH results indicated that strain SPS-2 harbored 22 SM biosynthetic gene clusters (BGCs), 14 of which are cryptic and unknown. LS–MS/MS and GNPS-based analyses suggested that this endophytic fungus is a potential producer of bioactive SMs and merits further exploration and development.

Characterization of NRPS and PKS genes involved in the biosynthesis of SMs in Alternaria dauci including the phytotoxic polyketide aldaulactone

Alternaria dauci is a Dothideomycete fungus, causal agent of carrot leaf blight. As a member of the Alternaria genus, known to produce a lot of secondary metabolite toxins, A. dauci is also supposed to synthetize host specific and non-host specific toxins playing a crucial role in pathogenicity. This study provides the first reviewing of secondary metabolism genetic basis in the Alternaria genus by prediction of 55 different putative core genes. Interestingly, aldaulactone, a phytotoxic benzenediol lactone from A. dauci, was demonstrated as important in pathogenicity and in carrot partial resistance to this fungus. As nothing is known about aldaulactone biosynthesis, bioinformatic analyses on a publicly available A. dauci genome data set that were reassembled, thanks to a transcriptome data set described here, allowed to identify 19 putative secondary metabolism clusters. We exploited phylogeny to pinpoint cluster 8 as a candidate in aldaulactone biosynthesis. This cluster contains AdPKS7 and AdPKS8, homologs with genes encoding a reducing and a non-reducing polyketide synthase. Clusters containing such a pair of PKS genes have been identified in the biosynthesis of resorcylic acid lactones or dihydroxyphenylacetic acid lactones. AdPKS7 and AdPKS8 gene expression patterns correlated with aldaulactone production in different experimental conditions. The present results highly suggest that both genes are responsible for aldaulactone biosynthesis.

Fungal Melanin Biosynthesis Pathway as Source for Fungal Toxins

Contamination of food and feed with toxin-producing fungi is a major threat in agriculture and for human health. The filamentous fungus Alternaria alternata is one of the most widespread postharvest contaminants and a weak plant pathogen. It produces a large variety of secondary metabolites with alternariol and its derivatives as characteristic mycotoxin. Other important phyto- and mycotoxins are perylene quinones (PQs), some of which have anticancer properties. Here, we discovered that the PQ altertoxin (ATX) biosynthesis shares most enzymes with the 1,8-dihydroxynaphthalene (1,8-DHN) melanin pathway. However, melanin was formed in aerial hyphae and spores, and ATXs were synthesized in substrate hyphae. This spatial separation is achieved through the promiscuity of a polyketide synthase, presumably producing a pentaketide (T4HN), a hexaketide (AT4HN), and a heptaketide (YWA1) as products. T4HN directly enters the altertoxin and DHN melanin pathway, whereas AT4HN and YWA1 can be converted only in aerial hyphae, which probably leads to a higher T4HN concentration, favoring 1,8-DHN melanin formation. Whereas the production of ATXs was strictly dependent on the CmrA transcription factor, melanin could still be produced in the absence of CmrA to some extent. This suggests that different cues regulate melanin and toxin formation. Since DHN melanin is produced by many fungi, PQs or related compounds may be produced in many more fungi than so far assumed.

Morphological and Molecular Characterization of Some Alternaria Species Isolated from Tomato Fruits Concerning Mycotoxin Production and Polyketide Synthase Genes

Tomatoes (Lycopersicon esculentum) are one of the main crops grown in Egypt. The fungal black spot illness of fruits is usually associated with the secretion of mycotoxin by Alternaria toxigenic species. Twenty Alternaria isolates were isolated from infected tomatoes fruits by baiting technique, morphologically identified to species level, and confirmed using Internal Transcribed Spacer (ITS) gene sequencing. ITS gene sequencing of fragments obtained 547, 547, 542, 554, and 547 bp for A. alternata, A. brassicicola, A. citri, A. radicina, and A. tenuissima, respectively. Alternaria species were investigated for mycotoxin production using the high-performance liquid chromatography (HPLC) technique. The data from the HPLC analysis showed that the mycotoxins were determined in four out of five Alternaria species, with the incidence ranging from 0.89-9.85 µg/mL of fungal extract at different retention times. Alternaria alternata was the most active species and produced three types of toxins. Polyketide synthase genes (pksH and pksJ) which are involved in the Alternaria toxin's biosynthesis were also amplified from the DNA of Alternaria species.

Roles of AaVeA on Mycotoxin Production via Light in Alternaria alternata

Alternaria alternata is a principal plant pathogen responsible for the biosynthesis of mycotoxins, including tenuazonic acid (TeA), alternariol (AOH), and alternariol monomethyl ether (AME). The velvet gene VeA is involved in fungal growth, development, and secondary metabolism, including mycotoxin biosynthesis via light regulation. In this study, the detailed regulatory roles of AaVeA in A. alternata with various light sources were investigated from the comparative analyses between the wild type and the gene knockout strains. In fungal growth and conidiation, mycelial extension was independent of light regulation in A. alternata. Red light favored conidiation, but blue light repressed it. The absence of AaVeA caused the marked reduction of hyphae extension and conidiophore formation even though red light could not induce more spores in ΔAaVeA mutant. The differentially expressed genes (DEGs) enriched in hyphal growth and conidiation were drastically transcribed from the comparatively transcriptomic profile between the wild type and ΔAaVeA mutant strains with or without light. In mycotoxin production, TeA biosynthesis seems no obvious effect by light regulation, but AOH and AME formation was significantly stimulated by blue light. Nevertheless, the disruption of AaVeA resulted in a marked decrease in mycotoxin production and the action of the stimulation was lost via blue light for the abundant accumulation of AOH and AME in the ΔAaVeA strain. From DEG expression and further verification by RT-qPCR, the loss of AaVeA caused the discontinuous supply of the substrates for mycotoxin biosynthesis and the drastic decline of biosynthetic gene expression. In addition, pathogenicity depends on AaVeA regulation in tomato infected by A. alternata in vivo. These findings provide a distinct understanding of the roles of AaVeA in fungal growth, development, mycotoxin biosynthesis, and pathogenicity in response to various light sources.

Recent Advances in Alternaria Phytotoxins: A Review of Their Occurrence, Structure, Bioactivity and Biosynthesis

Alternaria is a ubiquitous fungal genus in many ecosystems, consisting of species and strains that can be saprophytic, endophytic, or pathogenic to plants or animals, including humans. Alternaria species can produce a variety of secondary metabolites (SMs), especially low molecular weight toxins. Based on the characteristics of host plant susceptibility or resistance to the toxin, Alternaria phytotoxins are classified into host-selective toxins (HSTs) and non-host-selective toxins (NHSTs). These Alternaria toxins exhibit a variety of biological activities such as phytotoxic, cytotoxic, and antimicrobial properties. Generally, HSTs are toxic to host plants and can cause severe economic losses. Some NHSTs such as alternariol, altenariol methyl-ether, and altertoxins also show high cytotoxic and mutagenic activities in the exposed human or other vertebrate species. Thus, Alternaria toxins are meaningful for drug and pesticide development. For example, AAL-toxin, maculosin, tentoxin, and tenuazonic acid have potential to be developed as bioherbicides due to their excellent herbicidal activity. Like altersolanol A, bostrycin, and brefeldin A, they exhibit anticancer activity, and ATX V shows high activity to inhibit the HIV-1 virus. This review focuses on the classification, chemical structure, occurrence, bioactivity, and biosynthesis of the major Alternaria phytotoxins, including 30 HSTs and 50 NHSTs discovered to date.

Fatal attraction of Caenorhabditis elegans to predatory fungi through 6-methyl-salicylic acid

Salicylic acid is a phenolic phytohormone which controls plant growth and development. A methyl ester (MSA) derivative thereof is volatile and involved in plant-insect or plant-plant communication. Here we show that the nematode-trapping fungus Duddingtonia flagrans uses a methyl-salicylic acid isomer, 6-MSA as morphogen for spatiotemporal control of trap formation and as chemoattractant to lure Caenorhabditis elegans into fungal colonies. 6-MSA is the product of a polyketide synthase and an intermediate in the biosynthesis of arthrosporols. The polyketide synthase (ArtA), produces 6-MSA in hyphal tips, and is uncoupled from other enzymes required for the conversion of 6-MSA to arthrosporols, which are produced in older hyphae. 6-MSA and arthrosporols both block trap formation. The presence of nematodes inhibits 6-MSA and arthrosporol biosyntheses and thereby enables trap formation. 6-MSA and arthrosporols are thus morphogens with some functions similar to quorum-sensing molecules. We show that 6-MSA is important in interkingdom communication between fungi and nematodes.

Review: Holistic pest management against early blight disease towards sustainable agriculture

Alternaria species are well-known aggressive pathogens that are widespread globally and warmer temperatures caused by climate change might increase their abundance more drastically. Early blight (EB) disease, caused mainly by Alternaria solani, and brown spot, caused by Alternaria alternata, are major concerns in potato, tomato and eggplant production. The development of EB is strongly linked to varieties, crop development stages, environmental factors, cultivation and field management. Several forecasting models for pesticide application to control EB were created in the last century and more recent scientific advances have included modern breeding technology to detect resistant genes and precision agriculture with hyperspectral sensors to pinpoint damage locations on plants. This paper presents an overview of the EB disease and provides an evaluation of recent scientific advances to control the disease. First of all, we describe the outline of this disease, encompassing biological cycles of the Alternaria genus, favorite climate and soil conditions as well as resistant plant species. Second, versatile management practices to minimize the effect of this pathogen at field level are discussed, covering their limitations and pitfalls. A better understanding of the underlying factors of this disease and the potential of novel research can contribute to implementing integrated pest management systems for an ecofriendly farming system. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mycotoxin Profile and Phylogeny of Pathogenic Alternaria Species Isolated from Symptomatic Tomato Plants in Lebanon

The tomato is one of the most consumed agri-food products in Lebanon. Several fungal pathogens, including Alternaria species, can infect tomato plants during the whole growing cycle. Alternaria infections cause severe production and economic losses in field and during storage. In addition, Alternaria species represent a serious toxicological risk since they are able to produce a wide range of mycotoxins, associated with different toxic activities on human and animal health. Several Alternaria species were detected on tomatoes, among which the most important are A. solani, A. alternata, and A. arborescens. A set of 49 Alternaria strains isolated from leaves and stems of diseased tomato plants were characterised by using a polyphasic approach. All strains were included in the recently defined phylogenetic Alternaria section and grouped in three well-separated sub-clades, namely A. alternata (24 out of 49), A. arborescens (12 out of 49), and A. mali morpho-species (12 out of 49). One strain showed high genetic similarity with an A.limoniasperae reference strain. Chemical analyses showed that most of the Alternaria strains, cultured on rice, were able to produce alternariol (AOH), alternariol methyl ether (AME), altenuene (ALT) and tenuazonic acid (TA), with values up to 5634, 16,006, 5156, and 4507 mg kg⁻¹, respectively. In addition, 66% of the strains were able to co-produce simultaneously the four mycotoxins investigated. The pathogenicity test carried out on 10 Alternaria strains, representative of phylogenetic sub-clades, revealed that they were all pathogenic on tomato fruits. No significant difference among strains was observed, although A. alternata and A. arborescens strains were slightly more aggressive than A. mali morpho-species strains. This paper reports new insights on mycotoxin profiles, genetic variability, and pathogenicity of Alternaria species on tomatoes.

Fungal phytochrome chromophore biosynthesis at mitochondria

Mitochondria are essential organelles because of their function in energy conservation. Here, we show an involvement of mitochondria in phytochrome‐dependent light sensing in fungi. Phytochrome photoreceptors are found in plants, bacteria, and fungi and contain a linear, heme‐derived tetrapyrrole as chromophore. Linearization of heme requires heme oxygenases (HOs) which reside inside chloroplasts in planta. Despite the poor degree of conservation of HOs, we identified two candidates in the fungus Alternaria alternata. Deletion of either one phenocopied phytochrome deletion. The two enzymes had a cooperative effect and physically interacted with phytochrome, suggesting metabolon formation. The metabolon was attached to the surface of mitochondria with a C‐terminal anchor (CTA) sequence in HoxA. The CTA was necessary and sufficient for mitochondrial targeting. The affinity of phytochrome apoprotein to HoxA was 57,000‐fold higher than the affinity of the holoprotein, suggesting a “kiss‐and‐go” mechanism for chromophore loading and a function of mitochondria as assembly platforms for functional phytochrome. Hence, two alternative approaches for chromophore biosynthesis and insertion into phytochrome evolved in plants and fungi.

Unveiling the spatial distribution of aflatoxin B1 and plant defense metabolites in maize using AP-SMALDI mass spectrometry imaging

In order to cope with the presence of unfavorable compounds, plants can biotransform xenobiotics, translocate both parent compounds and metabolites, and perform compartmentation and segregation at the cellular or tissue level. Such a scenario also applies to mycotoxins, fungal secondary metabolites with a pre-eminent role in plant infection. In this work, we aimed to describe the effect of the interplay between Zea mays (maize) and aflatoxin B1 (AFB1) at the tissue and organ level. To address this challenge, we used atmospheric pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) to investigate the biotransformation, localization and subsequent effects of AFB1 on primary and secondary metabolism of healthy maize plants, both in situ and from a metabolomics standpoint. High spatial resolution (5 µm) provided fine localization of AFB1, which was located within the root intercellular spaces, and co-localized with its phase-I metabolite aflatoxin M2. We provided a parallel visualization of maize metabolic changes, induced in different organs and tissues by an accumulation of AFB1. According to our untargeted metabolomics investigation, anthocyanin biosynthesis and chlorophyll metabolism in roots are most affected. The biosynthesis of these metabolites appears to be inhibited by AFB1 accumulation. On the other hand, metabolites found in above-ground organs suggest that the presence of AFB1 may also activate the biochemical response in the absence of an actual fungal infection; indeed, several plant secondary metabolites known for their antimicrobial or antioxidant activities were localized in the outer tissues, such as phenylpropanoids, benzoxazinoids, phytohormones and lipids.

The Good, the Bad, and the Ugly: Mycotoxin Production During Postharvest Decay and Their Influence on Tritrophic Host-Pathogen-Microbe Interactions

Mycotoxins are a prevalent problem for stored fruits, grains, and vegetables. Alternariol, aflatoxin, and patulin, produced by Alternaria spp., Aspergillus spp., and Penicillium spp., are the major mycotoxins that negatively affect human and animal health and reduce fruit and produce quality. Control strategies for these toxins are varied, but one method that is increasing in interest is through host microbiome manipulation, mirroring a biocontrol approach. While the majority of mycotoxins and other secondary metabolites (SM) produced by fungi impact host–fungal interactions, there is also an interplay between the various organisms within the host microbiome. In addition to SMs, these interactions involve compounds such as signaling molecules, plant defense and growth hormones, and metabolites produced by both the plants and microbial community. Therefore, studies to understand the impact of the various toxins impacting the beneficial and harmful microorganisms that reside within the microbiome is warranted, and could lead to identification of safe analogs for antimicrobial activity to reduce fruit decay. Additionally, exploring the composition of the microbial carposphere of host plants is likely to shed light on developing a microbial consortium to maintain quality during storage and abate mycotoxin contamination.

Characterization of Alternaria Species Associated with Heart Rot of Pomegranate Fruit

This study was aimed at identifying Alternaria species associated with heart rot disease of pomegranate fruit in southern Italy and characterizing their mycotoxigenic profile. A total of 42 Alternaria isolates were characterized. They were obtained from pomegranate fruits with symptoms of heart rot sampled in Apulia and Sicily and grouped into six distinct morphotypes based on macro- and microscopic features. According to multigene phylogenetic analysis, including internal transcribed spacer (ITS), translation elongation factor 1-α (EF-1α), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and a SCAR marker (OPA10-2), 38 isolates of morphotypes 1 to 5 were identified as Alternaria alternata, while isolates of morphotype 6, all from Sicily, clustered within the Alternaria arborescens species complex. In particular, isolates of morphotype 1, the most numerous, clustered with the ex-type isolate of A. alternata, proving to belong to A. alternata. No difference in pathogenicity on pomegranate fruits was found between isolates of A. alternata and A. arborescens and among A. alternata isolates of different morphotypes. The toxigenic profile of isolates varied greatly: in vitro, all 42 isolates produced tenuazonic acid and most of them other mycotoxins, including alternariol, alternariol monomethyl ether, altenuene and tentoxin.

The HOG Pathway Plays Different Roles in Conidia and Hyphae During Virulence of Alternaria alternata

The black mold Alternaria alternata causes dramatic losses in agriculture due to postharvest colonization and mycotoxin formation and is a weak pathogen on living plants. Fungal signaling processes are crucial for successful colonization of a host plant. Because the mitogen-activated protein kinase HogA is important for the expression of stress-associated genes, we tested a ∆hogA-deletion strain for pathogenicity. When conidia were used as inoculum, the ∆hogA-deletion strain was largely impaired in colonizing tomato and apple. In comparison, hyphae as inoculum colonized the fruit very well. Hence, HogA appears to be important only in the initial stages of plant colonization. A similar difference between conidial inoculum and hyphal inoculum was observed on artificial medium in the presence of different stress agents. Whereas wild-type conidia adapted well to different stresses, the ∆hogA-deletion strain failed to grow under the same conditions. With hyphae as inoculum, the wild type and the ∆hogA-deletion strain grew in a very similar way. At the molecular level, we observed upregulation of several catalase (catA, -B, and -D) and superoxide dismutase (sodA, -B, and -E) genes in germlings but not in hyphae after exposure to 4 mM hydrogen peroxide. The upregulation required the high osmolarity glycerol (HOG) pathway. In contrast, in mycelia, catD, sodA, sodB, and sodE were upregulated upon stress in the absence of HogA. Several other stress-related genes behaved in a similar way.

Transcriptomic Insights into the Antifungal Effects of Magnolol on the Growth and Mycotoxin Production of Alternaria alternata

Alternaria alternata is an important phytopathogen causing fruit black rot and also producing a variety of mycotoxins, such as alternariol (AOH) and alternariol monomethyl ether (AME) as two main contaminants. This could lead to economic losses of agricultural products as well as human health risks. In this study, magnolol extracted from the traditional Chinese herb, Mangnolia officinalis, exhibited an obvious antifungal property and could completely suppress the mycelial growth at 100 μM. Morphological differences of A. alternata were observed to be significantly shrunk and wrinkled after the exposure to magnolol. Furthermore, AOH and AME were no longer produced in response to 50 μM of magnolol. To uncover the antifungal and antimycotoxigenic mechanisms, the transcriptomic profiles of A. alternata—treated with or without magnolol—were evaluated. The clustered genes responsible for AOH and AME biosynthesis were obviously less transcribed under magnolol stress and this was further confirmed by qRT-PCR. The global regulators of carbon and nitrogen utilization, such as CreA and NmrA, were significantly down-regulated and this possibly caused the reduction in mycotoxins. In addition, fatty acid β-oxidation was regarded to contribute to polyketide mycotoxin production for the supply of precursor acetyl-CoA while the expression of these related genes was inhibited. The response to magnolol led to the marked alteration of oxidative stress and the down-expression of the mitogen-activated protein kinase (MAPK) signaling pathway from the transcriptome data and the determination of peroxidase (POD), superoxide dismutase (SOD) and glutathione (GSH) assays. This above might be the very reason for the growth supression and mycotoxin production of A. alternata by magnolol. This study provides new insights into its potential as an important active ingredient for the control of A. alternata and its mycotoxins in fruits and their products.

Genetic diversity of Alternaria species associated with black point in wheat grains

The genus Alternaria is a widely distributed major plant pathogen that can act as a saprophyte in plant debris. Fungi of this genus frequently infect cereal crops and cause such diseases as black point and wheat leaf blight, which decrease the yield and quality of cereal products. A total of 25 Alternaria sp. isolates were collected from germ grains of various wheat cultivars from different geographic regions in Kazakhstan. We investigated the genetic relationships of the main Alternaria species related to black point disease of wheat in Kazakhstan, using the inter-primer binding site (iPBS) DNA profiling technique. We used 25 retrotransposon-based iPBS primers to identify the differences among and within Alternaria species populations, and analyzed the variation using clustering (UPGMA) and statistical approaches (AMOVA). Isolates of Alternaria species clustered into two main genetic groups, with species of A.alternata and A.tennuissima forming one cluster, and isolates of A. infectoria forming another. The genetic diversity found using retrotransposon profiles was strongly correlated with geographic data. Overall, the iPBS fingerprinting technique is highly informative and useful for the evaluation of genetic diversity and relationships of Alternaria species.

Molecular Identification and Mycotoxin Production by Alternaria Species Occurring on Durum Wheat, Showing Black Point Symptoms

Black point is a fungal disease of wheat, mainly associated with mycotoxigenic Alternaria species. Affected wheat kernels are characterized by dark brown discolouration of the embryo region and reduction of grain quality. Potential risk is the possible accumulation of Alternaria mycotoxins, alternariol (AOH), alternariol-monomethyl ether (AME), tenuazonic acid (TA), and altenuene (ALT), provided by haemato-toxic, genotoxic, and mutagenic activities. One hundred and twenty durum wheat samples belonging to 30 different genotypes grown in Bologna and Modena areas, in Italy, showing black point symptoms, were analyzed for Alternaria species and their mycotoxin contamination. Alternariol was selected as an indicator of the capability of the Alternaria species to produce mycotoxin in vivo in field conditions. The data showed that Alternaria species occurred in 118 out of 120 wheat kernels samples, with the incidence of infected kernels ranging between 1% and 26%. Moreover, AOH was detected by using a HPLC with a diode array detector (LC-DAD) in 98 out of 120 samples with values ranging between 24 and 262 µg Kg⁻¹. Ninety-two Alternaria representative strains, previously identified morphologically, were identified at species/section level using gene sequencing, and therefore were analyzed for their mycotoxin profiles. Eighty-four strains, phylogenetically grouped in the Alternaria section, produced AOH, AME, and TA with values up to 8064, 14,341, and 3683 µg g⁻¹, respectively, analyzed by using a LC-DAD. On the other hand, eight Alternaria strains, included in Infectoriae Section, showed a very low or no capability to produce mycotoxins.

Alternaria alternata uses two siderophore systems for iron acquisition

Iron is one of the most abundant elements on earth and essential for life. However, Fe³⁺ ions are rather insoluble and microorganisms such as fungi may use siderophores as strong chelators for uptake. In addition, free cytoplasmic iron is rather toxic and intracellular siderophores are used to control the toxicity. Siderophores are also important for iron storage. We studied two siderophore systems in the plant necrotrophic fungus Alternaria alternata and show that the non-ribosomal peptide synthase, Nps2, is required for the biosynthesis of intracellular ferricrocin, whereas Nps6 is needed for the formation of extracellular coprogen and coprogen B. Whereas nps2 was dispensable for growth on iron-depleted medium, nps6 was essential under those conditions. nps2 deletion caused an increase in spore formation and reduced pathogenicity on tomato. Our results suggest that A. alternata employs an external and an internal siderophore system to adapt to low iron conditions.

Effects of Essential Oil Citral on the Growth, Mycotoxin Biosynthesis and Transcriptomic Profile of Alternaria alternata

Alternaria alternata is a critical phytopathogen that causes foodborne spoilage and produces a polyketide mycotoxin, alternariol (AOH), and its derivative, alternariol monomethyl ether (AME). In this study, the inhibitory effects of the essential oil citral on the fungal growth and mycotoxin production of A. alternata were evaluated. Our findings indicated that 0.25 μL/mL (222.5 μg/mL) of citral completely suppressed mycelial growth as the minimum inhibitory concentration (MIC). Moreover, the 1/2MIC of citral could inhibit more than 97% of the mycotoxin amount. Transcriptomic profiling was performed by comparative RNA-Seq analysis of A. alternata with or without citral treatment. Out of a total of 1334 differentially expressed genes (DEGs), 621 up-regulated and 713 down-regulated genes were identified under citral stress conditions. Numerous DEGs for cell survival, involved in ribosome and nucleolus biogenesis, RNA processing and metabolic processes, and protein processing, were highly expressed in response to citral. However, a number of DEGs responsible for the metabolism of several carbohydrates and amino acids, sulfate and glutathione metabolism, the metabolism of xenobiotics and transporter activity were significantly more likely to be down-regulated. Citral induced the disturbance of cell integrity through the disorder of gene expression, which was further confirmed by the fact that exposure to citral caused irreversibly deleterious disruption of fungal spores and the inhibition of ergosterol biosynthesis. Citral perturbed the balance of oxidative stress, which was likewise verified by a reduction of total antioxidative capacity. In addition, citral was able to modulate the down-regulation of mycotoxin biosynthetic genes, including pksI and omtI. The results provide new insights for exploring inhibitory mechanisms and indicate citral as a potential antifungal and antimytoxigenic alternative for cereal storage.