HOG MAP kinase regulation of alternariol biosynthesis in Alternaria alternata is important for substrate colonization

The characteristics, occurrence, and toxicological effects of alternariol: a mycotoxin

Alternaria species are mycotoxin-producing fungi known to infect fresh produce and to cause their spoilage. Humans get exposed to fungal secondary metabolites known as mycotoxin via the ingestion of contaminated food. Alternariol (AOH) (C14H10O5) is an isocoumarins produced by different species of Alternaria including Alternaria alternata. AOH is often found in grain, fruits and fruits-based food products with high levels in legumes, nuts, and tomatoes. AOH was first discovered in 1953, and it is nowadays linked to esophagus cancer and endocrine disruption due to its similarity to estrogen. Although considered as an emerging mycotoxin with no regulated levels in food, AOH occurs in highly consumed dietary products and has been detected in various masked forms, which adds to its occurrence. Therefore, this comprehensive review was developed to give an overview on recent literature in the field of AOH. The current study summarizes published data on occurrence levels of AOH in different food products in the last ten years and evaluates those levels in comparison to recommended levels by the regulating entities. Such surveillance facilitates the work of health risk assessors and highlights commodities that are most in need of AOH levels regulation. In addition, the effects of AOH on cells and animal models were summarized in two tables; data include the last two-year literature studies. The review addresses also the main characteristics of AOH and the possible human exposure routes, the populations at risk, and the effect of anthropogenic activities on the widespread of the mycotoxin. The commonly used detection and control methods described in the latest literature are also discussed to guide future researchers to focus on mitigating mycotoxins contamination in the food industry. This review aims mainly to serve as a guideline on AOH for mycotoxin regulation developers and health risk assessors.

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.

The AwHog1 Transcription Factor Influences the Osmotic Stress Response, Mycelium Growth, OTA Production, and Pathogenicity in Aspergillus westerdijkiae fc-1

Aspergillus westerdijkiae, known as the major ochratoxin A (OTA) producer, usually occurs on agricultural crops, fruits, and dry-cured meats. Microorganisms produce OTA to adapt to the high osmotic pressure environment that is generated during food processing and storage. To investigate the relationship between OTA biosynthesis and the high osmolarity glycerol (HOG) pathway, the transcription factor AwHog1 gene in A. westerdijkiae was functionally characterised by means of a loss-of-function mutant. Our findings demonstrated that the growth and OTA production of a mutant lacking AwHog1 decreased significantly and was more sensitive to high osmotic media. The ΔAwHog1 mutant displayed a lower growth rate and a 73.16% reduction in OTA production in the wheat medium compared to the wild type. After three days of culture, the growth rate of the ΔAwHog1 mutant in medium with 60 g/L NaCl and 150 g/L glucose was slowed down 19.57% and 13.21%, respectively. Additionally, the expression of OTA biosynthesis genes was significantly reduced by the deletion of the AwHog1 gene. The infection ability of the ΔAwHog1 mutant was decreased, and the scab diameter of the pear was 6% smaller than that of the wild type. These data revealed that transcription factor AwHog1 plays a key role in the osmotic response, growth, OTA production, and pathogenicity in A. westerdijkiae.

SakA Regulates Morphological Development, Ochratoxin A Biosynthesis and Pathogenicity of Aspergillus westerdijkiae and the Response to Different Environmental Stresses

Ochratoxin A (OTA), as a common mycotoxin, has seriously harmful effects on agricultural products, livestock and humans. There are reports on the regulation of SakA in the MAPK pathway, which regulates the production of mycotoxins. However, the role of SakA in the regulation of Aspergillus westerdijkiae and OTA production is not clear. In this study, a SakA deletion mutant (ΔAwSakA) was constructed. The effects of different concentrations of D-sorbitol, NaCl, Congo red and H₂O₂ on the mycelia growth, conidia production and biosynthesis of OTA were investigated in A. westerdijkiae WT and ΔAwSakA. The results showed that 100 g/L NaCl and 3.6 M D-sorbitol significantly inhibited mycelium growth and that a concentration of 0.1% Congo red was sufficient to inhibit the mycelium growth. A reduction in mycelium development was observed in ΔAwSakA, especially in high concentrations of osmotic stress. A lack of AwSakA dramatically reduced OTA production by downregulating the expression of the biosynthetic genes otaA, otaY, otaB and otaD. However, otaC and the transcription factor otaR1 were slightly upregulated by 80 g/L NaCl and 2.4 M D-sorbitol, whereas they were downregulated by 0.1% Congo red and 2 mM H₂O₂. Furthermore, ΔAwSakA showed degenerative infection ability toward pears and grapes. These results suggest that AwSakA is involved in the regulation of fungal growth, OTA biosynthesis and the pathogenicity of A. westerdijkiae and could be influenced by specific environmental stresses.

AnAzf1 acts as a positive regulator of ochratoxin A biosynthesis in Aspergillus niger

Aspergillus niger produces genotoxic and carcinogenic ochratoxin A (OTA) that severely threatens human and animal health. Transcription factor Azf1 is essential in regulating fungal cell development and primary metabolism. However, its effect and mechanism on secondary metabolism are unclear. Here, we characterized and deleted a Azf1 homolog gene, An15g00120 (AnAzf1), in A. niger, which completely blocked OTA production, and repressed the OTA cluster genes, p450, nrps, hal, and bzip at the transcriptional level. The results indicated that AnAzf1 was a positive regulator of OTA biosynthesis. Transcriptome sequencing results showed that the AnAzf1 deletion significantly upregulated antioxidant genes and downregulated oxidative phosphorylation genes. Enzymes involved in reactive oxygen species (ROS) scavenging, including catalase (CAT) and peroxidase (POD) were increased, and the corresponding ROS levels were decreased. Upregulation of genes (cat, catA, hog1, and gfd) in the MAPK pathway and downregulation of genes in iron homeostasis were associated with decreased ROS levels, linking the altered MAPK pathway and iron homeostasis to lower ROS levels caused by AnAzf1 deletion. Additionally, enzymes including complex I (NADH-ubiquinone oxidoreductase), and complex V (ATP synthase), as well as ATP levels, were significantly decreased, indicating impaired oxidative phosphorylation caused by the AnAzf1-deletion. During lower ROS levels and impaired oxidative phosphorylation, OTA was not produced in ∆AnAzf1. Together, these results strongly suggested that AnAzf1 deletion blocked OTA production in A. niger by a synergistic interference of ROS accumulation and oxidative phosphorylation. KEY POINTS: • AnAzf1 positively regulated OTA biosynthesis in A. niger. • Deletion of AnAzf1 decreased ROS levels and impaired oxidative phosphorylation. • An altered MAPK pathway and iron homeostasis were associated with lower ROS levels.

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.

Morphological and chemical characterization of Alternaria populations from apple fruit

Alternaria is a frequent contaminant of apple fruit, causing severe economic losses. It can produce external lesions and mouldy core, characterised by a rotten area in the apple core. In the present study, morphological and chemical characterization of Alternaria from apples was performed, evaluating differences related to agricultural practices and type of disease. A low morphological diversity was observed; most of the isolates were identified as A. tenuissima sp.-grp. (95 %). A. arborescens sp.-grp. and A. gaisen sp.-grp. were present in a proportion of 1 %, and 3 % of the isolates showed intermediate characteristics between these sp.-grps. and were identified as Alternaria sp. The chemical diversity was greater; 27 secondary metabolites were produced by the apple isolates. The most frequents were altertoxin-I (85 %), altechromone A (76 %), tentoxin (69 %), and tenuazonic acid (68 %). The alternariols were produced in a lower frequency when comparing with isolates from other crops; alternariol, 58 % and alternariol monomethyl ether, 57 %. The predominant secondary metabolite profile included compounds from different chemical families, such as dibenzopyrones, tetramic acids, perylene quinones, and cyclic tetrapeptides. A wider metabolomic capacity was observed in isolates from conventional apples when compared to those from organic fruit, with the predominance of strong producers of altertoxins and alternariols. The isolates from mouldy core showed higher ability to produce metabolites from different chemical families than those from external lesions. The wide chemical diversity of the Alternaria apple population should be considered to assess the health risk associated with apple by-products.

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.

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.

Effect of Debaryomyces hansenii and the antifungal PgAFP protein on Alternaria spp. growth, toxin production, and RHO1 gene expression in a tomato-based medium

Tomato fruit is susceptible to Alternaria spp. spoilage, which poses a health risk due to their mycotoxin production. Biopreservation relies on the use of whole microorganisms or their metabolites to manage spoilage microorganisms including filamentous fungi. However, the use of treatments at fungistatic level might activate intracellular pathways, which can cause an increment in mycotoxin accumulation. The objective of this work was to evaluate the effect of two strains of Debaryomyces hansenii and the antifungal protein PgAFP at 10 and 40 μg/mL. Both growth and production of two of the most common mycotoxins (tenuazonic acid and alternariol monomethyl ether) by Alternaria tenuissima sp.-grp. and Alternaria arborescens sp.-grp. on a tomato-based matrix, were analysed at 12 °C. Additionally, the impact of these biocontrol agents on the stress-related RHO1 gene expression was assessed. All treatments reduced mycotoxin accumulation (from 27 to 92% of inhibition). Their mode of action against Alternaria spp. in tomato seems unrelated to damages to fungal cell wall integrity at the genomic level. Therefore, the two D. hansenii strains (CECT 10352 and CECT 10353) and the antifungal protein PgAFP at 10 μg/mL are suggested as biocontrol strategies in tomato fruit at postharvest stage.

Influence of Light and Water Activity on Growth and Mycotoxin Formation of Selected Isolates of Aspergillus flavus and Aspergillus parasiticus

Aspergillus flavus and A. parasiticus are the main causes of aflatoxin contamination in various foods, particularly grains, as they can thrive in environments with lower water activity and higher temperatures. The growth of Aspergillus and the formation of the mycotoxins aflatoxin and cyclopiazonic acid are strongly influenced by environmental stimuli and can be reduced by modulating parameters such as water activity, pH, temperature and light during the storage. This study has two objectives—on the one hand, to assess how global warming and an increase in exposure to sunlight affect growth and mycotoxin formation, and on the other hand, how the findings from these experiments can be used to reduce fungal growth and mycotoxin formation in stored foods. Using growth substrates with two different water activities (a_w 0.95, a_w 0.98), together with a light incubation device consisting of different chambers equipped with diodes emitting visible light of five different wavelengths (455 nm, 470 nm, 530 nm, 590 nm, 627 nm) plus white light, we analyzed the growth and mycotoxin formation of selected Aspergillus flavus and A. parasiticus isolates. It was shown that light with a wavelength of 455/470 nm alone, but especially in combination with a lower water activity of a_w 0.95, leads to a significant reduction in growth and mycotoxin formation, which was accompanied by reduced transcriptional activity of the responsible mycotoxin biosynthetic genes. Therefore, these results can be used to significantly reduce the growth and the mycotoxin formation of the analyzed fungi during storage and to estimate the trend of fungal infestation by Aspergillus flavus and A. parasiticus in water activity- and light exposure-equivalent climate change scenarios. Mycotoxin-producing aspergilli can be effective and sustainably inhibited using a combination of short-wave light and lowered water activity in the substrate. A higher annual mean temperature accompanying climate change may lead to an increased spread of aflatoxin-producing fungi in areas that were previously too cold for them. On the other hand, there will be regions in the world where contamination with aflatoxin-producing fungi will be reduced due to increased drought and sun exposure.

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.

cAMP-PKA and HOG1 signaling pathways regulate liamocin production by different ways via the transcriptional activator Msn2 in Aureobasidium melanogenum

Liamocins, as the secondary metabolites synthesized and secreted by Aureobasidium spp., consist of a single mannitol or a single arabitol head group partially O-acylated with three 3,5-dihydroxydecanoic ester groups or directly esterified with three or four 3,5-dihydroxydecanoic ester tails. Very recently, the whole synthetic pathway of liamocins in A. melanogenum 6-1-2 has been elucidated. It was found that the promoter sequences of all the genes related to liamocin synthesis in A. melanogenum 6-1-2 had stress regulatory elements with core sequences of AGGGG or CCCCT. Therefore, expression of all the genes would be regulated by the Msn2. In this study, it was found that removal of the single one MSN2 gene in A. melanogenum 6-1-2 made the mutant decrease yield of extracellular liamocin by 92.28 %, while complementation of the MSN2 gene in the mutant rendered liamocin synthesis to be restored. When A. melanogenum 6-1-2 was cultured in the liamocin fermentation medium with high glucose and low nitrogen, the Msn2 was localized in the nucleus and positively regulated the expression of the genes related to liamocin biosynthesis. Furthermore, when the key BCY1 gene encoding regulatory subunit of the cAMP-PKA signaling pathway in A. melanogenum 6-1-2 was knocked out, the amount of extracellular liamocins synthesized by the mutant was decreased by 96.73 % and the Msn2 was localized in the cytoplasm. Similarly, when the key HOG1 gene in the HOG1 signaling pathway was deleted, liamocin biosynthesis in the knockout strain was decreased by 98.09 %. However, it was found that the Hog1 may be one part of the general transcription complex to regulate the transcription of the MSN2 gene, leading to the reduced Msn2 and liamocin synthesis in the mutant. In addition, the key TOR1 gene and SNF1 gene in the TOR1 signaling pathway and the SNF1 signaling pathway were not involved in the regulation of the Msn2 activity and liamocin synthesis. It was concluded that the transcriptional activator Msn2, the HOG1 signaling pathway and the cAMP-PKA signaling pathway were involved in the regulation of liamocin biosynthesis and production.

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.

Understanding the effect of postharvest tomato temperatures on two toxigenic Alternaria spp. strains: growth, mycotoxins and cell-wall integrity-related gene expression

BACKGROUND

Tomato fruit is susceptible to Alternaria spp. spoilage. Correct postharvest management is necessary to prevent mold growth and mycotoxin accumulation, temperature being one of the main factors associated with these problems. The effect of different postharvest temperatures (5, 12, 25, and 35 °C) on growth, mycotoxin production, and stress-related gene expression by two Alternaria spp. was assessed.

RESULTS

Growth rates decreased rapidly when temperature was higher than the optimum (25 °C), while a gradual reduction was detected at lower temperatures. Tenuazonic acid (TeA) was strongly synthesized at all the temperatures that were evaluated, with a maximum between 12 and 25 °C. Alternariol monomethyl ether (AME) was produced only at the two lowest temperatures, with a peak at 12 °C. Regarding the expression of the stress-related RHO1 gene, during active fungal growth both Alternaria spp. showed more copies of the gene as temperature increased. At the stationary phase, RHO1 gene expression was significantly higher at 12 °C, coinciding with the highest accumulation of AME.

CONCLUSION

Changes in temperature related to different postharvest stages of tomato fruits markedly affect toxigenic Alternaria spp. The highest levels of both mycotoxins were recorded at 12 °C, a common storage temperature for tomato fruit. An association between alternariol biosynthesis and the cell wall integrity pathway was also noticed in relation to temperature, suggesting that temperature may act as a stressor stimulating the RHO1 gene expression, which in turn triggers this mycotoxin synthesis. These results will be useful in developing new strategies to control Alternaria spoilage efficiently in tomato fruit and by-products. © 2019 Society of Chemical Industry.

Contamination of fresh and dried tomato by Alternaria toxins in southern Italy

In the present investigation, fresh and dried tomato samples from markets and packinghouses located in Apulia region (southern Italy) were analysed for Alternaria toxins. All samples proved to be contaminated by tenuazonic acid (TeA); in particular, dried tomatoes were contaminated in the range 425-81,592 µg/kg, while fresh tomatoes were in the range 11-4560 µg/kg. The second most abundant toxin was alternariol monomethyl ether (AME), followed by tentoxin (TEN) and alternariol (AOH). Overall dried tomatoes were more contaminated than fresh ones, although this seemed not directly related to the presence of sodium chloride, utilized in the drying process. Five representative Alternaria isolates within those collected from samples proved to be one Alternaria arborescens (A215) and four Alternaria alternata. Within the latter species, one strain belonged to morphotype tenuissima (A216), and three to alternata (A214, A217 and A218). They were confirmed to produce TeA, AOH, and AME in vitro. This study demonstrates the possible risk for consumers' health related to the consumption of contaminated fresh and dried tomatoes, and thus the need for suitable control strategies.

Chemotaxonomy of Mycotoxigenic Small-Spored Alternaria Fungi - Do Multitoxin Mixtures Act as an Indicator for Species Differentiation?

Necrotrophic as well as saprophytic small-spored Alternaria (A.) species are annually responsible for major losses of agricultural products, such as cereal crops, associated with the contamination of food and feedstuff with potential health-endangering Alternaria toxins. Knowledge of the metabolic capabilities of different species-groups to form mycotoxins is of importance for a reliable risk assessment. 93 Alternaria strains belonging to the four species groups Alternaria tenuissima, A. arborescens, A. alternata, and A. infectoria were isolated from winter wheat kernels harvested from fields in Germany and Russia and incubated under equal conditions. Chemical analysis by means of an HPLC-MS/MS multi-Alternaria-toxin-method showed that 95% of all strains were able to form at least one of the targeted 17 non-host specific Alternaria toxins. Simultaneous production of up to 15 (modified) Alternaria toxins by members of the A. tenuissima, A. arborescens, A. alternata species-groups and up to seven toxins by A. infectoria strains was demonstrated. Overall tenuazonic acid was the most extensively formed mycotoxin followed by alternariol and alternariol mono methylether, whereas altertoxin I was the most frequently detected toxin. Sulfoconjugated modifications of alternariol, alternariol mono methylether, altenuisol and altenuene were frequently determined. Unknown perylene quinone derivatives were additionally detected. Strains of the species-group A. infectoria could be segregated from strains of the other three species-groups due to significantly lower toxin levels and the specific production of infectopyrone. Apart from infectopyrone, alterperylenol was also frequently produced by 95% of the A. infectoria strains. Neither by the concentration nor by the composition of the targeted Alternaria toxins a differentiation between the species-groups A. alternata, A. tenuissima and A. arborescens was possible.

Detection of changes in mould cell wall stress-related gene expression by a novel reverse transcription real-time PCR method

The cell wall integrity (CWI) pathway is activated in response to cell wall stresses due to different food-related environments. Rho1 is one of the main regulators within such pathway. The objective of this work was to design an easy-to-use RT-qPCR technique for the evaluation of the Rho1 gene expression useful to measure responses to the presence of cell wall stressors such as the antifungal protein PgAFP. Two primer pairs were designed from published conserved regions. Their specificity initially was determined by in silico analysis for several fungal species. After optimising the qPCR, the primer pair Rho1-F1/R2 was selected due to the lowest Cq values obtained and its specificity. The qPCR method showed efficiencies between 97.5% and 100.5%. Applicability of the designed qPCR method was evaluated in the presence of the stressor PgAFP. The PgAFP-resistant Penicillium polonicum and the PgAFP-sensitive Aspergillus flavus showed Rho1 gene over- and under- expression, respectively, indicating that the CWI pathway is activated in the former species but not activated in the latter one in response to the stress caused by PgAFP. This novel qPCR methodology able to detect changes in CWI-related gene expression in filamentous fungi will be useful in future studies to evaluate physiological mould responses to different food environmental challenges.

Isolation, characterization and toxicological potential of Alternaria-mycotoxins (TeA, AOH and AME) in different Alternaria species from various regions of India

Alternaria species produce various sorts of toxic metabolites during their active growth and causes severe diseases in many plants by limiting their productivity. These toxic metabolites incorporate various mycotoxins comprising of dibenzo-α-pyrone and some tetramic acid derivatives. In this study, we have screened out total 48 isolates of Alternaria from different plants belonging to different locations in India, on the basis of their pathogenic nature. Pathogenicity testing of these 48 strains on susceptible tomato variety (CO-3) showed 27.08% of the strains were highly pathogenic, 35.41% moderately pathogenic and 37.5% were less pathogenic. Phylogenetic analysis showed the presence of at least eight evolutionary cluster of the pathogen. Toxins (TeA, AOH and AME) were isolated, purified on the basis of column chromatography and TLC, and further confirmed by the HPLC-UV chromatograms using standards. The final detection of toxins was done by the LC-MS/MS analysis by their mass/charge ratio. The present study develops an approach to classify the toxicogenic effect of each of the individual mycotoxins on tomato plant and focuses their differential susceptibility to develop disease symptoms. This study represents the report of the natural occurrence and distribution of Alternaria toxins in various plants from India.

Spotlight on the Underdogs-An Analysis of Underrepresented Alternaria Mycotoxins Formed Depending on Varying Substrate, Time and Temperature Conditions

Alternaria (A.) is a genus of widespread fungi capable of producing numerous, possibly health-endangering Alternaria toxins (ATs), which are usually not the focus of attention. The formation of ATs depends on the species and complex interactions of various environmental factors and is not fully understood. In this study the influence of temperature (7 °C, 25 °C), substrate (rice, wheat kernels) and incubation time (4, 7, and 14 days) on the production of thirteen ATs and three sulfoconjugated ATs by three different Alternaria isolates from the species groups A. tenuissima and A. infectoria was determined. High-performance liquid chromatography coupled with tandem mass spectrometry was used for quantification. Under nearly all conditions, tenuazonic acid was the most extensively produced toxin. At 25 °C and with increasing incubation time all toxins were formed in high amounts by the two A. tenuissima strains on both substrates with comparable mycotoxin profiles. However, for some of the toxins, stagnation or a decrease in production was observed from day 7 to 14. As opposed to the A. tenuissima strains, the A. infectoria strain only produced low amounts of ATs, but high concentrations of stemphyltoxin III. The results provide an essential insight into the quantitative in vitro AT formation under different environmental conditions, potentially transferable to different field and storage conditions.

PiHOG1, a stress regulator MAP kinase from the root endophyte fungus Piriformospora indica, confers salinity stress tolerance in rice plants

In this study, yeast HOG1 homologue from the root endophyte Piriformospora indica (PiHOG1) was isolated and functionally characterized. Functional expression of PiHOG1 in S. cerevisiae ∆hog1 mutant restored osmotolerance under high osmotic stress. Knockdown (KD) transformants of PiHOG1 generated by RNA interference in P. indica showed that genes for the HOG pathway, osmoresponse and salinity tolerance were less stimulated in KD-PiHOG1 compared to the wild-type under salinity stress. Furthermore, KD lines are impaired in the colonization of rice roots under salinity stress of 200 mM NaCl, and the biomass of the host plants, their shoot and root lengths, root number, photosynthetic pigment and proline contents were reduced as compared to rice plants colonized by WT P. indica. Therefore, PiHOG1 is critical for root colonisation, salinity tolerance and the performance of the host plant under salinity stress. Moreover, downregulation of PiHOG1 resulted not only in reduced and delayed phosphorylation of the remaining PiHOG1 protein in colonized salinity-stressed rice roots, but also in the downregulation of the upstream MAP kinase genes PiPBS2 and PiSSK2 involved in salinity tolerance signalling in the fungus. Our data demonstrate that PiHOG1 is not only involved in the salinity response of P. indica, but also helping host plant to overcome salinity stress.

Characterization of Citrus-Associated Alternaria Species in Mediterranean Areas

Alternaria brown spot is one of the most important diseases of tangerines and their hybrids worldwide. Recently, outbreaks in Mediterranean areas related to susceptible cultivars, refocused attention on the disease. Twenty representatives were selected from a collection of 180 isolates of Alternaria spp. from citrus leaves and fruit. They were characterized along with reference strains of Alternaria spp. Micro- and macroscopic characteristics separated most Alternaria isolates into six morphotypes referable to A. alternata (5) and A. arborescens (1). Phylogenetic analyses, based on endopolygalacturonase (endopg) and internal transcribed spacer (ITS), confirmed this finding. Moreover, a five-gene phylogeny including two anonymous genomics regions (OPA 1-3 and OPA 2-1), and the beta-tubulin gene (ß-tub), produced a further clustering of A. alternata into three clades. This analysis suggested the existence of intra-species molecular variability. Investigated isolates showed different levels of virulence on leaves and fruit. In particular, the pathogenicity on fruit seemed to be correlated with the tissue of isolation and the clade. The toxigenic behavior of Alternaria isolates was also investigated, with tenuazonic acid (TeA) being the most abundant mycotoxin (0.2-20 mg/L). Isolates also synthesized the mycotoxins alternariol (AOH), its derivate alternariol monomethyl ether (AME), and altenuene (ALT), although to a lesser extent. AME production significantly varied among the six morphotypes. The expression of pksJ/pksH, biosynthetic genes of AOH/AME, was not correlated with actual toxin production, but it was significantly different between the two genotypes and among the four clades. Finally, ten isolates proved to express the biosynthetic genes of ACTT1 phytotoxin, and thus to be included in the Alternaria pathotype tangerine. A significant correlation between pathogenicity on leaves and ACTT1 gene expression was recorded. The latter was significantly dependent on geographical origin. The widespread occurrence of Alternaria spp. on citrus fruit and their ability to produce mycotoxins might represent a serious concern for producers and consumers.

Natural occurrence of mycotoxins and toxigenic capacity of Alternaria strains from mouldy peppers

Sweet pepper (Capsicum annuum) is an important crop cultivated worldwide, with Argentina being one of the major producers in South America. The fruit is susceptible to several fungal diseases, leading to severe economic losses for producers. In this study, Alternaria was found as the prevalent genus in mouldy peppers (50% fruits infected). Morphological identification revealed that all 64 Alternaria isolates belonged to small-spored species, most of them corresponding to A. tenuissima, A. arborescens and A. alternata species-groups. Their secondary metabolite profile was evaluated in vitro; alternariols were synthesized by most of the isolates (91% for alternariol and 92% for alternariol monomethyl ether). A high number of Alternaria spp. also produced tenuazonic acid (64%), altenuene (84%) and tentoxin (72%). In addition, damaged pepper fruits were analysed for the presence of tenuazonic acid and alternariols. A total 32 out of 48 spoiled pepper fruits were contaminated with at least one of these metabolites. Half of the samples were positive for tenuazonic acid (range 8-11,422μg/kg), while alternariol and its monomethyl ether were less frequently detected (21 and 29%, respectively) and at lower concentrations. This is the first report on the natural occurrence of Alternaria mycotoxins in Argentinean sweet pepper, and highlights a consumer risk when mouldy fruits are used in industrialized products because these compounds are not destroyed by conventional heat treatments.

Light regulation of mycotoxin biosynthesis: new perspectives for food safety

Mycotoxins are secondary metabolites produced by toxigenic fungi contaminating foods and feeds in pre-, post-harvest and processing, and represent a great concern worldwide, both for the economic implications and for the health of the consumers. Many environmental conditions are involved in the regulation of mycotoxin biosynthesis. Among these, light represents one of the most important signals for fungi, influencing several physiological responses such as pigmentation, sexual development and asexual conidiation, primary and secondary metabolism, including mycotoxin biosynthesis. In this review we summarise some recent findings on the effect of specific light wavelength and intensity on mycotoxin biosynthesis in the main toxigenic fungal genera. We describe the molecular mechanism underlying light perception and its involvement in the regulation of secondary metabolism, focusing on VeA, global regulator in Aspergillus nidulans, and the White-Collar proteins, key components of light response in Neurospora crassa. Light of specific wavelength and intensity exerts different effects both on growth and on toxin production depending on the fungal genus. In Penicillium spp. red (627 nm) and blue wavelengths (455-470 nm) reduce ochratoxin A (OTA) biosynthesis by modulating the level of expression of the ochratoxin polyketide synthase. Furthermore a mutual regulation between citrinin and OTA production is reported in Penicillium toxigenic species. In Aspergillus spp. the effect of light treatment is strongly dependent on the species and culture conditions. Royal blue wavelength (455 nm) of high intensity (1,700 Lux) is capable of completely inhibit fungal growth and OTA production in Aspergillus stenyii and Penicillum verrucosum. In Fusarium spp. the effect of light exposure is less effective; mycotoxin-producing species, such as Fusarium verticillioides and Fusarium proliferatum, grow better under light conditions, and fumonisin production increased. This review provides a comprehensive picture on light regulation of mycotoxin biosynthesis and discusses possible new applications of this resource in food safety.

Oxidative stress induces the biosynthesis of citrinin by Penicillium verrucosum at the expense of ochratoxin

Penicillium verrucosum is a fungus that can produce ochratoxin A and citrinin, two structurally related nephrotoxic mycotoxins. P. verrucosum usually occurs on wheat but can occasionally also be found in NaCl rich habitats such as salted cheeses or olives, indicating that this fungus can adapt to different environments. The ratio of ochratoxin A to citrinin produced by P. verrucosum is shifted to one of either mycotoxin at the expense of the other dependent on the environmental conditions. High NaCl concentrations shift secondary metabolite biosynthesis towards ochratoxin A production. P. verrucosum copes with NaCl stress by increased ochratoxin A biosynthesis, ensuring chloride homeostasis. Ochratoxin A carries chlorine in its molecule and can excrete chlorine from the cell. It was further shown that the regulation of ochratoxin A by high NaCl conditions is mediated by the HOG MAP kinase signal transduction pathway. Here it is shown that high oxidative stress conditions, evoked for example by increasing concentrations of Cu(2+) cations in the growth medium, shift secondary metabolite biosynthesis of P. verrucosum from ochratoxin A to citrinin. The production of citrinin normalizes the oxidative status of the fungal cell under oxidative stress conditions leading to an adaptation to these environmental conditions and protects against increased oxidative stress caused by increased Cu(2+) concentrations. Moreover citrinin also protects against light of short wavelength, which may also increase the oxidative status of the environment. The biosynthesis of citrinin is apparently regulated by a cAMP/PKA signaling pathway, because increasing amounts of external cAMP reduce citrinin biosynthesis in a concentration dependent manner. These conditions lead to the cross-regulation of the ochratoxin A/citrinin secondary metabolite pair and support the adaptation of P. verrucosum to different environments.

Partial resistance of carrot to Alternaria dauci correlates with in vitro cultured carrot cell resistance to fungal exudates

Although different mechanisms have been proposed in the recent years, plant pathogen partial resistance is still poorly understood. Components of the chemical warfare, including the production of plant defense compounds and plant resistance to pathogen-produced toxins, are likely to play a role. Toxins are indeed recognized as important determinants of pathogenicity in necrotrophic fungi. Partial resistance based on quantitative resistance loci and linked to a pathogen-produced toxin has never been fully described. We tested this hypothesis using the Alternaria dauci-carrot pathosystem. Alternaria dauci, causing carrot leaf blight, is a necrotrophic fungus known to produce zinniol, a compound described as a non-host selective toxin. Embryogenic cellular cultures from carrot genotypes varying in resistance against A. dauci were confronted with zinniol at different concentrations or to fungal exudates (raw, organic or aqueous extracts). The plant response was analyzed through the measurement of cytoplasmic esterase activity, as a marker of cell viability, and the differentiation of somatic embryos in cellular cultures. A differential response to toxicity was demonstrated between susceptible and partially resistant genotypes, with a good correlation noted between the resistance to the fungus at the whole plant level and resistance at the cellular level to fungal exudates from raw and organic extracts. No toxic reaction of embryogenic cultures was observed after treatment with the aqueous extract or zinniol used at physiological concentration. Moreover, we did not detect zinniol in toxic fungal extracts by UHPLC analysis. These results suggest that strong phytotoxic compounds are present in the organic extract and remain to be characterized. Our results clearly show that carrot tolerance to A. dauci toxins is one component of its partial resistance.

The fungal phytotoxin alternariol 9-methyl ether and some of its synthetic analogues inhibit the photosynthetic electron transport chain

Alternariol and monomethylalternariol are natural phytotoxins produced by some fungal strains, such as Nimbya and Alternaria. These substances confer virulence to phytopathogens, yet no information is available concerning their mode of action. Here we show that in the micromolar range alternariol 9-methyl ether is able to inhibit the electron transport chain (IC50 = 29.1 ± 6.5 μM) in isolated spinach chloroplasts. Since its effectiveness is limited by poor solubility in water, several alternariol analogues were synthesized using different aromatic aldehydes. The synthesized 6H-benzo[c]cromen-6-ones, 5H-chromene[4,3-b]pyridin-5-one, and 5H-chromene[4,3-c]pyridin-5-one also showed inhibitory properties, and three 6H-benzo[c]cromen-6-ones were more effective (IC50 = 12.8-22.8 μM) than the lead compound. Their addition to the culture medium of a cyanobacterial model strain was found to inhibit algal growth, with a relative effectiveness that was consistent with their activity in vitro. In contrast, the growth of a nonphotosynthetic plant cell culture was poorly affected. These compounds may represent a novel lead for the development of new active principles targeting photosynthesis.

Differences in the regulation of ochratoxin A by the HOG pathway in Penicillium and Aspergillus in response to high osmolar environments

Penicillium verrucosum, P. nordicum and Aspergillus carbonarius are three important ochratoxin A producing species. P. verrucosum is in addition able to produce citrinin. It has been shown earlier that P. nordicum is adapted to NaCl rich environments like salt rich dry cured foods or even salines. In this organism, the biosynthesis of ochratoxin A plays an adaptive role in this habitat. P. verrucosum generally can be found on cereals, but occasionally also on salt rich dry cured foods. In contrast A. carbonarius usually cannot be found in NaCl rich environments, but it occurs in another environment with high concentration of solutes, e.g., in sugar rich substrates like grapes and grape juices. Usually osmotic challenging conditions activate the HOG MAP kinase signal cascade, which in turn activates various osmo-regulated genes. In the current analysis, it could be demonstrated that in case of P. nordicum and P. verrucosum the NaCl induced production of ochratoxin A is correlated to the phosphorylation status of the HOG MAP kinase. Just the opposite was true for A. carbonarius. In this case, also higher amounts of NaCl in the medium lead to an increased phosphorylation status of HOG, but no increase in ochratoxin biosynthesis was observed. In contrast to the Penicillia, higher NaCl concentrations lead to a rapid cessation of growth by A. carbonarius. High glucose concentrations have much less impact on growth and the phosphorylation of HOG.