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

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.

The Genus Cladosporium: A Prospective Producer of Natural Products

Cladosporium, a genus of ascomycete fungi in the Dematiaceae family, is primarily recognized as a widespread environmental saprotrophic fungus or plant endophyte. Further research has shown that the genus is distributed in various environments, particularly in marine ecosystems, such as coral reefs, mangroves and the polar region. Cladosporium, especially the marine-derived Cladosporium, is a highly resourceful group of fungi whose natural products have garnered attention due to their diverse chemical structures and biological activities, as well as their potential as sources of novel leads to compounds for drug production. This review covers the sources, distribution, bioactivities, biosynthesis and structural characteristics of compounds isolated from Cladosporium in the period between January 2000 and December 2022, and conducts a comparative analysis of the Cladosporium isolated compounds derived from marine and terrestrial sources. Our results reveal that 34% of Cladosporium-derived natural products are reported for the first time. And 71.79% of the first reported compounds were isolated from marine-derived Cladosporium. Cladosporium-derived compounds exhibit diverse skeletal chemical structures, concentrating in the categories of polyketides (48.47%), alkaloids (19.21%), steroids and terpenoids (17.03%). Over half of the natural products isolated from Cladosporium have been found to have various biological activities, including cytotoxic, antibacterial, antiviral, antifungal and enzyme-inhibitory activities. These findings testify to the tremendous potential of Cladosporium, especially the marine-derived Cladosporium, to yield novel bioactive natural products, providing a structural foundation for the development of new drugs.

Alternaria alternata Isolated from Infected Pears (Pyrus communis) in Italy Produces Non-Host Toxins and Hydrolytic Enzymes as Infection Mechanisms and Exhibits Competitive Exclusion against Botrytis cinerea in Co-Infected Host Fruits

Alternaria alternata is one of the most devastating phytopathogenic fungi. This microorganism causes black spots in many fruits and vegetables worldwide, generating significant post-harvest losses. In this study, an A. alternata strain, isolated from infected pears (Pyrus communis) harvested in Italy, was characterized by focusing on its pathogenicity mechanisms and competitive exclusion in the presence of another pathogen, Botrytis cinerea. In in vitro assays, the fungus produces strong enzymatic activities such as amylase, xylanase, and cellulase, potentially involved during the infection. Moreover, it secretes four different toxins purified and identified as altertoxin I, alteichin, alternariol, and alternariol 4-methyl ether. Only alteichin generated necrotic lesions on host-variety pears, while all the compounds showed moderate to slight necrotic activity on non-host pears and other non-host fruit (lemon, Citrus limon), indicating they are non-host toxins. Interestingly, A. alternata has shown competitive exclusion to the competitor fungus Botrytis cinerea when co-inoculated in host and non-host pear fruits, inhibiting its growth by 70 and 65%, respectively, a result not observed in a preliminary characterization in a dual culture assay. Alteichin and alternariol 4-methyl ether tested against B. cinerea had the best inhibition activity, suggesting that the synergism of these toxins and enzymatic activities of A. alternata are probably involved in the competitive exclusion dynamics in host and non-host pear fruits.

Deciphering the antimicrobial activity of multifaceted rhizospheric biocontrol agents of solanaceous crops viz., Trichoderma harzianum MC2, and Trichoderma harzianum NBG

The Solanaceae family is generally known to be the third most economically important plant taxon, but also harbors a host of plant pathogens. Diseases like wilt and fruit rot of solanaceous crops cause huge yield losses in the field as well as in storage. In the present study, eight isolates of Trichoderma spp. were obtained from rhizospheric micro-flora of three solanaceous crops: tomato, brinjal, and chili plants, and were subsequently screened for pre-eminent biocontrol activity against three fungal (Fusarium oxysporum f. sp. lycopersicum, Colletotrichum gloeosporioides, and Rhizoctonia solani) and one bacterial (Ralstonia solanacearum) pathogen. Morphological, ITS, and tef1α marker-based molecular identification revealed eight isolates were different strains of Trichoderma. Seven isolates were distinguished as T. harzianum while one was identified as T. asperellum. In vitro antagonistic and biochemical assays indicated significant biocontrol activity governed by all eight isolates. Two fungal isolates, T. harzianum MC2 and T. harzianum NBG were further evaluated to decipher their best biological control activity. Preliminary insights into the secondary metabolic profile of both isolates were retrieved by liquid chromatography-mass spectrometry (LC-MS). Further, a field experiment was conducted with the isolates T. harzianum MC2 and T. harzianum NBG which successfully resulted in suppression of bacterial wilt disease in tomato. Which possibly confer biocontrol properties to the identified isolates. The efficacy of these two strains in suppressing bacterial wilt and promoting plant growth in the tomato crop was also tested in the field. The disease incidence was significantly reduced by 47.50% and yield incremented by 54.49% in plants treated in combination with both the bioagents. The results of scanning electron microscopy were also in consensus with the in planta results. The results altogether prove that T. harzianum MC2 and T. harzianum NBG are promising microbes for their prospective use in agricultural biopesticide formulations.

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.

Phytotoxic Secondary Metabolites from Fungi

Fungal phytotoxic secondary metabolites are poisonous substances to plants produced by fungi through naturally occurring biochemical reactions. These metabolites exhibit a high level of diversity in their properties, such as structures, phytotoxic activities, and modes of toxicity. They are mainly isolated from phytopathogenic fungal species in the genera of Alternaria, Botrytis, Colletotrichum, Fusarium, Helminthosporium, and Phoma. Phytotoxins are either host specific or non-host specific phytotoxins. Up to now, at least 545 fungal phytotoxic secondary metabolites, including 207 polyketides, 46 phenols and phenolic acids, 135 terpenoids, 146 nitrogen-containing metabolites, and 11 others, have been reported. Among them, aromatic polyketides and sesquiterpenoids are the main phytotoxic compounds. This review summarizes their chemical structures, sources, and phytotoxic activities. We also discuss their phytotoxic mechanisms and structure-activity relationships to lay the foundation for the future development and application of these promising metabolites as herbicides.

Synthesis of 7-hydroxy-6H-naphtho[2,3-c]coumarin via a TsOH-mediated tandem reaction

A concise and efficient method for the synthesis of 7-hydroxy-6H-naphtho[2,3-c]coumarin using available 1-(2-hydroxyphenyl)-2-phenylethanone and Meldrum's acid has been developed. This transformation involved a tandem aldol reaction/lactonization/Friedel-Crafts reaction to form a lactone ring and a benzene ring. It showed high atom economy with water and acetone as the byproducts. Mechanism studies demonstrated two roles of Meldrum's acid: (i) as the reagent for the tandem reaction, and (ii) as the catalyst for the Friedel-Crafts reaction. Moreover, the hydroxyl group of 7-hydroxy-6H-naphtho[2,3-c]coumarin was further functionalized efficiently by arylethynyl, aryl, and cyano groups to furnish D-π-A compounds with excellent fluorescence emissions (Φ_F = 0.14-0.78).

Cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids

The first cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids has been developed. This operationally simple protocol allows rapid access to synthetically useful coumarins on gram scale by employing CeCl₃ as a photocatalyst and O₂ as a terminal oxidant. Overall, this delivers an economical and environmentally amiable entry to diversely substituted coumarins, important structural motifs in bioactive molecules.

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.

Rhodium(iii)-catalysed decarbonylative annulation through C–H activation: expedient access to aminoisocoumarins by weak coordination

Rh-Catalysed decarbonylative annulation of isatoic anhydrides with alkynes through C–H activation for the direct synthesis of 8-aminoisocoumarins has been developed.

Natural Abenquines and Their Synthetic Analogues Exert Algicidal Activity against Bloom-Forming Cyanobacteria

Abenquines are natural quinones, produced by some Streptomycetes, showing the ability to inhibit cyanobacterial growth in the 1 to 100 μM range. To further elucidate their biological significance, the synthesis of several analogues (4f-h, 5a-h) allowed us to identify some steric and electronic requirements for bioactivity. Replacing the acetyl by a benzoyl group in the quinone core and also changing the amino acid moiety with ethylpyrimidinyl or ethylpyrrolidinyl groups resulted in analogues 25-fold more potent than the natural abenquines. The two most effective analogues inhibited the proliferation of five cyanobacterial strains tested, with IC₅₀ values ranging from 0.3 to 3 μM. These compounds may be useful leads for the development of an effective strategy for the control of cyanobacterial blooms.

Nanostructured lipid carriers (NLC) for the delivery of natural molecules with antimicrobial activity: production, characterisation and in vitro studies

This study describes the preparation, characterisation and in vitro activity of nanostructured lipid carriers (NLCs) encapsulating natural molecules with antimicrobial activity, such as plumbagin, hydroquinon, eugenol, alpha-asarone and alpha-tocopherol. NLCs were prepared by melt and ultrasonication method, characterised by Cryo-TEM for morphology and SdFFF for dimensional distribution and active encapsulation yields. In vitro tests were conducted on bacteria, fungi and human cell cultures. In vitro tests demonstrated that plumbagin is strongly toxic towards F. oxysporum especially when active molecules are loaded on NLC. Plumbagin was completely non toxic on cyanobacterial model strain up to a threshold over which cell viability was completely lost. NLC loaded with active molecules showed a lower toxicity as compared to their free form on human cultured cells. Although further studies need to be performed, these systems can be potentially proposed to control phytopathogenic organisms.

An efficient one-pot synthesis of functionalized chromeno[4,3-b]pyridine derivatives under catalyst-free conditions

A concise, efficient one-pot synthesis of functionalized chromeno[4,3-b]pyridine derivatives via a three-component reaction of 4-oxo-4H-chromene-3-carbaldehydes, malononitrile or cyanoacetates, and aromatic amines under catalyst-free conditions in an environmentally friendly medium (ethanol-water, 3:1 v/v) is described. This synthesis involves a group-assisted purification process, which avoids traditional recrystallization and chromatographic purification methods.

A metal-free one-pot synthesis of benzo[c]chromen-6-ones from 3,4-dichlorocoumarins and butadienes using tandem photo-thermal-photo reactions

An efficient, simple and versatile synthesis of biologically valuable benzo[c]chromen-6-ones is achieved using a tandem photo-thermal-photo reaction sequence starting from 3,4-dichlorocoumarins and a 1,3-butadiene. In this concise one-pot protocol, neither metal catalyst nor peroxide promoter is needed and the products can be purified through simple recrystallization in most cases. The synthesis consists of a reaction sequence of photo-induced [4 + 2] and [2 + 2] cycloadditions, silica gel promoted elimination of HCl and electrocyclic cyclobutene ring opening followed by a photo-induced 6π electrocyclization. The reactions proceed well with a range of dichlorocoumarins and some typical butadienes to provide the corresponding annulated products in 70-80% yield.

Acridone Alkaloids from Swinglea glutinosa (Rutaceae) and Their Effects on Photosynthesis

Continuing our search for herbicide models based on natural products, we investigated the action mechanisms of five alkaloids isolated from Swinglea glutinosa (Rutaceae): Citrusinine-I (1), glycocitrine-IV (2), 1,3,5-trihydroxy-10-methyl- 2,8-bis(3-methylbut-2-en-1-yl)-9(10H)-acridinone (3), (2R)-2-tert-butyl-3,10-dihydro-4,9-dihydroxy-11-methoxy-10-methylfuro[3,2-b]acridin-5(2H)-one (4), and (3R)-2,3,4,7-tetrahydro-3,5,8-trihydroxy-6-methoxy-2,2,7-trimethyl-12H-pyrano[2,3-a]acridin-12-one (5) on several photosynthetic activities in an attempt to find new compounds that affect photosynthesis. Through polarographic techniques, the compounds inhibited the non-cyclic electron transport in the basal, phosphorylating, and uncoupled conditions from H2 O to methylviologen (=MV). Therefore, they act as Hill reaction inhibitors. This approach still suggested that the compounds 4 and 5 had their interaction site located at photosystem I. Studies on fluorescence of chlorophyll a suggested that acridones (1-3) have different modes of interaction and inhibition sites on the photosystem II electron transport chain.

Highly potent anti-leishmanial derivatives of hederagenin, a triperpenoid from Sapindus saponaria L

Leishmaniasis is a neglected tropical disease (NTDs), endemic in 88 countries that affect more than 12 million people. Current drugs are limited due to their toxicity, development of biological resistance, length of treatment and high cost. Thus, the search for new effective and less toxic treatments is an urgent need. In this study, we report the synthesis of 3 new amide derivatives of hederagenin (22-24) with yields between 70% and 90%, along with 57 other derivatives of hederagenin (1-21, 25-60) carrying different groups at C-28 previously reported by our group, and the results of their in vitro ability to inhibit the growth of Leishmania infantum. Some derivatives (3, 4, 44, 49 and 52), showed activity at micromolar level and low toxicity against BGM and HepG2 cells. Moreover, the ability of hederagenin derivatives 3 (IC₅₀ = 9.7 μM), 4 (12 μM), 44 (11 μM) and 49 (2 μM), to prevent proliferation of intracellular amastigote forms of L. infantum and their higher selectivity index and low toxicity compared to commercial positive drug control of choice (potassium antimonyl tartrate trihydrate) (IC₅₀ = 80 μM, SI = 0.1), make these compounds promising candidates for the treatment of leishmaniasis.

Mechanisms of thaxtomin A-induced root toxicity revealed by a thaxtomin A sensitive Arabidopsis mutant (ucu2-2/gi-2)

The Arabidopsis mutant ( ucu2 - 2/gi - 2 ) is thaxtomin A, isoxaben and NPA-sensitive indicated by root growth and ion flux responses providing new insights into these compounds mode of action and interactions. Thaxtomin A (TA) is a cellulose biosynthetic inhibitor (CBI) that promotes plant cell hypertrophy and cell death. Electrophysiological analysis of steady-state K(+) and Ca(2+) fluxes in Arabidopsis thaliana roots pretreated with TA for 24 h indicated a disturbance in the regulation of ion movement across the plant cell membrane. The observed inability to control solute movement, recorded in rapidly growing meristematic and elongation root zones, may partly explain typical root toxicity responses to TA treatment. Of note, the TA-sensitive mutant (ucu2-2/gi-2) was more susceptible with K(+) and Ca(2+) fluxes altered between 1.3 and eightfold compared to the wild-type control where fluxes altered between 1.2 and threefold. Root growth inhibition assays showed that the ucu2-2/gi-2 mutant had an increased sensitivity to the auxin 2,4-D, but not IAA or NAA; it also had increased sensitivity to the auxin efflux transport inhibitor, 1-naphthylphthalamic acid (NPA), but not 2,3,5- Triiodobenzoic acid (TIBA), when compared to the WT. The NPA sensitivity data were supported by electrophysiological analysis of H(+) fluxes in the mature (but not elongation) root zone. Increased sensitivity to the CBI, isoxaben (IXB), but not dichlobenil was recorded. Increased sensitivity to both TA and IXB corresponded with higher levels of accumulation of these toxins in the root tissue, compared to the WT. Further root growth inhibition assays showed no altered sensitivity of ucu2-2/gi-2 to two other plant pathogen toxins, alternariol and fusaric acid. Identification of a TA-sensitive Arabidopsis mutant provides further insight into how this CBI toxin interacts with plant cells.

Evolution of Chemical Diversity in a Group of Non-Reduced Polyketide Gene Clusters: Using Phylogenetics to Inform the Search for Novel Fungal Natural Products

Fungal polyketides are a diverse class of natural products, or secondary metabolites (SMs), with a wide range of bioactivities often associated with toxicity. Here, we focus on a group of non-reducing polyketide synthases (NR-PKSs) in the fungal phylum Ascomycota that lack a thioesterase domain for product release, group V. Although widespread in ascomycete taxa, this group of NR-PKSs is notably absent in the mycotoxigenic genus Fusarium and, surprisingly, found in genera not known for their secondary metabolite production (e.g., the mycorrhizal genus Oidiodendron, the powdery mildew genus Blumeria, and the causative agent of white-nose syndrome in bats, Pseudogymnoascus destructans). This group of NR-PKSs, in association with the other enzymes encoded by their gene clusters, produces a variety of different chemical classes including naphthacenediones, anthraquinones, benzophenones, grisandienes, and diphenyl ethers. We discuss the modification of and transitions between these chemical classes, the requisite enzymes, and the evolution of the SM gene clusters that encode them. Integrating this information, we predict the likely products of related but uncharacterized SM clusters, and we speculate upon the utility of these classes of SMs as virulence factors or chemical defenses to various plant, animal, and insect pathogens, as well as mutualistic fungi.

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

SnPKS19 Encodes the Polyketide Synthase for Alternariol Mycotoxin Biosynthesis in the Wheat Pathogen Parastagonospora nodorum

Alternariol (AOH) is an important mycotoxin from the Alternaria fungi. AOH was detected for the first time in the wheat pathogen Parastagonospora nodorum in a recent study. Here, we exploited reverse genetics to demonstrate that SNOG_15829 (SnPKS19), a close homolog of Penicillium aethiopicum norlichexanthone (NLX) synthase gene gsfA, is required for AOH production. We further validate that SnPKS19 is solely responsible for AOH production by heterologous expression in Aspergillus nidulans. The expression profile of SnPKS19 based on previous P. nodorum microarray data correlated with the presence of AOH in vitro and its absence in planta. Subsequent characterization of the ΔSnPKS19 mutants showed that SnPKS19 and AOH are not involved in virulence and oxidative stress tolerance. Identification and characterization of the P. nodorum SnPKS19 cast light on a possible alternative AOH synthase gene in Alternaria alternata and allowed us to survey the distribution of AOH synthase genes in other fungal genomes. We further demonstrate that phylogenetic analysis could be used to differentiate between AOH synthases and the closely related NLX synthases. This study provides the basis for studying the genetic regulation of AOH production and for development of molecular diagnostic methods for detecting AOH-producing fungi in the future.