Substrate specificity and inhibition of brassinin hydrolases, detoxifying enzymes from the plant pathogens Leptosphaeria maculans and Alternaria brassicicola

Interactions of fungi with non-isothiocyanate products of the plant glucosinolate pathway: A review on product formation, antifungal activity, mode of action and biotransformation

The glucosinolate pathway, which is present in the order Brassicales, is one of the most researched defensive natural product biosynthesis pathways. Its core molecules, the glucosinolates are broken down upon pathogen challenge or tissue damage to yield an array of natural products that may help plants defend against the stressor. Though the most widely known glucosinolate decomposition products are the antimicrobial isothiocyanates, there is a wide range of other volatile and non-volatile natural products that arise from this biosynthetic pathway. This review summarizes our current knowledge on the interaction of these much less examined, non-isothiocyanate products with fungi. It deals with compounds including (1) glucosinolates and their biosynthesis precursors; (2) glucosinolate-derived nitriles (e.g. derivatives of 1H-indole-3-acetonitrile), thiocyanates, epithionitriles and oxazolidine-2-thiones; (3) putative isothiocyanate downstream products such as raphanusamic acid, 1H-indole-3-methanol (= indole-3-carbinol) and its oligomers, 1H-indol-3-ylmethanamine and ascorbigen; (4) 1H-indole-3-acetonitrile downstream products such as 1H-indole-3-carbaldehyde (indole-3-carboxaldehyde), 1H-indole-3-carboxylic acid and their derivatives; and (5) indole phytoalexins including brassinin, cyclobrassinin and brassilexin. Herein, a literature review on the following aspects is provided: their direct antifungal activity and the proposed mechanisms of antifungal action, increased biosynthesis after fungal challenge, as well as data on their biotransformation/detoxification by fungi, including but not limited to fungal myrosinase activity.

Responses of the Necrotrophic Fungus Alternaria brassisicola to the Indolic Phytoalexin Brassinin

Alternaria brassicicola causes black spot disease in Brassicaceae. During host infection, this necrotrophic fungus is exposed to various antimicrobial compounds, such as the phytoalexin brassinin which is produced by many cultivated Brassica species. To investigate the cellular mechanisms by which this compound causes toxicity and the corresponding fungal adaptive strategies, we first analyzed fungal transcriptional responses to short-term exposure to brassinin and then used additional functional approaches. This study supports the hypothesis that indolic phytoalexin primarily targets mitochondrial functions in fungal cells. Indeed, we notably observed that phytoalexin treatment of A. brassicicola disrupted the mitochondrial membrane potential and resulted in a significant and rapid decrease in the oxygen consumption rates. Secondary effects, such as Reactive oxygen species production, changes in lipid and endoplasmic reticulum homeostasis were then found to be induced. Consequently, the fungus has to adapt its metabolism to protect itself against the toxic effects of these molecules, especially via the activation of high osmolarity glycerol and cell wall integrity signaling pathways and by induction of the unfolded protein response.

Disarming the Host: Detoxification of Plant Defense Compounds During Fungal Necrotrophy

While fungal biotrophs are dependent on successfully suppressing/subverting host defenses during their interaction with live cells, necrotrophs, due to their lifestyle are often confronted with a suite of toxic metabolites. These include an assortment of plant defense compounds (PDCs) which can demonstrate broad antifungal activity. These PDCs can be either constitutively present in plant tissue or induced in response to infection, but are nevertheless an important obstacle which needs to be overcome for successful pathogenesis. Fungal necrotrophs have developed a number of strategies to achieve this goal, from the direct detoxification of these compounds through enzymatic catalysis and modification, to the active transport of various PDCs to achieve toxin sequestration and efflux. Studies have shown across multiple pathogens that the efficient detoxification of host PDCs is both critical for successful infection and often a determinant factor in pathogen host range. Here, we provide a broad and comparative overview of the various mechanisms for PDC detoxification which have been identified in both fungal necrotrophs and fungal pathogens which depend on detoxification during a necrotrophic phase of infection. Furthermore, the effect that these mechanisms have on fungal host range, metabolism, and disease control will be discussed.

Complexity of Brassica oleracea-Alternaria brassicicola Susceptible Interaction Reveals Downregulation of Photosynthesis at Ultrastructural, Transcriptional, and Physiological Levels

Black spot disease, caused by Alternaria brassicicola in Brassica species, is one of the most devastating diseases all over the world, especially since there is no known fully resistant Brassica cultivar. In this study, the visualization of black spot disease development on Brassica oleracea var. capitata f. alba (white cabbage) leaves and subsequent ultrastructural, molecular and physiological investigations were conducted. Inter- and intracellular hyphae growth within leaf tissues led to the loss of host cell integrity and various levels of organelle disintegration. Severe symptoms of chloroplast damage included the degeneration of chloroplast envelope and grana, and the loss of electron denseness by stroma at the advanced stage of infection. Transcriptional profiling of infected leaves revealed that photosynthesis was the most negatively regulated biological process. However, in infected leaves, chlorophyll and carotenoid content did not decrease until 48 hpi, and several chlorophyll a fluorescence parameters, such as photosystem II quantum yield (Fv/Fm), non-photochemical quenching (NPQ), or plant vitality parameter (Rdf) decreased significantly at 24 and 48 hpi compared to control leaves. Our results indicate that the initial stages of interaction between B. oleracea and A. brassicicola are not uniform within an inoculation site and show a complexity of host responses and fungal attempts to overcome host cell defense mechanisms. The downregulation of photosynthesis at the early stage of this susceptible interaction suggests that it may be a part of a host defense strategy, or, alternatively, that chloroplasts are targets for the unknown virulence factor(s) of A. brassicicola. However, the observed decrease of photosynthetic efficiency at the later stages of infection is a result of the fungus-induced necrotic lesion expansion.

Pathogen inactivation of cruciferous phytoalexins: detoxification reactions, enzymes and inhibitors

This review covers the detoxification pathways of cruciferous phytoalexins, the corresponding detoxifying enzymes and their natural and synthetic inhibitors. Paldoxins are examined as a potentially sustainable strategy to control plant pathogenic fungi.

Synthesis and Antifungal Activity against Fusarium oxysporum of Some Brassinin Analogs Derived from l-tryptophan: A DFT/B3LYP Study on the Reaction Mechanism

An efficient methodology to obtain novel antifungal analogs of brassinin 1 is described. Starting from l-tryptophan 2, N,N′-dialkylthiourea 4, 4-[(1H-indol-3-yl)methylene]-2-sulfanylidene-1,3-thiazolidin-5-one 5 and alkyl (2S)-3-(1H-indol-3-yl)-2-{[(alkylsulfanyl)carbonothioyl]amino}propanoate 6 type compounds were obtained as main products in different ratios depending on the reaction conditions via a tandem dithiocarbamate formation and Michael addition reaction. In order to understand the dependence of the reaction conditions on the mechanism pathway, a DFT/B3LYP study was performed. The results suggested the existence of competitive mechanistic routes which involve the presence of an ionic dithiocarbamate intermediate 9. Antifungal activities of all products were then evaluated against Fusarium oxysporum through mycelial growth inhibition using a microscale amended-medium assay. IC₅₀ values were thus determined for each compound. These results showed that 6-related compounds can be considered as promissory antifungal agents.

How the necrotrophic fungus Alternaria brassicicola kills plant cells remains an enigma

Alternaria species are mainly saprophytic fungi, but some are plant pathogens. Seven pathotypes of Alternaria alternata use secondary metabolites of host-specific toxins as pathogenicity factors. These toxins kill host cells prior to colonization. Genes associated with toxin synthesis reside on conditionally dispensable chromosomes, supporting the notion that pathogenicity might have been acquired several times by A. alternata. Alternaria brassicicola, however, seems to employ a different mechanism. Evidence on the use of host-specific toxins as pathogenicity factors remains tenuous, even after a diligent search aided by full-genome sequencing and efficient reverse-genetics approaches. Similarly, no individual genes encoding lipases or cell wall-degrading enzymes have been identified as strong virulence factors, although these enzymes have been considered important for fungal pathogenesis. This review describes our current understanding of toxins, lipases, and cell wall-degrading enzymes and their roles in the pathogenesis of A. brassicicola compared to those of other pathogenic fungi. It also describes a set of genes that affect pathogenesis in A. brassicicola. They are involved in various cellular functions that are likely important in most organisms and probably indirectly associated with pathogenesis. Deletion or disruption of these genes results in weakly virulent strains that appear to be sensitive to the defense mechanisms of host plants. Finally, this review discusses the implications of a recent discovery of three important transcription factors associated with pathogenesis and the putative downstream genes that they regulate.

Transcriptional responses of the Bdtf1-deletion mutant to the phytoalexin brassinin in the necrotrophic fungus Alternaria brassicicola

Brassica species produce the antifungal indolyl compounds brassinin and its derivatives, during microbial infection. The fungal pathogen Alternaria brassicicola detoxifies brassinin and possibly its derivatives. This ability is an important property for the successful infection of brassicaceous plants. Previously, we identified a transcription factor, Bdtf1, essential for the detoxification of brassinin and full virulence. To discover genes that encode putative brassinin-digesting enzymes, we compared gene expression profiles between a mutant strain of the transcription factor and wild-type A. brassicicola under two different experimental conditions. A total of 170 and 388 genes were expressed at higher levels in the mutants than the wild type during the infection of host plants and saprophytic growth in the presence of brassinin, respectively. In contrast, 93 and 560 genes were expressed, respectively, at lower levels in the mutant than the wild type under the two conditions. Fifteen of these genes were expressed at lower levels in the mutant than in the wild type under both conditions. These genes were assumed to be important for the detoxification of brassinin and included Bdtf1 and 10 putative enzymes. This list of genes provides a resource for the discovery of enzyme-coding genes important in the chemical modification of brassinin.

Red card for pathogens: phytoalexins in sorghum and maize

Cereal crop plants such as maize and sorghum are constantly being attacked by a great variety of pathogens that cause large economic losses. Plants protect themselves against pathogens by synthesizing antimicrobial compounds, which include phytoalexins. In this review we summarize the current knowledge on phytoalexins produced by sorghum (luteolinidin, apigeninidin) and maize (zealexin, kauralexin, DIMBOA and HDMBOA). For these molecules, we highlight biosynthetic pathways, known intermediates, proposed enzymes, and mechanisms of elicitation. Finally, we discuss the involvement of phytoalexins in plant resistance and their possible application in technology, medicine and agriculture. For those whose world is round we tried to set the scene in the context of a hypothetical football game in which pathogens fight with phytoalexins on the different playing fields provided by maize and sorghum.

The effects of glucosinolates and their breakdown products on necrotrophic fungi

Glucosinolates are a diverse class of S- and N-containing secondary metabolites that play a variety of roles in plant defense. In this study, we used Arabidopsis thaliana mutants that contain different amounts of glucosinolates and glucosinolate-breakdown products to study the effects of these phytochemicals on phytopathogenic fungi. We compared the fungus Botrytis cinerea, which infects a variety of hosts, with the Brassicaceae-specific fungus Alternaria brassicicola. B. cinerea isolates showed variable composition-dependent sensitivity to glucosinolates and their hydrolysis products, while A. brassicicola was more strongly affected by aliphatic glucosinolates and isothiocyanates as decomposition products. We also found that B. cinerea stimulates the accumulation of glucosinolates to a greater extent than A. brassicicola. In our work with A. brassicicola, we found that the type of glucosinolate-breakdown product is more important than the type of glucosinolate from which that product was derived, as demonstrated by the sensitivity of the Ler background and the sensitivity gained in Col-0 plants expressing epithiospecifier protein both of which accumulate simple nitrile and epithionitriles, but not isothiocyanates. Furthermore, in vivo, hydrolysis products of indole glucosinolates were found to be involved in defense against B. cinerea, but not in the host response to A. brassicicola. We suggest that the Brassicaceae-specialist A. brassicicola has adapted to the presence of indolic glucosinolates and can cope with their hydrolysis products. In contrast, some isolates of the generalist B. cinerea are more sensitive to these phytochemicals.

Metabolism and metabolites of dithiocarbamates in the plant pathogenic fungus Leptosphaeria maculans

Synthetic compounds containing a dithiocarbamate group are known to have a variety of biological effects and applications including antifungal, herbicidal, and insecticidal application. Leptosphaeria maculans is a fungal pathogen of crucifers able to detoxify efficiently the only plant natural product containing a dithiocarbamate group, the phytoalexin brassinin. To evaluate the effects of dithiocarbamates on L. maculans, a number of structurally diverse S-methyl dithiocarbamates containing indolyl, biphenyl, and benzimidazolyl moieties were synthesized, and their antifungal activities and metabolism by L. maculans were investigated. All dithiocarbamates were transformed by L. maculans through hydrolysis to the corresponding amines, which were less antifungal than the parent compounds. Two dithiocarbonates were shown to be much less antifungal than the corresponding dithiocarbamates. Results of this investigation indicate that S-methyl dithiocarbamates are not useful inhibitors of L. maculans and that their rates of transformation by L. maculans did not correlate with the antifungal activity of the particular compound.

Differential protein expression in response to the phytoalexin brassinin allows the identification of molecular targets in the phytopathogenic fungus Alternaria brassicicola

The effects of the cruciferous phytoalexin brassinin on the protein expression patterns of the phytopathogenic fungus Alternaria brassicicola were investigated. Cell-free protein extracts of mycelia of A. brassicicola induced with brassinin at 0.50 and 0.10 mm were fractionated, and the proteins in soluble fractions were separated by two-dimensional electrophoresis. Spots corresponding to differentially expressed proteins were digested and analysed by liquid chromatography-electrospray ionization-mass spectrometry. The number of differentially expressed proteins was significantly higher in mycelia treated with brassinin at 0.50 mm (96 protein spots) than in mycelia treated with brassinin at 0.10 mm (18 protein spots). The majority of differentially expressed proteins included proteins involved in metabolism, processing, synthesis and several heat shock proteins (HSPs). Brassinin concentrations below 0.30 mm induced HSP90, a protein involved in the regulation of morphogenetic signalling in fungi, suggesting that 0.30 mm is a minimal concentration of brassinin necessary for the protection of brassicas against A. brassicicola. These results reveal that HSP90 is a potential target for inhibition in stressed A. brassicicola and confirm that brassinin has strong detrimental effects on A. brassicicola, suggesting that its detoxification by the fungus suppresses an important defence layer of the plant.

The phytoalexins from cultivated and wild crucifers: chemistry and biology

Phytoalexins are antimicrobial secondary metabolites produced de novo by plants in response to stress, including microbial attack. In general, phytoalexins are important components of plant defenses against fungal and bacterial pathogens. The phytoalexins of crucifers are indole alkaloids derived from (S)-tryptophan, most of which contain a sulfur atom derived from cysteine. Beside their antimicrobial activity against different plant pathogenic species, cruciferous phytoalexins have shown anticarcinogenic effects on various human cell lines. This review focuses on the phytoalexins produced by cruciferous plants reported to date, with particular emphasis on their chemical synthesis, biosynthesis, metabolism by plant fungal pathogens and biological activities. A summary table containing all phytoalexins, their cultivated and wild cruciferous sources, their synthetic starting materials, biotransformation products and biological activities is provided.

Cell wall integrity and high osmolarity glycerol pathways are required for adaptation of Alternaria brassicicola to cell wall stress caused by brassicaceous indolic phytoalexins

Camalexin, the characteristic phytoalexin of Arabidopsis thaliana, inhibits growth of the fungal necrotroph Alternaria brassicicola. This plant metabolite probably exerts its antifungal toxicity by causing cell membrane damage. Here we observed that activation of a cellular response to this damage requires cell wall integrity (CWI) and the high osmolarity glycerol (HOG) pathways. Camalexin was found to activate both AbHog1 and AbSlt2 MAP kinases, and activation of the latter was abrogated in a AbHog1 deficient strain. Mutant strains lacking functional MAP kinases showed hypersensitivity to camalexin and brassinin, a structurally related phytoalexin produced by several cultivated Brassica species. Enhanced susceptibility to the membrane permeabilization activity of camalexin was observed for MAP kinase deficient mutants. These results suggest that the two signalling pathways have a pivotal role in regulating a cellular compensatory response to preserve cell integrity during exposure to camalexin. AbHog1 and AbSlt2 deficient mutants had reduced virulence on host plants that may, at least for the latter mutants, partially result from their inability to cope with defence metabolites such as indolic phytoalexins. This constitutes the first evidence that a phytoalexin activates fungal MAP kinases and that outputs of activated cascades contribute to protecting the fungus against antimicrobial plant metabolites.

Phytoalexins from Brassicaceae: news from the front

The chemical structures, syntheses, metabolism and biological activities of the cruciferous phytoalexins discovered to date, with particular focus on the latest results dealing with their biosynthesis and detoxification are reviewed.

Impact of Cruciferous Phytoalexins on the Detoxification of Brassilexin by the Blackleg Fungus Pathogenic to Brown Mustard

The biotransformation of brassilexin, a potent phytoalexin produced by brown mustard (Brassica juncea L.), in the presence of various cruciferous phytoalexins was investigated. An important group of isolates of the fungal species Leptosphaeria maculans (Laird 2 and Mayfair 2), which is virulent to brown mustard, but not to canola, was used in this investigation. Brassilexin was detoxified by the fungus, but none of the phytoalexins seemed to affect substantially the rate of brassilexin detoxification; after 12 h of incubation, the amounts of brassilexin remaining in culture were as low as in controls, except in co-incubations with cyclobrassinin and sinalexin, which afforded intermediates that in solution oxidized spontaneously to brassilexin.