Molecular cloning of AbGst1 encoding a glutathione transferase differentially expressed during exposure of Alternaria brassicicola to isothiocyanates

Sclerotinia sclerotiorum Response to Long Exposure to Glucosinolate Hydrolysis Products by Transcriptomic Approach

White mold disease, caused by the necrotrophic fungus Sclerotinia sclerotiorum, affects Brassica crops. Brassica crops produce a broad array of compounds, such as glucosinolates, which contribute to the defense against pathogens. From their hydrolysis, several products arise that have antimicrobial activity (GHPs) whose toxicity is structure dependent. S. sclerotiorum may overcome the toxic effect of moderate GHP concentrations after prolonged exposure to their action. Our objective was to identify the molecular mechanism underlying S. sclerotiorum response to long exposure to two chemically diverse GHPs: aliphatic GHP allyl-isothiocyanate (AITC) and indole GHP indol-3-carbinol (I3C). We found that the transcriptomic response is dependent on the type of GHP and on their initial target, involving cell membranes in the case of AITC or DNA in the case of I3C. Response mechanisms include the reorganization of chromatin, mediated by histone chaperones hip4 and cia1, ribosome synthesis controlled by the kinase-phosphatase pair aps1-ppn1, catabolism of proteins, ergosterol synthesis, and induction of detoxification systems. These mechanisms probably help S. sclerotiorum to grow and survive in an environment where GHPs are constantly produced by Brassica plants upon glucosinolate breakdown. IMPORTANCEBrassica species, including important vegetable crops, such as cabbage, cauliflower, or broccoli, or oil crops, such as rapeseed, produce specific chemical compounds useful to protect them against pests and pathogens. One of the most destructive Brassica diseases in temperate areas around the world is Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum. This is a generalist pathogen that causes disease over more than 400 plant species, being a serious threat to economically important crops worldwide, including potato, bean, soybean, and sunflower, among many others. Understanding the mechanisms utilized by pathogens to overcome specific plant defensive compounds can be useful to increase plant resistance. Our study demonstrated that Sclerotinia shows different adaptation mechanisms, including detoxification systems, to grow and survive when plant protective compounds are present.

Effects of Glucosinolate-Derived Isothiocyanates on Fungi: A Comprehensive Review on Direct Effects, Mechanisms, Structure-Activity Relationship Data and Possible Agricultural Applications

Plants heavily rely on chemical defense systems against a variety of stressors. The glucosinolates in the Brassicaceae and some allies are the core molecules of one of the most researched such pathways. These natural products are enzymatically converted into isothiocyanates (ITCs) and occasionally other defensive volatile organic constituents (VOCs) upon fungal challenge or tissue disruption to protect the host against the stressor. The current review provides a comprehensive insight on the effects of the isothiocyanates on fungi, including, but not limited to mycorrhizal fungi and pathogens of Brassicaceae. In the review, our current knowledge on the following topics are summarized: direct antifungal activity and the proposed mechanisms of antifungal action, QSAR (quantitative structure-activity relationships), synergistic activity of ITCs with other agents, effects of ITCs on soil microbial composition and allelopathic activity. A detailed insight into the possible applications is also provided: the literature of biofumigation studies, inhibition of post-harvest pathogenesis and protection of various products including grains and fruits is also reviewed herein.

Metabolic and biotransformation effects on dietary glucosinolates, their bioavailability, catabolism and biological effects in different organisms

Glucosinolate-producing plants have long been recognized for both their distinctive benefits to human nutrition and their resistance traits against pathogens and herbivores. Despite the accumulation of glucosinolates (GLS) in plants is associated with their resistance to various biotic and abiotic stresses, the defensive and biological activities of GLS are commonly conveyed by their metabolic products. In view of this, metabolism is considered the driving factor upon the interactions of GLS-producing plants with other organisms, also influenced by plant and plant attacking or digesting organism characteristics. Several microbial pathogens and insects have evolved the capacity to detoxify GLS-hydrolysis products or inhibit their formation via different means, highlighting the relevance of their metabolic abilities for the plants' defense system activation and target organism detoxification. Strikingly, some bacteria, fungi and insects can likewise produce their own myrosinase (MYR)-like enzymes in one of the most important adaptation strategies against the GLS-MYR plant defense system. Knowledge of GLS metabolic pathways in herbivores and pathogens can impact plant protection efforts and may be harnessed upon for genetically modified plants that are more resistant to predators. In humans, the interest in the implementation of GLS in diets for the prevention of chronic diseases has grown substantially. However, the efficiency of such approaches is dependent on GLS bioavailability and metabolism, which largely involves the human gut microbiome. Among GLS-hydrolytic products, isothiocyanates (ITC) have shown exceptional properties as chemical plant defense agents against herbivores and pathogens, along with their health-promoting benefits in humans, at least if consumed in reasonable amounts. Deciphering GLS metabolic pathways provides critical information for catalyzing all types of GLS towards the generation of ITCs as the biologically most active metabolites. This review provides an overview on contrasting metabolic pathways in plants, bacteria, fungi, insects and humans towards GLS activation or detoxification. Further, suggestions for the preparation of GLS containing plants with improved health benefits are presented.

Selection of suitable reference genes for quantitative real time PCR in different Tulasnella isolates and orchid-fungus symbiotic germination system

Under natural conditions, mycorrhizal symbiosis accompanies nearly the entire life cycle of orchids from seed germination through to flowering and fruiting. Tulasnella-like orchid mycorrhizal fungi are the most common mycorrhizal fungi found in association with orchid species. Presently suitable reference genes have not been systematically selected for the quantification of gene expression via Real-Time Quantitative Reverse Transcription PCR (RT-qPCR). We evaluated 12 candidate Tulasnella genes in nine different Tulasnella isolates and in the Dendrobium-fungal symbiotic germination associations followed by statistical analysis using the programs Bestkeeper, geNorm, and Normfinder to analyze the expression stability of the individual genes. The results showed that the EF2, UBC, and PP2A genes had the highest rankings with relatively stable expression levels across the different genotypes and during the symbiotic seed germination process by the three programs, and may be suitable for RT-qPCR normalization. Furthermore, the gene encoding C-5 Sterol desaturase (C5SD) was selected to verify the reliability of EF2, UBC, and PP2A expression during the Tulasnella-Dendrobium symbiotic seed germination process. This study is the first systematic exploration of optimal reference genes for gene expression studies during the colonization of orchid seeds by the mycorrhizal fungus Tulasnella.

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.

The phytopathogenic fungus Sclerotinia sclerotiorum detoxifies plant glucosinolate hydrolysis products via an isothiocyanate hydrolase

Brassicales plants produce glucosinolates and myrosinases that generate toxic isothiocyanates conferring broad resistance against pathogens and herbivorous insects. Nevertheless, some cosmopolitan fungal pathogens, such as the necrotrophic white mold Sclerotinia sclerotiorum, are able to infect many plant hosts including glucosinolate producers. Here, we show that S. sclerotiorum infection activates the glucosinolate-myrosinase system, and isothiocyanates contribute to resistance against this fungus. S. sclerotiorum metabolizes isothiocyanates via two independent pathways: conjugation to glutathione and, more effectively, hydrolysis to amines. The latter pathway features an isothiocyanate hydrolase that is homologous to a previously characterized bacterial enzyme, and converts isothiocyanate into products that are not toxic to the fungus. The isothiocyanate hydrolase promotes fungal growth in the presence of the toxins, and contributes to the virulence of S. sclerotiorum on glucosinolate-producing plants.

Some plants produce toxic isothiocyanates that protect them against pathogens. Here, Chen et al. show that the plant pathogenic fungus Sclerotinia sclerotiorum converts isothiocyanates into non-toxic compounds via glutathione conjugation and, more effectively, via hydrolysis to amines using an isothiocyanate hydrolase.

The Evolutionary and Molecular Features of Broad Host-Range Necrotrophy in Plant Pathogenic Fungi

Necrotrophic fungal pathogens cause considerable disease on numerous economically important crops. Some of these pathogens are specialized to one or a few closely related plant species, whereas others are pathogenic on many unrelated hosts. The evolutionary and molecular bases of broad host-range necrotrophy in plant pathogens are not very well-defined and form an on-going area of research. In this review, we discuss what is known about broad host-range necrotrophic pathogens and compare them with their narrow host-range counterparts. We discuss the evolutionary processes associated with host generalism, and highlight common molecular features of the broad host-range necrotrophic lifestyle, such as fine-tuning of host pH, modulation of host reactive oxygen species and metabolic degradation of diverse host antimicrobials. We conclude that broad host-range necrotrophic plant pathogens have evolved a range of diverse and sometimes convergent responses to a similar selective regime governed by interactions with a highly heterogeneous host landscape.

MFS transporter from Botrytis cinerea provides tolerance to glucosinolate-breakdown products and is required for pathogenicity

Glucosinolates accumulate mainly in cruciferous plants and their hydrolysis-derived products play important roles in plant resistance against pathogens. The pathogen Botrytis cinerea has variable sensitivity to glucosinolates, but the mechanisms by which it responds to them are mostly unknown. Exposure of B. cinerea to glucosinolate-breakdown products induces expression of the Major Facilitator Superfamily transporter, mfsG, which functions in fungitoxic compound efflux. Inoculation of B. cinerea on wild-type Arabidopsis thaliana plants induces mfsG expression to higher levels than on glucosinolate-deficient A. thaliana mutants. A B. cinerea strain lacking functional mfsG transporter is deficient in efflux ability. It accumulates more isothiocyanates (ITCs) and is therefore more sensitive to this compound in vitro; it is also less virulent to glucosinolates-containing plants. Moreover, mfsG mediates ITC efflux in Saccharomyces cerevisiae cells, thereby conferring tolerance to ITCs in the yeast. These findings suggest that mfsG transporter is a virulence factor that increases tolerance to glucosinolates.

Plant glucosinolates are important in defense against fungal pathogens. Here, the authors identify a major facilitator superfamily transporter protein of the pathogen Botrytis cinerea, mfsG, that plays a role in efflux and detoxification of glucosinolate-breakdown products during plant–pathogen interactions.

Glutathione S-Transferase Enzymes in Plant-Pathogen Interactions

Plant glutathione S-transferases (GSTs) are ubiquitous and multifunctional enzymes encoded by large gene families. A characteristic feature of GST genes is their high inducibility by a wide range of stress conditions including biotic stress. Early studies on the role of GSTs in plant biotic stress showed that certain GST genes are specifically up-regulated by microbial infections. Later numerous transcriptome-wide investigations proved that distinct groups of GSTs are markedly induced in the early phase of bacterial, fungal and viral infections. Proteomic investigations also confirmed the accumulation of multiple GST proteins in infected plants. Furthermore, functional studies revealed that overexpression or silencing of specific GSTs can markedly modify disease symptoms and also pathogen multiplication rates. However, very limited information is available about the exact metabolic functions of disease-induced GST isoenzymes and about their endogenous substrates. The already recognized roles of GSTs are the detoxification of toxic substances by their conjugation with glutathione, the attenuation of oxidative stress and the participation in hormone transport. Some GSTs display glutathione peroxidase activity and these GSTs can detoxify toxic lipid hydroperoxides that accumulate during infections. GSTs can also possess ligandin functions and participate in the intracellular transport of auxins. Notably, the expression of multiple GSTs is massively activated by salicylic acid and some GST enzymes were demonstrated to be receptor proteins of salicylic acid. Furthermore, induction of GST genes or elevated GST activities have often been observed in plants treated with beneficial microbes (bacteria and fungi) that induce a systemic resistance response (ISR) to subsequent pathogen infections. Further research is needed to reveal the exact metabolic functions of GST isoenzymes in infected plants and to understand their contribution to disease resistance.

Validation of reference genes for normalization of gene expression by qRT-PCR in a resveratrol-producing entophytic fungus (Alternaria sp. MG1)

Alternaria sp. MG1, an endophytic fungus isolated from Vitis vinifera, can independently produce resveratrol, indicating that this species contains the key genes for resveratrol biosynthesis. Identification of these key genes is essential to understand the resveratrol biosynthesis pathway in this strain, which is currently unknown in microorganisms. qRT-PCR is an efficient and widely used method to identify the key genes related to unknown pathways at the level of gene expression. Verification of stable reference genes in this strain is essential for qRT-PCR data normalization, although results have been reported for other Alternaria sp. strains. In this study, nine candidate reference genes including TUBA, EF1, EF2, UBC, UFD, RPS5, RPS24, ACTB and 18S were evaluated for expression stability in a diverse set of six samples representing different growth periods. We compared cell culture conditions and an optimized condition for resveratrol production. The comparison of the results was performed using four statistical softwares. A combination of TUBA and EF1 was found to be suitable for normalization of Alternaria sp. MG1 in different developmental stages, and 18S was found to be the least stable. The reference genes verified in this study will facilitate further research to explore gene expression and molecular mechanisms as well as the improvement of secondary metabolite yields in Alternaria sp. MG1. To our knowledge, this is the first validation of reference genes in Alternaria with the capability to produce resveratrol. Additionally, these results provide useful guidelines for the selection of reference genes in other Alternaria species.

Infection and Gene Expression of the Clubroot Pathogen Plasmodiophora brassicae in Resistant and Susceptible Canola Cultivars

Infection by the clubroot pathogen Plasmodiophora brassicae on resistant and susceptible canola cultivars was investigated at various times following inoculation. Primary infection occurred on more than 90% of root hairs in both cultivars at 7 days after inoculation (dai), and thereafter declined to less than 20% at 14 to 35 dai. The amount of primary infection on the two cultivars was similar at each time point. Secondary infections were rare in both cultivars at 5 and 7 dai but became common after 14 dai. At 14 to 28 dai, the level of secondary infection was greater in the resistant cultivar than in the susceptible one. The in planta expression of 12 selected P. brassicae genes was investigated by reverse-transcription quantitative polymerase chain reaction. All genes were upregulated at 5 or 7 dai in the resistant cultivar. In the susceptible cultivar, the 12 genes could be classified into three groups according to their expression patterns: 2 genes showed an expression peak at 14 dai, 3 showed two expression peaks at 14 and 35 dai, and the others showed an expression peak at 35 dai. Results from this study will be useful in breeding for resistance and in selecting candidate pathogenicity genes for further studies.

Characterization of glutathione transferases involved in the pathogenicity of Alternaria brassicicola

BACKGROUND

Glutathione transferases (GSTs) represent an extended family of multifunctional proteins involved in detoxification processes and tolerance to oxidative stress. We thus anticipated that some GSTs could play an essential role in the protection of fungal necrotrophs against plant-derived toxic metabolites and reactive oxygen species that accumulate at the host-pathogen interface during infection.

RESULTS

Mining the genome of the necrotrophic Brassica pathogen Alternaria brassicicola for glutathione transferase revealed 23 sequences, 17 of which could be clustered into the main classes previously defined for fungal GSTs and six were 'orphans'. Five isothiocyanate-inducible GSTs from five different classes were more thoroughly investigated. Analysis of their catalytic properties revealed that two GSTs, belonging to the GSTFuA and GTT1 classes, exhibited GSH transferase activity with isothiocyanates (ITC) and peroxidase activity with cumene hydroperoxide, respectively. Mutant deficient for these two GSTs were however neither more susceptible to ITC nor less aggressive than the wild-type parental strain. By contrast mutants deficient for two other GSTs, belonging to the Ure2pB and GSTO classes, were distinguished by their hyper-susceptibility to ITC and low aggressiveness against Brassica oleracea. In particular AbGSTO1 could participate in cell tolerance to ITC due to its glutathione-dependent thioltransferase activity. The fifth ITC-inducible GST belonged to the MAPEG class and although it was not possible to produce the soluble active form of this protein in a bacterial expression system, the corresponding deficient mutant failed to develop normal symptoms on host plant tissues.

CONCLUSIONS

Among the five ITC-inducible GSTs analyzed in this study, three were found essential for full aggressiveness of A. brassicicola on host plant. This, to our knowledge is the first evidence that GSTs might be essential virulence factors for fungal necrotrophs.

The antibacterial properties of isothiocyanates

Isothiocyanates (ITCs) are natural plant products generated by the enzymic hydrolysis of glucosinolates found in Brassicaceae vegetables. These natural sulfur compounds and their dithiocarbamate conjugates have been previously evaluated for their anti-cancerous properties. Their antimicrobial properties have been previously studied as well, mainly for food preservation and plant pathogen control. Recently, several revelations concerning the mode of action of ITCs in prokaryotes have emerged. This review addresses these new studies and proposes a model to summarize the current knowledge and hypotheses for the antibacterial effect of ITCs and whether they may provide the basis for the design of novel antibiotics.

Methyl jasmonate and 1-methylcyclopropene treatment effects on quinone reductase inducing activity and post-harvest quality of broccoli

Effect of pre-harvest methyl jasmonate (MeJA) and post-harvest 1-methylcyclopropene (1-MCP) treatments on broccoli floret glucosinolate (GS) concentrations and quinone reductase (QR, an in vitro anti-cancer biomarker) inducing activity were evaluated two days prior to harvest, at harvest and at 10, 20, and 30 days of post-harvest storage at 4 °C. MeJA treatments four days prior to harvest of broccoli heads was observed to significantly increase floret ethylene biosynthesis resulting in chlorophyll catabolism during post-harvest storage and reduced product quality. Post-harvest treatment with 1-methylcyclopropene (1-MCP), which competitively binds to protein ethylene receptors, maintained post-harvest floret chlorophyll concentrations and product visual quality in both control and MeJA-treated broccoli. Transcript abundance of BoPPH, a gene which is responsible for the synthesis of pheophytinase, the primary enzyme associated with chlorophyll catabolism in broccoli, was reduced by 1-MCP treatment and showed a significant, negative correlation with floret chlorophyll concentrations. The GS, glucobrassicin, neoglucobrassicin, and gluconasturtiin were significantly increased by MeJA treatments. The products of some of the GS from endogenous myrosinase hydrolysis [sulforaphane (SF), neoascorbigen (NeoASG), N-methoxyindole-3-carbinol (NI3C), and phenethyl isothiocyanate (PEITC)] were also quantified and found to be significantly correlated with QR. Sulforaphane, the isothiocyanate hydrolysis product of the GS glucoraphanin, was found to be the most potent QR induction agent. Increased sulforaphane formation from the hydrolysis of glucoraphanin was associated with up-regulated gene expression of myrosinase (BoMyo) and the myrosinase enzyme co-factor gene, epithiospecifier modifier1 (BoESM1). This study demonstrates the combined treatment of MeJA and 1-MCP increased QR activity without post-harvest quality loss.

Sulfur and Sulfur Compounds in Plant Defence

The multiplicity of chemical structures of sulfur containing compounds, influenced in part by the element's several oxidation states, directly results in diverse modes of action for sulfur-containing natural products synthesized as secondary metabolites in plants. Sulfur-containing natural products constitute a formidable wall of defence against a wide range of pathogens and pests. Steady progress in the development of new technologies have advanced research in this area, helping to uncover the role of such important plant defence molecules like endogenously-released elemental sulphur, but also deepening current understanding of other better-studied compounds like the glucosinolates. As studies continue in this area, it is becoming increasingly evident that sulfur and sulfur compounds play far more important roles in plant defence than perhaps previously suspected.

Arabidopsis cell death in compatible and incompatible interactions with Alternaria brassicicola

Two strains of necrotrophic Alternaria brassicicola, Ab40857 and Ab42464, are virulent on Korean cabbage and several wild types of Arabidopsis thaliana. Interaction between Ab42464 and Col-0 was compatible, whereas interaction between Ab40857 and Col-0 was incompatible. The loss of defense, no death (dnd) 1 function abrogated the compatibility between Ab42464 and Col-0, and the accelerated cell death (acd) 2 mutation attenuated the Col-0's resistance against Ab40857. These two fungal strains induced PR1 transcription in Col-0. Ab40857 accelerated transcription of PDF1.2, THI2.1, CAT, and POX by 12 h compared to those challenged with Ab42464. More abundant cell death was observed in Col-0 infected with Ab42464, however, callose deposition was evident in the incompatible interaction. Remarkably, Ab40857-infected areas of acd2-2 underwent rampant cell death and Ab42464 triggered callose production in dnd1-1. Furthermore, the incompatibility between Ab40857 and Col-0 was nullified by the coronatine-insensitive 1 (coi1) and phytoalexin-deficient 3 (pad3) mutations but not by nonexpresser of PR genes (npr1) and pad4. Ab40857 induced abundant cell death in pad3. Taken together, cell death during the early infection stage is a key determinant that discriminates between a compatible interaction and an incompatible one, and the resistance within Col-0 against Ab40857 is dependent on a defense-signaling pathway mediated by jasmonic acid and PAD3.

Differentially expressed cDNAs in Alternaria alternata treated with 2-propenyl isothiocyanate

The molecular mechanism of the fungal tolerance phenotype to fungicides is not completely understood. This knowledge would allow for the development of environmentally friendly strategies for the control of fungal infection. With the goal of determining genes induced by 2p-ITC, a forward suppressive subtractive hybridization was performed using cDNAs from ITC-treated Alternaria alternata as a "tester" and from untreated fungus as a "driver." Using this approach, a library containing 102 ESTs was generated that resulted in 50 sequences after sequence assembly (17 contigs and 33 singletons). Blastx analysis revealed that 38% and 40% of the sequences showed significant similarity with known and hypothetical proteins, respectively, whereas 18% were not similar to known genes. These last sequences could represent novel genes that play an unknown role in the molecular responses of fungi during adaptation to 2p-ITC. Clones similar to opsins, ABC transporters, calmodulin, ATPases and SNOG proteins were identified. Using real-time RT-PCR analysis, significant inductions of an ABC transporter and a Ca(++) ATPase during 2p-ITC treatment were discovered. These results suggest that the fungal resistance phenotype to 2p-ITC involves calcium ions and 2p-ITC efflux via an ABC transporter.

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.

Gene expression profile of the plant pathogen Fusarium graminearum under the antagonistic effect of Pantoea agglomerans

The pathogenic fungus Fusarium graminearum is an ongoing threat to agriculture, causing losses in grain yield and quality in diverse crops. Substantial progress has been made in the identification of genes involved in the suppression of phytopathogens by antagonistic microorganisms; however, limited information regarding responses of plant pathogens to these biocontrol agents is available. Gene expression analysis was used to identify differentially expressed transcripts of the fungal plant pathogen F. graminearum under antagonistic effect of the bacterium Pantoea agglomerans. A macroarray was constructed, using 1014 transcripts from an F. graminearum cDNA library. Probes consisted of the cDNA of F. graminearum grown in the presence and in the absence of P. agglomerans. Twenty-nine genes were either up (19) or down (10) regulated during interaction with the antagonist bacterium. Genes encoding proteins associated with fungal defense and/or virulence or with nutritional and oxidative stress responses were induced. The repressed genes coded for a zinc finger protein associated with cell division, proteins containing cellular signaling domains, respiratory chain proteins, and chaperone-type proteins. These data give molecular and biochemical evidence of response of F. graminearum to an antagonist and could help develop effective biocontrol procedures for pathogenic plant fungi.

Exogenously applied isothiocyanates enhance glutathione S-transferase expression in Arabidopsis but act as herbicides at higher concentrations

Isothiocyanates (ITCs) are sulfur-containing compounds that are generated by the glucosinolate-myrosinase system in plants. Although previous greenhouse studies have demonstrated the phytotoxicity of ITCs, their action modes are still unknown. In this study, we report the physiological responses of Arabidopsis thaliana treated with three exogenous ITCs: methyl ITC, allyl ITC, and phenethyl ITC. Administration of a high dose of each ITC inhibited plant growth and induced severe bleaching in the rosette leaves. The bleaching was concomitant with the elevation of electrolyte leakage and the generation of hydrogen peroxide. Although the three ITCs showed bleaching symptoms, phenethyl ITC was the most potent. A low dose of phenethyl ITC, at which the ITC did not promote leaf bleaching, enhanced the accumulation of transcripts of glutathione S-transferases (GSTs) in Arabidopsis. When 16 GST genes were tested, the levels of transcripts corresponding to 5 of the GST genes were enhanced in response to the phenethyl ITC treatment. In particular, the expression of a Tau class gene (AtGSTU19, At1g78380) responded to the phenethyl ITC treatment. Enhancement of the AtGSTU19 gene expression also occurred in the treatment of both allyl ITC and methyl ITC. These results suggest that the administration of ITCs to Arabidopsis at high doses has an herbicidal effect by inducing oxidative burst-like responses, but that administration at lower doses enhances the expression of specific GST genes in Arabidopsis.

Novel class of glutathione transferases from cyanobacteria exhibit high catalytic activities towards naturally occurring isothiocyanates

In the present paper, we report a novel class of GSTs (glutathione transferases), called the Chi class, originating from cyanobacteria and with properties not observed previously in prokaryotic enzymes. GSTs constitute a widespread multifunctional group of proteins, of which mammalian enzymes are the best characterized. Although GSTs have their origin in prokaryotes, few bacterial representatives have been characterized in detail, and the catalytic activities and substrate specificities observed have generally been very modest. The few well-studied bacterial GSTs have largely unknown physiological functions. Genome databases reveal that cyanobacteria have an extensive arsenal of glutathione-associated proteins. We have studied two cyanobacterial GSTs which are the first examples of bacterial enzymes that are as catalytically efficient as the best mammalian enzymes. GSTs from the thermophile Thermosynechococcus elongatus BP-1 and from Synechococcus elongatus PCC 6301 were found to catalyse the conjugation of naturally occurring plant-derived isothiocyanates to glutathione at high rates. The cyanobacterial GSTs studied are smaller than previously described members of this enzyme family, but display many of the typical structural features that are characteristics of GSTs. They are also active towards several classical substrates, but at the same moderate rates that have been observed for other GSTs derived from prokaryotes. The cloning, expression and characterization of two cyanobacterial GSTs are described. The possible significance of the observed catalytic properties is discussed in the context of physiological relevance and GST evolution.

Transcriptional responses to exposure to the brassicaceous defence metabolites camalexin and allyl-isothiocyanate in the necrotrophic fungus Alternaria brassicicola

SUMMARY Alternaria brassicicola is the causative agent of black spot disease of Brassicaceae belonging to the genera Brassica and Raphanus. During host infection, A. brassicicola is exposed to high levels of antimicrobial defence compounds such as indolic phytoalexins and glucosinolate breakdown products. To investigate the transcriptomic response of A. brassicicola when challenged with brassicaceous defence metabolites, suppression subtractive hybridization (SSH) was performed to generate two cDNA libraries from germinated conidia treated either with allyl isothiocyanate (Al-ITC) or with camalexin. Following exposure to Al-ITC, A. brassicicola displayed a response similar to that experienced during oxidative stress. Indeed, a substantial subset of differentially expressed genes was related to cell protection against oxidative damage. Treatment of A. brassicicola conidia with the phytoalexin camalexin appeared to activate a compensatory mechanism to preserve cell membrane integrity and, among the camalexin-elicited genes, several were involved in sterol and sphingolipid biosynthesis. The transcriptomic analysis suggested that protection against the two tested compounds also involved mechanisms aimed at limiting their intracellular accumulation, such as melanin biosynthesis (in the case of camalexin exposure only) and drug efflux. From the Al-ITC and the camalexin differentially expressed genes identified here, 25 were selected to perform time-course studies during interactions with brassicaceous hosts. In planta, up-regulation of all the selected genes was observed during infection of Raphanus sativus whereas only a subset were over-expressed during the incompatible interaction with Arabidopsis thaliana ecotype Columbia.