Peroxidation of tobacco membrane lipids by the photosensitizing toxin, cercosporin

A Secondary Metabolite of Cercospora sp., Associated with Rosa damascena Mill., Inhibits Proliferation, Biofilm Production, Ergosterol Synthesis and Other Virulence Factors in Candida albicans

Here we describe the antimicrobial potential of secondary metabolites, fulvic acid (F.A.) and anhydrofulvic acid (AFA), produced by RDE147, an endophyte of Rosa damascena Mill. The endophyte was identified as Cercospora piaropi by ITS and β-tubulin-based phylogenetic analyses, while chemoprofiling of the endophyte by column chromatography and spectroscopy yielded two pure compounds, F.A. and AFA. The compounds demonstrated different antimicrobial profiles, with AFA suppressing the growth of C. albicans at 7.3 µg ml^-1 IC₅₀. Further studies revealed that AFA strongly restricted the biofilm production and hyphae formation in C. albicans by down-regulating several biofilm and morphogenesis-related genes. The time-kill assays confirmed the fungicidal activity of AFA against C. albicans, killing 83.6% of the pathogen cells in 24 h at the MIC concentration, and the post-antibiotic effect (PAE) experiments established the suppression of C. albicans growth for extended time periods. The compound acted synergistically with amphotericin B and nystatin and reduced ergosterol biosynthesis by the pathogen, confirmed by ergosterol estimation and comparative expression profiling of selected genes and molecular docking of AFA with C. albicans squalene epoxidase. AFA also suppressed the expression of several other virulence genes of the fungal pathogen. The study determines the anti-C. albicans potential of AFA and its impact on the biology of the pathogen. It also indicates that Cercospora species may yield potential bioactive molecules, especially fulvic acid derivatives. However, it is imperative to conduct in vivo studies to explore this molecule's therapeutic potential further.

Photoantimicrobials in agriculture

Classical approaches for controlling plant pathogens may be impaired by the development of pathogen resistance to chemical pesticides and by limited availability of effective antimicrobial agents. Recent increases in consumer awareness of and/or legislation regarding environmental and human health, and the urgent need to improve food security, are driving increased demand for safer antimicrobial strategies. Therefore, there is a need for a step change in the approaches used for controlling pre- and post-harvest diseases and foodborne human pathogens. The use of light-activated antimicrobial substances for the so-called antimicrobial photodynamic treatment is known to be effective not only in a clinical context, but also for use in agriculture to control plant-pathogenic fungi and bacteria, and to eliminate foodborne human pathogens from seeds, sprouted seeds, fruits, and vegetables. Here, we take a holistic approach to review and re-evaluate recent findings on: (i) the ecology of naturally-occurring photoantimicrobials, (ii) photodynamic processes including the light-activated antimicrobial activities of some plant metabolites, and (iii) fungus-induced photosensitization of plants. The inhibitory mechanisms of both natural and synthetic light-activated substances, known as photosensitizers, are discussed in the contexts of microbial stress biology and agricultural biotechnology. Their modes-of-antimicrobial action make them neither stressors nor toxins/toxicants (with specific modes of poisonous activity), but a hybrid/combination of both. We highlight the use of photoantimicrobials for the control of plant-pathogenic fungi and quantify their potential contribution to global food security.

Photodynamic treatment of pathogens

The current viral pandemic has highlighted the compelling need for effective and versatile treatments, that can be quickly tuned to tackle new threats, and are robust against mutations. Development of such treatments is made even more urgent in view of the decreasing effectiveness of current antibiotics, that makes microbial infections the next emerging global threat. Photodynamic effect is one such method. It relies on physical processes proceeding from excited states of particular organic molecules, called photosensitizers, generated upon absorption of visible or near infrared light. The excited states of these molecules, tailored to undergo efficient intersystem crossing, interact with molecular oxygen and generate short lived reactive oxygen species (ROS), mostly singlet oxygen. These species are highly cytotoxic through non-specific oxidation reactions and constitute the basis of the treatment. In spite of the apparent simplicity of the principle, the method still has to face important challenges. For instance, the short lifetime of ROS means that the photosensitizer must reach the target within a few tens nanometers, which requires proper molecular engineering at the nanoscale level. Photoactive nanostructures thus engineered should ideally comprise a functionality that turns the system into a theranostic means, for instance, through introduction of fluorophores suitable for nanoscopy. We discuss the principles of the method and the current molecular strategies that have been and still are being explored in antimicrobial and antiviral photodynamic treatment.

In Vitro Screening of Various Bacterially Produced Double-Stranded RNAs for Silencing Cercospora cf. flagellaris Target Genes and Suppressing Cercosporin Production

Cercospora leaf blight (CLB), primarily caused by Cercospora cf. flagellaris, is one of the most important diseases of soybean (Glycine max) in Louisiana. The pathogen produces cercosporin, a nonspecific toxin and an important virulence factor. There are no commercial cultivars with CLB resistance, and the pathogen has developed substantial resistance to the frequently used fungicides. Consequently, alternative methods are needed to manage CLB. One possibility is the RNA interference-based topical application of double-stranded (ds)RNA. The present study addressed the two most critical steps for this novel approach to be practical: inexpensively producing large quantities of dsRNA and identifying the right target genes for silencing. A screening method was developed to compare the effectiveness of Escherichia coli-produced dsRNAs targeting five fungal genes involved in cercosporin production for silencing in liquid culture. As much as 151.6 mg of dsRNA-containing total nucleic acids (TNAs) was produced from 1 liter of E. coli Luria broth culture using the L4440 vector. All tested dsRNAs reduced cercosporin production. However, significant target gene suppression was only detected in the cultures treated with dsRNAs from Avr4 and CTB8. The most potent dsRNA was from Avr4, which reduced 50% of cercosporin production at an estimated TNA concentration of 10.4 µg/ml (half maximal effective concentration [EC₅₀]), and the least potent dsRNA was from HN-2, with an estimated EC₅₀ of 46.7 µg/ml TNA. The present study paves the road for managing CLB under field conditions using dsRNA. Additionally, this approach could be adapted to identify the best dsRNAs to manage other fungal diseases.

Phytopathogenic Cercosporoid Fungi-From Taxonomy to Modern Biochemistry and Molecular Biology

Phytopathogenic cercosporoid fungi have been investigated comprehensively due to their important role in causing plant diseases. A significant amount of research has been focused on the biology, morphology, systematics, and taxonomy of this group, with less of a focus on molecular or biochemical issues. Early and extensive research on these fungi focused on taxonomy and their classification based on in vivo features. Lately, investigations have mainly addressed a combination of characteristics such as morphological traits, host specificity, and molecular analyses initiated at the end of the 20th century. Some species that are important from an economic point of view have been more intensively investigated by means of genetic and biochemical methods to better understand the pathogenesis processes. Cercosporin, a photoactivated toxin playing an important role in Cercospora diseases, has been extensively studied. Understanding cercosporin toxicity in relation to reactive oxygen species (ROS) production facilitated the discovery and regulation of the cercosporin biosynthesis pathway, including the gene cluster encoding pathway enzymes. Furthermore, these fungi may be a source of other biotechnologically important compounds, e.g., industrially relevant enzymes. This paper reviews methods and important results of investigations of this group of fungi addressed at different levels over the years.

Engineering Cercospora disease resistance via expression of Cercospora nicotianae cercosporin-resistance genes and silencing of cercosporin production in tobacco

Fungi in the genus Cercospora cause crop losses world-wide on many crop species. The wide host range and success of these pathogens has been attributed to the production of a photoactivated toxin, cercosporin. We engineered tobacco for resistance to Cercospora nicotianae utilizing two strategies: 1) transformation with cercosporin autoresistance genes isolated from the fungus, and 2) transformation with constructs to silence the production of cercosporin during disease development. Three C. nicotianae cercosporin autoresistance genes were tested: ATR1 and CFP, encoding an ABC and an MFS transporter, respectively, and 71cR, which encodes a hypothetical protein. Resistance to the pathogen was identified in transgenic lines expressing ATR1 and 71cR, but not in lines transformed with CFP. Silencing of the CTB1 polyketide synthase and to a lesser extent the CTB8 pathway regulator in the cercosporin biosynthetic pathway also led to the recovery of resistant lines. All lines tested expressed the transgenes, and a direct correlation between the level of transgene expression and disease resistance was not identified in any line. Resistance was also not correlated with the degree of silencing in the CTB1 and CTB8 silenced lines. We conclude that expression of fungal cercosporin autoresistance genes as well as silencing of the cercosporin pathway are both effective strategies for engineering resistance to Cercospora diseases where cercosporin plays a critical role.

The AVR4 effector is involved in cercosporin biosynthesis and likely affects the virulence of Cercospora cf. flagellaris on soybean

One of the most devastating fungal diseases of soybean in the southern USA is Cercospora leaf blight (CLB), which is caused mainly by Cercospora cf. flagellaris. Recent studies found that the fungal effector AVR4, originally identified in Cladosporium fulvum as a chitin-binding protein, is highly conserved among other Cercospora species. We wanted to determine whether it is present in C. cf. flagellaris and, if so, whether it plays a role in the pathogen infection of soybean. We cloned the Avr4 gene and created C. cf. flagellaris ∆avr4 mutants, which produced little cercosporin and significantly reduced expression of cercosporin biosynthesis genes. The ∆avr4 mutants were also more sensitive to chitinase and showed reduced virulence on soybean compared to the wild-type. The observed reduced virulence of C. cf. flagellaris ∆avr4 mutants on detached soybean leaves is likely due to reduced cercosporin biosynthesis. The phenotypes of reduced cercosporin production and cercosporin pathway gene expression, similar to those of the ∆avr4 mutants, were reproduced when wild-type C. cf. flagellaris was treated with double-stranded RNA targeting Avr4 in vitro. These two independent approaches demonstrated for the first time the direct involvement of AVR4 in the biosynthesis of cercosporin.

Influences of light on growth, reproduction and hypocrellin production by Shiraia sp. SUPER-H168

Light is a very important signal for fungi since it influences many different physiological responses. The effects of dark or light at different wavelengths on growth, reproduction and hypocrellins of Shiraia sp. SUPER-H168 were studied: dark, white, red, yellow, green, blue and purple. All incubations under different light conditions had significant stimulating effects on aerial hyphae and suppressing effects on hypocrellin biosynthesis compared with dark incubation. Under blue and purple light especially blue light, the colonies with profuse growth of aerial mycelium were formed. Hypocrellin production reached 13.73 mg per dish under dark condition, and decreased to 4.01 mg and 2.83 mg per dish under white and blue light, respectively. Light condition not only influenced hypocrellin production but also influenced the composition of hypocrellins. Four types of hyphae, namely surface, aerial, biofilm and penetrative hyphae, were observed by light microscopy and SEM. This study found that biofilm hyphae was so closely connected with production of secondary metabolites, and hypocrellins were only produced by biofilm hyphae. Light promoted sexual development and inhibited asexual reproduction, especially blue light strongly inhibited asexual development.

Plant Photodynamic Stress: What's New?

In the 1970's, an unconventional stressful photodynamic treatment applied to plants was investigated in two directions. Exogenous photosensitizer treatment underlies direct photodynamic stress while treatment mediating endogenous photosensitizer over-accumulation pinpoints indirect photodynamic stress. For indirect photodynamic treatment, tetrapyrrole biosynthesis pathway was deregulated by 5-aminolevulenic acid or diphenyl ether. Overall, photodynamic stress involves the generation of high amount of reactive oxygen species leading to plant cell death. All these investigations were mainly performed to gain insight into new herbicide development but they were rapidly given up or limited due to the harmfulness of diphenyl ether and the high cost of 5-aminolevulinic acid treatment. Twenty years ago, plant photodynamic stress came back by way of crop transgenesis where for example protoporphyrin oxidases from human or bacteria were overexpressed. Such plants grew without dramatic effects of photodamage suggesting that plants tolerated induced photodynamic stress. In this review, we shed light on the occurrence of plant photodynamic stress and discuss challenging issues in the context of agriculture focusing on direct photodynamic modality. Indeed, we highlighted applications of exogenous PS especially porphyrins on plants, to further develop an emerged antimicrobial photodynamic treatment that could be a new strategy to kill plant pathogens without disturbing plant growth.

Cercosporin From Pseudocercosporella capsellae and its Critical Role in White Leaf Spot Development

Pseudocercosporella capsellae, the causative agent of white leaf spot disease in Brassicaceae, can produce a purple-pink pigment on artificial media resembling, but not previously confirmed as, the toxin cercosporin. Chemical extraction with ethyl acetate from growing hyphae followed by quantitative (thin-layer chromatography [TLC] and high-performance liquid chromatography [HPLC]) and qualitative methods showed an identical absorption spectrum, with similar retardation factor (Rf) values on TLC papers and an identical peak with the same retention time in HPLC as for a standard for cercosporin. We believe this is the first report to confirm that the purple-pink pigment produced by P. capsellae is cercosporin. Confocal microscopy detected green autofluorescence of cercosporin-producing hyphae, confirming the presence of cercosporin inside hyphae. The highly virulent UWA Wlra-7 isolate of P. capsellae produced the greatest quantity of cercosporin (10.69 mg g^-1). The phytotoxicity and role of cercosporin in disease initiation across each of three Brassicaceae host species (Brassica juncea, B. napus, and Raphanus raphanistrum) was also studied. Culture filtrates containing cercosporin were phytotoxic to all three host plant species, producing large, white lesions on highly sensitive B. juncea, only water-soaked areas on least sensitive R. raphanistrum, and intermediate lesions on B. napus. It is noteworthy that sensitivity to cercosporin of these three host species was analogous to their susceptibility to the pathogen, viz., B. juncea the most susceptible, R. raphanistrum the least susceptible, and B. napus intermediate. The presence of cercosporin in the inoculum significantly increased disease severity on the highly cercosporin-sensitive B. juncea. We believe that this is the first study to demonstrate that P. capsellae produces cercosporin in liquid culture rather than agar media. Finally, this study highlights an important role of cercosporin as a pathogenicity factor in white leaf spot disease on Brassicaceae as evidenced by the ability of the cercosporin-rich culture filtrate to reproduce white leaf spot lesions on host plants and by the enhanced virulence of P. capsellae in the presence of cercosporin.

Molecular Characterization of the Cercosporin Biosynthetic Pathway in the Fungal Plant Pathogen Cercospora nicotianae

Perylenequinones are a class of photoactivated polyketide mycotoxins produced by fungal plant pathogens that notably produce reactive oxygen species with visible light. The best-studied perylenequinone is cercosporin-a product of the Cercospora species. While the cercosporin biosynthetic gene cluster has been described in the tobacco pathogen Cercospora nicotianae, little is known of the metabolite's biosynthesis. Furthermore, in vitro investigations of the polyketide synthase central to cercosporin biosynthesis identified the naphthopyrone nor-toralactone as its direct product-an observation in conflict with published biosynthetic proposals. Here, we present an alternative biosynthetic pathway to cercosporin based on metabolites characterized from a series of biosynthetic gene knockouts. We show that nor-toralactone is the key polyketide intermediate and the substrate for the unusual didomain protein CTB3. We demonstrate the unique oxidative cleavage activity of the CTB3 monooxygenase domain in vitro. These data advance our understanding of perylenequinone biosynthesis and expand the biochemical repertoire of flavin-dependent monooxygenases.

Soybean Resistance to Cercospora sojina Infection Is Reduced by Silicon

Frogeye leaf spot, caused by Cercospora sojina, is one of the most important leaf diseases of soybean worldwide. Silicon (Si) is known to increase the resistance of several plant species to pathogens. The cultivars Bossier and Conquista, which are susceptible and resistant, respectively, to frogeye leaf spot, supplied and nonsupplied with Si were examined for the activities of defense enzymes and the concentrations of total soluble phenolics (TSP) and lignin-thioglycolic acid (LTGA) derivatives at 8, 14, and 16 days after inoculation (dai) with C. sojina. The importance of cell wall degrading enzymes (CWDE) to the infection process of C. sojina and the effect of Si on their activities were also determined. Soybean plants were grown in hydroponic culture containing either 0 or 2 mM Si (-Si and +Si, respectively) and noninoculated or C. sojina inoculated. Severity of frogeye leaf spot was higher in cultivar Bossier plants than cultivar Conquista and also in the +Si plants compared with their -Si counterparts. Except for the concentrations of TSP and LTGA derivatives, activities of defense enzymes and the CWDE did not change for +Si noninoculated plants regardless of the cultivar. The activities of lipoxygenases, phenylalanine ammonia-lyases, chitinases, and polyphenoloxidases as well as the activities of CWDE decreased for the +Si inoculated plants. The results from this study demonstrated that defense enzyme activities decreased in soybean plants supplied with Si, which compromised resistance to C. sojina infection.

Natural compounds as next-generation herbicides

Herbicides with new modes of action (MOAs) are badly needed due to the rapidly evolving resistance to commercial herbicides, but a new MOA has not been introduced in over 20 years. The greatest pest management challenge for organic agriculture is the lack of effective natural product herbicides. The structural diversity and evolved biological activity of natural phytotoxins offer opportunities for the development of both directly used natural compounds and synthetic herbicides with new target sites based on the structures of natural phytotoxins. Natural phytotoxins are also a source for the discovery of new herbicide target sites that can serve as the focus of traditional herbicide discovery efforts. There are many examples of strong natural phytotoxins with MOAs other than those used by commercial herbicides, which indicates that there are molecular targets of herbicides that can be added to the current repertoire of commercial herbicide MOAs.

Reactive oxygen species in plant pathogenesis: the role of perylenequinone photosensitizers

SIGNIFICANCE

Reactive oxygen species (ROS) play multiple roles in interactions between plants and microbes, both as host defense mechanisms and as mediators of pathogenic and symbiotic associations. One source of ROS in these interactions are photoactivated, ROS-generating perylenequinone pigments produced via polyketide metabolic pathways in plant-associated fungi. These natural products, including cercosporin, elsinochromes, hypocrellins, and calphostin C, are being utilized as medicinal agents, enzyme inhibitors, and in tumor therapy, but in nature, they play a role in the establishment of pathogenic associations between fungi and their plant hosts.

RECENT ADVANCES

Photoactivated perylenequinones are photosensitizers that use light energy to form singlet oxygen (¹O₂) and free radical oxygen species which damage cellular components based on localization of the perylenequinone molecule. Production of perylenequinones during infection commonly results in lipid peroxidation and membrane damage, leading to leakage of nutrients from cells into the intercellular spaces colonized by the pathogen. Perylenequinones show almost universal toxicity against organisms, including plants, mice, bacteria, and most fungi. The producing fungi are resistant, however, and serve as models for understanding resistance mechanisms.

CRITICAL ISSUES

Studies of resistance mechanisms by perylenequinone-producing fungi such as Cercospora species are leading to an understanding of cellular resistance to ¹O₂ and oxidative stress. Recent studies show commonalities between resistance mechanisms in these fungi with extensive studies of ¹O₂ and oxidative stress responses in photosynthetic organisms.

FUTURE DIRECTIONS

Such studies hold promise both for improved medical use and for engineering crop plants for disease resistance.

Analysis of the cercosporin polyketide synthase CTB1 reveals a new fungal thioesterase function

The polyketide synthase CTB1 is demonstrated to catalyze pyrone formation thereby expanding the known biosynthetic repertoire of thioesterase domains in iterative, non-reducing polyketide synthases.

Phytotoxic secondary metabolites and peptides produced by plant pathogenic Dothideomycete fungi

Many necrotrophic plant pathogenic fungi belonging to the class of Dothideomycetes produce phytotoxic metabolites and peptides that are usually required for pathogenicity. Phytotoxins that affect a broad range of plant species are known as non-host-specific toxins (non-HSTs), whereas HSTs affect only a particular plant species or more often genotypes of that species. For pathogens producing HSTs, pathogenicity and host specificity are largely defined by the ability to produce the toxin, while plant susceptibility is dependent on the presence of the toxin target. Non-HSTs are not the main determinants of pathogenicity but contribute to virulence of the producing pathogen. Dothideomycetes are remarkable for the production of toxins, particularly HSTs because they are the only fungal species known so far to produce them. The synthesis, regulation, and mechanisms of action of the most important HSTs and non-HSTs will be discussed. Studies on the mode of action of HSTs have highlighted the induction of programed cell death (PCD) as an important mechanism. We discuss HST-induced PCD and the plant hypersensitive response upon recognition of avirulence factors that share common pathways. In this respect, although nucleotide-binding-site-leucine-rich repeat types of resistance proteins mediate resistance against biotrophs, they can also contribute to susceptibility toward necrotrophs.

The antioxidative effect of de novo generated vitamin B6 in Plasmodium falciparum validated by protein interference

The malaria parasite Plasmodium falciparum is able to synthesize de novo PLP (pyridoxal 5'-phosphate), the active form of vitamin B6. In the present study, we have shown that the de novo synthesized PLP is used by the parasite to detoxify 1O2 (singlet molecular oxygen), a highly destructive reactive oxygen species arising from haemoglobin digestion. The formation of 1O2 and the response of the parasite were monitored by live-cell fluorescence microscopy, by transcription analysis and by determination of PLP levels in the parasite. Pull-down experiments of transgenic parasites overexpressing the vitamin B6-biosynthetic enzymes PfPdx1 and PfPdx2 clearly demonstrated an interaction of the two proteins in vivo which results in an elevated PLP level from 12.5 μM in wild-type parasites to 36.6 μM in the PfPdx1/PfPdx2-overexpressing cells and thus to a higher tolerance towards 1O2. In contrast, by applying the dominant-negative effect on the cellular level using inactive mutants of PfPdx1 and PfPdx2, P. falciparum becomes susceptible to 1O2. Our results demonstrate clearly the crucial role of vitamin B6 biosynthesis in the detoxification of 1O2 in P. falciparum. Besides the known role of PLP as a cofactor of many essential enzymes, this second important task of the vitamin B6 de novo synthesis as antioxidant emphasizes the high potential of this pathway as a target of new anti-malarial drugs.

Proteomic analysis reveals an aflatoxin-triggered immune response in cotyledons of Arachis hypogaea infected with Aspergillus flavus

An immune response is triggered in host cells when host receptors recognize conserved molecular motifs, pathogen-associated molecular patterns (PAMPs), such as β-glucans, and chitin at the cell surface of a pathogen. Effector-triggered immunity occurs when pathogens deliver effectors into the host cell to suppress the first immune signaling. Using a differential proteomic approach, we identified an array of proteins responding to aflatoxins in cotyledons of peanut (Arachis hypogaea) infected with aflatoxin-producing (toxigenic) but not nonaflatoxin-producing (atoxigenic) strains of Aspergillus flavus. These proteins are involved in immune signaling and PAMP perception, DNA and RNA stabilization, induction of defense, innate immunity, hypersensitive response, biosynthesis of phytoalexins, cell wall responses, peptidoglycan assembly, penetration resistance, condensed tannin synthesis, detoxification, and metabolic regulation. Gene expression analysis confirmed the differential abundance of proteins in peanut cotyledons supplemented with aflatoxins, with or without infection with the atoxigenic strain. Similarly, peanut germination and A. flavus growth were altered in response to aflatoxin B1. These findings show an additional immunity initiated by aflatoxins. With the PAMP- and effector-triggered immune responses, this immunity constitutes the third immune response of the immune system in peanut cotyledon cells. The system is also a three-grade coevolution of plant-pathogen interaction.

Regulation of stomatal tropism and infection by light in Cercospora zeae-maydis: evidence for coordinated host/pathogen responses to photoperiod?

Cercospora zeae-maydis causes gray leaf spot of maize, which has become one of the most widespread and destructive diseases of maize in the world. C. zeae-maydis infects leaves through stomata, which is predicated on the ability of the pathogen to perceive stomata and reorient growth accordingly. In this study, the discovery that light was required for C. zeae-maydis to perceive stomata and infect leaves led to the identification of CRP1, a gene encoding a putative blue-light photoreceptor homologous to White Collar-1 (WC-1) of Neurospora crassa. Disrupting CRP1 via homologous recombination revealed roles in multiple aspects of pathogenesis, including tropism of hyphae to stomata, the formation of appressoria, conidiation, and the biosynthesis of cercosporin. CRP1 was also required for photoreactivation after lethal doses of UV exposure. Intriguingly, putative orthologs of CRP1 are central regulators of circadian clocks in other filamentous fungi, raising the possibility that C. zeae-maydis uses light as a key environmental input to coordinate pathogenesis with maize photoperiodic responses. This study identified a novel molecular mechanism underlying stomatal tropism in a foliar fungal pathogen, provides specific insight into how light regulates pathogenesis in C. zeae-maydis, and establishes a genetic framework for the molecular dissection of infection via stomata and the integration of host and pathogen responses to photoperiod.

Chemical Constituents of the New Endophytic Fungus Mycosphaerella sp. nov. and Their Anti-parasitic Activity

Chemical investigation of a new endophytic fungus, Mycosphaerella sp. nov. strain F2140, associated with the foliage of the plant Psychotria horizontalis (Rubiaceae) in Panama, resulted in the isolation of cercosporin (1) and a new cercosporin analog (3) as the major components. The structures of minor compounds in the extract were elucidated by detailed spectroscopic analysis as 2-(2-butyl)-6-ethyl-3-hydroxy-6-methylcyclohex-2-ene-1, 5-dione (4), 3-(2-butyl)-6-ethyl-5-hydroxy-2-methoxy-6-methyl-cyclohex-2-enone (5), and an isomer of 5 (6). To study the influence of the hydroxy groups on the antiparasitic activity of cercosporin, compound 1 was acetylated to obtain derivative 2. The isolated compounds 1-6 were tested in vitro to determine their antiparasitic activity against the causal agents of malaria ( Plasmodium falciparum), leishmaniasis ( Leishmania donovani), and Chagas disease ( Trypanosoma cruzi). Cytotoxicity and potential anticancer activity of these compounds were evaluated using mammalian Vero cells and MCF7 cancer cell lines, respectively. Compounds 1 and 2 displayed high potency against L. donovani (IC50 0.46 and 0.64 μM), T. cruzi (IC50 1.08 and 0.78 μM), P. falciparum (IC50 1.03 and 2.99 μM), and MCF7 cancer cell lines (IC50 4.68 and 3.56 μM). Compounds 3-6 were not active in these assays at a concentration of 10 μg/mL.

Chemical constituents of the new endophytic fungus Mycosphaerella sp. nov. and their anti-parasitic activity

Chemical investigation of a new endophytic fungus, Mycosphaerella sp. nov. strain F2140, associated with the foliage of the plant Psychotria horizontalis (Rubiaceae) in Panama, resulted in the isolation of cercosporin (1) and a new cercosporin analog (3) as the major components. The structures of minor compounds in the extract were elucidated by detailed spectroscopic analysis as 2-(2-butyl)-6-ethyl-3-hydroxy-6-methylcyclohex-2-ene-1,5-dione (4), 3-(2-butyl)-6-ethyl-5-hydroxy-2-methoxy-6-methyl-cyclohex-2-enone (5), and an isomer of 5 (6). To study the influence of the hydroxy groups on the anti-parasitic activity of cercosporin, compound 1 was acetylated to obtain derivative 2. The isolated compounds 1- 6 were tested in vitro to determine their anti-parasitic activity against the causal agents of malaria (Plasmodium falciparum), leishmaniasis (Leishmania donovani), and Chagas disease (Trypanosoma cruzi). Cytotoxicity and potential anticancer activity of these compounds were evaluated using mammalian Vero cells and MCF7 cancer cell lines, respectively. Compounds 1 and 2 displayed high potency against L. donovani (IC50 0.46 and 0.64 microM), T. cruzi (IC50 1.08 and 0.78 microM), P. falciparum (IC50 1.03 and 2.99 microM), and MCF7 cancer cell lines (IC50 4.68 and 3.56 microM). Compounds 3-6 were not active in these assays at a concentration of 10 microg/mL.

Isolation, characterization, and production of red pigment from Cercospora piaropi a biocontrol agent for waterhyacinth

A red pigment produced by a Mexican isolate of Cercospora piaropi (waterhyacinth pathogen) has been isolated and identified as cercosporin. The kinetic of cercosporin production in culture media during dark/light regimes was evaluated. When C. piaropi was cultivated in continuous light and potato dextrose broth culture, a maximum of cercosporin production was observed (72.59 mg/l). Despite other reports, C piaropi Mexican isolate produce cercosporin in dark conditions (25.70 mg/l). The results suggest that production of cercosporin in C. piaropi-waterhyacinth pathogenesis is an important factor to take into account in biocontrol strategies.

Assessment of Lineages of Cercospora kikuchii in Louisiana for Aggressiveness and Screening Soybean Cultivars for Resistance to Cercospora Leaf Blight

Cercospora leaf blight (CLB) became much more prevalent in Louisiana beginning in 1999. A previous study showed that the Louisiana population of Cercospora kikuchii, the causal agent, was dominated by a new lineage that differed from those collected in other locations at earlier times. In this study, we tested whether the dominance of the new lineage was caused by higher aggressiveness and screened soybean cultivars for resistance to CLB. Representative isolates from both lineages were used individually to inoculate six soybean cultivars in the greenhouse. Contrary to expectations, the new lineage was less aggressive. Three virulence groups were defined in this pathogen based on correlation of the aggressiveness of individual isolates on soybean cultivars. Other possible causes that led to the dominance of the new lineage in Louisiana are discussed. Eleven soybean cultivars were tested for disease reaction at two locations over 3 years in the field. Two cultivars, AG5701 (Asgrow) and TV59R85 (Terral), were among the more resistant cultivars to CLB both in the greenhouse and in the field.

Production of cercosporin toxin by the phytopathogenic Cercospora fungi is affected by diverse environmental signals

Cercosporin is a polyketide phytotoxin produced by many phytopathogenic Cercospora spp. We investigated environmental signals that have elaborate control of cercosporin production. Light is the most critical factor for cercosporin production. Cercospora nicotianae accumulated substantial quantities of cercosporin only when grown on a particular potato dextrose agar under light but produced little cercosporin on other brands of potato dextrose agar or media with defined ingredients. In addition to light regulation, numerous factors including salts, buffers, and ions markedly affected cercosporin production. By contrast, pH had little effect on cercosporin production. Depletion or alteration of the carbon or nitrogen sources also affected cercosporin production. Production of cercosporin was elevated to varying levels by metal ions, such as cobalt, ferric, manganese, and zinc. Significant differences in cercosporin production were observed among various Cercospora species. Further, regulation of cercosporin production by phosphate buffer, ammonium, LiCl, but not metal ions appeared to occur at transcriptional levels. Expression of the genes involved in cercosporin biosynthesis and regulation decreased markedly and was closely concomitant with the amounts of cercosporin reduced as the fungus was grown on medium containing phosphate, LiCl, ammonium, or dimethyl sulfoxide. The results reveal the complexity of cercosporin production at the physiological and genetic levels. A model delineating regulatory controls of cercosporin biosynthesis is proposed and discussed.

Functional characterization of three genes encoding putative oxidoreductases required for cercosporin toxin biosynthesis in the fungus Cercospora nicotianae

Cercosporin is a non-host-selective, photoactivated polyketide toxin produced by many phytopathogenic Cercospora species, which plays a crucial role during pathogenesis on host plants. Upon illumination, cercosporin converts oxygen molecules to toxic superoxide and singlet oxygen that damage various cellular components and induce lipid peroxidation and electrolyte leakage. Three genes (CTB5, CTB6 and CTB7) encoding putative FAD/FMN- or NADPH-dependent oxidoreductases in the cercosporin toxin biosynthetic pathway of C. nicotianae were functionally analysed. Replacement of each gene via double recombination was utilized to create null mutant strains that were completely impaired in cercosporin production as a consequence of specific interruption at the CTB5, CTB6 or CTB7 locus. Expression of CTB1, CTB5, CTB6, CTB7 and CTB8 was drastically reduced or nearly abolished when CTB5, CTB6 or CTB7 was disrupted. Production of cercosporin was revived when a functional gene cassette was introduced into the respective mutants. All ctb5, ctb6 and ctb7 null mutants retained wild-type levels of resistance against toxicity of cercosporin or singlet-oxygen-generating compounds, indicating that none of the genes plays a role in self-protection.

Cellular toxicity of elsinochrome phytotoxins produced by the pathogenic fungus, Elsinoë fawcettii causing citrus scab

Elsinochromes are the red/orange pigments produced by many Elsinoë fungal species and are structurally similar to the phytotoxin, cercosporin. Here, pigments were extracted from cultures of a citrus pathogen, Elsinoë fawcettii and tested for cellular toxicity. On irradiation with light, elsinochromes rapidly killed suspension cultured citrus and tobacco cells. The toxicity was decreased by adding the singlet oxygen ((1)O(2)) quenchers (bixin (carotenoid carboxylic acid), DABCO (1, 4-diazabicyco octane), ascorbate or reduced glutathione). Application of elsinochromes onto rough lemon leaves resulted in necrotic lesions, whereas lesion development was inhibited by the addition of bixin, DABCO or ascorbate, but not a-tocopherol. Incubation of rough lemon leaf discs with elsinochromes in the light induced a steady increase of electrolyte leakage. Compared with two photosensitizing compounds, hematoporphyrin and cercosporin, the accumulation of (1)O(2) induced by elsinochromes after irradiation was indicated by successful detection of the cholesterol oxidation product, 5a-hydroperoxide. Addition of a potent quencher, beta-carotene prevented 5alpha-hydroperoxide production. Elsinochromes generated superoxide ions (O(2)(*-)), whereas accumulation of O(2)(*-)was blocked by addition of the superoxide dismutase, a scavenger of O(2)(*-), but not the (1)O(2)-quencher, DABCO. Our study indicated that elsinochromes are functioning as photosensitizing compounds that produce (1)O(2)and O(2)(*-), and exert toxicity to plant cells.

Molecular analysis of the cercosporin biosynthetic gene cluster in Cercospora nicotianae

We describe a core gene cluster, comprised of eight genes (designated CTB1-8), and associated with cercosporin toxin production in Cercospora nicotianae. Sequence analysis identified 10 putative open reading frames (ORFs) flanking the previously characterized CTB1 and CTB3 genes that encode, respectively, the polyketide synthase and a dual methyltransferase/monooxygenase required for cercosporin production. Expression of eight of the genes was co-ordinately induced under cercosporin-producing conditions and was regulated by the Zn(II)Cys(6) transcriptional activator, CTB8. Expression of the genes, affected by nitrogen and carbon sources and pH, was also controlled by another transcription activator, CRG1, previously shown to regulate cercosporin production and resistance. Disruption of the CTB2 gene encoding a methyltransferase or the CTB8 gene yielded mutants that were completely defective in cercosporin production and inhibitory expression of the other CTB cluster genes. Similar 'feedback' transcriptional inhibition was observed when the CTB1, or CTB3 but not CTB4 gene was inactivated. Expression of four ORFs located on the two distal ends of the cluster did not correlate with cercosporin biosynthesis and did not show regulation by CTB8, suggesting that the biosynthetic cluster was limited to CTB1-8. A biosynthetic pathway and a regulatory network leading to cercosporin formation are proposed.

The Cercospora nicotianae gene encoding dual O-methyltransferase and FAD-dependent monooxygenase domains mediates cercosporin toxin biosynthesis

Cercosporin, a photo-activated, non-host-selective phytotoxin produced by many species of the plant pathogenic fungus Cercospora, causes peroxidation of plant cell membranes by generating reactive oxygen species and is an important virulence determinant. Here we report a new gene, CTB3 that is involved in cercosporin biosynthesis in Cercospora nicotianae. CTB3 is adjacent to a previously identified CTB1 encoding a polyketide synthase which is also required for cercosporin production. CTB3 contains a putative O-methyltransferase domain in the N-terminus and a putative flavin adenine dinucleotide (FAD)-dependent monooxygenase domain in the C-terminus. The N-terminal amino acid sequence also is similar to that of the transcription enhancer AFLS (formerly AFLJ) involved in aflatoxin biosynthesis. Expression of CTB3 was differentially regulated by light, medium, nitrogen and carbon sources and pH. Disruption of the N- or C-terminus of CTB3 yielded mutants that failed to accumulate the CTB3 transcript and cercosporin. The Deltactb3 disruptants produced a yellow pigment that is not toxic to tobacco suspension cells. Production of cercosporin in a Deltactb3 null mutant was fully restored when transformed with a functional CTB3 clone or when paired with a Deltactb1-null mutant (defective in polyketide synthase) by cross feeding of the biosynthetic intermediates. Pathogenicity assays using detached tobacco leaves revealed that the Deltactb3 disruptants drastically reduced lesion formation.

Deletion of a MFS transporter-like gene in Cercospora nicotianae reduces cercosporin toxin accumulation and fungal virulence

Many phytopathogenic Cercospora species produce a host-nonselective polyketide toxin, called cercosporin, whose toxicity exclusively relies on the generation of reactive oxygen species. Here, we describe a Cercospora nicotianae CTB4 gene that encodes a putative membrane transporter and provide genetic evidence to support its role in cercosporin accumulation. The predicted CTB4 polypeptide has 12 transmembrane segments with four conserved motifs and has considerable similarity to a wide range of transporters belonging to the major facilitator superfamily (MFS). Disruption of the CTB4 gene resulted in a mutant that displayed a drastic reduction of cercosporin production and accumulation of an unknown brown pigment. Cercosporin was detected largely from fungal hyphae of ctb4 disruptants, but not from the surrounding medium, suggesting that the mutants were defective in both cercosporin biosynthesis and secretion. Cercosporin purified from the ctb4 disruptants exhibited toxicity to tobacco suspension cells, insignificantly different from wild-type, whereas the disruptants formed fewer lesions on tobacco leaves. The ctb4 null mutants retained normal resistance to cercosporin and other singlet oxygen-generating photosensitizers, indistinguishable from the parental strain. Transformation of a functional CTB4 clone into a ctb4 null mutant fully revived cercosporin production. Thus, we propose that the CTB4 gene encodes a putative MFS transporter responsible for secretion and accumulation of cercosporin.

PDX1 is essential for vitamin B6 biosynthesis, development and stress tolerance in Arabidopsis

Vitamin B6 is an essential coenzyme for numerous metabolic enzymes and is a potent antioxidant. In plants, very little is known about its contribution to viability, growth and development. The de novo pathway of vitamin B6 biosynthesis has only been described recently and involves the protein PDX1 (pyridoxal phosphate synthase protein). Arabidopsis thaliana has three homologs of PDX1, two of which, PDX1.1 and PDX1.3, have been demonstrated as functional in vitamin B6 biosynthesis in vitro and by yeast complementation. In this study, we show that the spatial and temporal expression patterns of PDX1.1 and PDX1.3, investigated at the transcript and protein level, largely overlap, but PDX1.3 is more abundant than PDX1.1. Development of single pdx1.1 and pdx1.3 mutants is partially affected, whereas disruption of both genes causes embryo lethality at the globular stage. Detailed examination of the single mutants, in addition to those that only have a single functional copy of either gene, indicates that although these genes are partially redundant in vitamin B6 synthesis, PDX1.3 is more requisite than PDX1.1. Developmental distinctions correlate with the vitamin B6 content. Furthermore, we provide evidence that in addition to being essential for plant growth and development, vitamin B6 also plays a role in stress tolerance and photoprotection of plants.

An oxidoreductase is involved in cercosporin degradation by the bacterium Xanthomonas campestris pv. zinniae

The polyketide toxin cercosporin plays a key role in pathogenesis by fungal species of the genus Cercospora. The bacterium Xanthomonas campestris pv. zinniae is able to rapidly degrade this toxin. Growth of X. campestris pv. zinniae strains in cercosporin-containing medium leads to the breakdown of cercosporin and to the formation of xanosporic acid, a nontoxic breakdown product. Five non-cercosporin-degrading mutants of a strain that rapidly degrades cercosporin (XCZ-3) were generated by ethyl methanesulfonate mutagenesis and were then transformed with a genomic library from the wild-type strain. All five mutants were complemented with the same genomic clone, which encoded a putative transcriptional regulator and an oxidoreductase. Simultaneous expression of these two genes was necessary to complement the mutant phenotype. Sequence analysis of the mutants showed that all five mutants had point mutations in the oxidoreductase gene and no mutations in the regulator. Quantitative reverse transcription-PCR (RT-PCR) showed that the expression of both of these genes in the wild-type strain is upregulated after exposure to cercosporin. Both the oxidoreductase and transcriptional regulator genes were transformed into three non-cercosporin-degrading bacteria to determine if they are sufficient for cercosporin degradation. Quantitative RT-PCR analysis confirmed that the oxidoreductase was expressed in all transconjugants. However, none of the transconjugants were able to degrade cercosporin, suggesting that additional factors are required for cercosporin degradation. Further study of cercosporin degradation in X. campestris pv. zinniae may allow for the engineering of Cercospora-resistant plants by using a suite of genes.

Photoactivated perylenequinone toxins in fungal pathogenesis of plants

Several genera of plant pathogenic fungi produce photoactivated perylenequinone toxins involved in pathogenesis of their hosts. These toxins are photosensitizers, absorbing light energy and generating reactive oxygen species that damage the membranes of the host cells. Studies with toxin-deficient mutants and on the involvement of light in symptom development have documented the importance of these toxins in successful pathogenesis of plants. This review focuses on the well studied perylenequinone toxin, cercosporin, produced by species in the genus Cercospora. Significant progress has been made recently on the biosynthetic pathway of cercosporin, with the characterization of genes encoding a polyketide synthase and a major facilitator superfamily transporter, representing the first and last steps of the biosynthetic pathway, as well as important regulatory genes. In addition, the resistance of Cercospora fungi to cercosporin and to the singlet oxygen that it generates has led to the use of these fungi as models for understanding cellular resistance to photosensitizers and singlet oxygen. These studies have shown that resistance is complex, and have documented a role for transporters, transient reductive detoxification, and quenchers in cercosporin resistance.

The CTB1 gene encoding a fungal polyketide synthase is required for cercosporin biosynthesis and fungal virulence of Cercospora nicotianae

Cercosporin is a light-activated, non-host-selective toxin produced by many Cercospora fungal species. In this study, a polyketide synthase gene (CTB1) was functionally identified and molecularly characterized to play a key role in cercosporin biosynthesis by Cercospora nicotianae. We also provide conclusive evidence to confirm the crucial role of cercosporin in fungal pathogenesis. CTB1 encoded a polypeptide with a deduced length of 2,196 amino acids containing a keto synthase (KS), an acyltransferase (AT), a thioesterase/claisen cyclase (TE/CYC), and two acyl carrier protein (ACP) domains, and had high levels of similarity to many fungal type I polyketide synthases. Expression of a 6.8-kb CTB1 transcript was highly regulated by light and medium composition, consistent with the conditions required for cercosporin biosynthesis in cultures. Targeted disruption of CTB1 resulted in the loss of both CTB1 transcript and cercosporin biosynthesis in C. nicotianae. The ctb1-null mutants incited fewer necrotic lesions on inoculated tobacco leaves compared with the wild type. Complementation of ctb1-null mutants with a full-length CTB1 clone restored wild-type levels of cercosporin production as well as the ability to induce lesions on tobacco. Thus, we have demonstrated conclusively that cercosporin is synthesized via a polyketide pathway, and cercosporin is an important virulence factor in C. nicotianae. The results also suggest that strategies that avoid the toxicity of cercosporin will be useful in reduction of disease incidence caused by Cercospora spp.

Expression of the cercosporin toxin resistance gene ( CRG1) as a dicistronic mRNA in the filamentous fungus Cercospora nicotianae

The CRG1 gene in Cercospora nicotianae encodes a transcription factor and is required for cercosporin toxin resistance and production. Cloning and sequencing of the downstream region of the CRG1 gene led to the discovery of an adjacent gene ( PUT1) encoding a putative uracil transporter. Expression of CRG1 and PUT1 as assessed by Northern analysis indicated that, in addition to the expected monocistronic mRNAs (2.6 kb and 2.0 kb, respectively), a common 4.5-kb mRNA could be identified, using either a CRG1 or a PUT1 gene probe. The 2.6-kb transcript identified only by the CRG1 probe was expressed constitutively, whereas the 2.0-kb transcript identified only by the PUT1 probe was differentially expressed in various media. Four cDNA clones containing CRG1, PUT1, and the CRG1- PUT1 intergenic region were identified as part of the products from the 4.5-kb transcript. Both the 4.5-kb and 2.6-kb transcripts were not detectable in three crg1-disrupted mutants, using the CRG1 probe. The 2.0-kb transcript, but not the 4.5-kb one was detected using the PUT1 probe in the three crg1-disrupted mutants. Taken together, we conclude that the 4.5-kb transcript is a dicistronic mRNA of both CRG1 and PUT1 in the fungus C. nicotianae. This is the first example of a dicistronic mRNA identified in filamentous fungi.

The CRG1 gene required for resistance to the singlet oxygen-generating cercosporin toxin in Cercospora nicotianae encodes a putative fungal transcription factor

The Cercospora nicotianae CRG1 gene is involved in cellular resistance to the perylenequinone toxin, cercosporin, that generates highly toxic singlet oxygen upon exposure to light. The entire open reading frame (ORF) of CRG1 was isolated and sequenced. The gene contains an ORF of 1950bp including a 65-bp intron. The predicted 650 amino acid CRG1 protein contains a Cys(6)Zn(2) binuclear cluster DNA-binding motif with homology to various fungal regulatory proteins, indicating that CRG1 may act functionally as a transcription activator. Targeted gene disruption of CRG1 resulted in mutants that are partially sensitive to cercosporin and reduced in cercosporin production. Genetic complementation revealed that CRG1 fully restored cercosporin resistance, but only slightly restored cercosporin production in a UV-derived mutant (CS10) containing a single nucleotide substitution in crg1. Complementation of a crg1-null mutant, however, yielded strains that are similar to the wild-type in both phenotypes. These results indicate that the transcription regulator CRG1 is involved in the activation of genes associated with cercosporin resistance and production in the fungus Cercospora nicotianae.

Involvement of calcium/calmodulin signaling in cercosporin toxin biosynthesis by Cercospora nicotianae

Cercosporin is a non-host-selective, perylenequinone toxin produced by many phytopathogenic Cercospora species. The involvement of Ca(2+)/calmodulin (CaM) signaling in cercosporin biosynthesis was investigated by using pharmacological inhibitors. The results suggest that maintaining endogenous Ca(2+) homeostasis is required for cercosporin biosynthesis in Cercospora nicotianae. The addition of excess Ca(2+) to the medium slightly increased fungal growth but resulted in a reduction in cercosporin production. The addition of Ca(2+) chelators [EGTA and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] also reduced cercosporin production. Ca(2+) channel blockers exhibited a strong inhibition of cercosporin production only at higher concentrations (>2 mM). Cercosporin production was reduced greatly by Ca(2+) ionophores (A23187 and ionomycin) and internal Ca(2+) blocker [3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester]. Phospholipase C inhibitors (lithium, U73122, and neomycin) led to a concentration-dependent inhibition of cercosporin biosynthesis. Furthermore, the addition of CaM inhibitors (compound 48/80, trifluoperazine, W-7, and chlorpromazine) also markedly reduced cercosporin production. In contrast to W-7, W-5, with less specificity for CaM, led to only minor inhibition of cercosporin production. The inhibitory effects of Ca(2+)/CaM inhibitors were partially or completely reversed by the addition of external Ca(2+). As assessed with Fluo-3/AM (a fluorescent Ca(2+) indicator), the Ca(2+) content in the cytoplasm decreased significantly when fungal cultures were grown in a medium containing Ca(2+)/CaM antagonists, confirming the specificity of those Ca(2+)/CaM antagonists in C. nicotianae. Taken together, the results suggest that Ca(2+)/CaM signal transduction may play a pivotal role in cercosporin biosynthesis in C. nicotianae.

Biodegradation of the polyketide toxin cercosporin

Cercosporin is a non-host-specific polyketide toxin produced by many species of plant pathogens belonging to the genus Cercospora. This red-pigmented, light-activated toxin is an important pathogenicity determinant for Cercospora species. In this study, we screened 244 bacterial isolates representing 12 different genera for the ability to degrade cercosporin. Cercosporin degradation was determined by screening for the presence of cleared zones surrounding colonies on cercosporin-containing culture medium and was confirmed by assaying the kinetics of degradation in liquid medium. Bacteria belonging to four different genera exhibited the cercosporin-degrading phenotype. The isolates with the greatest cercosporin-degrading activity belonged to Xanthomonas campestris pv. zinniae and X. campestris pv. pruni. Isolates of these pathovars removed over 90% of the cercosporin from culture medium within 48 h. Bacterial degradation of red cercosporin was accompanied by a shift in the color of the growth medium to brown and then green. The disappearance of cercosporin was accompanied by the appearance of a transient green product, designated xanosporic acid. Xanosporic acid and its more stable lactone derivative, xanosporolactone, are nontoxic to cercosporin-sensitive fungi and to plant tissue and are labile in the presence of light. Detailed spectroscopic analysis (to be reported in a separate publication) of xanosporolactone revealed that cercosporin loses one methoxyl group and gains one oxygen atom in the bacterial conversion. The resulting chromophore (4,9-dihydroxy-3-oxaperlylen-10H-10-one) has never been reported before but is biosynthetically plausible via oxygen insertion by a cytochrome P-450 enzyme.

The Photoactivated Cercospora Toxin Cercosporin: Contributions to Plant Disease and Fundamental Biology

Plant pathogenic fungi in eight genera produce light-activated perylenequinone toxins that are toxic to plants via the generation of activated oxygen species, particularly singlet oxygen. Studies on the cercosporin toxin produced by Cercospora species have documented an important role for this toxin in pathogenesis of host plants. Cercosporin-generated active oxygen species destroy the membranes of host plants, providing nutrients to support the growth of these intercellular pathogens. Resistance of Cercospora species to the toxic effects of their own toxin has allowed these organisms to be used as a model for understanding the cellular basis of resistance to singlet oxygen and to general oxidative stress. In particular, the recent discovery that pyridoxine (vitamin B6) quenches singlet oxygen has led to the understanding of a novel role for this vitamin in cells as well as the discovery of a novel pathway of biosynthesis.

Dihydrocercosporin singlet oxygen production and subcellular localization: a possible defense against cercosporin phototoxicity in Cercospora

Fungi in the genus Cercospora produce cercosporin, a potent singlet oxygen (1O2)-generating photosensitizer that plays a critical role in the ability of these fungi to parasitize plants. Although plants, mice, bacteria and many fungi are sensitive to cercosporin, Cercospora species are resistant to its toxicity. The cellular resistance of these fungi to cercosporin has been correlated with fungal cell surface reducing ability and the ability to maintain cercosporin in a chemically reduced state. As a model for reduced cercosporin we employed a reduced, acetylated derivative (hexaacetyl-dihydrocercosporin, HAC) that we tested for 1O2 production in a range of solvents. We found that as a 1O2 photosensitizer, HAC was only moderately effective in organic solvents (phi SO = 0.14-0.18) and very poor in water (phi SO = 0.02-0.04). By contrast, the 1O2 quantum yield of cercosporin itself was unaffected by solvent (phi SO = 0.84-0.97). To investigate the localization of reduced cercosporin in fungal cells, we developed a fluorescence assay using laser scanning confocal microscopy. This assay showed a uniform green fluorescence, indicative of reduced cercosporin, in the cytoplasm of hyphal cells treated with cercosporin. We hypothesize that the main protection mechanism against cercosporin phototoxicity in the fungus consists of transformation of cercosporin to a reduced state and localization of this reduced form in the aqueous compartment of the cell, thus decreasing intracellular 1O2 production to levels that can be tolerated by the fungus. In addition, we have, for the first time, directly detected 1O2 phosphorescence from fungal culture, either stained with the photosensitizer rose bengal or actively synthesizing cercosporin, demonstrating 1O2 production in vivo and from cercosporin in culture.

CFP, the putative cercosporin transporter of Cercospora kikuchii, is required for wild type cercosporin production, resistance, and virulence on soybean

Many species of the fungal genus Cercospora, including the soybean pathogen C. kikuchii, produce the phytotoxic polyketide cercosporin. Cercosporin production is induced by light. Previously, we identified several cDNA clones of mRNA transcripts that exhibited light-enhanced accumulation in C. kikuchii. Targeted disruption of the genomic copy of one of these, now designated CFP (cercosporin facilitator protein), results in a drastic reduction in cercosporin production, greatly reduced virulence of the fungus to soybean, and increased sensitivity to exogenous cercosporin. Sequence analysis of CFP reveals an 1,821-bp open reading frame encoding a 65.4-kDa protein similar to several members of the major facilitator superfamily (MFS) of integral membrane transporter proteins known to confer resistance to various antibiotics and toxins in fungi and bacteria. We propose that CFP encodes a cercosporin transporter that contributes resistance to cercosporin by actively exporting cercosporin, thus maintaining low cellular concentrations of the toxin.

Cell surface redox potential as a mechanism of defense against photosensitizers in fungi

The phytotoxin cercosporin, a singlet oxygen-generating photosensitizer, is toxic to plants, mice, and many fungi, yet the fungi that produce it, Cercospora spp., are resistant. We hypothesize that resistance to cercosporin may result from a reducing environment at the cell surface. Twenty tetrazolium dyes differing in redox potential were used as indicators of cell surface redox potential of seven fungal species differing in resistance to cercosporin. Resistant fungi were able to reduce significantly more dyes than were sensitive fungi. A correlation between dye reduction and cercosporin resistance was also observed when resistance levels of Cercospora species were manipulated by growth on different media. The addition of the reducing agents ascorbate, cysteine, and reduced glutathione (GSH) to growth media decreased cercosporin toxicity for sensitive fungi. None of these agents directly reduced cercosporin at the concentrations at which they protected fungi. Spectral and thin-layer chromatographic analyses of cercosporin solutions containing the different reducing agents indicated that GSH, but not cysteine or ascorbate, reacted with cercosporin. Resistant and sensitive fungi did not differ in endogenous levels of cysteine, GSH, or total thiols. On the basis of data from this and other studies, this report presents a model which proposes that cercosporin resistance results from the production of reducing power at the surfaces of resistant cells, leading to transient reduction and detoxification of the cercosporin molecule.

Mutants of Cercospora kikuchii Altered in Cercosporin Synthesis and Pathogenicity

We have obtained spontaneous and UV-induced stable mutants, altered in the synthesis of cercosporin, of the fungal soybean pathogen Cercospora kikuchii . The mutants were isolated on the basis of colony color on minimal medium. The UV-induced mutants accumulated, at most, 2% of wild-type cercosporin levels on all media tested. In contrast, cercosporin accumulation by the spontaneous mutants was strongly medium regulated, occurring only on potato dextrose medium but at concentrations comparable to those produced by the wild-type strain. UV-induced mutants unable to synthesize cercosporin on any medium were unable to incite lesions when inoculated onto the soybean host. Cercosporin was reproducibly isolated from all inoculated leaves showing lesions. Although cercosporin involvement in disease has been indirectly suggested by many previous studies, this is the first report in which mutants blocked in cercosporin synthesis have been used to demonstrate that cercosporin is a crucial pathogenicity factor for this fungal genus.

Electrolyte Leakage, Lipoxygenase, and Lipid Peroxidation Induced in Tomato Leaf Tissue by Specific and Nonspecific Elicitors from Cladosporium fulvum

Glycoprotein nonspecific elicitor (NSE) and a specific elicitor preparation from intercellular fluids (SE) of tomato (Lycopersicon esculentum Mill. cv Bonny Best or Potentate) infected with race 2.4.5 of Cladosporium fulvum Cooke [syn. Fulvia fulva (Cooke) Ciferri] were injected into cv Sonatine (resistant to race 2.4.5) to compare electrolyte leakage, lipoxygenase activity, and lipid peroxidation induced in response to these elicitors. Increased electrolyte leakage was induced by NSE or SE; the leakage due to NSE but not to SE was inhibited by the nonsteroidal antiinflammatory drug (NSAID) piroxicam. Under normal photoperiod conditions, higher levels of lipoxygenase activity were detected 6 hours after injection with either elicitor. This activity peaked by 12 hours with both elicitors and declined to control levels by 24 hours when visible necrosis could be detected. Both NSE and SE-induced lipoxygenase was inhibited by piroxicam in vitro. Lipid peroxidation in elicitor-treated tissue was also assayed at 6, 12, and 24 hours after injection using the TBA test for malonaldehyde. Increased peroxidation was detected in response to NSE or SE at 12 hours with similar values obtained at 24 hours. With plants incubated in the dark, lipoxygenase, and lipid peroxidation were similarly induced in SE-injected tissue whereas necrosis induction by SE was light dependent.