(-)-BOTRYODIPLODIN, A UNIQUE RIBOSE-ANALOG TOXIN

Molecular interactions between the soilborne pathogenic fungus Macrophomina phaseolina and its host plants

Mentioned for the first time in an article 1971, the occurrence of the term "Macrophomina phaseolina" has experienced a steep increase in the scientific literature over the past 15 years. Concurrently, incidences of M. phaseolina-caused crop diseases have been getting more frequent. The high levels of diversity and plasticity observed for M. phasolina genomes along with a rich equipment of plant cell wall degrading enzymes, secondary metabolites and putative virulence effectors as well as the unusual longevity of microsclerotia, their asexual reproduction structures, make this pathogen very difficult to control and crop protection against it very challenging. During the past years several studies have emerged reporting on host defense measures against M. phaseolina, as well as mechanisms of pathogenicity employed by this fungal pathogen. While most of these studies have been performed in crop systems, such as soybean or sesame, recently interactions of M. phaseolina with the model plant Arabidopsis thaliana have been described. Collectively, results from various studies are hinting at a complex infection cycle of M. phaseolina, which exhibits an early biotrophic phase and switches to necrotrophy at later time points during the infection process. Consequently, responses of the hosts are complex and seem coordinated by multiple defense-associated phytohormones. However, at this point no robust and strong host defense mechanism against M. phaseolina has been described.

Status of Phytotoxins Isolated from Necrotrophic Fungi Causing Diseases on Grain Legumes

Fungal phytotoxins can be defined as secondary metabolites toxic to host plants and are believed to be involved in the symptoms developed of a number of plant diseases by targeting host cellular machineries or interfering with host immune responses. As any crop, legumes can be affected by a number of fungal diseases, causing severe yield losses worldwide. In this review, we report and discuss the isolation, chemical, and biological characterization of fungal phytotoxins produced by the most important necrotrophic fungi involved in legume diseases. Their possible role in plant-pathogen interaction and structure-toxicity relationship studies have also been reported and discussed. Moreover, multidisciplinary studies on other prominent biological activity conducted on reviewed phytotoxins are described. Finally, we explore the challenges in the identification of new fungal metabolites and their possible applications in future experiments.

Soybean Seed Sugars: A Role in the Mechanism of Resistance to Charcoal Rot and Potential Use as Biomarkers in Selection

Charcoal rot, caused by Macrophomina phaseolina, is a major soybean disease resulting in significant yield loss and poor seed quality. Currently, no resistant soybean cultivar is available in the market and resistance mechanisms to charcoal rot are unknown, although the disease is believed to infect plants from infected soil through the roots by unknown toxin-mediated mechanisms. The objective of this research was to investigate the association between seed sugars (sucrose, raffinose, stachyose, glucose, and fructose) and their role as biomarkers in the soybean defense mechanism in the moderately resistant (MR) and susceptible (S) genotypes to charcoal rot. Seven MR and six S genotypes were grown under irrigated (IR) and non-irrigated (NIR) conditions. A two-year field experiment was conducted in 2012 and 2013 at Jackson, TN, USA. The main findings in this research were that MR genotypes generally had the ability to maintain higher seed levels of sucrose, glucose, and fructose than did S genotypes. Conversely, susceptible genotypes showed a higher level of stachyose and lower levels of sucrose, glucose, and fructose. This was observed in 6 out of 7 MR genotypes and in 4 out of 6 S genotypes in 2012; and in 5 out of 7 MR genotypes and in 5 out of 6 S genotypes in 2013. The response of S genotypes with higher levels of stachyose and lower sucrose, glucose, and fructose, compared with those of MR genotypes, may indicate the possible role of these sugars in a defense mechanism against charcoal rot. It also indicates that nutrient pathways in MR genotypes allowed for a higher influx of nutritious sugars (sucrose, glucose, and fructose) than did S genotypes, suggesting these sugars as potential biomarkers for selecting MR soybean plants after harvest. This research provides new knowledge on seed sugars and helps in understanding the impact of charcoal rot on seed sugars in moderately resistant and susceptible genotypes.

Pigment Produced by Glycine-Stimulated Macrophomina Phaseolina Is a (−)-Botryodiplodin Reaction Product and the Basis for an In-Culture Assay for (−)-Botryodiplodin Production

An isolate of Macrophomina phaseolina from muskmelons (Cucumis melo) was reported by Dunlap and Bruton to produce red pigment(s) in melons and in culture in the presence of added glycine, alanine, leucine, or asparagine in the medium, but not with some other amino acids and nitrogen-containing compounds. We explored the generality and mechanism of this pigment production response using pathogenic M. phaseolina isolates from soybean plants expressing symptoms of charcoal rot disease. A survey of 42 M. phaseolina isolates growing on Czapek-Dox agar medium supplemented with glycine confirmed pigment production by 71% of isolates at the optimal glycine concentration (10 g/L). Studies in this laboratory have demonstrated that some pathogenic isolates of M. phaseolina produce the mycotoxin (−)-botryodiplodin, which has been reported to react with amino acids, proteins, and other amines to produce red pigments. Time course studies showed a significant positive correlation between pigment and (−)-botryodiplodin production by selected M. phaseolina isolates with maximum production at seven to eight days. Pigments produced in agar culture medium supplemented with glycine, beta-alanine, or other amines exhibited similar UV-vis adsorption spectra as did pigments produced by (±)-botryodiplodin reacting in the same agar medium. In a separate study of 39 M. phaseolina isolates, red pigment production (OD₅₂₀) on 10 g/L glycine-supplemented Czapek-Dox agar medium correlated significantly with (−)-botryodiplodin production (LC/MS analysis of culture filtrates) in parallel cultures on un-supplemented medium. These results support pigment production on glycine-supplemented agar medium as a simple and inexpensive in-culture method for detecting (−)-botryodiplodin production by M. phaseolina isolates.

Molecular interaction of charcoal rot pathogenesis in soybean: a complex interaction

Charcoal rot (CR) is a major disease of soybean, which is caused by Macrophomina phaseolina (Mp). Increasing temperatures and low rainfall in recent years have immensely benefitted the pathogen. Hence, the search for genetically acquired resistance to this pathogen is essential. The pathogen is a hemibiotroph, which germinates on the root surface and colonizes epidermal tissue. Several surface receptors initiate pathogenesis, followed by the secretion of various enzymes that provide entry to host tissue. Several enzymes and other converging cascades in the pathogen participate against host defensive responses. β-glucan of the fungal cell wall is recognized as MAMPs (microbe-associated molecular patterns) in plants, which trigger host immune responses. Kinase receptors, resistance, and pathogenesis-related genes correspond to host defense response. They work in conjunction with hormone-mediated defense pathway especially, the systemic acquired resistance, calcium-signaling, and production of phytoalexins. Due to its quantitative nature, limited QTLs have been identified in soybean for CR resistance. The present review attempts to provide a functional link between M. phaseolina pathogenicity and soybean responses. Elucidation of CR resistance responses would facilitate improved designing of breeding programs, and may help in the selection of corresponding genes to introgress CR resistant traits.

First Report of the Production of Mycotoxins and Other Secondary Metabolites by Macrophomina phaseolina (Tassi) Goid. Isolates from Soybeans (Glycine max L.) Symptomatic with Charcoal Rot Disease

Macrophomina phaseolina (Tassi) Goid., the causal agent of charcoal rot disease of soybean, is capable of causing disease in more than 500 other commercially important plants. This fungus produces several secondary metabolites in culture, including (-)-botryodiplodin, phaseolinone and mellein. Given that independent fungal isolates may differ in mycotoxin and secondary metabolite production, we examined a collection of 89 independent M. phaseolina isolates from soybean plants with charcoal rot disease using LC-MS/MS analysis of culture filtrates. In addition to (-)-botryodiplodin and mellein, four previously unreported metabolites were observed in >19% of cultures, including kojic acid (84.3% of cultures at 0.57–79.9 µg/L), moniliformin (61.8% of cultures at 0.011–12.9 µg/L), orsellinic acid (49.4% of cultures at 5.71–1960 µg/L) and cyclo[L-proline-L-tyrosine] (19.1% of cultures at 0.012–0.082 µg/L). In addition, nine previously unreported metabolites were observed at a substantially lower frequency (<5% of cultures), including cordycepin, emodin, endocrocin, citrinin, gliocladic acid, infectopyron, methylorsellinic acid, monocerin and N-benzoyl-L-phenylalanine. Further studies are needed to investigate the possible effects of these mycotoxins and metabolites on pathogenesis by M. phaseolina and on food and feed safety, if any of them contaminate the seeds of infected soybean plants.

Secondary Metabolites of Lasiodiplodia theobromae: Distribution, Chemical Diversity, Bioactivity, and Implications of Their Occurrence

Lasiodiplodia theobromae is a plant pathogenic fungus from the family Botryosphaeriaceae that is commonly found in tropical and subtropical regions. It has been associated with many hosts, causing diverse diseases and being responsible for serious damages on economically important crops. A diverse array of bioactive low molecular weight compounds has been described as being produced by L. theobromae cultures. In this review, the existing literature on secondary metabolites of L. theobromae, their bioactivity, and the implications of their occurrence are compiled. Moreover, the effects of abiotic factors (e.g., temperature, nutrient availability) on secondary metabolites production are highlighted, and possible avenues for future research are presented. Currently, a total of 134 chemically defined compounds belonging to the classes of secondary metabolites and fatty acids have been reported from over 30 L. theobromae isolates. Compounds reported include cyclohexenes and cyclohexenones, indoles, jasmonates, lactones, melleins, phenols, and others. Most of the existing bioactivity studies of L. theobromae metabolites have assessed their potential phytotoxic, cytotoxic, and antimicrobial activities. In fact, its host adaptability and its ability to cause diseases in plants as well as in humans may be related to the capacity to produce bioactive compounds directly involved in host–fungus interactions.

Nitro compounds as the core structures of promising energetic materials and versatile reagents for organic synthesis

This review addresses some promising areas of chemistry of nitro compounds extensively developed in recent years in Russia (particularly at the N.D.Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences) and worldwide. The most important results in the synthesis of novel energetic N-, C- and O-nitro compounds are summarized. New environmentally friendly approaches to the preparation of known compounds of this series, used as components of energetic compositions, are considered. Methods for selective transformations of various nitro compounds to valuable products of organic synthesis, primarily biologically active products and their precursors, are systematically analyzed.

The bibliography includes 446 references.

Phytotoxic Responses of Soybean (Glycine max L.) to Botryodiplodin, a Toxin Produced by the Charcoal Rot Disease Fungus, Macrophomina phaseolina

Toxins have been proposed to facilitate fungal root infection by creating regions of readily-penetrated necrotic tissue when applied externally to intact roots. Isolates of the charcoal rot disease fungus, Macrophomina phaseolina, from soybean plants in Mississippi produced a phytotoxic toxin, (-)-botryodiplodin, but no detectable phaseolinone, a toxin previously proposed to play a role in the root infection mechanism. This study was undertaken to determine if (-)-botryodiplodin induces toxic responses of the types that could facilitate root infection. (±)-Botryodiplodin prepared by chemical synthesis caused phytotoxic effects identical to those observed with (-)-botryodiplodin preparations from M. phaseolina culture filtrates, consistent with fungus-induced phytotoxicity being due to (-)-botryodiplodin, not phaseolinone or other unknown impurities. Soybean leaf disc cultures of Saline cultivar were more susceptible to (±)-botryodiplodin phytotoxicity than were cultures of two charcoal rot-resistant genotypes, DS97-84-1 and DT97-4290. (±)-Botryodiplodin caused similar phytotoxicity in actively growing duckweed (Lemna pausicostata) plantlet cultures, but at much lower concentrations. In soybean seedlings growing in hydroponic culture, (±)-botryodiplodin added to culture medium inhibited lateral and tap root growth, and caused loss of root caps and normal root tip cellular structure. Thus, botryodiplodin applied externally to undisturbed soybean roots induced phytotoxic responses of types expected to facilitate fungal root infection.

Toxin Production in Soybean (Glycine max L.) Plants with Charcoal Rot Disease and by Macrophomina phaseolina, the Fungus that Causes the Disease

Charcoal rot disease, caused by the fungus Macrophomina phaseolina, results in major economic losses in soybean production in southern USA. M. phaseolina has been proposed to use the toxin (-)-botryodiplodin in its root infection mechanism to create a necrotic zone in root tissue through which fungal hyphae can readily enter the plant. The majority (51.4%) of M. phaseolina isolates from plants with charcoal rot disease produced a wide range of (-)-botryodiplodin concentrations in a culture medium (0.14-6.11 µg/mL), 37.8% produced traces below the limit of quantification (0.01 µg/mL), and 10.8% produced no detectable (-)-botryodiplodin. Some culture media with traces or no (-)-botryodiplodin were nevertheless strongly phytotoxic in soybean leaf disc cultures, consistent with the production of another unidentified toxin(s). Widely ranging (-)-botryodiplodin levels (traces to 3.14 µg/g) were also observed in the roots, but not in the aerial parts, of soybean plants naturally infected with charcoal rot disease. This is the first report of (-)-botryodiplodin in plant tissues naturally infected with charcoal rot disease. No phaseolinone was detected in M. phaseolina culture media or naturally infected soybean tissues. These results are consistent with (-)-botryodiplodin playing a role in the pathology of some, but not all, M. phaseolina isolates from soybeans with charcoal rot disease in southern USA.

Lasiojasmonates A-C, three jasmonic acid esters produced by Lasiodiplodia sp., a grapevine pathogen

In this study, a strain (BL 101) of a species of Lasiodiplodia, not yet formally described, which was isolated from declining grapevine plants showing wedge-shaped cankers, was investigated for its ability to produce in vitro bioactive secondary metabolites. From culture filtrates of this strain three jasmonic acid esters, named lasiojasmonates A-C and 16-O-acetylbotryosphaerilactones A and C were isolated together with (1R,2R)-jasmonic acid, its methyl ester, botryosphaerilactone A, (3S,4R,5R)-4-hydroxymethyl-3,5-dimethyldihydro-2-furanone and (3R,4S)-botryodiplodin. The structures of lasiojasmonates A-C were established by spectroscopic methods as (1R*,2R*,3'S*,4'R*,5'R*)-4-hydroxymethyl-3,5-dimethyldihydro-2-furanone, (1R*,2R*,3'S*,4'R*,5'R*,10'R*,12'R*,13'R*,14'S*) and (1R*,2R*,3'S*,4'R*,5'R*,10'S*,12'R*,13'R*,14'S*)-4-(4-hydroxymethyl-3,5-dimethyltetrahydro-furan-2-yloxymethyl)-3,5-dimethyldihydro-2-furanones jasmonates (1, 4 and 5). The structures of 16-O-acetylbotryosphaerilactones A and C were determined by comparison of their spectral data with those of the corresponding acetyl derivatives obtained by acetylation of botryosphaerilactone A. The metabolites isolated, except 4 and 5, were tested at 1mg/mL on leaves of grapevine cv. Cannonau and cork oak using the leaf puncture assay. They were also tested on detached grapevine leaves at 0.5mg/mL and tomato cuttings at 0.1mg/mL. In all phytotoxic assays only jasmonic acid was found to be active. All metabolites were inactive in the zootoxic assay at 50 μg/mL.

Emerging phytopathogen Macrophomina phaseolina: biology, economic importance and current diagnostic trends

Macrophomina phaseolina (Tassi) Goid. is an important phytopathogenic fungus, infecting a large number of plant species and surviving for up to 15 years in the soil as a saprophyte. Although considerable research related to the biology and ecology of Macrophomina has been conducted, it continues to cause huge economic losses in many crops. Research is needed to improve the identification and characterization of genetic variability within their epidemiological and pathological niches. Better understanding of the variability within the pathogen population for traits that influence fitness and soil survival will certainly lead to improved management strategies for Macrophomina. In this context, the present review discusses various biological aspects and distribution of M. phaseolina throughout the world and their importance to different plant species. Accurate identification of the fungus has been aided with the use of nucleic acid-based molecular techniques. The development of PCR-based methods for identification and detection of M. phaseolina are highly sensitive and specific. Early diagnosis and accurate detection of pathogens is an essential step in plant disease management as well as quarantine. The progress in the development of various molecular tools used for the detection, identification and characterization of Macrophomina isolates were also discussed.