Phytotoxic Secondary Metabolites Research Articles

Introgression of early shoot vigour in wheat modifies root systems, increases competitiveness and provides options for integrated weed management

Abstract Background and aims Weeds are a major biotic stressor impacting crop production. Improving the competitiveness of wheat (Triticum aestivum L.) could provide a useful tool in integrated weed management. While wheat typically exhibits conservative early growth, early vigour has been increased through long-term recurrent selection for greater early biomass and leaf area. However, the influence of integrating such vigour into breeding lines for improving competitive ability remains to be explored. Methods In replicated controlled environment experiments, the effect of breeding early shoot vigour on root development and below-ground competitiveness was carefully examined. Physical and chemical characteristics of wheat vigour lines were assessed and compared with commercial cultivars in hydroponics and field soil experiments. Measurements included early root growth, rhizosheath size and growth responses in the presence of annual ryegrass, a major weed in wheat production. Results The vigorous lines exhibited larger leaf widths, increased cell file number, increased total root length and larger rhizosheaths compared with commercial parents. Numerous secondary metabolites with known allelopathic effects on weeds were detected in the roots and the rhizosphere, and significant allelochemical level differences observed between distilled water and soil water extract-treated plants. Although the vigour lines were significantly more competitive than the commercial cultivars against ryegrass, they produced similar levels of phytotoxic secondary metabolites. Conclusion Competition below-ground was strongly suppressive of ryegrass for the more vigorous genotypes suggesting that breeding with shoot vigour had pleiotropic effects on key root traits for below-ground wheat competitiveness.

Isolation, Characterization, and Bioherbicidal Potential of the 16-Residue Peptaibols from Emericellopsis sp. XJ1056.

Eco-friendly bioherbicides are urgently needed for managing the problematic weed Amaranthus retroflexus. A mass spectrometry- and bioassay-guided screening approach was employed to identify phytotoxic secondary metabolites from fungi for the development of such bioherbicides. This effort led to the discovery of six phytotoxic 16-residue peptaibols, including five new compounds (2-6) and a known congener (1), from Emericellopsis sp. XJ1056. Their planar structures were elucidated through the analysis of tandem mass and NMR spectroscopic data. The absolute configurations of the chiral amino acids were determined by advanced Marfey's method and chiral-phase liquid chromatography-mass spectrometry (LC-MS) analysis. Bioinformatic analysis and targeted gene disruption identified the biosynthetic gene cluster for these peptaibols. Compounds 1 and 2 significantly inhibited the radicle growth of A. retroflexus seedlings, and 1 demonstrated potent postemergence herbicidal activity against A. retroflexus while exhibiting minimal toxicity to Sorghum bicolor. Structure-activity relationship analysis underscored the importance of trans-4-hydroxy-l-prolines at both the 10th and 13th positions for the herbicidal activities of these peptaibols.

Comparative Pangenomic Insights into the Distinct Evolution of Virulence Factors Among Grapevine Trunk Pathogens.

The permanent organs of grapevines (Vitis vinifera L.), like those of other woody perennials, are colonized by various unrelated pathogenic ascomycete fungi secreting cell wall-degrading enzymes and phytotoxic secondary metabolites that contribute to host damage and disease symptoms. Trunk pathogens differ in the symptoms they induce and the extent and speed of damage. Isolates of the same species often display a wide virulence range, even within the same vineyard. This study focuses on Eutypa lata, Neofusicoccum parvum, and Phaeoacremonium minimum, causal agents of Eutypa dieback, Botryosphaeria dieback, and Esca, respectively. We sequenced 50 isolates from viticulture regions worldwide and built nucleotide-level, reference-free pangenomes for each species. Through examination of genomic diversity and pangenome structure, we analyzed intraspecific conservation and variability of putative virulence factors, focusing on functions under positive selection and recent gene family dynamics of contraction and expansion. Our findings reveal contrasting distributions of putative virulence factors in the core, dispensable, and private genomes of each pangenome. For example, carbohydrate active enzymes (CAZymes) were prevalent in the core genomes of each pangenome, whereas biosynthetic gene clusters were prevalent in the dispensable genomes of E. lata and P. minimum. The dispensable fractions were also enriched in Gypsy transposable elements and virulence factors under positive selection (polyketide synthase genes in E. lata and P. minimum, glycosyltransferases in N. parvum). Our findings underscore the complexity of the genomic architecture in each species and provide insights into their adaptive strategies, enhancing our understanding of the underlying mechanisms of virulence. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Biosynthesis of Rubellins in Ramularia collo-cygni-Genetic Basis and Pathway Proposition.

The important disease Ramularia leaf spot of barley is caused by the fungus Ramularia collo-cygni. The disease causes yield and quality losses as a result of a decrease in photosynthesis efficiency due to the appearance of necrotic spots on the leaf surface. The development of these typical Ramularia leaf spot symptoms is thought to be linked with the release of phytotoxic secondary metabolites called rubellins in the host. However, to date, neither the biosynthetic pathways leading to the production of these metabolites nor their exact role in disease development are known. Using a combined in silico genetic and biochemistry approach, we interrogated the genome of R. collo-cygni to identify a putative rubellin biosynthetic gene cluster. Here we report the identification of a gene cluster containing homologues of genes involved in the biosynthesis of related anthraquinone metabolites in closely related fungi. A putative pathway to rubellin biosynthesis involving the genes located on the candidate cluster is also proposed.

Secondary Metabolites Produced by Xylaria necrophora Are Responsible for Foliar Symptoms Associated with Taproot Decline of Soybean

Taproot decline (TRD) of soybean ( Glycine max [L] Merr.) is an emerging disease caused by Xylaria necrophora and currently observed in the southern United States. X. necrophora infects soybean roots, causing necrosis and foliar interveinal chlorosis followed by necrosis. The most recent estimates of yield losses associated with this disease (0.57 to 1.18 million bushels) highlight its potential to become a major problem for producers in the region. The mechanism by which X. necrophora affects soybean foliage remains unknown, but a plausible explanation is that phytotoxic secondary metabolites (SMs) are produced by X. necrophora in the roots. To test this hypothesis, cell-free culture filtrates (CFCFs) from three X. necrophora strains and one putative Colletotrichum siamense isolate were used to challenge soybean stem cuttings to determine if foliar symptoms of TRD are caused by SMs, tolerance to SMs among potentially resistant soybean cultivars identified in greenhouse and field experiments, and specificity of SMs across plant species (cotton, peanut, tomato, and soybean). Measurements of chlorophyll content and root length were obtained at 7 and 14 days of exposure. Lower chlorophyll content and shorter roots were observed on all soybean cultivars treated with CFCFs of pathogenic isolates, and no effects on other plant species were observed, suggesting that X. necrophora produces SMs that are detrimental to soybean exclusively and resistance upon direct exposure to SMs does not exist. These results suggest that soybean-specific SMs are produced by X. necrophora in the roots, causing the interveinal chlorosis and subsequent necrosis observed on plants affected by TRD.

Impact of temperature variation on the phytotoxic secondary metabolite production by Lasiodiplodia theobromae

AbstractBotryosphaeriaceae species are associated with grapevine trunk diseases. Lasiodiplodia theobromae plays a significant role as one of the most virulent grapevine pathogens in the world. In this study, we evaluate the effects of temperature on phytotoxin‐induced virulence in Lasiodiplodia theobromae. The fungal strain is grown in Czapek‐Dox liquid medium at four different temperatures (20°C, 25°C, 30°C and 35°C) for 14 days under dark conditions. The culture filtrates were screened for three selected phytotoxins, indole‐3‐carboxylic acid, jasmonic acid, and lasiodiplodin using UPLC MS/MS techniques. Phytotoxicity of the crude culture filtrates was tested on detached grapevine leaves. With UPLC MS/MS analysis, we identified that indole‐3‐carboxylic acid and jasmonic acid production varied with the increasing temperature, and did not detect any lasiodiplodin. At high temperatures, we observed a significant increase in the phytotoxicity of crude culture filtrates on the detached grapevine leaves. The most severe necrotic leaf lesions were observed when inoculated with crude culture filtrates extracted at 35°C.

Phytotoxic Secondary Metabolites from Fungi.

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

Identification and characterization of Cercospora beticola necrosis-inducing effector CbNip1.

Cercospora beticola is a hemibiotrophic fungus that causes cercospora leaf spot disease of sugar beet (Beta vulgaris). After an initial symptomless biotrophic phase of colonization, necrotic lesions appear on host leaves as the fungus switches to a necrotrophic lifestyle. The phytotoxic secondary metabolite cercosporin has been shown to facilitate fungal virulence for several Cercospora spp. However, because cercosporin production and subsequent cercosporin‐initiated formation of reactive oxygen species is light‐dependent, cell death evocation by this toxin is only fully ensured during a period of light. Here, we report the discovery of the effector protein CbNip1 secreted by C. beticola that causes enhanced necrosis in the absence of light and, therefore, may complement light‐dependent necrosis formation by cercosporin. Infiltration of CbNip1 protein into sugar beet leaves revealed that darkness is essential for full CbNip1‐triggered necrosis, as light exposure delayed CbNip1‐triggered host cell death. Gene expression analysis during host infection shows that CbNip1 expression is correlated with symptom development in planta. Targeted gene replacement of CbNip1 leads to a significant reduction in virulence, indicating the importance of CbNip1 during colonization. Analysis of 89 C. beticola genomes revealed that CbNip1 resides in a region that recently underwent a selective sweep, suggesting selection pressure exists to maintain a beneficial variant of the gene. Taken together, CbNip1 is a crucial effector during the C. beticola–sugar beet disease process.

Morphological and phytochemical analysis of leaves and stems of Tetrapterys multiglandulosa CAV.

This study aimed to carry out morphoanatomical, histochemical and phytochemical analysis of leaves and stems of Tetrapterys multiglandulosa Cav. (Malpighiaceae), a native plant responsible for abortion and sudden death in beef cattle. Plant specimens underwent anatomical study, in which leaves and stems were dried and ground with aqueous and ethanol extracts subjected to histochemical and phytochemical analysis. Anatomical observation of vegetative organs detected the presence of simple columnar cells in the adaxial epidermis, palisade mesophyll, parenchyma with a layer of cells and paracitic type stomata. Stem cross-sections presented circular section, single layer (uniseriate) epidermis with thick cuticle and Malpighi’s trichomes, the phelogen was in differentiation. The cortex showed angular collenchyma, parenchyma with exogenous channels and druse shaped calcium oxalate crystals. Phytochemical tests of aqueous and ethanol extracts of leaves and stems indicated the presence of phenolic compounds, tannins, flavonoids, saponins, alkaloids and cardiotonic heterosides. Potentially phytotoxic secondary metabolites evidenced in the leaves and stems, including saponins and cardiotonic heterosides were correlated with clinical signs observed in intoxicated animals.

Rapid Evolution of Plant-Bacterium Mutualism in the Rhizosphere

Plant roots are associated with a diverse microbial community that consists of antagonistic, commensal and mutualistic organisms. While beneficial plant-microbe interactions are important for plant performance, direct evidence for the evolution of rhizosphere mutualism remains elusive. Here we experimentally show that initially plant-antagonistic Pseudomonas protegens bacterium can rapidly evolve into a mutualist in the rhizosphere of Arabidopsis thaliana only within six plant growth cycles (6 months). Using an in vivo experimental evolution system, we observed that P. protegens rapidly diversified into distinct phenotypes, with some having clearly positive effects on the plant performance. Crucially, this evolution of plant facilitation was also accompanied with increased bacterial fitness indicative of a mutualistic interaction. At the genetic level, P. protegens mutualism was associated with mutations in the GacS/GacA two-component global regulator system, which is known to affect several aspects of bacterial physiology including carbon metabolism, host-microbe signalling, and downregulation of phytotoxic secondary metabolites. At the phenotypic level, the evolution of mutualism could be explained via two mechanisms: improved competition for root exudates and enhanced capacity for activating the root-specific transcription factor gene MYB72, which is essential for the production and excretion of the antimicrobial scopoletin for which the mutualists evolved enhanced tolerance. Together, these results show that plant-bacteria mutualism can rapidly evolve at an agriculturally relevant evolutionary timescale in the rhizosphere.

Gramillin A and B: Cyclic Lipopeptides Identified as the Nonribosomal Biosynthetic Products of Fusarium graminearum.

The virulence and broad host range of Fusarium graminearum is associated with its ability to secrete an arsenal of phytotoxic secondary metabolites, including the regulated mycotoxins belonging to the deoxynivalenol family. The TRI genes responsible for the biosynthesis of deoxynivalenol and related compounds are usually expressed during fungal infection. However, the F. graminearum genome harbors an array of unexplored biosynthetic gene clusters that are also co-induced with the TRI genes, including the nonribosomal peptide synthetase 8 ( NRPS8) gene cluster. Here, we identify two bicyclic lipopeptides, gramillin A (1) and B (2), as the biosynthetic end products of NRPS8. Structural elucidation by high-resolution LC-MS and NMR, including 1H-15N-13C HNCO and HNCA on isotopically enriched compounds, revealed that the gramillins possess a fused bicyclic structure with ring closure of the main peptide macrocycle occurring via an anhydride bond. Through targeted gene disruption, we characterized the GRA1 biosynthetic gene and its transcription factor GRA2 in the NRPS8 gene cluster. Further, we show that the gramillins are produced in planta on maize silks, promoting fungal virulence on maize but have no discernible effect on wheat head infection. Leaf infiltration of the gramillins induces cell death in maize, but not in wheat. Our results show that F. graminearum deploys the gramillins as a virulence agent in maize, but not in wheat, thus displaying host-specific adaptation.

Phytotoxic Secondary Metabolites from the Endolichenic Fungus Myxotrichum sp.

One new macrolide myxotrilactone A (1), along with six known compounds (2–7), were isolated from the extract of solid-substrate cultures of the endolichenic fungus Myxotrichum sp. by various chromatography methods. The planar structure of compound 1 was determined by HR-ESI-MS and NMR data, and its relative and absolute configurations were established by NOESY experiment and comparison with the known compound 2. Selected isolates were evaluated for their allelopathic activities, and compounds 1 and 2 significantly retarded shoot elongation of Arabidopsis thaliana.

Aldaulactone - An Original Phytotoxic Secondary Metabolite Involved in the Aggressiveness of Alternaria dauci on Carrot.

Qualitative plant resistance mechanisms and pathogen virulence have been extensively studied since the formulation of the gene-for-gene hypothesis. The mechanisms involved in the quantitative traits of aggressiveness and plant partial resistance are less well-known. Nevertheless, they are prevalent in most plant-necrotrophic pathogen interactions, including the Daucus carota–Alternaria dauci interaction. Phytotoxic metabolite production by the pathogen plays a key role in aggressiveness in these interactions. The aim of the present study was to explore the link between A. dauci aggressiveness and toxin production. We challenged carrot embryogenic cell cultures from a susceptible genotype (H1) and two partially resistant genotypes (I2 and K3) with exudates from A. dauci strains with various aggressiveness levels. Interestingly, A. dauci-resistant carrot genotypes were only affected by exudates from the most aggressive strain in our study (ITA002). Our results highlight a positive link between A. dauci aggressiveness and the fungal exudate cell toxicity. We hypothesize that the fungal exudate toxicity was linked with the amount of toxic compounds produced by the fungus. Interestingly, organic exudate production by the fungus was correlated with aggressiveness. Hence, we further analyzed the fungal organic extract using HPLC, and correlations between the observed peak intensities and fungal aggressiveness were measured. One observed peak was closely correlated with fungal aggressiveness. We succeeded in purifying this peak and NMR analysis revealed that the purified compound was a novel 10-membered benzenediol lactone, a polyketid that we named ‘aldaulactone’. We used a new automated image analysis method and found that aldaulactone was toxic to in vitro cultured plant cells at those concentrations. The effects of both aldaulactone and fungal organic extracts were weaker on I2-resistant carrot cells compared to H1 carrot cells. Taken together, our results suggest that: (i) aldaulactone is a new phytotoxin, (ii) there is a relationship between the amount of aldaulactone produced and fungal aggressiveness, and (iii) carrot resistance to A. dauci involves mechanisms of resistance to aldaulactone.

Fusarium fujikuroi : A novel source of metabolites with herbicidal activity

Abstract This study presents the isolation and identification of a fungus (Fusarium fujikuroi) from the Brazilian Pampa biome that is able to produce phytotoxic secondary metabolites with herbicidal activity. Before selecting such promising fungus, 132 microorganisms were isolated from weed plants with infections symptoms. Each sample of isolated material was used in submerged fermentation to produce a fermented broth. Thereafter, the cells were separated by centrifugation and the supernatant was filtered. Each filtered sample (culture filtrate) was used to evaluate the herbicidal activity in the bioassays with two target plants: Cucumis sativus and Sorghum bicolor. The selection criteria consisted of analyzing germination in pre-emergence, phytotoxicity, plant height and root length in post-emergence, and lesions in detached leaf-punctured assay. The pre-emergence was the primary screening, where 11 samples of culture filtrate provided 100% inhibition of germination of both plants. These samples underwent a secondary screening, which the sample SO210 presented 25% phytotoxicity, 40% reduction of plant height, 28% reduction of root length, and necrosis and chlorosis in 70% of foliar area. The microorganism coded as SO210 was identified as a strain of Fusarium fujikuroi. Up to now, studies that attribute herbicidal activity to this fungus are scarce in the literature. Therefore, this work shows a potential activity of metabolites produced by this fungus toward the inhibition of germination and development of plant weeds.

Effect of phytotoxic secondary metabolites and semisynthetic compounds from endophytic fungus Xylaria feejeensis strain SM3e-1b on spinach chloroplast photosynthesis

We investigated the mechanism of action on the photosynthesis light reactions of three major secondary metabolites produced by the endophytic fungus Xylaria feejeensis strain SM3e-1b, isolated from Sapium macrocarpum; and four novel derivatives of coriloxine, a major compound produced by X. feejeensis. The natural phytotoxins include one epoxycyclohexenone derivative, coriloxine (1), and two quinone derivatives (2–3). The semisynthetic derivatives of coriloxine are two cyclohexenone (4–6) and two quinone compounds (5–7). Cyclohexenone (4), (4R,5S,6R)-6-chloro-4,5-dihydroxy-3-methoxy-5-methylcyclohex-2-enone, inhibited ATP synthesis in freshly lysed spinach chloroplasts from water to MV; it also partly inhibited the basal and uncoupled photosynthetic electron transport, and significantly enhanced the phosphorylating electron transport and Mg2+-ATPase activity, thus demonstrating its action as an uncoupler agent. On the other hand, quinone (7), 2-((4-butylphenyl)amino)-5-methoxy-3-methylcyclohexa-2,5-diene-1,4-dione, inhibited ATP synthesis, and non-cyclic electron transport from water to MV in basal, phosphorylating and uncoupled conditions in a concentration-dependent manner. Hence, (7) behaves as a Hill reaction inhibitor at the PSII electron transport on the water splitting enzyme (OEC), and on the acceptor side between P680 and QA. This mechanism of action was confirmed by chlorophyll a fluorescence measurements. These results indicate that coriloxine derivatives 4 and 7 could work as prototype structures for the development of new herbicides. Contrastingly, natural products 1–3, and derivatives 5 and 6 did not show a significant inhibitory effect on ATP synthesis.

Seasonality affects phytotoxic potential of five native species of Neotropical savanna

Phytotoxic secondary metabolites produced by plants have been studied as possible sources of bioherbicides. However, several environmental factors can change the secondary metabolism of plants, and thus, the production of these compounds. Among these factors, seasonality can be a source of variation in the content of all classes of secondary metabolites. In this study, we evaluated the phytotoxic effect of aqueous extracts of leaves from Byrsonima intermedia A. Juss. (Malpighiaceae), Gochnatia polymorpha (Less.) Cabrera (Asteraceae), Luehea candicans Mart. (Tiliaceae), Miconia chamissois Naudin (Melastomataceae), and Qualea cordata Spreng (Vochysiaceae) (species of the Brazilian savanna), collected both during the dry and the rainy season, on germination and seedling growth of maize and cucumber. The analysed parameters were affected by all leaf extracts collected during both seasons. However, a greater phytotoxic effect was observed when leaves were collected during the dry or during the rainy season, depending on the savanna species studied, on the target species, and on the parameters evaluated. The phytochemical screening of all extracts was also evaluated and allowed the identification of anthraquinones, triterpenoids, saponins, and tannins. The composition of extracts differed between the seasons for all species. This study highlights the importance of considering savanna seasonality when studying the phytotoxicity of the species of this biome.

Impact of macrocidins, produced by Phoma macrostoma, on carotenoid profiles of plants

Phoma macrostoma (Montagne) is a bioherbicide that controls broadleaf weeds such as Canada thistle (Cirsium arvense L. (Scop.)) and dandelion (Taraxacum officinale Weber ex F.H. Wigg.), while leaving crop plants, such as wheat (Triticum aestivum L.) and pumpkin (Cucurbita L. spp), unharmed. The bioherbicidal activity of P. macrostoma is largely due to the production of phytotoxic secondary metabolites, macrocidins. While macrocidins produce photobleaching symptoms in susceptible plants, the mechanism(s) by which this occurs are not known. The hypothesis that macrocidins inhibit carotenoid biosynthesis was tested by comparing the carotenoid profiles and other physiological parameters of macrocidin-treated Canada thistle, dandelion, pumpkin and wheat with control plants and plants treated with the known carotenoid biosynthesis inhibitor, diflufenican. Only in susceptible plants, macrocidins induced photobleaching symptoms, lowered total chlorophyll content and reduced photosynthetic gas exchange while increasing the percentage of the carotenoid precursor phytoene. While diflufenican also led to chlorosis in dandelion and thistles, its impacts differed from those of macrocidins in that diflufenican-treated dandelion, thistle and pumpkin had increased total carotenoid and carotenoid precursor content compared to the control. Diflufenican-treated dandelion and thistle contained 88±3% and 94±2% phytoene, respectively. In contrast, macrocidins-treated dandelion and thistle (at the higher dose) contained 54±12% and 80±3% phytoene, respectively, while the controls contained only 4±0.5% and 10±1%. The β-carotene to lutein ratio in diflufenican-treated dandelion or thistles did not differ from that of the control, while this ratio was reduced in thistles treated with the higher dose of macrocidins. This paper provides the first proof that macrocidins impact the carotenoid biosynthesis by partially inhibiting the carotenoid biosynthetic enzyme phytoene desaturase (PDS) and at one or more other steps in carotenoid biogenesis.

Characterization of the Coronatine-Like Phytotoxins Produced by the Common Scab Pathogen Streptomyces scabies.

Streptomyces scabies is an important causative agent of common scab disease of potato tubers and other root crops. The primary virulence factor produced by this pathogen is a phytotoxic secondary metabolite called thaxtomin A, which is essential for disease development. In addition, the genome of S. scabies harbors a virulence-associated biosynthetic gene cluster called the coronafacic acid (CFA)-like gene cluster, which was previously predicted to produce metabolites that resemble the Pseudomonas syringae coronatine (COR) phytotoxin. COR consists of CFA linked to an ethylcyclopropyl amino acid called coronamic acid, which is derived from L-allo-isoleucine. Using a combination of genetic and chemical analyses, we show that the S. scabies CFA-like gene cluster is responsible for producing CFA-L-isoleucine as the major product as well as other minor COR-like metabolites. Production of the metabolites was shown to require the cfl gene, which is located within the CFA-like gene cluster and encodes an enzyme involved in ligating CFA to its amino acid partner. CFA-L-isoleucine purified from S. scabies cultures was shown to exhibit bioactivity similar to that of COR, though it was found to be less toxic than COR. This is the first report demonstrating the production of coronafacoyl phytotoxins by S. scabies, which is the most prevalent scab-causing pathogen in North America.

Regulation of coronafacoyl phytotoxin production by the PAS-LuxR family regulator CfaR in the common scab pathogen Streptomyces scabies.

Potato common scab is an economically important crop disease that is characterized by the formation of superficial, raised or pitted lesions on the potato tuber surface. The most widely distributed causative agent of the disease is Streptomyces scabies, which produces the phytotoxic secondary metabolite thaxtomin A that serves as a key virulence factor for the organism. Recently, it was demonstrated that S. scabies can also produce the phytotoxic secondary metabolite coronafacoyl-L-isoleucine (CFA-L-Ile) as well as other related metabolites in minor amounts. The expression of the biosynthetic genes for CFA-L-Ile production is dependent on a PAS-LuxR family transcriptional regulator, CfaR, which is encoded within the phytotoxin biosynthetic gene cluster in S. scabies. In this study, we show that CfaR activates coronafacoyl phytotoxin production by binding to a single site located immediately upstream of the putative -35 hexanucleotide box within the promoter region for the biosynthetic genes. The binding activity of CfaR was shown to require both the LuxR and PAS domains, the latter of which is involved in protein homodimer formation. We also show that CFA-L-Ile production is greatly enhanced in S. scabies by overexpression of both cfaR and a downstream co-transcribed gene, orf1. Our results provide important insight into the regulation of coronafacoyl phytotoxin production, which is thought to contribute to the virulence phenotype of S. scabies. Furthermore, we provide evidence that CfaR is a novel member of the PAS-LuxR family of regulators, members of which are widely distributed among actinomycete bacteria.

Characterization of amygdalin-degrading Lactobacillus species.

Cyanogenic glycosides are phytotoxic secondary metabolites produced by some crop plants. The aim of this study was to identify lactic acid bacteria (LAB) capable of catabolizing amygdalin, a model cyanogenic glycoside, for use in the biodetoxification of amygdalin-containing foods and feeds. Amygdalin-catabolizing lactobacilli were characterized using a combination of cultivation-dependent and molecular assays. Lactobacillus paraplantarum and Lactobacillus plantarum grew robustly on amygdalin (Amg(+)), while other LAB species typically failed to catabolize amygdalin (Amg(-)). Interestingly, high concentrations of amygdalin and two of its metabolic derivatives (mandelonitrile and benzaldehyde) inhibited the growth of Lact. plantarum RENO 0093. The differential regulation of genes tentatively involved in cyanohydrin metabolism illustrated that the metabolism of amygdalin- and glucose-grown cultures also differed significantly. Amygdalin fermentation was a relatively uncommon phenotype among the LAB and generally limited to strains from the Lact. plantarum group. Phenotype microarrays (PM) enabled strain-level discrimination between closely related strains within a species and suggested that phenotypic differences might affect niche specialization. Amygdalin-degrading lactobacilli with practical application in the biodetoxification of amygdalin were characterized. These strains show potential for use as starter cultures to improve the safety of foods and feeds.