Metachelins, mannosylated and N-oxidized coprogen-type siderophores from Metarhizium robertsii

Genome comparisons reveal accessory genes crucial for the evolution of apple Glomerella leaf spot pathogenicity in Colletotrichum fungi

Apple Glomerella leaf spot (GLS) is an emerging fungal disease caused by Colletotrichum fructicola and other Colletotrichum species. These species are polyphyletic and it is currently unknown how these pathogens convergently evolved to infect apple. We generated chromosome-level genome assemblies of a GLS-adapted isolate and a non-adapted isolate in C. fructicola using long-read sequencing. Additionally, we resequenced 17 C. fructicola and C. aenigma isolates varying in GLS pathogenicity using short-read sequencing. Genome comparisons revealed a conserved bipartite genome architecture involving minichromosomes (accessory chromosomes) shared by C. fructicola and other closely related species within the C. gloeosporioides species complex. Moreover, two repeat-rich genomic regions (1.61 Mb in total) were specifically conserved among GLS-pathogenic isolates in C. fructicola and C. aenigma. Single-gene deletion of 10 accessory genes within the GLS-specific regions of C. fructicola identified three that were essential for GLS pathogenicity. These genes encoded a putative non-ribosomal peptide synthetase, a flavin-binding monooxygenase and a small protein with unknown function. These results highlight the crucial role accessory genes play in the evolution of Colletotrichum pathogenicity and imply the significance of an unidentified secondary metabolite in GLS pathogenesis.

Inventory of the Secondary Metabolite Biosynthetic Potential of Members within the Terminal Clade of the Fusarium solani Species Complex

The Fusarium solani species complex (FSSC) constitutes at least 77 phylogenetically distinct species including several agriculturally important and clinically relevant opportunistic pathogens. As with other Fusaria, they have been well documented to produce many secondary metabolites-compounds that are not required for the fungus to grow or develop but may be beneficial to the organism. An analysis of ten genomes from fungi within the terminal clade (clade 3) of the FSSC revealed each genome encoded 35 (F. cucurbitcola) to 48 (F. tenucristatum) secondary metabolite biosynthetic gene clusters (BGCs). A total of seventy-four different BGCs were identified from the ten FSSC genomes including seven polyketide synthases (PKS), thirteen nonribosomal peptide synthetases (NRPS), two terpene synthase BGCs, and a single dimethylallytryptophan synthase (DMATS) BGC conserved in all the genomes. Some of the clusters that were shared included those responsible for producing naphthoquinones such as fusarubins, a red pigmented compound, squalestatin, and the siderophores malonichrome, ferricrocin, and triacetylfusarinine. Eight novel NRPS and five novel PKS BGCs were identified, while BGCs predicted to produce radicicol, gibberellin, and fusaoctaxin were identified, which have not previously described in members of the FSSC. The diversity of the secondary metabolite repertoire of the FSSC may contribute to the expansive host range of these fungi and their ability to colonize broad habitats.

Entomopathogenic Fungi-Mediated Solubilization and Induction of Fe Related Genes in Melon and Cucumber Plants

Endophytic insect pathogenic fungi have a multifunctional lifestyle; in addition to its well-known function as biocontrol agents, it may also help plants respond to other biotic and abiotic stresses, such as iron (Fe) deficiency. This study explores M. brunneum EAMa 01/58-Su strain attributes for Fe acquisition. Firstly, direct attributes include siderophore exudation (in vitro assay) and Fe content in shoots and in the substrate (in vivo assay) were evaluated for three strains of Beauveria bassiana and Metarhizium bruneum. The M. brunneum EAMa 01/58-Su strain showed a great ability to exudate iron siderophores (58.4% surface siderophores exudation) and provided higher Fe content in both dry matter and substrate compared to the control and was therefore selected for further research to unravel the possible induction of Fe deficiency responses, Ferric Reductase Activity (FRA), and relative expression of Fe acquisition genes by qRT-PCR in melon and cucumber plants.. In addition, root priming by M. brunneum EAMa 01/58-Su strain elicited Fe deficiency responses at transcriptional level. Our results show an early up-regulation (24, 48 or 72 h post inoculation) of the Fe acquisition genes FRO1, FRO2, IRT1, HA1, and FIT as well as the FRA. These results highlight the mechanisms involved in the Fe acquisition as mediated by IPF M. brunneum EAMa 01/58-Su strain.

Inductive Production of the Iron-Chelating 2-Pyridones Benefits the Producing Fungus To Compete for Diverse Niches

Diverse 2-pyridone alkaloids have been identified with an array of biological and pharmaceutical activities, including the development of drugs. However, the biosynthetic regulation and chemical ecology of 2-pyridones remain largely elusive. Here, we report the inductive activation of the silent polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) (tenS) gene cluster for the biosynthesis of the tenellin-type 2-pyridones in the insect-pathogenic fungus Beauveria bassiana when cocultured with its natural competitor fungus Metarhizium robertsii. A pathway-specific transcription factor, tenR, was identified, and the overexpression of tenR well expanded the biosynthetic mechanism of 15-hydroxytenellin (15-HT) and its derivatives. In particular, a tandemly linked glycosyltransferase-methyltransferase gene pair located outside the tenS gene cluster was verified to mediate the rare and site-specific methylglucosylation of 15-HT at its N-OH residue. It was evident that both tenellin and 15-HT can chelate iron, which could benefit B. bassiana to outcompete M. robertsii in cocultures and to adapt to iron-replete and -depleted conditions. Relative to the wild-type strain, the deletion of tenS had no obvious negative effect on fungal virulence, but the overexpression of tenR could substantially increase fungal pathogenicity toward insect hosts. The results of this study well advance the understanding of the biosynthetic machinery and chemical ecology of 2-pyridones.

Transcriptional Differences Guided Discovery and Genetic Identification of Coprogen and Dimerumic Acid Siderophores in Metarhizium robertsii

Siderophores are small molecular iron chelators and participate in the multiple cellular processes in fungi. In this study, biosynthesis gene clusters of coprogens and dimerumic acids were identified by transcriptional level differences of genes related to iron deficiency conditions in Metarhizium robertsii. This leads to the characterization of new coprogen metachelin C (1) and five known siderophores metachelin A (2), metachelin A-CE (3), metachelin B (4), dimerumic acid 11-mannoside (5), and dimerumic acid (6). The structure of metachelin C (1) was elucidated by NMR spectroscopy and HR-ESI-MS analysis. Genetic deletions of mrsidA, and mrsidD abolished the production of compounds 1–6 that implied their involvement in the biosynthesis of coprogen and dimerumic acid. Interestingly, NRPS gene mrsidD is responsible for biosynthesis of both coprogen and dimerumic acid, thus we proposed plausible biosynthetic pathways for the synthesis of coprogen and dimerumic acid siderophores. Therefore, our study provides the genetic basis for understanding the biosynthetic pathway of coprogen and dimerumic acid in Metarhizium robertsii.

Discovery and genetic identification of amphiphilic coprogen siderophores from Trichoderm hypoxylon

Siderophores are small molecular iron chelators and participate in the multiple cellular processes in fungi. In this study, we discovered and identified five amphiphilic coprogen siderophores including three new natural products according to LC-MS-guided separation strategy from Trichoderm hypoxylon. The structures of three new coprogens were elucidated by NMR spectroscopy, and high-resolution (HR)-ESI-MS analysis. Genetic deletions of dfcA and dfcB abolished the production of compounds 1-5 that implied their involvement in the biosynthesis of coprogens. Interestingly, cultivations of ΔdfcA and ΔdfcB mutants with pathogenic fungi Fusarium oxysporum and Mucor corcinelloides showed the weaker inhibitions in comparison to wild type that demonstrated coprogen's role in combating the pathogenic fungi. Our study not only enriched the diversities of siderophores but also provided an approach for finding the rare amphiphilic coprogen siderophores in fungi. Furthermore, this work provided a basis for investigation on the biosynthesis of fungal amphiphilic siderophores and their ecological roles in nature. KEY POINTS: • A series of amphiphilic coprogens were found. • The gene cluster of amphiphilic coprogens and ecological roles were elucidated.

A new iron(III) chelator of coprogen-type siderophore from the deep-sea-derived fungus Mycosphaerella sp. SCSIO z059

Mycosphazine A (1), a new iron(III) chelator of coprogen-type siderophore, and mycosphamide A (2), a new cyclic amide benzoate, together with six known aryl amides (3-8), were isolated from the fermentation broth of the deep-sea-derived fungus Mycosphaerella sp. SCSIO z059. Alkaline hydrolysis of 1 afforded a new epimer of dimerum acid, mycosphazine B (1a), and a new bi-fusarinine-type siderophore, mycosphazine C (1b). The planar structures of the new compounds were elucidated by extensive spectroscopic data analysis. The absolute configurations of amino acid residues in 1a and 1b were determined by acid hydrolysis. And the absolute configuration of 2 was established by quantum chemical calculations of the electronic circular dichroism (ECD) spectra. Compound 1 is the first siderophore-Fe(III) chelator incorporating both L-ornithine and D-ornithine unites. Compounds 3-8 were reported as natural products for the first time, and the ¹H and ¹³C NMR data of 6 and 8 were assigned for the first time. Compounds 1 and 1a could greatly promote the biofilm formation of bacterium Bacillus amyloliquefaciens with the rate of about 249% and 524% at concentration of 100 μg·mL^-1, respectively.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

Beauveria bassiana Multifunction as an Endophyte: Growth Promotion and Biologic Control of Trialeurodes vaporariorum, (Westwood) (Hemiptera: Aleyrodidae) in Tomato

Simple Summary

The tomato, Solanum lycopersicum L. has great importance worldwide for its nutritional characteristics and its antioxidant content. It is cultivated in different geographical areas, under field and greenhouse conditions, and it can be subjected to abiotic and abiotic factors that negatively affect production and quality. In this study, we evaluated the effect of five native endophytic strains of Beauveria bassiana on the reproduction of greenhouse whiteflies and the growth of tomatoes. The endophyte was inoculated in the substrate, and plants were afterward exposed to adult populations of the insect. For plant-growth promoter activity, the effect of endophytic strains on phosphate solubilization, iron siderophores production, plant height, and biomass was determined. The RGM-557 strain reduced the number of eggs and nymphs per cm² on leaflet by 66 and 65%, respectively, compared with the control (water); while in comparison with the chemical insecticide the reduction was 32 and 55%, respectively. Most strains showed some degree of phosphate solubilization and siderophores production. Plants inoculated with strains RGM-557 and RGM-731 produced the greatest plant heights; RGM-731 obtained the greatest plant biomass. Endophytic B. bassiana provide important protection levels against whiteflies in tomato—in addition to promoting their growth.

Abstract

The tomato, Solanum lycopersicum L. is one of the most consumed vegetables in the world; nevertheless, it is affected by biotic and abiotic factors that reduce its productivity. The whitefly is globally considered as the main pest under protected crop conditions, where biologic control using endophytic fungi emerges as a sustainable alternative. We evaluated the indirect effects of five native endophytic strains of Beauveria bassiana on the reproduction of greenhouse whiteflies and the growth of tomatoes. The plant growth substrate was inoculated with five strains of this endophyte and the resulting plants were then exposed to whiteflies afterwards. The effect that endophytic strains had on phosphate solubilization, iron siderophore production, plant height, and plant biomass were evaluated. The evaluated endophytes reduced the number of eggs per cm² on leaflets compared to the control and behaved similarly to the commercial synthetic insecticide. Leaflets inoculated with strains RGM-557, RGM-644 and RGM-731 showed fewer nymphs than the control and those treated with insecticide. RGM-557 and RGM-731 produced the greatest plant heights; RGM-731 obtained the greatest plant biomass. Our study provides evidence that native endophytic strains of B. bassiana have a biocontrol effect on whiteflies and could be used to promote tomato growth.

Xyloneside A: A New Glycosylated Incisterol Derivative from Xylaria sp. FB

Xylaria species are prolific natural product producers. Here, we report the characterization of a new glycosylated incisterol derivative, called xyloneside A (1) and two known lignans (2 and 3) from the ascomycetous Xylaria sp. FB. The structure of xyloneside A (1) was determined by 1D and 2D NMR spectroscopy, high-resolution electrospray ionization mass spectrometry and electronic circular dichroism measurements. Xyloneside A is composed of a 1,2,3,4,5,10,19-heptanorergosterane skeleton and a β-D-mannopyranose moiety. This is the first report of an incisterol derivative from an Ascomycete. The biological effects of the isolated metabolites on cytotoxicity, autophagy, cell-migration, and angiogenesis were evaluated.

Fungal isolate and crop cultivar influence the beneficial effects of root inoculation with entomopathogenic fungi in strawberry

BACKGROUND

Root inoculations of crop plants with beneficial fungi constitute a promising strategy for growth promotion and control of above-ground pests and diseases. Here, strawberry roots (cultivar 'Albion' and 'Pircinque') were inoculated with 25 different Brazilian entomopathogenic fungal isolates of three genera and the effects on Tetranychus urticae oviposition and plant growth were evaluated in greenhouse experiments.

RESULTS

Reductions in the number of T. urticae eggs compared to control treatments were observed on both cultivars inoculated with almost all isolates. For the cultivar 'Albion', Metarhizium anisopliae (ESALQ 1604, ESALQ 1669), M. robertsii (ESALQ 1622, ESALQ 1635), Metarhizium sp. Indet. (ESALQ 1684) and Beauveria bassiana (ESALQ 3323) increased dry weight of roots and leaves, and fruit yield, while M. robertsii (ESALQ 1634), Metarhizium sp. Indet. (ESALQ 1637) and (ESALQ 1636) enhanced fruit yield and dry weight of leaves, respectively. For the cultivar 'Pircinque', M. anisopliae (ESALQ 1669) was the only isolate observed to increase dry weight of roots.

CONCLUSION

The results suggest that inoculation of strawberry roots with entomopathogenic fungi may be an innovative strategy for pest management above ground. Furthermore, these inoculations may also stimulate plant growth and strawberry production, but the effects depend on fungal strains and crop cultivar. © 2019 Society of Chemical Industry.

Siderophores from the Entomopathogenic Fungus Beauveria bassiana

We report NMR- and MS-based structural characterizations of siderophores and related compounds from Beauveria bassiana (Balsamo-Crivelli) Vuillemin, including ten new chemical entities (2-4, 6-9, 11-12, and 15) and five known compounds, (1, 5, 10, 13, and 14). The siderophore mixture from ARSEF strain #2680 included two compounds in which N⁵-mevalonyl-N⁵-hydroxyornithine replaces both (2) or one (3) of the N⁵-anhydromevalonyl-N⁵-hydroxyornithine units of dimerumic acid (1). Mevalonolactone (14) was present as a degradation product of 2 and 3. ARSEF #2860 also produced compounds that have mannopyranose (5, 6) or 4-O-methyl-mannopyranose units (4, 7), two compounds (8, 9) that can be rationalized as 4-O-methyl-mannopyranosyl analogues of the esterifying acid moieties of metachelins A and B, respectively, and two probable decomposition products of 1, a nitro compound (11) and a formate (12). Beauverichelin A (15), a coprogen-type siderophore that represents the di-4-O-methyl-mannopyranosyl analogue of metachelin A, was detected in crude extracts of ARSEF #2860, but only in trace amounts. ARSEF strains #252 and #1955 yielded beauverichelin A in quantities that were sufficient for NMR analysis. Only the di- (1-7) and trihydroxamate (15) siderophores showed iron-binding activity in the CAS assay and, when ferrated, showed strong ESIMS signals consistent with 1:1 ligand/iron complexes.

Determination of the molecular basis for coprogen import by Gram-negative bacteria

In order to survive in mixed microbial communities, some species of fungi secrete coprogens, siderophores that facilitate capture of the scarce nutrient iron. The TonB-dependent transporter FhuE is integrated in the outer membrane of Gram-negative bacteria and has been reported to scavenge these fungally produced coprogens. In this work, an Escherichia coli strain was engineered that is dependent solely on FhuE for its access to siderophore-sequestered iron. Using this tool, it is shown that while FhuE is highly active in the import of coprogens, it has some level of promiscuity, acting as a low-affinity transporter for related siderophores. The crystal structure of FhuE in complex with coprogen was determined, providing a structural basis to explain this selective promiscuity. The structural data, in combination with functional analysis, presented in this work show that FhuE has evolved to specifically engage with planar siderophores. A potential evolutionary driver, and a critical consequence of this selectivity, is that it allows FhuE to exclude antibiotics that mimic nonplanar hydroxamate siderophores: these toxic molecules could otherwise cross the outer membrane barrier through a Trojan horse mechanism.

Tobacco Growth Promotion by the Entomopathogenic Fungus, Isaria javanica pf185

Isaria javanica pf185 is an important entomopathogenic fungus with potential for use as an agricultural biocontrol agent. However, the effect of I. javanica pf185 on plant growth is unknown. Enhanced tobacco growth was observed when tobacco roots were exposed to spores, cultures, and fungal cell-free culture supernatants of this fungus. Tobacco seedlings were also exposed to the volatiles of I. javanica pf185 in vitro using I-plates in which the plant and fungus were growing in separate compartments connected only by air space. The length and weight of seedlings, content of leaf chlorophyll, and number of root branches were significantly increased by the fungal volatiles. Heptane, 3-hexanone, 2,4-dimethylhexane, and 2-nonanone were detected, by solid-phase micro-extraction and gas chromatography-mass spectrophotometry, as the key volatile compounds produced by I. javanica pf185. These findings illustrate that I. javanica pf185 can be used to promote plant growth, and also as a biocontrol agent of insect and plant diseases. Further studies are necessary to elucidate the mechanisms by which I. javanica pf185 promotes plant growth.

A Highly Conserved Basidiomycete Peptide Synthetase Produces a Trimeric Hydroxamate Siderophore

The model white-rot basidiomycete, Ceriporiopsis (Gelatoporia) subvermispora B, encodes putative natural product biosynthesis genes. Among them is the gene for the seven-domain nonribosomal peptide synthetase CsNPS2. It is a member of the as-yet-uncharacterized fungal type VI siderophore synthetase family, which is highly conserved and widely distributed among the basidiomycetes. These enzymes include only one adenylation (A) domain, i.e., one complete peptide synthetase module, and two thiolation/condensation (T-C) didomain partial modules which together constitute an AT₁C₁T₂C₂T₃C₃ domain setup. The full-length CsNPS2 enzyme (274.5 kDa) was heterologously produced as a polyhistidine fusion in Aspergillus niger as a soluble and active protein. N ⁵-acetyl-N ⁵-hydroxy-l-ornithine (l-AHO) and N ⁵-cis-anhydromevalonyl-N ⁵ -hydroxy-l-ornithine (l-AMHO) were accepted as the substrates, based on results of an in vitro substrate-dependent [³²P]ATP-pyrophosphate radioisotope exchange assay. Full-length holo-CsNPS2 catalyzed amide bond formation between three l-AHO molecules to release the linear l-AHO trimer, called basidioferrin, as the product in vitro, which was verified by liquid chromatography-high-resolution electrospray ionization-mass spectrometry analysis. Phylogenetic analyses suggested that type VI family siderophore synthetases are widespread in mushrooms and evolved in a common ancestor of basidiomycetes.IMPORTANCE The basidiomycete nonribosomal peptide synthetase CsNPS2 represents a member of a widely distributed but previously uninvestigated class (type VI) of fungal siderophore synthetases. Genes orthologous to CsNPS2 are highly conserved across various phylogenetic clades of the basidiomycetes. Hence, our work serves as a broadly applicable model for siderophore biosynthesis and iron metabolism in higher fungi. Also, our results on the amino acid substrate preference of CsNPS2 support a further understanding of the substrate selectivity of fungal adenylation domains. Methodologically, this report highlights the Aspergillus niger/SM-Xpress-based system as a suitable platform to heterologously express multimodular basidiomycete biosynthesis enzymes in the >250-kDa range in soluble and active form.

Entomopathogenic fungi-based mechanisms for improved Fe nutrition in sorghum plants grown on calcareous substrates

Although entomopathogenic fungi (EPF) are best known for their ability to protect crops against insect pests, they may have other beneficial effects on their host plants. These effects, which include promoting plant growth and conferring resistance against abiotic stresses, have been examined in recent years to acquire a better understanding of them. The primary purposes of the present study were (i) to ascertain in vitro whether three different strains of EPF (viz., Metarhizium, Beauveria and Isaria) would increase the Fe bioavailability in calcareous or non-calcareous media containing various Fe sources (ferrihydrite, hematite and goethite) and (ii) to assess the influence of the EPF inoculation method (seed dressing, soil treatment or leaf spraying) on the extent of the endophytic colonization of sorghum and the improvement in the Fe nutrition of pot-grown sorghum plants on an artificial calcareous substrate. All the EPFs studied were found to increase the Fe availability during the in vitro assay. The most efficient EPF was M. brunneum EAMa 01/58-Su, which lowered the pH of the calcareous medium, suggesting that it used a different strategy (organic acid release) than the other two fungi that raised the pH of the non-calcareous medium. The three methods used to inoculate sorghum plants with B. bassiana and M. brunneum in the pot experiment led to differences in re-isolation from plant tissues and in the plant height. These three inoculation methods increased the leaf chlorophyll content of young leaves when the Fe deficiency symptoms were most apparent in the control plants (without fungal inoculation) as well as the Fe content of the above-ground biomass in the plants at the end of the experiment. The total root lengths and fine roots were also increased in response to fungal applications with the three inoculation methods. However, the soil treatment was the most efficient method; thus, its effect on the leaf chlorophyll content was the most persistent, and the effects on the total root length and fine roots were the most apparent. In conclusion, EPF improved the Fe nutrition of the sorghum plants, but their effects depended on the inoculation method.

Targeted disruption of the polyketide synthase gene pks15 affects virulence against insects and phagocytic survival in the fungus Beauveria bassiana

The reducing clade III polyketide synthase genes, including pks15, are highly conserved among entomopathogenic fungi. To examine the function of pks15, we used targeted disruption to investigate the impact of Beauveria bassiana pks15 on insect pathogenesis. Southern analysis verified that the Δpks15 mutant was disrupted by a single integration of the transformation cassette at the pks15 locus. The Δpks15 mutant had a slight reduction in radial growth, and it produced fewer spores. Our insect bioassays indicated the Δpks15 mutant to be significantly reduced in virulence against beet armyworms compared to wild type (WT), which could be partially accounted for by its markedly decreased ability to survive phagocytosis. Total haemocyte count decreased sharply by 50-fold from days 1-3 post-inoculation in insects infected with WT, compared to a 5-fold decrease in the Δpks15 mutant. The mutant also produced fewer hemolymph hyphal bodies than WT by 3-fold. In co-culture studies with amoebae that have phagocytic ability similar to that of insect haemocytes, at 48 h the mortality rate of amoebae engulfing Δpks15 decreased by 72 %, and Δpks15 CFU decreased by 83 % compared to co-culture with WT. Thus, the Δpks15 mutant had a reduced ability to cope with phagocytosis and highly reduced virulence in an insect host. These data elucidate a mechanism of insect pathogenesis associated with polyketide biosynthesis.

Disruptions of the genes involved in lysine biosynthesis, iron acquisition, and secondary metabolisms affect virulence and fitness in Metarhizium robertsii

Based on genomic analysis, polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) pathways account for biosynthesis of the majority of the secondary metabolites produced by the entomopathogenic fungus Metarhizium robertsii. To evaluate the contribution of these pathways to M. robertsii fitness and/or virulence, mutants deleted for mrpptA, the Sfp-type 4' phosphopantetheinyl transferase gene required for their activation were generated. ΔmrpptA strains were deficient in PKS and NRPS activity resulting in colonies that lacked the typical green pigment and failed to produce the nonribosomal peptides (destruxins, serinocylins, and the siderophores ferricrocin and metachelins) as well as the hybrid polyketide-peptides (NG-39x) that are all produced by the wild type (WT) M. robertsii. The ΔmrpptA colonies were also auxotrophic for lysine. Two other mutant strains were generated: ΔmraarA, in which the α-aminoadipate reductase gene critical for lysine biosynthesis was disrupted, and ΔmrsidA, in which the L-ornithine N⁵-oxygenase gene that is critical for hydroxamate siderophore biosynthesis was disrupted. The phenotypes of these mutants were compared to those of ΔmrpptA to separate effects of the loss of lysine or siderophore production from the overall effect of losing all polyketide and non-ribosomal peptide production. Loss of lysine biosynthesis marginally increased resistance to H₂O₂ while it had little effect on the sensitivity to the cell wall disruptor sodium dodecyl sulfate (SDS) and no effect on sensitivity to iron deprivation. In contrast, combined loss of metachelin and ferricrocin through the inactivation of mrsidA resulted in mutants that were as hypersensitive or slightly more sensitive to H₂O₂, iron deprivation, and SDS, and were either identical or marginally higher in ΔmrpptA strains. In contrast to ΔmrpptA, loss of mrsidA did not completely abolish siderophore activity, which suggests the production of one or more non-hydroxamate iron-chelating compounds. Deletion of mrpptA, mrsidA, and mraarA reduced conidium production and conidia of a GFP-tagged ΔmrpptA strain displayed a longer germination delay than WT on insect cuticles, a deficiency that was rescued by lysine supplementation. Compared with WT, ΔmrpptA strains displayed ∼19-fold reduction in virulence against Drosophila suzukii. In contrast, lysine auxotrophy and loss of siderophores accounted for ∼2 and ∼6-fold decreases in virulence, respectively. Deletion of mrpptA had no significant effect on growth inhibition of Bacillus cereus. Our results suggest that PKS and NRPS metabolism plays a significant role in M. robertsii virulence, depresses conidium production, and contributes marginally to resistance to oxidative stress and iron homeostasis, but has no significant antibacterial effect.

Intracellular siderophore but not extracellular siderophore is required for full virulence in Metarhizium robertsii

Efficient iron acquisition mechanisms are fundamental for microbial survival in the environment and for pathogen virulence within their hosts. M. robertsii produces two known iron-binding natural products: metachelins, which are used to scavenge extracellular iron, and ferricrocin, which is strictly intracellular. To study the contribution of siderophore-mediated iron uptake and storage to M. robertsii fitness, we generated null mutants for each siderophore synthase gene (mrsidD and mrsidC, respectively), as well as for the iron uptake transcriptional repressor mrsreA. All of these mutants showed impaired germination speed, differential sensitivity to hydrogen peroxide, and differential ability to overcome iron chelation on growth-limiting iron concentrations. RT-qPCR data supported regulation of mrsreA, mrsidC, and mrsidD by supplied iron in vitro and during growth within the insect host, Spodoptera exigua. We also observed strong upregulation of the insect iron-binding proteins, transferrins, during infection. Insect bioassays revealed that ferricrocin is required for full virulence against S. exigua; neither the loss of metachelin production nor the deletion of the transcription factor mrsreA significantly affected M. robertsii virulence.