Genome Sequence of the Chestnut Blight Fungus Cryphonectria parasitica EP155: A Fundamental Resource for an Archetypical Invasive Plant Pathogen

Cryphonectria parasitica is the causal agent of chestnut blight, a fungal disease that almost entirely eliminated mature American chestnut from North America over a 50-year period. Here, we formally report the genome of C. parasitica EP155 using a Sanger shotgun sequencing approach. After finishing and integration with simple-sequence repeat markers, the assembly was 43.8 Mb in 26 scaffolds (L₅₀ = 5; N₅₀ = 4.0Mb). Eight chromosomes are predicted: five scaffolds have two telomeres and six scaffolds have one telomere sequence. In total, 11,609 gene models were predicted, of which 85% show similarities to other proteins. This genome resource has already increased the utility of a fundamental plant pathogen experimental system through new understanding of the fungal vegetative incompatibility system, with significant implications for enhancing mycovirus-based biological control.

Resistant ticks inhibit Metarhizium infection prior to haemocoel invasion by reducing fungal viability on the cuticle surface

We studied disease progression of, and host responses to, four species in the Metarhizium anisopliae complex expressing green fluorescent protein (GFP). We compared development and determined their relative levels of virulence against two susceptible arthropods, the cattle tick Rhipicephalus annulatus and the lepidopteran Galleria mellonella, and two resistant ticks, Hyalomma excavatum and Rhipicephalus sanguineus. Metarhizium brunneum Ma7 caused the greatest mortality of R. annulatus, Metarhizium robertsii ARSEF 2575 and Metarhizium pingshaense PPRC51 exhibited intermediate levels of virulence, and Metarhizium majus PPRC27 caused low mortality of cattle ticks. Conidia of all four species germinated on all hosts examined, but on resistant hosts, sustained hyphal growth was inhibited and GFP emission steadily and significantly decreased over time, suggesting a loss of fungal viability. Cuticle penetration was observed only for the three most virulent species infecting susceptible hosts. Cuticles of resistant and susceptible engorged female ticks showed significant increases in red autofluorescence at sites immediately under fungal hyphae. This is the first report (i) of tick mortality occurring after cuticle penetration but prior to haemocoel colonization and (ii) that resistant ticks do not support development of Metarhizium germlings on the outer surface of the cuticle. Whether reduced Metarhizium viability on resistant tick cuticles is due to antibiosis or limited nutrient availability is unknown.

Genetic basis of destruxin production in the entomopathogen Metarhizium robertsii

Destruxins are among the most exhaustively researched secondary metabolites of entomopathogenic fungi, yet definitive evidence for their roles in pathogenicity and virulence has yet to be shown. To establish the genetic bases for the biosynthesis of this family of depsipeptides, we identified a 23,792-bp gene in Metarhizium robertsii ARSEF 2575 containing six complete nonribosomal peptide synthetase modules, with an N-methyltransferase domain in each of the last two modules. This domain arrangement is consistent with the positioning of the adjacent amino acids N-methyl-L: -valine and N-methyl-L: -alanine within the depsipeptide structure of destruxin. DXS expression levels in vitro and in vivo exhibited comparable patterns, beginning at low levels during the early growth phases and increasing with time. Targeted gene knockout using Agrobacterium-mediated transformation produced mutants that failed to synthesize destruxins, in comparison with wild type and ectopic control strains, indicating the involvement of this gene in destruxin biosynthesis. The destruxin synthetase (DXS) disruption mutant was as virulent as the control strain when conidial inoculum was topically applied to larvae of Spodoptera exigua, Galleria mellonella, and Tenebrio molitor indicating that destruxins are dispensable for virulence in these insect hosts. The DXS mutants exhibited no other detectable changes in morphology and development.

Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control

BACKGROUND

Banana (Musa spp.) is grown throughout the tropical and subtropical regions of the world. The fruits are a key staple food in many developing countries and a source of income for subsistence farmers. Bananas are also a major, multibillion-dollar export commodity for consumption primarily in developed countries, where few banana cultivars are grown. The fungal pathogen Mycosphaerella fijiensis causes black leaf streak disease (BLSD; aka black Sigatoka leaf spot) on the majority of edible banana cultivars grown worldwide. The fact that most of these cultivars are sterile and unsuitable for the breeding of resistant lines necessitates the extensive use of fungicides as the primary means of disease control. BLSD is a significant threat to the food security of resource-poor populations who cannot afford fungicides, and increases the environmental and health hazards where large-acreage monocultures of banana (Cavendish subgroup, AAA genome) are grown for export.

TAXONOMY

Mycosphaerella fijiensis M. Morelet is a sexual, heterothallic fungus having Pseudocercospora fijiensis (M. Morelet) Deighton as the anamorph stage. It is a haploid, hemibiotrophic ascomycete within the class Dothideomycetes, order Capnodiales and family Mycosphaerellaceae. Its taxonomic placement is based on DNA phylogeny, morphological analyses and cultural characteristics.

DISEASE SYMPTOMS AND HOST RANGE

Mycosphaerella fijiensis is a leaf pathogen that causes reddish-brown streaks running parallel to the leaf veins, which aggregate to form larger, dark-brown to black compound streaks. These streaks eventually form fusiform or elliptical lesions that coalesce, form a water-soaked border with a yellow halo and, eventually, merge to cause extensive leaf necrosis. The disease does not kill the plants immediately, but weakens them by decreasing the photosynthetic capacity of leaves, causing a reduction in the quantity and quality of fruit, and inducing the premature ripening of fruit harvested from infected plants. Although Musa spp. are the primary hosts of M. fijiensis, the ornamental plant Heliconia psittacorum has been reported as an alternative host.

NEW OPPORTUNITIES

Several valuable tools and resources have been developed to overcome some of the challenges of studying this host-pathogen system. These include a DNA-mediated fungal transformation system and the ability to conduct targeted gene disruptions, reliable quantitative plant bioassays, diagnostic probes to detect and differentiate M. fijiensis from related pathogens and to distinguish strains of different mating types, and a genome sequence that has revealed a wealth of gene sequences and molecular markers to be utilized in functional and population biology studies.

USEFUL WEBSITES

http://bananas.bioversityinternational.org/, http://genome.jgi-psf.org/Mycfi2/Mycfi2.home.html, http://www.isppweb.org/names_banana_pathogen.asp#fun, http://www.promusa.org/.

Fungal secondary metabolites as modulators of interactions with insects and other arthropods

Fungi share a diverse co-evolutionary history with animals, especially arthropods. In this review, we focus on the role of secondary metabolism in driving antagonistic arthropod-fungus interactions, i.e., where fungi serve as a food source to fungal grazers, compete with saprophagous insects, and attack insects as hosts for growth and reproduction. Although a wealth of studies on animal-fungus interactions point to a crucial role of secondary metabolites in deterring animal feeding and resisting immune defense strategies, causal evidence often remains to be provided. Moreover, it still remains an unresolved puzzle as to what extent the tight regulatory control of secondary metabolite formation in some model fungi represents an evolved chemical defense system favored by selective pressure through animal antagonists. Given these gaps in knowledge, we highlight some co-evolutionary aspects of secondary metabolism, such as induced response, volatile signaling, and experimental evolution, which may help in deciphering the ecological importance and evolutionary history of secondary metabolite production in fungi.

Identification of a hybrid PKS-NRPS required for the biosynthesis of NG-391 in Metarhizium robertsii

The fungal entomopathogen Metarhizium robertsii (formerly known as M. anisopliae var. anisopliae) is a prolific producer of secondary metabolites of which very little is known at the genetic level. To establish the genetic bases for the biosynthesis of the mutagenic compound NG- 391, we identified a 19,818 kb genomic region harboring the predicted hybrid polyketide synthase-nonribosomal peptide synthetase NGS1, plus five additional ORFs. NGS1 knockouts generated by Agrobacterium-mediated transformation failed to produce detectable levels of NG-391, indicating the involvement of this locus in its biosynthesis. NGS1 deletion mutants had no significant changes in virulence levels against larvae of Spodoptera exigua and in resistance to hydrogen peroxide-generated oxidative stress compared to the wild-type strain. All 6 ORFs were expressed in medium supporting production of NG-391, and NGS1 was expressed during the interaction with the S. exigua host. The use of an NGS1 promoter-GFP reporter fusion showed that during in vitro growth in still broth cultures, NGS1 expression is restricted to the early exponential phase and is affected by M. robertsii cell density.

Promoter analysis of the cell surface-abundant and hypoviral-regulated cryparin gene from Cryphonectria parasitica

Cryparin, encoded as a single copy gene (Crp) of the chestnut blight fungus Cryphonectria parasitica, is the most abundant protein produced by this fungus. However, its accumulation is decreased remarkably in C. parastica strains containing the double-stranded (ds) RNA virus Cryphonectria hypovirus 1. To characterize the transcriptional regulatory element(s) for strong expression and viral regulation, promoter analysis was conducted. Serial deletion of the Crp promoter region resulted in a step-wise decrease in promoter activity, indicating a localized distribution of genetic elements in the cryparin promoter. Promoter analysis indicated two positive and a repressive cis-acting elements. Among them, the promoter region between nt -1,282 and -907 appeared to be necessary for hypoviral-mediated down-regulation. An electrophoretic mobility shift assay (EMSA) on the corresponding promoter region (-1,282/-907) indicated two regions at (-1257/-1158) and (-1107/-1008) with the characteristic AGGAGGA-N42-GAGAGGA and its inverted repeat TCCTCTC-N54-TCCTCCT, respectively, appeared to be specific binding sites for cellular factors.

Serinocyclins A and B, cyclic heptapeptides from Metarhizium anisopliae

Two new cyclic heptapeptides, serinocyclins A (1) and B (2), were isolated from conidia of the entomopathogenic fungus Metarhizium anisopliae. Structures were elucidated by a combination of mass spectrometric, NMR, and X-ray diffraction techniques. Serinocyclin A (1) contains three serine units, a hydroxyproline (Hyp), a beta-alanine (beta-Ala), and two uncommon nonproteinogenic amino acids, 1-aminocyclopropane-1-carboxylic acid (Acc) and gamma-hydroxylysine (HyLys). The peptide sequence established for 1 by NMR is cyclo-(Acc-Hyp-Ser1-HyLys-beta-Ala-Ser2-Ser3). Serinocyclin B (2) has Lys in place of the HyLys unit found in 1. Chiral amino acid analysis indicated the presence in both compounds of one (2 S,4 R)-Hyp, two L-Ser, and one D-Ser residue. A Lys found in the hydrolyzate of 2 was established as D-configured. A crystal structure of 1 established the position of the D-Ser (Ser2) and the absolute configuration of the HyLys unit (2 R,4 S). The absence of methyl groups is unusual among fungal peptides and, along with the charged lysyl side chain and multiple hydroxyl groups, contributes to the polar nature of the compounds. Serinocyclin A produced a sublethal locomotory defect in mosquito larvae at an EC 50 of 59 ppm.

A Quantitative Assay Using Mycelial Fragments to Assess Virulence of Mycosphaerella fijiensis

ABSTRACT We describe a method to evaluate the virulence of Mycosphaerella fijiensis, the causal agent of black leaf streak disease (BLSD) of banana and plantain. The method is based on the delivery of weighed slurries of fragmented mycelia by camel's hair brush to 5-by-5-cm areas on the abaxial surface of banana leaf blades. Reliable BLSD development was attained in an environmental growth chamber with stringent lighting and humidity controls. By localizing inoculum onto small areas of large leaves, we achieved a dramatic increase in the number of strains that can be tested on each leaf and plant, which is critical for comparing the virulence of numerous strains concurrently. Image analysis software was used to measure the percentage of each inoculated leaf section showing BLSD symptoms over time. We demonstrated that the level of disease of four isolates was correlated with the weight of the mycelium applied and relatively insensitive to the degree of fragmentation of hyphae. This is the first report demonstrating that weighed mycelial inoculum, combined with image analysis software to measure disease severity, can be used to quantitatively assess the virulence of M. fijiensis under rigorously controlled environmental conditions.

Production of mutagenic metabolites by Metarhizium anisopliae

NG-391 (1) and NG-393 (2), previously reported from undescribed Fusarium species as nerve-cell growth stimulants, were identified from fermentation extracts of the entomopathogenic fungus Metarhizium anisopliae. These compounds are 7-desmethyl analogues of fusarin C and (8Z)-fusarin C, mutagenic toxins from Fusarium species that contaminate corn. A mutant strain of M. anisopliae (KOB1-3) overproduces 1 and 2 by ca. 10-fold relative to the wild-type strain, ARSEF 2575, from which it was derived. Overproduction of these compounds in KOB1-3 imparts a yellow pigmentation to the culture medium of the fungus. These compounds were inactive at 100 mug/disk in antimicrobial disk diffusion assays. Compound 1 was inactive at 100 ppm in a mosquitocidal assay. However, like their fusarin analogues, 1 and 2 exhibited potent S9-dependent mutagenic activity in the Salmonella mutagenicity test. Discovery of these highly mutagenic mycotoxins in M. anisopliae suggests that screening for production of NG-391 and NG-393 in strains that are used as biocontrol agents would be a prudent course of action. The impact of these findings on the use of M. anisopliae as a biocontrol agent is currently unknown and requires further investigation.

A novel class of gene controlling virulence in plant pathogenic ascomycete fungi

Insertional mutants of the fungal maize pathogen Cochliobolus heterostrophus were screened for altered virulence. One mutant had 60% reduction in lesion size relative to WT but no other detectable change in phenotype. Analysis of sequence at the insertion site revealed a gene (CPS1) encoding a protein with two AMP-binding domains. CPS1 orthologs were detected in all Cochliobolus spp. examined, in several other classes of ascomycete fungi, and in animals but not in basidiomycete fungi, bacteria, or plants. Phylogenetic analysis suggested that CPS1 represents a previously undescribed subset of adenylate-forming enzymes that have diverged from certain acyl-CoA ligases, which in bacteria are involved in biosynthesis of nonribosomal peptides or polyketidepeptide hybrids. Disruption of CPS1 caused reduced virulence of both race T and race O of C. heterostrophus on maize, of Cochliobolus victoriae on oats, and of Gibberella zeae on wheat. These results suggest that CPS1 functions as a general fungal virulence factor in plant pathogenic ascomycetes.