Six type-I PKS classes and highly conserved melanin and elsinochrome gene clusters found in diverse Elsinoë species.

Elsinoë species are phytopathogenic fungi that cause serious scab diseases on economically important plants. The disease symptoms arise from the effects of a group of phytotoxins known as elsinochromes, produced via a type-I polyketide synthase (PKS) biosynthetic pathway. The elsinochrome gene cluster was first annotated in Elsinoë fawcettii where the main type-I PKS gene was characterized as EfPKS1. A later study showed that this gene and the associated cluster had not been correctly annotated, and that EfPKS1 was actually the anchor gene of the melanin biosynthetic pathway. A new type-I PKS gene EfETB1 associated with elsinochrome production was also identified. The aim of this study was to identify all type-I PKS genes in the genomes of seven Elsinoë species with the goal of independently verifying the PKS containing clusters for both melanin and elsinochrome production. A total of six type-I PKS classes were identified, although there was variation between the species in the number and type of classes present. Genes similar to the E. fawcettii EfPKS1 and EfETB1 type-I PKS genes were associated with melanin and elsinochrome production respectively in all species. The complete melanin and elsinochrome PKS containing clusters were subsequently annotated in all the species with high levels of synteny across Elsinoë species. This study provides a genus-level overview of type-I PKS distribution in Elsinoë species, including an additional line of support for the annotation of the melanin and elsinochrome PKS containing clusters in these important plant pathogens.

Unidirectional mating-type switching is underpinned by a conserved MAT1 locus architecture.

Unidirectional mating-type switching is a form of homothallic reproduction known only in a small number of filamentous ascomycetes. Their ascospores can give rise to either self-sterile isolates that require compatible partners for subsequent sexual reproduction, or self-fertile individuals capable of completing this process in isolation. The limited studies previously conducted in these fungi suggest that the differences in mating specificity are determined by the architecture of the MAT1 locus. In self-fertile isolates that have not undergone unidirectional mating-type switching, the locus contains both MAT1-1 and MAT1-2 mating-type genes, typical of primary homothallism. In the self-sterile isolates produced after a switching event, the MAT1-2 genes are lacking from the locus, likely due to a recombination-mediated deletion of the MAT1-2 gene information. To determine whether these arrangements of the MAT1 locus support unidirectional mating-type switching in the Ceratocystidaceae, the largest known fungal assemblage capable of this reproduction strategy, a combination of genetic and genomic approaches were used. The MAT1 locus was annotated in representative species of Ceratocystis, Endoconidiophora, and Davidsoniella. In all cases, MAT1-2 genes interrupted the MAT1-1-1 gene in self-fertile isolates. The MAT1-2 genes were flanked by two copies of a direct repeat that accurately predicted the boundaries of the deletion event that would yield the MAT1 locus of self-sterile isolates. Although the relative position of the MAT1-2 gene region differed among species, it always disrupted the MAT1-1-1 gene and/or its expression in the self-fertile MAT1 locus. Following switching, this gene and/or its expression was restored in the self-sterile arrangement of the locus. This mirrors what has been reported in other species capable of unidirectional mating-type switching, providing the strongest support for a conserved MAT1 locus structure that is associated with this process. This study contributes to our understanding of the evolution of unidirectional mating-type switching.

Sporothrix and Sporotrichosis: A South African Perspective on a Growing Global Health Threat.

Sporotrichosis is a disease that arises from a fungal infection caused by members of the Ascomycete genus Sporothrix. The disease has a unique history in South Africa, due to an association with gold mines, where large numbers of mine workers were infected in the 1930s and 1940s. This was likely driven by hot humid conditions and timber supports used in these mine shafts. Furthermore, the disease is the most common subcutaneous fungal infection amongst the general population in South Africa, and the large number of immunocompromised individuals increases the public health risk in the country. Sporothrix is a genus in the Ophiostomatales, a fungal order primarily associated with environmental habitats. Unsurprisingly, sporotrichosis therefore has a documented history of sapronotic transmission from contaminated plant material. This review provides insights into the understanding of sporotrichosis and Sporothrix species, with a particular emphasis on the South African situation. We highlight knowledge gaps, particularly regarding the ecological factors influencing the occurrence and distribution of these species, which in turn affect the patterns of sporotrichosis. We also emphasise a need for ongoing proactive research and surveillance to prevent future outbreaks of sporotrichosis, an emerging disease with growing health implications worldwide.

LAMP Assay to Detect Elsinoë necatrix, an Important Eucalyptus Shoot and Leaf Pathogen.

Eucalyptus scab and shoot malformation caused by Elsinoë necatrix is an emerging disease and a serious threat to the global commercial forestry industry. The disease was first discovered in North Sumatra, Indonesia, and now requires a simple and effective method for early pathogen detection. In this study, a rapid and sensitive loop-mediated isothermal amplification (LAMP) assay was developed for E. necatrix. A unique region in a secondary metabolite gene cluster was used as a target for the assay. To test robustness of the assay, LAMP was verified in 15 strains of E. necatrix. A specificity test against 23 closely related Elsinoë species and three fungal species commonly isolated on Eucalyptus showed that the LAMP assay exclusively identified E. necatrix isolates. The assay had a high level of sensitivity, able to detect 0.01 ng (approximately 400 target copies) of pure E. necatrix DNA. Furthermore, using a simple DNA extraction method, it was possible to use this assay to detect E. necatrix in infected Eucalyptus leaves.

Characterization of the Fusarium circinatum biofilm environmental response role.

The capacity to form biofilms is a common trait among many microorganisms present on Earth. In this study, we demonstrate for the first time that the fatal pine pitch canker agent, Fusarium circinatum, can lead a biofilm-like lifestyle with aggregated hyphal bundles wrapped in extracellular matrix (ECM). Our research shows F. circinatum's ability to adapt to environmental changes by assuming a biofilm-like lifestyle. This was demonstrated by varying metabolic activities exhibited by the biofilms in response to factors like temperature and pH. Further analysis revealed that while planktonic cells produced small amounts of ECM per unit of the biomass, heat- and azole-exposed biofilms produced significantly more ECM than nonexposed biofilms, further demonstrating the adaptability of F. circinatum to changing environments. The increased synthesis of ECM triggered by these abiotic factors highlights the link between ECM production in biofilm and resistance to abiotic stress. This suggests that ECM-mediated response may be one of the key survival strategies of F. circinatum biofilms in response to changing environments. Interestingly, azole exposure also led to biofilms that were resistant to DNase, which typically uncouples biofilms by penetrating the biofilm and degrading its extracellular DNA; we propose that DNases were likely hindered from reaching target cells by the ECM barricade. The interplay between antifungal treatment and DNase enzyme suggests a complex relationship between eDNA, ECM, and antifungal agents in F. circinatum biofilms. Therefore, our results show how a phytopathogen's sessile (biofilm) lifestyle could influence its response to the surrounding environment.

Structure and number of mating pheromone genes is closely linked to sexual reproductive strategy in Huntiella.

BACKGROUND: Huntiella resides in the Ceratocystidaceae, a family of fungi that accommodates important plant pathogens and insect-associated saprotrophs. Species in the genus have either heterothallic or unisexual (a form of homothallism) mating systems, providing an opportunity to investigate the genetic mechanisms that enable transitions between reproductive strategies in related species. Two newly sequenced Huntiella genomes are introduced in this study and comparative genomics and transcriptomics tools are used to investigate the differences between heterothallism and unisexuality across the genus. RESULTS: Heterothallic species harbored up to seven copies of the a-factor pheromone, each of which possessed numerous mature peptide repeats. In comparison, unisexual Huntiella species had only two or three copies of this gene, each with fewer repeats. Similarly, while the heterothallic species expressed up to 12 copies of the mature α-factor pheromone, unisexual species had up to six copies. These significant differences imply that unisexual Huntiella species do not rely on a mating partner recognition system in the same way that heterothallic fungi do. CONCLUSION: While it is suspected that mating type-independent pheromone expression is the mechanism allowing for unisexual reproduction in Huntiella species, our results suggest that the transition to unisexuality may also have been associated with changes in the genes governing the pheromone pathway. While these results are specifically related to Huntiella, they provide clues leading to a better understanding of sexual reproduction and the fluidity of mating strategies in fungi more broadly.

Nonribosomal peptide synthetase gene clusters and characteristics of predicted NRPS-dependent siderophore synthetases in Armillaria and other species in the Physalacriaceae.

Fungal secondary metabolites are often pathogenicity or virulence factors synthesized by genes contained in secondary metabolite gene clusters (SMGCs). Nonribosomal polypeptide synthetase (NRPS) clusters are SMGCs which produce peptides such as siderophores, the high affinity ferric iron chelating compounds required for iron uptake under aerobic conditions. Armillaria spp. are mostly facultative necrotrophs of woody plants. NRPS-dependent siderophore synthetase (NDSS) clusters of Armillaria spp. and selected Physalacriaceae were investigated using a comparative genomics approach. Siderophore biosynthesis by strains of selected Armillaria spp. was evaluated using CAS and split-CAS assays. At least one NRPS cluster and other clusters were detected in the genomes studied. No correlation was observed between the number and types of SMGCs and reported pathogenicity of the species studied. The genomes contained one NDSS cluster each. All NDSSs were multi-modular with the domain architecture (ATC)3(TC)2. NDSS clusters of the Armillaria spp. showed a high degree of microsynteny. In the genomes of Desarmillaria spp. and Guyanagaster necrorhizus, NDSS clusters were more syntenic with NDSS clusters of Armillaria spp. than to those of the other Physalacriaceae species studied. Three A-domain orthologous groups were identified in the NDSSs, and atypical Stachelhaus codes were predicted for the A3 orthologous group. In vitro biosynthesis of mainly hydroxamate and some catecholate siderophores was observed. Hence, Armillaria spp. generally contain one highly conserved, NDSS cluster although some interspecific variations in the products of these clusters is expected. Results from this study lays the groundwork for future studies to elucidate the molecular biology of fungal phyto-pathogenicity.

A LAMP Assay for Rapid Detection of the Pitch Canker Pathogen Fusarium circinatum.

The pine pitch canker pathogen Fusarium circinatum is endemic in the southeastern United States and Central America and represents an invasive threat globally. This ecologically adaptable fungus readily infects all parts of its pine hosts, leading to widespread mortality of nursery seedlings and decline in the health and productivity of forest stands. Because trees infected by F. circinatum can remain asymptomatic for long periods of time, accurate and rapid tools are needed for real-time diagnostics and surveillance at ports, in nurseries, and in plantations. To meet this need and to limit the spread and impact of the pathogen, we developed a molecular test using loop-mediated isothermal amplification (LAMP), a technology that allows for the rapid detection of pathogen DNA on portable, field-capable devices. LAMP primers were designed and validated to amplify a gene region unique to F. circinatum. Using a globally representative collection of F. circinatum isolates and other closely related species, we have demonstrated that the assay can be used to identify F. circinatum across its genetic diversity and that it is sensitive to as few as 10 cells from purified DNA extracts. The assay can also be used with a simple, pipette-free DNA extraction method and is compatible with testing symptomatic pine tissues in the field. This assay has the potential to facilitate diagnostic and surveillance efforts both in the laboratory and in the field and, thus, to reduce the spread and impact of pitch canker worldwide.

Molecular basis of cycloheximide resistance in the Ophiostomatales revealed.

Resistance to the antibiotic Cycloheximide has been reported for a number of fungal taxa. In particular, some yeasts are known to be highly resistant to this antibiotic. Early research showed that this resulted from a transition mutation in one of the 60S ribosomal protein genes. In addition to the yeasts, most genera and species in the Ophiostomatales are highly resistant to this antibiotic, which is widely used to selectively isolate these fungi. Whole-genome sequences are now available for numerous members of the Ophiostomatales providing an opportunity to determine whether the mechanism of resistance in these fungi is the same as that reported for yeast genera such as Kluyveromyces. We examined all the available genomes for the Ophiostomatales and discovered that a transition mutation in the gene coding for ribosomal protein eL42, which results in the substitution of the amino acid Proline to Glutamine, likely confers resistance to this antibiotic. This change across all genera in the Ophiostomatales suggests that the mutation arose early in the evolution of these fungi.

Novel mating-type-associated genes and gene fragments in the genomes of Mycosphaerellaceae and Teratosphaeriaceae fungi.

The mating-type (MAT1) locus encodes transcription factors essential for the onset of the sexual cycle in ascomycete fungi. This locus has been characterised in only a few heterothallic, plant pathogenic Mycosphaerellaceae and Teratosphaeriaceae. We used available genome sequences for Mycosphaerellales species to investigate the presence of two unique mating-type-associated features. The accessory MAT1 genes, MAT1-1-10 (MATORF2) and MAT1-2-12 (MATORF1), typically occurred in both MAT idiomorphs of Mycosphaerellaceae species. In contrast, they were associated with only one idiomorph in Teratosphaeriaceae species. In Pseudocercospora, phylogenetic analyses showed that homologs present in different idiomorphs were paralogous and subject to different selective pressures, indicating that their evolution is linked to mating type. In almost half of the investigated Mycosphaerellales genomes, numerous short fragment sequences, almost identical to portions of the MAT1-1-1 and MAT1-2-1 genes, were present in multiple areas outside of the MAT1 locus. Aligned to the MAT1 genes, these sequences resembled an mRNA transcript. Fragment sequences were similar among species groups and occurred at the same genomic positions, implying that monophyletic groups have the same origins of these sequences. Although the functions of the MAT fragment sequences and accessory MAT1 genes remain unknown, both were expressed in the representative Mycosphaerellaceae and Teratosphaeriaceae species that were investigated.

Genetic response to nitrogen starvation in the aggressive Eucalyptus foliar pathogen Teratosphaeria destructans.

Teratosphaeria destructans is one of the most aggressive foliar pathogens of Eucalyptus. The biological factors underpinning T. destructans infections, which include shoot and leaf blight on young trees, have never been interrogated. Thus, the means by which the pathogen modifies its host environment to overcome host defences remain unknown. By applying transcriptome sequencing, the aim of this study was to compare gene expression in a South African isolate of T. destructans grown on nitrogen-deficient and complete media. This made it possible to identify upregulated genes in a nitrogen-starved environment, often linked to the pathogenicity of the fungus. The results support the hypothesis that nitrogen starvation in T. destructans likely mirrors an in planta genetic response. This is because 45% of genes that were highly upregulated under nitrogen starvation have previously been reported to be associated with infection in other pathogen systems. These included several CAZymes, fungal effector proteins, peptidases, kinases, toxins, lipases and proteins associated with detoxification of toxic compounds. Twenty-five secondary metabolites were identified and expressed in both nitrogen-deficient and complete conditions. Additionally, the most highly expressed genes in both growth conditions had pathogenicity-related functions. This study highlights the large number of expressed genes associated with pathogenicity and overcoming plant defences. As such, the generated baseline knowledge regarding pathogenicity and aggressiveness in T. destructans is a valuable reference for future in planta work.

Phylogenomic incongruence in Ceratocystis: a clue to speciation?

BACKGROUND: The taxonomic history of Ceratocystis, a genus in the Ceratocystidaceae, has been beset with questions and debate. This is due to many of the commonly used species recognition concepts (e.g., morphological and biological species concepts) providing different bases for interpretation of taxonomic boundaries. Species delineation in Ceratocystis primarily relied on genealogical concordance phylogenetic species recognition (GCPSR) using multiple standard molecular markers. RESULTS: Questions have arisen regarding the utility of these markers e.g., ITS, BT and TEF1-α due to evidence of intragenomic variation in the ITS, as well as genealogical incongruence, especially for isolates residing in a group referred to as the Latin-American clade (LAC) of the species. This study applied a phylogenomics approach to investigate the extent of phylogenetic incongruence in Ceratocystis. Phylogenomic analyses of a total of 1121 shared BUSCO genes revealed widespread incongruence within Ceratocystis, particularly within the LAC, which was typified by three equally represented topologies. Comparative analyses of the individual gene trees revealed evolutionary patterns indicative of hybridization. The maximum likelihood phylogenetic tree generated from the concatenated dataset comprised of 1069 shared BUSCO genes provided improved phylogenetic resolution suggesting the need for multiple gene markers in the phylogeny of Ceratocystis. CONCLUSION: The incongruence observed among single gene phylogenies in this study call into question the utility of single or a few molecular markers for species delineation. Although this study provides evidence of interspecific hybridization, the role of hybridization as the source of discordance will require further research because the results could also be explained by high levels of shared ancestral polymorphism in this recently diverged lineage. This study also highlights the utility of BUSCO genes as a set of multiple orthologous genes for phylogenomic studies.

The novel Huntiella omanensis mating gene, MAT1-2-7, is essential for ascomatal maturation.

Sexual reproduction is a highly conserved feature of the eukaryotes, yet sexual compatibility is determined by a wide variety of mechanisms. In ascomycete fungi, sexual development is controlled by genes at the mating type (MAT) locus that confer either MAT1-1 or MAT1-2 mating identity. Although the locus harbours, at minimum, a single gene, the individual MAT loci of certain species, including Huntiella omanensis, encode for two or more genes. The MAT1-2 idiomorph of H. omanensis is made up of MAT1-2-1, a primary MAT gene that is highly conserved in the Pezizomycotina and possesses a well-characterized DNA binding motif, the HMG-box domain. The idiomorph also harbours a novel secondary MAT gene, named MAT1-2-7, with no recognizable functional domains. In this study, we developed a transformation and CRISPR-Cas9-based genome editing protocol to characterize the MAT1-2-7 gene with respect to its function in mating. We have shown that MAT1-2-7 is essential for sexual reproduction and that isolates carrying the truncated MAT1-2-7 gene are incapable of ascomatal maturation and further sexual development. MAT1-2-7 was also shown to influence the vegetative radial growth rate of H. omanensis, illustrating the pleiotropic effects often associated with MAT genes.

Genome comparisons suggest an association between Ceratocystis host adaptations and effector clusters in unique transposable element families.

Ceratocystis fimbriata is a host specific fungal pathogen of sweet potato (Ipomoea batatas). The closely related species, C. manginecans, is an important pathogen of trees (e.g. Acacia mangium and Mangifera indica) but has never been isolated from tuber crops. The genetic factors that determine the host range and host specificity of these species have not been determined. The aim of this study was to compare the genomes of C. fimbriata and C. manginecans in order to identify species-specific genetic differences that could be associated with host specificity. This included whole-genome alignments as well as comparisons of gene content and transposable elements (TEs). The genomes of the two species were found to be very similar, sharing similar catalogues of CAZymes, peptidases and lipases. However, the genomes of the two species also varied, harbouring species-specific genes (e.g. small secreted effectors, nutrient processing proteins and stress response proteins). A portion of the TEs identified (17%) had a unique distribution in each species. Transposable elements appeared to have played a prominent role in the divergence of the two species because they were strongly associated with chromosomal translocations and inversions as well as with unique genomic regions containing species-specific genes. Two large effector clusters, with unique TEs in each species, were identified. These effectors displayed non-synonymous mutations and deletions, conserved within a species, and could serve as mutational hot-spots for the development of host specificity in the two species.

Mating strategy and mating type distribution in six global populations of the Eucalyptus foliar pathogen Teratosphaeria destructans.

Teratosphaeria destructans is an aggressive fungal pathogen causing leaf and shoot blight on young Eucalyptus trees in plantations. The disease occurs across tropical and subtropical regions of South East Asia and has recently been found in South Africa. Asexual structures of the pathogen are produced on infected tissues, but sexual structures have never been observed. The aim of this study was to investigate the reproductive biology of T. destructans by characterising its mating type (MAT1) locus and investigating its potential for sexual recombination. We found that T. destructans has a heterothallic mating system, with either the MAT1-1-1 and MAT1-1-10 genes (MAT1-1 idiomorph) or the MAT1-2-1 and MAT1-2-12 genes (MAT1-2 idiomorph) present in a single individual. With a multiplex PCR assay, it was possible to distinguish the two MAT idiomorphs in several Teratosphaeria species and this approach was applied to six global populations of T. destructans. Although both mating types occurred in the South East Asian populations, a single mating type dominated each population. Isolates from the recent disease outbreak in South Africa comprised only a single mating type. Attempts to induce a sexual cycle in vitro using strains of opposite mating type were not successful. The uneven distribution of mating types in populations of T. destructans and the presence of only an asexual state on infected tissues suggests the absence of or at least a minor role for sexual reproduction where the pathogen occurs on non-native Eucalyptus in plantations.

The mating system of the Eucalyptus canker pathogen Chrysoporthe austroafricana and closely related species.

Fungi in the genus Chrysoporthe are economically important canker pathogens of commercially grown Eucalyptus species and native Myrtales. Before the current study, homothallism was widely accepted as the mating system of these species, but this hypothesis could not be fully tested. Using whole genome sequences, we characterized the MAT locus of two C. austroafricana isolates and its sibling species, C. cubensis and C. deuterocubensis. A unique MAT1-2 idiomorph containing a truncated MAT1-1-1 gene, and a MAT1-1-2 gene, was identified in one isolate of C. austroafricana and a MAT1-1 idiomorph was found in the other. The presence of a single idiomorph in each isolate suggests that this fungus is heterothallic. Screening for MAT genes in 65 C. austroafricana isolates revealed a bias towards MAT1-2 idiomorphs suggesting a recent introduction in Eucalyptus species. Chrysoporthe cubensis and C. deuterocubensis are apparently homothallic since all the expected MAT genes were identified in their genome sequences. These findings were corroborated by the expression profiles of pheromone genes and their receptors, which conformed to the expected patterns observed in heterothallic and homothallic isolates. Long terminal repeat sequences (LTRs) and specifically retrotransposons were identified in the MAT locus of C. deuterocubensis and C. cubensis, indicating that the evolution of mating systems in Chrysoporthe species could be mediated by these elements.

QTL mapping of mycelial growth and aggressiveness to distinct hosts in Ceratocystis pathogens.

Some species of Ceratocystis display strong host specificity, such as C. fimbriata sensu stricto that is restricted to sweet potato (Ipomoea batatas) as host. In contrast, the closely related C. manginecans, infects Acacia mangium and Mangifera indica but is not pathogenic to I. batatas. Despite the economic importance of these fungi, knowledge regarding the genetic factors that influence their pathogenicity and host specificity is limited. A recent inheritance study, based on an interspecific cross between C. fimbriata and C. manginecans and the resultant 70 F1 progeny, confirmed that traits such as mycelial growth rate, spore production and aggressiveness on A. mangium and I. batatas are regulated by multiple genes. In the present study, a quantitative trait locus (QTL) analysis was performed to determine the genomic loci associated with these traits. All 70 progeny isolates were genotyped with SNP markers and a linkage map was constructed. The map contained 467 SNPs, distributed across nine linkage groups, with a total length of 1203 cm. Using the progeny genotypes and phenotypes, one QTL was identified on the linkage map for mycelial growth rate, one for aggressiveness to A. mangium and two for aggressiveness to I. batatas (P < 0.05). Two candidate genes, likely associated with mycelial growth rate, were identified in the QTL region. The three QTLs associated with aggressiveness to different hosts contained candidate genes involved in protein processing, detoxification and regions with effector genes and high transposable element density. The results provide a foundation for studies considering the function of genes regulating various quantitative traits in Ceratocystis.

Unexpected placement of the MAT1-1-2 gene in the MAT1-2 idiomorph of Thielaviopsis.

Sexual reproduction in the Ascomycota is controlled by genes encoded at the mating-type or MAT1 locus. The two allelic versions of this locus in heterothallic species, referred to as idiomorphs, are defined by the MAT1-1-1 (for the MAT1-1 idiomorph) and MAT1-2-1 (for the MAT1-2 idiomorph) genes. Both idiomorphs can contain additional genes, although the contents of each is typically specific to and conserved within particular Pezizomycotina lineages. Using full genome sequences, complemented with conventional PCR and Sanger sequencing, we compared the mating-type idiomorphs in heterothallic species of Thielaviopsis (Ceratocystidaceae). The analyses showed that the MAT1-1 idiomorph of T. punctulata, T. paradoxa, T. euricoi, T. ethacetica and T. musarum harboured only the expected MAT1-1-1 gene. In contrast, the MAT1-2 idiomorph of T. punctulata, T. paradoxa and T. euricoi encoded the MAT1-2-1, MAT1-2-7 and MAT1-1-2 genes. Of these, MAT1-2-1 and MAT1-2-7 are genes previously reported in this idiomorph, while MAT1-1-2 is known only in the MAT1-1 idiomorph. Phylogenetic analysis showed that the Thielaviopsis MAT1-1-2 groups with the known homologues of this gene in other Microascales, thus confirming its annotation. Previous work suggests that MAT1-1-2 is involved in fruiting body development, a role that would be unaffected by its idiomorphic position. This notion is supported by our findings for the MAT1 locus structure in Thielaviopsis species. This also serves as the first example of a MAT1-1-specific gene restricted to only the MAT1-2 idiomorph.

The Bark-Beetle-Associated Fungus, Endoconidiophora polonica, Utilizes the Phenolic Defense Compounds of Its Host as a Carbon Source.

Norway spruce (Picea abies) is periodically attacked by the bark beetle Ips typographus and its fungal associate, Endoconidiophora polonica, whose infection is thought to be required for successful beetle attack. Norway spruce produces terpenoid resins and phenolics in response to fungal and bark beetle invasion. However, how the fungal associate copes with these chemical defenses is still unclear. In this study, we investigated changes in the phenolic content of Norway spruce bark upon E. polonica infection and the biochemical factors mediating these changes. Although genes encoding the rate-limiting enzymes in Norway spruce stilbene and flavonoid biosynthesis were actively transcribed during fungal infection, there was a significant time-dependent decline of the corresponding metabolites in fungal lesions. In vitro feeding experiments with pure phenolics revealed that E. polonica transforms both stilbenes and flavonoids to muconoid-type ring-cleavage products, which are likely the first steps in the degradation of spruce defenses to substrates that can enter the tricarboxylic acid cycle. Four genes were identified in E. polonica that encode catechol dioxygenases carrying out these reactions. These enzymes catalyze the cleavage of phenolic rings with a vicinal dihydroxyl group to muconoid products accepting a wide range of Norway spruce-produced phenolics as substrates. The expression of these genes and E. polonica utilization of the most abundant spruce phenolics as carbon sources both correlated positively with fungal virulence in several strains. Thus, the pathways for the degradation of phenolic compounds in E. polonica, initiated by catechol dioxygenase action, are important to the infection, growth, and survival of this bark beetle-vectored fungus and may play a major role in the ability of I. typographus to colonize spruce trees.