Structural features of genomes, including the three-dimensional arrangement of DNA in the nucleus, are increasingly seen as key contributors to the regulation of gene expression. However, studies on how genome structure and nuclear organisation influence transcription have so far been limited to a handful of model species. This narrow focus limits our ability to draw general conclusions about the ways in which three-dimensional structures are encoded, and to integrate information from three-dimensional data to address a broader gamut of biological questions. Here, we generate a complete and gapless genome sequence for the filamentous fungus, Epichloë festucae. We use Hi-C data to examine the three-dimensional organisation of the genome, and RNA-seq data to investigate how Epichloë genome structure contributes to the suite of transcriptional changes needed to maintain symbiotic relationships with the grass host. Our results reveal a genome in which very repeat-rich blocks of DNA with discrete boundaries are interspersed by gene-rich sequences that are almost repeat-free. In contrast to other species reported to date, the three-dimensional structure of the genome is anchored by these repeat blocks, which act to isolate transcription in neighbouring gene-rich regions. Genes that are differentially expressed in planta are enriched near the boundaries of these repeat-rich blocks, suggesting that their three-dimensional orientation partly encodes and regulates the symbiotic relationship formed by this organism.

Peramine is an insect-feeding deterrent produced by Epichloë species in symbiotic association with C 3 grasses. The perA gene responsible for peramine synthesis encodes a two-module nonribosomal peptide synthetase. Alleles of perA are found in most Epichloë species; however, peramine is not produced by many perA-containing Epichloë isolates. The genetic basis of these peramine-negative chemotypes is often unknown. Using PCR and DNA sequencing, we analyzed the perA genes from 72 Epichloë isolates and identified causative mutations of perA null alleles. We found nonfunctional perA-⌬R* alleles, which contain a transposon-associated deletion of the perA region encoding the C-terminal reductase domain, are widespread within the Epichloë genus and represent a prevalent mutation found in nonhybrid species. Disparate phylogenies of adjacent A2 and T2 domains indicated that the deletion of the reductase domain (R*) likely occurred once and early in the evolution of the genus, and subsequently there have been several recombinations between those domains. A number of novel point, deletion, and insertion mutations responsible for abolishing peramine production in full-length perA alleles were also identified. The regions encoding the first and second adenylation domains (A1 and A2, respectively) were common sites for such mutations. Using this information, a method was developed to predict peramine chemotypes by combining PCR product size polymorphism analysis with sequencing of the perA adenylation domains. Fungal secondary metabolites are a diverse group of important but often nonessential organic compounds with a wide range of properties that are likely to be advantageous for the producing organism or in some cases essential for pathogenicity or developmental stages (1-3). These low-molecular-weight compounds tend to only be produced under certain environmental or growth conditions. The biosynthetic pathways for production of any particular class of secondary metabolites are common to many fungi, but production of a specific secondary metabolite is often unique to a small phylogenetic group of species (4). Epichloë species are fungal endophytes of C 3 grasses that are known to produce several bioactive alkaloids that provide bioprotective properties to the host plant (5). These secondary metabolites include the indolediterpenes, ergot alkaloids, lolines, and peramine ( Fig. 1) (6, 7). The indole-diterpene lolitrem B and ergot alkaloid ergovaline have significant detrimental effects on the health and production of stock animals that graze infected pastures (7,8). The lolines are insecticidal (9), and peramine is a potent deterrent of feeding by insects, including the agriculturally important invertebrate pest Listronotus bonariensis (Argentine stem weevil) (10-12).Peramine synthesis is catalyzed by the two-module nonribosomal peptide synthetase (NRPS), peramine synthetase (PerA), encoded by the 8.3-kb gene perA (12). The first module of PerA contains an adenylation (A1) domain responsible for selection and activation of t...

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Nonribosomal peptide synthetases (NRPSs) generate the core peptide scaffolds of many natural products. These include small cyclic dipeptides such as the insect feeding deterrent peramine, which is a pyrrolopyrazine (PPZ) produced by grass-endophyticEpichloëfungi. Biosynthesis of peramine is catalyzed by the 2-module NRPS, PpzA-1, which has a C-terminal reductase (R) domain that is required for reductive release and cyclization of the NRPS-tethered dipeptidyl-thioester intermediate. However, some PpzA variants lack this R domain due to insertion of a transposable element into the 3′ end ofppzA. We demonstrate here that these truncated PpzA variants utilize nonenzymatic cyclization of the dipeptidyl thioester to a 2,5-diketopiperazine (DKP) to synthesize a range of novel PPZ products. Truncation of the R domain is sufficient to subfunctionalize PpzA-1 into a dedicated DKP synthetase, exemplified by the truncated variant, PpzA-2, which has also evolved altered substrate specificity and reducedN-methyltransferase activity relative to PpzA-1. Further allelic diversity has been generated by recombination-mediated domain shuffling betweenppzA-1andppzA-2, resulting in theppzA-3andppzA-4alleles, each of which encodes synthesis of a unique PPZ metabolite. This research establishes that efficient NRPS-catalyzed DKP biosynthesis can occur in vivo through nonenzymatic dipeptidyl cyclization and presents a remarkably clean example of NRPS evolution through recombinant exchange of functionally divergent domains. This work highlights that allelic variants of a single NRPS can result in a surprising level of secondary metabolite diversity comparable to that observed for some gene clusters.

Summary Epichloë festucae is an endophytic fungus that forms a symbiotic association with Lolium perenne. Here we analysed how the metabolome of the ryegrass apoplast changed upon infection of this host with sexual and asexual isolates of E. festucae. A metabolite fingerprinting approach was used to analyse the metabolite composition of apoplastic wash fluid from uninfected and infected L. perenne. Metabolites enriched or depleted in one or both of these treatments were identified using a set of interactive tools. A genetic approach in combination with tandem MS was used to identify a novel product of a secondary metabolite gene cluster. Metabolites likely to be present in the apoplast were identified using MarVis in combination with the BioCyc and KEGG databases, and an in‐house Epichloë metabolite database. We were able to identify the known endophyte‐specific metabolites, peramine and epichloëcyclins, as well as a large number of unknown markers. To determine whether these methods can be applied to the identification of novel Epichloë‐derived metabolites, we deleted a gene encoding a NRPS (lgsA) that is highly expressed in planta. Comparative MS analysis of apoplastic wash fluid from wild‐type‐ vs mutant‐infected plants identified a novel Leu/Ile glycoside metabolite present in the former.

Summary Peramine is a non‐ribosomal peptide‐derived pyrrolopyrazine (PPZ)‐containing molecule with anti‐insect properties. Peramine is known to be produced by fungi from genus Epichloë, which form mutualistic endophytic associations with cool‐season grass hosts. Peramine biosynthesis has been proposed to require only the two‐module non‐ribosomal peptide synthetase (NRPS) peramine synthetase (PerA), which is encoded by the 8.3 kb gene perA, though this has not been conclusively proven. Until recently, both peramine and perA were thought to be exclusive to fungi of genus Epichloë; however, a putative perA homologue was recently identified in the genome of the insect‐pathogenic fungus Metarhizium rileyi. We use a heterologous expression system and a hydrophilic interaction chromatography‐based analysis method to confirm that PerA is the only pathway‐specific protein required for peramine biosynthesis. The perA homologue from M. rileyi (MR_perA) is shown to encode a functional peramine synthetase, establishing a precedent for distribution of perA orthologs beyond genus Epichloë. Furthermore, perA is part of a larger seven‐gene PPZ cluster in M. rileyi, Metarhizium majus and the stalked‐cup lichen fungus Cladonia grayi. These PPZ genes encode proteins predicted to derivatize peramine into more complex PPZ metabolites, with the orphaned perA gene of Epichloë spp. representing an example of reductive evolution.

Calcineurin is a conserved calcium/calmodulin-dependent protein phosphatase, consisting of a catalytic subunit A and a regulatory subunit B, which is involved in calcium-dependent signalling and regulation of various important cellular processes. In this study, we functionally characterized the catalytic subunit A (CnaA) of the endophytic fungus Epichloë festucae which forms a symbiotic association with the grass host Lolium perenne. We deleted the CnaA-encoding gene cnaA in E. festucae and examined its role in hyphal growth, cell wall integrity and symbiosis. This ΔcnaA strain had a severe growth defect with loss of radial growth and hyper-branched hyphae. Transmission electron microscopy and confocal microscopy analysis of the mutant revealed cell wall defects, aberrant septation and the formation of intrahyphal hyphae, both in culture and in planta. The mutant strain also showed a reduced infection rate in planta. The fluorescence of mutant hyphae stained with WGA-AF488 was reduced, indicating reduced chitin accessibility. Together, these results show that E. festucae CnaA is required for fungal growth, maintaining cell wall integrity and host colonization.

Fungi from the genus Epichloë form systemic endobiotic infections of cool season grasses, producing a range of host-protective natural products in return for access to nutrients. These infections are asymptomatic during vegetative host growth, with associations between asexual Epichloë spp. and their hosts considered mutualistic. However, the sexual cycle of Epichloë spp. involves virulent growth, characterized by the envelopment and sterilization of a developing host inflorescence by a dense sheath of mycelia known as a stroma. Microscopic analysis of stromata revealed a dramatic increase in hyphal propagation and host degradation compared to asymptomatic tissues. RNAseq was used to identify differentially expressed genes (DEGs) in asymptomatic vs. stromatized tissues from three diverse Epichloë–host associations. Comparative analysis identified a core set of 135 DEGs that exhibited conserved transcriptional changes across all three associations. The core DEGs more strongly expressed during virulent growth encode proteins associated with host suppression, digestion, adaptation to the external environment, a biosynthetic gene cluster (BGC), and five transcription factors that may regulate Epichloë stroma formation. An additional five transcription factor encoding DEGs were suppressed during virulent growth, suggesting they regulate mutualistic processes. Expression of BGCs for natural products that suppress herbivory was universally suppressed during virulent growth, and additional BGCs that may encode production of novel host-protective natural products were identified. A comparative analysis of 26 Epichloë genomes found a general decrease in core DEG conservation among asexual species, and a specific decrease in conservation for the BGC expressed during virulent growth and an unusual uncharacterized gene.