Surrogate transformation of perennial ryegrass, Lolium perenne, using genetically modified Acremonium endophyte

Model Application of Entomopathogenic Fungi as Alternatives to Chemical Pesticides: Prospects, Challenges, and Insights for Next-Generation Sustainable Agriculture

In the past few decades, the control of pests and diseases of cultivated plants using natural and biological measures has drawn increasing attention in the quest to reduce the level of dependence on chemical products for agricultural production. The use of living organisms, predators, parasitoids, and microorganisms, such as viruses, bacteria, and fungi, has proven to be a viable and sustainable pest management technique. Among the aforementioned, fungi, most importantly the insect-pathogenic species, have been in use for more than 150years. These include the most popular strains belonging to the genera Beauveria, Metarhizium, Isaria, Hirsutella, and Lecanicillium. Their application is usually through an inundative approach, which inherently involves exposure of the fungal spores to unfavorable humidity, temperature, and solar radiation conditions. These abiotic factors reduce the persistence and efficacy of these insect-pathogenic fungi. Despite these limitations, over 170 strains have been formulated as mycopesticides and are available for commercial use. In the last few decades, numerous studies have suggested that these species of entomopathogenic fungi (EPF) offer far more benefits and have broader ecological functions than hitherto presumed. For instance, aside from their roles as insect killers, it has been well established that they also colonize various host plants and, hence, provide other benefits including plant pathogen antagonism and plant growth promotion and serve as sources of novel bioactive compounds and secondary metabolites, etc. In this light, the potential of EPF as alternatives or perhaps as supplements to chemical pesticides in plant protection is discussed in this review. The paper highlights the numerous benefits associated with endophytic fungal entomopathogen and host plant associations, the mechanisms involved in mediating plant defense against pests and pathogens, and the general limitations to the use of EPF in plant protection. A deeper understanding of these plant host-fungus-insect relationships could help unveil the hidden potentials of fungal endophytes, which would consequently increase the level of acceptance and adoption by users as an integral part of pest management programs and as a suitable alternative to chemical inputs toward sustainable crop production.

Epichloë Fungal Endophytes-From a Biological Curiosity in Wild Grasses to an Essential Component of Resilient High Performing Ryegrass and Fescue Pastures

The relationship between Epichloë endophytes found in a wide range of temperate grasses spans the continuum from antagonistic to mutualistic. The diversity of asexual mutualistic types can be characterised by the types of alkaloids they produce in planta. Some of these are responsible for detrimental health and welfare issues of ruminants when consumed, while others protect the host plant from insect pests and pathogens. In many temperate regions they are an essential component of high producing resilient tall fescue and ryegrass swards. This obligate mutualism between fungus and host is a seed-borne technology that has resulted in several commercial products being used with high uptake rates by end-user farmers, particularly in New Zealand and to a lesser extent Australia and USA. However, this has not happened by chance. It has been reliant on multi-disciplinary research teams undertaking excellent science to understand the taxonomic relationships of these endophytes, their life cycle, symbiosis regulation at both the cellular and molecular level, and the impact of secondary metabolites, including an understanding of their mammalian toxicity and bioactivity against insects and pathogens. Additionally, agronomic trials and seed biology studies of these microbes have all contributed to the delivery of robust and efficacious products. The supply chain from science, through seed companies and retailers to the end-user farmer needs to be well resourced providing convincing information on the efficacy and ensuring effective quality control to result in a strong uptake of these Epichloë endophyte technologies in pastoral agriculture.

DNA demethylation overcomes attenuation of colchicine biosynthesis in an endophytic fungus Diaporthe

Fungal endophytes, a major component of the plant host microbiome, are known to synthesize plant-derived metabolites in vitro. However, attenuation of metabolite production upon repeated sub-culturing is a major drawback towards utilizing them as an alternative for plant-derived metabolites. In this study, we isolated Diaporthe perseae, a fungal endophyte from Gloriosa superba tubers, which showed the production of colchicine in axenic cultures. Mass spectrometry, Nuclear Magnetic Resonance spectroscopy, and tubulin polymerization assays confirmed the compound to be colchicine. Repeated sub-culturing of the endophyte for 10 generations led to a reduction in the yield of the metabolite from 55.25 μg/g to 2.32 μg/g of mycelial dry weight. Treatment of attenuated cultures with DNA methylation inhibitor 5-azacytidine resulted in increased metabolite concentration (39.68 μg/g mycelial dry weight) in treated samples compared to control (2.61 μg/g mycelial dry weight) suggesting that 5-azacytidine can induce demethylation of the fungal genome to overcome the phenomenon of attenuation of metabolite synthesis. Reduced levels of global methylation were observed upon 5-azacytidine treatment in attenuated cultures (0.41 % of total cytosines methylated) as compared to untreated control (0.78 % of total cytosines methylated). The results provide a significant breakthrough in utilizing fungal endophytes as a veritable source of plant-derived metabolites from critically endangered plants.

Restoration of camptothecine production in attenuated endophytic fungus on re-inoculation into host plant and treatment with DNA methyltransferase inhibitor

Fungal endophytes inhabit living tissues of plants without any apparent symptoms and in many cases are known to produce secondary metabolites similar to those produced by their respective host plants. However on sub-culture, the endophytic fungi gradually attenuate their ability to produce the metabolites. Attenuation has been a major constraint in realizing the potential of endophytic fungi as an alternative source of plant secondary metabolites. In this study, we report attempts to restore camptothecine (CPT) production in attenuated endophytic fungi isolated from CPT producing plants, Nothapodytes nimmoniana and Miquelia dentata when they are passed through their host plant or plants that produce CPT and when treated with a DNA methyl transferase inhibitor. Attenuated endophytic fungi that traversed through their host tissue or plants capable of synthesizing CPT, produced significantly higher CPT compared to the attenuated fungi. Attenuated fungus cultured in the presence of 5-azacytidine, a DNA methyltransferase inhibitor, had an enhanced CPT content compared to untreated attenuated fungus. These results indicate that the attenuation of CPT production in endophytic fungi could in principle be reversed by eliciting some signals from plant tissue, most likely that which prevents the methylation or silencing of the genes responsible for CPT biosynthesis.

Alternaria sp. MG1, a resveratrol-producing fungus: isolation, identification, and optimal cultivation conditions for resveratrol production

Due to its potential in preventing or slowing the occurrence of many diseases, resveratrol (3,5,4'-trihydroxystilbene) has attracted great research interest. The objective of this study was to identify microorganisms from selected plants that produce resveratrol and to optimize the conditions for resveratrol production. Endophytes from Merlot wine grapes (Vitis vinifera L. cv. Merlot), wild Vitis (Vitis quinquangularis Rehd.), and Japanese knotweed (Polygonum cuspidatum Siebold & Zucc.) were isolated, and their abilities to produce resveratrol were evaluated. A total of 65 isolates were obtained and 21 produced resveratrol (6-123 μg/L) in liquid culture. The resveratrol-producing isolates belonged to seven genera, Botryosphaeria, Penicillium, Cephalosporium, Aspergillus, Geotrichum, Mucor, and Alternaria. The resveratrol-producing capability decreased or was completely lost in most isolates after three rounds of subculture. It was found that only the strain Alternaria sp. MG1 (isolated from cob of Merlot using GA1 medium) had stable and high resveratrol-producing capability in all subcultures. During liquid cultivation of Alternaria sp. MG1 in potato dextrose medium, the synthesis of resveratrol began on the first day, increased to peak levels on day 7, and then decreased sharply thereafter. Cell growth increased during cultivation and reached a stable and high level of biomass after 5 days. The best fermentation conditions for resveratrol production in liquid cultures of Alternaria sp. MG1 were an inoculum size of 6 %, a medium volume of 125 mL in a 250-mL flask, a rotation speed of 101 rpm, and a temperature of 27 °C.

An old yellow enzyme gene controls the branch point between Aspergillus fumigatus and Claviceps purpurea ergot alkaloid pathways

Ergot fungi in the genus Claviceps and several related fungal groups in the family Clavicipitaceae produce toxic ergot alkaloids. These fungi produce a variety of ergot alkaloids, including clavines as well as lysergic acid derivatives. Ergot alkaloids are also produced by the distantly related, opportunistic human pathogen Aspergillus fumigatus. However, this fungus produces festuclavine and fumigaclavines A, B, and C, which collectively differ from clavines of clavicipitaceous fungi in saturation of the last assembled of four rings in the ergoline ring structure. The two lineages are hypothesized to share early steps of the ergot alkaloid pathway before diverging at some point after the synthesis of the tricyclic intermediate chanoclavine-I. Disruption of easA, a gene predicted to encode a flavin-dependent oxidoreductase of the old yellow enzyme class, in A. fumigatus led to accumulation of chanoclavine-I and chanoclavine-I-aldehyde. Complementation of the A. fumigatus easA mutant with a wild-type allele from the same fungus restored the wild-type profile of ergot alkaloids. These data demonstrate that the product of A. fumigatus easA is required for incorporation of chanoclavine-I-aldehyde into more-complex ergot alkaloids, presumably by reducing the double bond conjugated to the aldehyde group, thus facilitating ring closure. Augmentation of the A. fumigatus easA mutant with a homologue of easA from Claviceps purpurea resulted in accumulation of ergot alkaloids typical of clavicipitaceous fungi (agroclavine, setoclavine, and its diastereoisomer isosetoclavine). These data indicate that functional differences in the easA-encoded old yellow enzymes of A. fumigatus and C. purpurea result in divergence of their respective ergot alkaloid pathways.

Study on the diversity of endophytic communities from rice (Oryza sativa L.) and their antagonistic activities in vitro

Endophytic populations were isolated from 2400 segments of Oryza sativa collected from Bhadra River Project Area, Southern India during December 2005 (Winter) and April 2006 (Summer). Overall colonization rates from surface sterilized tissues were 40.3% in roots and 25.83% in leaves during winter season, 20.15% in roots and 8.66% in leaves during summer season. Nineteen different fungal taxa, a Streptomyces sp. and bacterial species were isolated. Streptomyces sp., Chaetomium globosum, Penicillium chrysogenum, Fusarium oxysporum and Cladosporium cladosporioides were dominant endophytes in this study. Frequency of colonization between the sites, seasons and rice varieties were found to differ significantly. Dual culture studies revealed that C. globosum, P. chrysogenum and Streptomyces sp. are suitable candidates for extraction of biologically active compounds. Rice harbors many endophytic organisms and some of them have antagonistic properties against fungal pathogens.

Transformation of the ryegrass endophyte Neotyphodium lolii can alter its in planta mycelial morphology

The fungus Neotyphodium lolii grows in the intercellular spaces of perennial ryegrass as a mutualistic endosymbiont. One of the benefits it conveys to the plant is the production of alkaloids toxic to herbivores. We wanted to determine in planta expression patterns of the N. lolii 3-hydroxy-3-methylglutaryl-CoA reductase (HMG CoA reductase) gene, believed to be involved in the synthesis of two of these alkaloid toxins, lolitrem B and ergovaline. We transformed the N. lolii strain Lp19 with plasmids, in which DNA fragments upstream of the open reading frame of the N. lolii HMG CoA reductase gene controlled expression of the GUS (gusA; Escherichia coli beta-glucuronidase) reporter gene. In exponentially growing cultures, the GUS gene was not expressed if the length of upstream sequence was less than 400 bp, and >1100 bp were required for maximum expression. When reintroduced into ryegrass plants, transformants often showed highly increased hyphal branching compared to the wild-type parent strain, although in culture their growth kinetics and morphology were indistinguishable from that of the wild-type. Deterioration of hyphae and the hypha-plant interface occurred and in one transformant reduced tillering (formation of new plants, referred to in agronomy as tillers) and death of infected plants. We found no evidence that these abnormalities were caused by interference of the construct with the function of the native gene, as judged by analysis of the site of integration of the promoter-GUS cassette, expression of the native gene and lolitrem B and ergovaline levels in infected plants. However, there was some correlation between GUS expression and the degree of hyphal branching, suggesting that high levels of beta-glucuronidase may disturb the symbiotic interaction. Levels of another alkaloid, peramine, were also not significantly affected by transformation. In previous studies increased in planta branching of the endophyte has been shown to be associated with a severe reduction of alkaloid production. Our results show that a plant-endophyte association in which increased branching occurs is still able to produce alkaloids.

An ergot alkaloid biosynthesis gene and clustered hypothetical genes from Aspergillus fumigatus

The ergot alkaloids are a family of indole-derived mycotoxins with a variety of significant biological activities. Aspergillus fumigatus, a common airborne fungus and opportunistic human pathogen, and several fungi in the relatively distant taxon Clavicipitaceae (clavicipitaceous fungi) produce different sets of ergot alkaloids. The ergot alkaloids of these divergent fungi share a four-member ergoline ring but differ in the number, type, and position of the side chains. Several genes required for ergot alkaloid production are known in the clavicipitaceous fungi, and these genes are clustered in the genome of the ergot fungus Claviceps purpurea. We investigated whether the ergot alkaloids of A. fumigatus have a common biosynthetic and genetic origin with those of the clavicipitaceous fungi. A homolog of dmaW, the gene controlling the determinant step in the ergot alkaloid pathway of clavicipitaceous fungi, was identified in the A. fumigatus genome. Knockout of dmaW eliminated all known ergot alkaloids from A. fumigatus, and complementation of the mutation restored ergot alkaloid production. Clustered with dmaW in the A. fumigatus genome are sequences corresponding to five genes previously proposed to encode steps in the ergot alkaloid pathway of C. purpurea, as well as additional sequences whose deduced protein products are consistent with their involvement in the ergot alkaloid pathway. The corresponding genes have similarities in their nucleotide sequences, but the orientations and positions within the cluster of several of these genes differ. The data indicate that the ergot alkaloid biosynthetic capabilities in A. fumigatus and the clavicipitaceous fungi had a common origin.

Indole-diterpene gene cluster from Aspergillus flavus

Aflatrem is a potent tremorgenic mycotoxin produced by the soil fungus Aspergillus flavus and is a member of a large structurally diverse group of secondary metabolites known as indole-diterpenes. By using degenerate primers for conserved domains of fungal geranylgeranyl diphosphate synthases, we cloned two genes, atmG and ggsA (an apparent pseudogene), from A. flavus. Adjacent to atmG are two other genes, atmC and atmM. These three genes have 64 to 70% amino acid sequence similarity and conserved synteny with a cluster of orthologous genes, paxG, paxC, and paxM, from Penicillium paxilli which are required for indole-diterpene biosynthesis. atmG, atmC, and atmM are coordinately expressed, with transcript levels dramatically increasing at the onset of aflatrem biosynthesis. A genomic copy of atmM can complement a paxM deletion mutant of P. paxilli, demonstrating that atmM is a functional homolog of paxM. Thus, atmG, atmC, and atmM are necessary, but not sufficient, for aflatrem biosynthesis by A. flavus. This provides the first genetic evidence for the biosynthetic pathway of aflatrem in A. flavus.

The determinant step in ergot alkaloid biosynthesis by an endophyte of perennial ryegrass

Many cool-season grasses harbor fungal endophytes in the genus Neotyphodium, which enhance host fitness, but some also produce metabolites--such as ergovaline--believed to cause livestock toxicoses. In Claviceps species the first step in ergot alkaloid biosynthesis is thought to be dimethylallyltryptophan (DMAT) synthase, encoded by dmaW, previously cloned from Claviceps fusiformis. Here we report the cloning and characterization of dmaW from Neotyphodium sp. isolate Lp1, an endophyte of perennial ryegrass (Lolium perenne). The gene was then disrupted, and the mutant failed to produce any detectable ergovaline or simpler ergot and clavine alkaloids. The disruption was complemented with the C. fusiformis gene, which restored ergovaline production. Thus, the biosynthetic role of DMAT synthase was confirmed, and a mutant was generated for future studies of the ecological and agricultural importance of ergot alkaloids in endophytes of grasses.

Suppression of tandem-multimer formation during genetic transformation of the mycotoxin-producing fungus Penicillium paxilli by disrupting an orthologue of Aspergillus nidulans uvsC

An orthologue of Aspergillus nidulans uvsC and Saccharomyces cerevisiae RAD51 was cloned from the filamentous fungus, Penicillium paxilli. A mutation in uvsC causes UV sensitivity during germination. The product of RAD51 is involved in meiotic recombination and DNA damage repair. The deduced amino acid sequence of the product of this gene (Pprad51) shared 92% identity with UVSC. Site-specific disruption of pprad51 showed a significant effect for extra-cellular DNA integration. Transformation of the null mutant with pII99, which confers geneticin resistance, resulted in a shift from a predominance of direct repeats at a single site to single copies when compared with a control strain. A copy-number effect of integrated pII99 for geneticin selection was suggested as the frequency of direct repeat formation was less when selected at a lower concentration in the control strain. However, such an effect was not observed in the null mutant, further supporting an involvement of Pprad51 in direct repeat formation.

Concerted evolution in the ribosomal RNA genes of an Epichloë endophyte hybrid: comparison between tandemly arranged rDNA and dispersed 5S rrn genes

We examined ribosomal RNA concerted evolution in an Epichloë endophyte interspecific hybrid (Lp1) and its progenitors (Lp5 and E8). We show that the 5S rrn genes are organized as dispersed copies. Cloned 5S gene sequences revealed two subfamilies exhibiting 12% sequence divergence, with substitutions forming coevolving pairs that maintain secondary structure and presumably function. Observed sequence patterns are not fully consistent with either concerted or classical evolution. The 5S rrn genes are syntenic with the tandemly arranged rDNA genes, despite residing outside the rDNA arrays. We also examined rDNA concerted evolution. Lp1 has rDNA sequence from only one progenitor and contains multiple rDNA arrays. Using 5S rrn genes as chromosomal markers, we propose that interlocus homogenization has replaced all Lp5 rDNA sequence with E8 sequence in the hybrid. This interlocus homogenization appears to have been rapid and efficient and is the first demonstration of hybrid interlocus homogenization in the Fungi.

In planta regulation of extension of an endophytic fungus and maintenance of high metabolic rates in its mycelium in the absence of apical extension

The fungus Neotyphodium lolii is an endophytic symbiont. It grows in the intercellular spaces of the perennial ryegrass Lolium perenne, producing secondary metabolites which enhance the fitness of the association over that of uninfected L. perenne. We report that the average number of hyphal strands in a given section of a leaf remains constant during the life of a leaf, indicating synchrony of leaf and hyphal extension, including cessation of hyphal extension when leaf extension ceases. We used a constitutively expressed reporter gene as an indicator of the mycelium's metabolic activity during and after hyphal extension. Reporter gene activity decreased when the mycelium stopped extending in liquid culture but not in planta. This indicates that in planta endophyte hyphae remain metabolically highly active when extension has ceased and throughout the life of the leaf they are colonizing. The behavior of the fungus in planta indicates the existence of signaling pathways which (i) synchronize the extension of leaf and hypha by regulating hyphal extension, (ii) suppress hyphal branching, and (iii) stop apical extension of fungal hyphae, without reducing the mycelium's metabolic activity. These signals may be crucial for the symbiosis, by allowing the endophyte to switch the focus of its metabolic activity from extension to the production of secondary metabolites.

Elimination of ergovaline from a grass-Neotyphodium endophyte symbiosis by genetic modification of the endophyte

The fungal endophytes Neotyphodium lolii and Neotyphodium sp. Lp1 from perennial ryegrass (Lolium perenne), and related endophytes in other grasses, produce the ergopeptine toxin ergovaline, among other alkaloids, while also increasing plant fitness and resistance to biotic and abiotic stress. In the related fungus, Claviceps purpurea, the biosynthesis of ergopeptines requires the activities of two peptide synthetases, LPS1 and LPS2. A peptide synthetase gene hypothesized to be important for ergopeptine biosynthesis was identified in C. purpurea by its clustering with another ergot alkaloid biosynthetic gene, dmaW. Sequence analysis conducted independently of the research presented here indicates that this gene encodes LPS1 [Tudzynski, P., Holter, K., Correia, T., Arntz, C., Grammel, N. & Keller, U. (1999) Mol. Gen. Genet. 261, 133-141]. We have cloned a similar peptide synthetase gene from Neotyphodium lolii and inactivated it by gene knockout in Neotyphodium sp. Lp1. The resulting strain retained full compatibility with its perennial ryegrass host plant as assessed by immunoblotting of tillers and quantitative PCR. However, grass-endophyte associations containing the knockout strain did not produce detectable quantities of ergovaline as analyzed by HPLC with fluorescence detection. Disruption of this gene provides a means to manipulate the accumulation of ergovaline in endophyte-infected grasses for the purpose of determining the roles of ergovaline in endophyte-associated traits and, potentially, for ameliorating toxicoses in livestock.

Epichloë festucae and related mutualistic symbionts of grasses

Epichloë and Neotyphodium species (Ascomycota) are mutualistic symbionts (endophytes) of temperate grasses, to which they impart numerous and profound fitness benefits. Epichloë festucae, a common symbiont of Festuca, Lolium,and Koeleria spp., is a model for endophyte research that is amenable to Mendelian and molecular genetic analysis. Characteristics of E. festucae include: (i) production of the anti-insect alkaloids peramine and lolines, (ii) production of the anti-vertebrate alkaloids lolitrem B and ergovaline, (iii) efficient vertical transmission via host seeds, (iv) a mildly pathogenic state associated with the E. festucae sexual cycle, and (v) a clear role in enhancing survival of host plants. Genetic analysis of alkaloid production has recently begun. Also, physiological and ultrastructural studies suggest that signals communicated between E. festucae and host plants ensure an exquisitely balanced interaction to the mutual benefit of both partners. Several mutualistic Neotyphodium species are hybrids between E. festucae and other endophyte species.

Identification of Epichloë endophytes in planta by a microsatellite-based PCR fingerprinting assay with automated analysis

Epichloë endophytes are a group of filamentous fungi that include both sexual (Epichloë) and asexual (Neotyphodium) species. As a group they are genetically diverse and form both antagonistic and mutualistic associations with temperate grasses. We report here on the development of a microsatellite-based PCR system for fingerprinting this group of fungi with template isolated from either culture or infected plant material. M13mp19 partial genomic libraries were constructed for size-fractionated genomic DNA from two endophyte strains. These libraries were screened with a mixture of DIG-labeled dinucleotide and trinucleotide repeat probes. Positive clones were sequenced, and nine unique microsatellite loci were identified. An additional microsatellite was serendipitously identified in the 3' untranscribed region of the 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase gene from N. lolii Lp19. Primers were designed for each locus and a panel of endophytes, from different taxonomic groupings, was screened to determine the degree of polymorphism. On the basis of these results a multiplex assay was developed for strain identification with fluorescently labeled primers for five of these loci. Using this system the size of the products amplified can be precisely determined by automated analysis, and an allele profile for each strain can be readily generated. The assay was shown to resolve endophyte groupings to the level of known isozyme phenotype groupings. In a blind test the assay was used successfully to identify a set of endophytes in planta. A reference database of allele sizes has been established for the panel of endophytes examined, and this will be expanded as new strains are analyzed.

Quantitative assessment of in planta distribution of metabolic activity and gene expression of an endophytic fungus

Using perennial ryegrass infected with an Acremonium transformant carrying the Escherichia coli beta-D-glucuronidase gene (gusA) (GUS system) under control of a constitutive promoter, we have developed methods for the quantitative extraction of endophyte-associated GUS activity from plant material. Fluorometric assays of these extracts allow quantitative assessment of the distribution of endophyte-associated GUS activity within single plants (tillers) with high resolution. Fluorescence microscopy with the dye Imagene Green can in addition visualize individual GUS-expressing hyphae. Since the transformant expresses the GUS gene constitutively, GUS activity can be used as an indicator of in planta endophyte metabolic activity. Using this approach we found that (i) the concentration of endophyte metabolic activity in plant tissue decreases with increasing plant size, (ii) approximately 70% of endophyte metabolic activity present in a plant is located in the leaf sheaths, (iii) basal-apical gradients and lateral (younger to older tissue) gradients of endophyte metabolic activity exist and (iv) basal-apical gradients are established early in leaf development. Our data suggest that the concentration of endophyte in each part of the plant is regulated so that a predetermined threshold of total endophyte activity per plant is not exceeded and a consistent distribution pattern is maintained.

EPICHLOE SPECIES: fungal symbionts of grasses

Epichloë species and their asexual descendants (Acremonium endophytes) are fungal symbionts of C3 grasses that span the symbiotic continuum from antagonism to mutualism depending on the relative importance, respectively, of horizontal transmission of sexual spores versus vertical clonal transmission in healthy grass seeds. At least seven sexual Epichloë species are identifiable by mating tests, and many asexual genotypes are interspecific hybrids. Benefits conferred by the symbionts on host plants include protection from biotic factors and abiotic stresses such as drought. Four classes of beneficial alkaloids are associated with the symbionts: ergot alkaloids, indolediterpenes (lolitrems), peramine, and saturated aminopyrrolizidines (lolines). These alkaloids protect host plants from insect and vertebrate herbivores, including livestock. Genetic engineering of the fungal symbionts as more suitable biological protectants for forage grasses requires identification of fungal genes for alkaloid biosynthesis, and DNA-mediated transformation of the fungi.

A mutualistic fungal symbiont of perennial ryegrass contains two different pyr4 genes, both expressing orotidine-5'-monophosphate decarboxylase

A fragment of the Claviceps purpurea pyr4 gene, encoding orotidine-5'-monophosphate decarboxylase (OMP decarboxylase), was used to screen a genomic library from an isolate of a fungus, Acremonium sp. (designated Lp1), which grows as an endophyte in perennial ryegrass (Lolium perenne). Three positive clones, lambda MC11, lambda MC12 and lambda MC14, were isolated. Two of these clones, lambda MC12 and lambda MC14, were overlapping clones from the same locus, while lambda MC11 was from a different locus. Fragments of these clones which hybridised with C. purpurea pyr4 were sequenced and found to have similarity with pyr4 from other Pyrenomycete fungi. The pyr4 gene from lambda MC12 and lambda MC14 was designated pyr4-1 and that from lambda MC11 was designated pyr4-2. The predicted ORFs of the two genes were highly conserved, with 97.5% identity at the nucleotide level, the 5' non-coding sequences were the least conserved with 88.5% identity and the 3' non-coding sequences had 93.0% identity. RT-PCR analysis of total RNA from Lp1 demonstrated that transcripts from the two genes were present at similar levels, and hybridisation of pyr4-1 to Northern blots of total RNA from Lp1 showed that full-length transcripts were being produced. Genomic fragments containing pyr4 were transformed into a strain of Aspergillus nidulans which has a mutation in pyrG (encoding OMP decarboxylase). Both pyr4-1 and pyr4-2 complemented the pyrG mutation in A. nidulans, indicating that both encode functional OMP decarboxylases. It has been proposed [Schardl et al., Genetics 136 (1994) 1307-1317] that the two pyr4 in Lp1 arose by interspecific hybridisation, most likely between the ryegrass choke pathogen, Epichloë typhina, and another endophyte from perennial ryegrass, Acremonium lolii. Analysis by PCR amplification and direct sequencing of the variable 5' non-coding regions of pyr4, from possible ancestors to Lp1 supports this hypothesis. Comparisons of these sequences to the 5' non-coding sequences from pyr4-1 and pyr4-2 demonstrated that E. typhina and A. lolii were the most likely ancestors of the two pyr4 found in Lp1.

Heterologous and homologous plasmid integration at a spore-pigment locus in Penicillium paxilli generates large deletions

Mutations in a spore pigmentation locus (brs; brown spore) in Penicillium paxilli were isolated at a relatively-high frequency (0.17%) following integrative transformation of the hygromycin-resistance plasmid pAN7-1. A molecular analysis of four independently-isolated Brs- mutants showed that all contained pAN7-1 integrated at a single-site that was unique for each mutant. A previously-described Brs- mutant, YI-34 (Itoh et al. 1994), was a two-site integration. Three of the mutants had multiple copies of pAN7-1 arranged in head-to-tail tandem arrays. A 9.6-kb BamHI junction fragment was cloned from one of these, YI-33, by plasmid rescue and used to isolate two overlapping lambda clones, lambda WB33-1 and lambda WB33-2, that span about 30 kb in the region of the wild-type locus. When genomic digests of the five Brs- mutants were probed with these lambda clones all of them were found to contain an extensive deletion through a common region of the P. paxilli genome. Subsequent attempts to generate one-step gene replacements within a 4.5-kb EcoRI fragment at the wild-type locus resulted in the isolation of Brs- mutants at a frequency of 1.6%, but all mutants with this phenotype were also found to contain an extensive genomic deletion. Therefore, a common outcome of both heterologous and homologous plasmid integration at this locus is deletion formation.

Heterologous transformation of Zalerion arboricola

A heterologous DNA-mediated transformation system was developed for the pneumocandin-producing fungus Z. arboricola that was based on either conferral of hygromycin B resistance or complementation of a nitrate reductase mutant. Hygromycin-resistant transformants were selected with plasmid pCSN43 which contains the E. coli hygromycin B phosphotransferase gene under the control of Aspergillus nidulans trpC transcription signals. Transformation frequencies were about four transformants per microgram of circular DNA and could be improved four- to six-fold by linearizing the transforming DNA. The transformants differed from one another with respect to the copy number of the integrated plasmid and the site of integration. Adding an autonomously-replicating sequence (AMA1) from A. nidulans to pCSN43 enhanced transformation three-fold and produced, in addition, numerous abortive transformants. However, it is unlikely that the AMA1 sequence promoted plasmid replication in Z. arboricola. Nitrate reductase mutants of Z. arboricola were isolated by positive selection on chlorate-containing medium, and one mutant was subsequently transformed with pSTA700 which contains the nitrate reductase gene (niaD) from Cephalosporium acremonium. Introduction of the niaD gene restored sensitivity to chlorate in the mutant; therefore, using the niaD gene as a selectable marker provides a system for both positive and negative selection. To our knowledge, this is the first report describing transformation of a member of the genus Zalerion.

Integrative transformation of the mycotoxin-producing fungus, Penicillium paxilli

A high frequency transformation system has been developed for Penicillium paxilli using pAN7-1. Up to 44% of the primary transformants were heterokaryons. Loss of hygromycin resistance was observed in primary transformants that were sub-cultured on non-selective media, but single spores of these primary transformants were mitotically stable on both selective and non-selective media. A molecular analysis of the transformants generated showed that 78% had single-site integrations, with half of these containing a single copy of pAN7-1. CHEF-gel electrophoresis showed that P. paxilli has at least six chromosomes with a total genome size of about 23.4 Mb.

Fungal symbionts of grasses: evolutionary insights and agricultural potential

Some filamentous fungal endophytes confer on their grass hosts important biological properties including resistance to grazing herbivores and resistance to nematodes and some fungal pathogens, as well as drought tolerance and greater field persistence. The production of alkaloids toxic to grazing animals is an undesirable aspect of the association in agronomic situations. Consequently, genetic strategies are being pursued to manipulate fungal endophytes and their hosts for agricultural benefit.