Transformation of the rice blast fungus Magnaporthe grisea to hygromycin B resistance

Pyricularia oryzae: Lab star and field scourge

Pyricularia oryzae (syn. Magnaporthe oryzae), is a filamentous ascomycete that causes a major disease called blast on cereal crops, as well as on a wide variety of wild and cultivated grasses. Blast diseases have a tremendous impact worldwide particularly on rice and on wheat, where the disease emerged in South America in the 1980s, before spreading to Asia and Africa. Its economic importance, coupled with its amenability to molecular and genetic manipulation, have inspired extensive research efforts aiming at understanding its biology and evolution. In the past 40 years, this plant-pathogenic fungus has emerged as a major model in molecular plant-microbe interactions. In this review, we focus on the clarification of the taxonomy and genetic structure of the species and its host range determinants. We also discuss recent molecular studies deciphering its lifecycle.

TAXONOMY

Kingdom: Fungi, phylum: Ascomycota, sub-phylum: Pezizomycotina, class: Sordariomycetes, order: Magnaporthales, family: Pyriculariaceae, genus: Pyricularia.

HOST RANGE

P. oryzae has the ability to infect a wide range of Poaceae. It is structured into different host-specialized lineages that are each associated with a few host plant genera. The fungus is best known to cause tremendous damage to rice crops, but it can also attack other economically important crops such as wheat, maize, barley, and finger millet.

DISEASE SYMPTOMS

P. oryzae can cause necrotic lesions or bleaching on all aerial parts of its host plants, including leaf blades, sheaths, and inflorescences (panicles, spikes, and seeds). Characteristic symptoms on leaves are diamond-shaped silver lesions that often have a brown margin and whose appearance is influenced by numerous factors such as the plant genotype and environmental conditions. USEFUL WEBSITES Resources URL Genomic data repositories http://genome.jouy.inra.fr/gemo/ Genomic data repositories http://openriceblast.org/ Genomic data repositories http://openwheatblast.net/ Genome browser for fungi (including P. oryzae) http://fungi.ensembl.org/index.html Comparative genomics database https://mycocosm.jgi.doe.gov/mycocosm/home T-DNA mutant database http://atmt.snu.kr/ T-DNA mutant database http://www.phi-base.org/ SNP and expression data https://fungidb.org/fungidb/app/.

Metabolic efficiency reshapes the seminal relationship between pathogen growth rate and virulence

A cornerstone of classical virulence evolution theories is the assumption that pathogen growth rate is positively correlated with virulence, the amount of damage pathogens inflict on their hosts. Such theories are key for incorporating evolutionary principles into sustainable disease management strategies. Yet, empirical evidence raises doubts over this central assumption underpinning classical theories, thus undermining their generality and predictive power. In this paper, we identify a key component missing from current theories which redefines the growth-virulence relationship in a way that is consistent with data. By modifying the activity of a single metabolic gene, we engineered strains of Magnaporthe oryzae with different nutrient acquisition and growth rates. We conducted in planta infection studies and uncovered an unexpected non-monotonic relationship between growth rate and virulence that is jointly shaped by how growth rate and metabolic efficiency interact. This novel mechanistic framework paves the way for a much-needed new suite of virulence evolution theories.

A Botybirnavirus Isolated from Alternaria tenuissima Confers Hypervirulence and Decreased Sensitivity of Its Host Fungus to Difenoconazole

Alternaria alternata botybirnavirus 1 (AaBRV1) was isolated from a strain of Alternaria alternata, causing watermelon leaf blight in our previous research. The effect of AaBRV1 on the phenotype of its host fungus, however, was not determined. In the present study, a novel strain of AaBRV1 was identified in A. tenuissima strain TJ-NH-51S-4, the causal agent of cotton Alternaria leaf spot, and designated as AaBRV1-AT1. A mycovirus AaBRV1-AT1-free strain TJ-NH-51S-4-VF was obtained by protoplast regeneration, which eliminated AaBRV1-AT1 from the mycovirus AaBRV1-AT1-infected strain TJ-NH-51S-4. Colony growth rate, spore production, and virulence of strain TJ-NH-51S-4 were greater than they were in TJ-NH-51S-4-VF, while the sensitivity of strain TJ-NH-51S-4 to difenoconazole, as measured by the EC50, was lower. AaBRV1-AT1 was capable of vertical transmission via asexual spores and horizontal transmission from strain TJ-NH-51S-4 to strain XJ-BZ-5-1hyg (another strain of A. tenuissima) through hyphal contact in pairing cultures. A total of 613 differentially expressed genes (DEGs) were identified in a comparative transcriptome analysis between TJ-NH-51S-4 and TJ-NH-51S-4-VF. Relative to strain TJ-NH-51S-4-VF, the number of up-regulated and down-regulated DEGs in strain TJ-NH-51S-4 was 286 and 327, respectively. Notably, the expression level of one DEG-encoding cytochrome P450 sterol 14α-demethylase and four DEGs encoding siderophore iron transporters were significantly up-regulated. To our knowledge, this is the first documentation of hypervirulence and reduced sensitivity to difenoconazole induced by AaBRV1-AT1 infection in A. tenuissima.

Fungal oxysterol-binding protein-related proteins promote pathogen virulence and activate plant immunity

Oxysterol-binding protein-related proteins (ORPs) are a conserved class of lipid transfer proteins that are closely involved in multiple cellular processes in eukaryotes, but their roles in plant-pathogen interactions are mostly unknown. We show that transient expression of ORPs of Magnaporthe oryzae (MoORPs) in Nicotiana benthamina plants triggered oxidative bursts and cell death; treatment of tobacco Bright Yellow-2 suspension cells with recombinant MoORPs elicited the production of reactive oxygen species. Despite ORPs being normally described as intracellular proteins, we detected MoORPs in fungal culture filtrates and intercellular fluids from barley plants infected with the fungus. More importantly, infiltration of Arabidopsis plants with recombinant Arabidopsis or fungal ORPs activated oxidative bursts, callose deposition, and PR1 gene expression, and enhanced plant disease resistance, implying that ORPs may function as endogenous and exogenous danger signals triggering plant innate immunity. Extracellular application of fungal ORPs exerted an opposite impact on salicylic acid and jasmonic acid/ethylene signaling pathways. Brassinosteroid Insensitive 1-associated Kinase 1 was dispensable for the ORP-activated defense. Besides, simultaneous knockout of MoORP1 and MoORP3 abolished fungal colony radial growth and conidiation, whereas double knockout of MoORP1 and MoORP2 compromised fungal virulence on barley and rice plants. These observations collectively highlight the multifaceted role of MoORPs in the modulation of plant innate immunity and promotion of fungal development and virulence in M. oryzae.

Optimization of Agrobacterium tumefaciens-Mediated Transformation of Xylaria grammica EL000614, an Endolichenic Fungus Producing Grammicin

An endolichenic fungus Xylaria grammica EL000614 produces grammicin, a potent nematicidal pyrone derivative that can serve as a new control option for root-knot nematodes. We optimized an Agrobacterium tumefaciens-mediated transformation (ATMT) protocol for X. grammica to support genetic studies. Transformants were successfully generated after co-cultivation of homogenized young mycelia of X. grammica with A. tumefaciens strain AGL-1 carrying a binary vector that contains the bacterial hygromycin B phosphotransferase (hph) gene and the eGFP gene in T-DNA. The resulting transformants were mitotically stable, and PCR analysis showed the integratin of both genes in the genome of transformants. Expression of eGFP was confirmed via fluorescence microscopy. Southern analysis showed that 131 (78.9%) out of 166 transformants contained a single T-DNA insertion. Crucial factors for producing predominantly single T-DNA transformants include 48 h of co-cultivation, pre-treatment of A. tumefaciens cells with acetosyringone before co-cultivation, and using freshly prepared mycelia. The established ATMT protocol offers an efficient tool for random insertional mutagenesis and gene transfer in studying the biology and ecology of X. grammica.

Establishment of Agrobacterium tumefaciens-Mediated Transformation of Cladonia macilenta, a Model Lichen-Forming Fungus

Despite the fascinating biology of lichens, such as the symbiotic association of lichen-forming fungi (mycobiont) with their photosynthetic partners and their ability to grow in harsh habitats, lack of genetic tools manipulating mycobiont has hindered studies on genetic mechanisms underpinning lichen biology. Thus, we established an Agrobacterium tumefaciens-mediated transformation (ATMT) system for genetic transformation of a mycobiont isolated from Cladonia macilenta. A set of combinations of ATMT conditions, such as input biomass of mycobiont, co-cultivation period with Agrobacterium cells, and incubation temperature, were tested to identify an optimized ATMT condition for the C. macilenta mycobiont. As a result, more than 10 days of co-cultivation period and at least 2 mg of input biomass of the mycobiont were recommended for an efficient ATMT, owing to extremely slow growth rate of mycobionts in general. Moreover, we examined T-DNA copy number variation in a total of 180 transformants and found that 88% of the transformants had a single copy T-DNA insertion. To identify precise T-DNA insertion sites that interrupt gene function in C. macilenta, we performed TAIL-PCR analyses for selected transformants. A hypothetical gene encoding ankyrin repeats at its C-terminus was interrupted by T-DNA insertion in a transformant producing dark-brown colored pigment. Although the identification of the pigment awaits further investigation, this proof-of-concept study demonstrated the feasibility of use of ATMT in construction of a random T-DNA insertion mutant library in mycobionts for studying genetic mechanisms behind the lichen symbiosis, stress tolerance, and secondary metabolite biosynthesis.

Characterization of cmcp Gene as a Pathogenicity Factor of Ceratocystis manginecans

Ceratocystis manginecans causes mango wilt with significant economic losses. In the infection court, cerato-platanin (CP) family proteins (CPPs) are believed to involve in pathogenesis but has not been determined in C. manginecans. To confirm this function, a CP protein (CmCP) of C. manginecans was characterized in this study. A protoplast of C. manginecans was prepared by treating its mycelia with driselase and lysing enzymes. The cmcp gene was edited using CRISPR/Cas-U6-1 expression vectors in 60% PEG and 50 μg/mL hygromycin B in the medium, resulting in mutants with cmcp deletion (Δcmcp). A complemented mutant (Δcmcp-C) was obtained by transforming cmcp to Δcmcp. Both Δcmcp and Δcmcp-C were characterized by comparing them with a wild-type strain on morphology, mycelial growth, conidial production and pathogenicity. Additionally, cmcp was transformed and expressed in Pichia pastoris, and the derived recombinant protein CmCP caused a severe necrosis on Nicotiana tabacum leaves. CmCP-treated plant leaves showed symptoms of hypersensitive response including electrolyte leakage, reactive oxygen species generation and overexpression of defense-related genes PR-1, PAD3, ERF1, HSR203J, and HIN1. All those results suggested that cmcp gene was required for the growth development of C. manginecans and functioned as a major pathogenicity factor in mango infection.

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.

MoRBP9 Encoding a Ran-Binding Protein Microtubule-Organizing Center Is Required for Asexual Reproduction and Infection in the Rice Blast Pathogen Magnaporthe oryzae

Like many fungal pathogens, the conidium and appressorium play key roles during polycyclic dissemination and infection of Magnaporthe oryzae. Ran-binding protein microtubule-organizing center (RanBPM) is a highly conserved nucleocytoplasmic protein. In animalia, RanBPM has been implicated in apoptosis, cell morphology, and transcription. However, the functional roles of RanBPM, encoded by MGG_00753 (named MoRBP9) in M. oryzae, have not been elucidated. Here, the deletion mutant ΔMorbp9 for MoRBP9 was generated via homologous recombination to investigate the functions of this gene. The ΔMorbp9 exhibited normal conidial germination and vegetative growth but dramatically reduced conidiation compared with the wild type, suggesting that MoRBP9 is involved in conidial production. ΔMorbp9 conidia failed to produce appressoria on hydrophobic surfaces, whereas ΔMorbp9 still developed aberrantly shaped appressorium-like structures at hyphal tips on the same surface, suggesting that MoRBP9 is involved in the morphology of appressorium-like structures from hyphal tips and is critical for development of appressorium from germ tubes. Taken together, our results indicated that MoRBP9 played a pleiotropic role in polycyclic dissemination and infection-related morphogenesis of M. oryzae.

Distinct roles of the YPEL gene family in development and pathogenicity in the ascomycete fungus Magnaporthe oryzae

Members of the Yippee-like (YPEL) gene family are highly conserved in eukaryotes and are homologous to the Drosophila yippee gene. In this study, we functionally characterized two YPEL-homologous genes, MoYPEL1 and MoYPEL2, in the rice blast pathogen Magnaporthe oryzae using the deletion mutants ΔMoypel1, ΔMoypel2, and ΔΔMoypel1,2. The MoYPEL1 deletion mutant was significantly defective in conidiation and unable to undergo appressorium development; however, deletion of MoYPEL2 resulted in a significant increase in conidiation and the abnormal development of two appressoria per conidium. These data demonstrate the opposite roles of each member of the YPEL gene family during the development of M. oryzae. The double mutant was phenotypically similar to the ΔMoypel1 mutant in conidiation, but similar to the ΔMoypel2 mutant in appressorium development. Subcellular localization of the MoYPEL1 protein was dynamic during appressorium development, while the MoYPEL2 protein consistently localized within the nuclei during developmental stages. Our studies indicate that the two YPEL gene family members play distinct roles in the developmental stages of M. oryzae, furthering our understanding of disease dissemination and development in fungi.

The Nep1-like protein family of Magnaporthe oryzae is dispensable for the infection of rice plants

The necrosis- and ethylene-inducing protein 1 (Nep1)-like proteins (NLPs) are a class of microbe-associated molecular patterns widely distributed across diverse groups of plant-associated microorganisms. In spite of the cytotoxic activity in dicot plants, the role of most NLPs in the virulence of plant pathogens is still largely unknown. We showed that the MoNLP family of rice blast fungus varied very little in amino acid sequence, transient expression of three MoNLPs induced cell death and the production of reactive oxygen species in Nicotiana benthamiana, and the expression of MoNLPs was induced during infection of susceptible rice plants. To further investigate the biological role of the MoNLP family, a marker-free gene replacement vector was developed and used to knock out the whole family in Magnaporthe oryzae. Results showed no significant difference in disease levels caused by wild type and the quadruple ΔMoNLP mutant strains. Likewise, the sporulation and radial growth of the two strains were similar under various unfavorable cultural conditions including malnutrition and abiotic stresses. These observations demonstrated that the MoNLP family is dispensable for the fungal tolerance to the tested adverse cultural conditions, and more importantly, for the virulence of blast fungus on susceptible rice plants.

Enhancement of yellow pigment production by intraspecific protoplast fusion of Monascus spp. yellow mutant (ade(-)) and white mutant (prototroph)

To breed industrially useful strains of a slow-growing, yellow pigment producing strain of Monascus sp., protoplasts of Monascus purpureus yellow mutant (ade(-)) and rapid-growing M. purpureus white mutant (prototroph) were fused and fusants were selected on minimal medium (MM). Preliminary conventional protoplast fusion of the two strains was performed and the result showed that only white colonies were detected on MM. It was not able to differentiate the fusants from the white parental prototroph. To solve this problem, the white parental prototroph was thus pretreated with 20mM iodoacetamide (IOA) for cytoplasm inactivation and subsequently taken into protoplast fusion with slow-growing Monascus yellow mutant. Under this development technique, only the fusants, with viable cytoplasm from Monascus yellow mutant (ade(-)), could thus grow on MM, whereas neither IOA pretreated white parental prototroph nor yellow auxotroph (ade(-)) could survive. Fifty-three fusants isolated from yellow colonies obtained through this developed technique were subsequently inoculated on complete medium (MY agar). Fifteen distinguished yellow colonies from their parental yellow mutant were then selected for biochemical, morphological and fermentative properties in cassava starch and soybean flour (SS) broth. Finally, three most stable fusants (F7, F10 and F43) were then selected and compared in rice solid culture. Enhancement of yellow pigment production over the parental yellow auxotroph was found in F7 and F10, while enhanced glucoamylase activity was found in F43. The formation of fusants was further confirmed by monacolin K content, which was intermediate between the two parents (monacolin K-producing yellow auxotroph and non-monacolin K producing white prototroph).

Harbouring public good mutants within a pathogen population can increase both fitness and virulence

Existing theory, empirical, clinical and field research all predict that reducing the virulence of individuals within a pathogen population will reduce the overall virulence, rendering disease less severe. Here, we show that this seemingly successful disease management strategy can fail with devastating consequences for infected hosts. We deploy cooperation theory and a novel synthetic system involving the rice blast fungus Magnaporthe oryzae. In vivo infections of rice demonstrate that M. oryzae virulence is enhanced, quite paradoxically, when a public good mutant is present in a population of high-virulence pathogens. We reason that during infection, the fungus engages in multiple cooperative acts to exploit host resources. We establish a multi-trait cooperation model which suggests that the observed failure of the virulence reduction strategy is caused by the interference between different social traits. Multi-trait cooperative interactions are widespread, so we caution against the indiscriminant application of anti-virulence therapy as a disease-management strategy.

Agrobacterium tumefaciens-mediated transformation of the lichen fungus, Umbilicaria muehlenbergii

Transformation-mediated mutagenesis in both targeted and random manners has been widely applied to decipher gene function in diverse fungi. However, a transformation system has not yet been established for lichen fungi, severely limiting our ability to study their biology and mechanism underpinning symbiosis via gene manipulation. Here, we report the first successful transformation of the lichen fungus, Umbilicaria muehlenbergii, via the use of Agrobacterium tumefaciens. We generated a total of 918 transformants employing a binary vector that carries the hygromycin B phosphotransferase gene as a selection marker and the enhanced green fluorescent protein gene for labeling transformants. Randomly selected transformants appeared mitotically stable, based on their maintenance of hygromycin B resistance after five generations of growth without selection. Genomic Southern blot showed that 88% of 784 transformants contained a single T-DNA insert in their genome. A number of putative mutants affected in colony color, size, and/or morphology were found among these transformants, supporting the utility of Agrobacterium tumefaciens-mediated transformation (ATMT) for random insertional mutagenesis of U. muehlenbergii. This ATMT approach potentially offers a systematic gene functional study with genome sequences of U. muehlenbergii that is currently underway.

Septin-mediated plant cell invasion by the rice blast fungus, Magnaporthe oryzae

To cause rice blast disease, the fungus Magnaporthe oryzae develops a pressurized dome-shaped cell called an appressorium, which physically ruptures the leaf cuticle to gain entry to plant tissue. Here, we report that a toroidal F-actin network assembles in the appressorium by means of four septin guanosine triphosphatases, which polymerize into a dynamic, hetero-oligomeric ring. Septins scaffold F-actin, via the ezrin-radixin-moesin protein Tea1, and phosphatidylinositide interactions at the appressorium plasma membrane. The septin ring assembles in a Cdc42- and Chm1-dependent manner and forms a diffusion barrier to localize the inverse-bin-amphiphysin-RVS-domain protein Rvs167 and the Wiskott-Aldrich syndrome protein Las17 at the point of penetration. Septins thereby provide the cortical rigidity and membrane curvature necessary for protrusion of a rigid penetration peg to breach the leaf surface.

The Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

The Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

Horizontal transfer of supernumerary chromosomes in fungi

Several species of filamentous fungi contain so-called dispensable or supernumerary chromosomes. These chromosomes are dispensable for the fungus to survive, but may carry genes required for specialized functions, such as infection of a host plant. It has been shown that at least some dispensable chromosomes are able to transfer horizontally (i.e., in the absence of a sexual cycle) from one fungal strain to another. In this paper, we describe a method by which this can be shown. Horizontal chromosome transfer (HCT) occurs during co-incubation of two strains. To document the actual occurrence of HCT, it is necessary to select for HCT progeny. This is accomplished by transforming two different drug-resistance genes into the two parent strains before their co-incubation. In one of the strains (the "donor"), a drug-resistance gene should be integrated in a chromosome of which the propensity for HCT is under investigation. In the "tester" or "recipient" strain, another drug-resistance gene should be integrated somewhere in the core genome. In this way, after co-incubation, HCT progeny can be selected on plates containing both drugs. HCT can be initiated with equal amounts of asexual spores of both strains, plated on regular growth medium for the particular fungus, followed by incubation until new asexual spores are formed. The new asexual spores are then harvested and plated on plates containing both drugs. Double drug-resistant colonies that appear should carry at least one chromosome from each parental strain. Finally, double drug-resistant strains need to be analysed to assess whether HCT has actually occurred. This can be done by various genome mapping methods, like CHEF-gels, AFLP, RFLP, PCR markers, optical maps, or even complete genome sequencing.

MoCRZ1, a gene encoding a calcineurin-responsive transcription factor, regulates fungal growth and pathogenicity of Magnaporthe oryzae

Ca(2+)-dependent signaling plays important roles in cellular development and metabolism in fungi. Pharmacological and molecular evidence clearly indicates that Ca(2+)-dependent signaling is required for infection-related development and pathogenicity in the rice blast fungus Magnaporthe oryzae. However, little information is available on downstream regulators in the Ca(2+)-dependent signaling pathway. To understand the role of a calcineurin-dependent transcription factor in the rice blast fungus, an ortholog of Saccharomyces cerevisiae CRZ1 in M. oryzae, MoCRZ1, was identified and functionally characterized. The Deltamocrz1 mutant exhibited impaired growth in the presence of Ca(2+) ions or cell wall perturbing agents. The Deltamocrz1 mutant also showed reduced conidiation and reduced pathogenicity, which is mainly due to a defect in host penetration. MoCRZ1 fused to EGFP was trans-localized into the nucleus in a Ca(2+)/calcineurin-dependent manner. The MoCRZ1 gene is also required for the calcineurin-dependent transcriptional induction of FKS1, a gene encoding a beta-1,3 glucan synthase, CHS2 and CHS4, genes encoding two chitin synthases, and PMC and PMR gene families encoding P-type ATPases in response to Ca(2+). These results suggest that MoCRZ1 is a downstream regulator in Ca(2+)-dependent signaling for pathogenicity in M. oryzae, and its biochemical mechanisms are well conserved among fungal species.

TheMagnaporthe griseasnodprot1 homolog, MSP1, is required for virulence

Secreted proteins play central roles in plant-microbe interactions acting as signals, toxins, and effectors. One important group of small secreted proteins is the snodprot1 family, members of which have demonstrated phytotoxic properties. A split-marker transformation system was applied for gene deletion of the snodprot1 homolog, MSP1, in the rice blast fungus Magnaporthe grisea. msp1 mutants were phenotypically indistinguishable from wild type and elaborated apparently normal appressoria. However, the deletion mutants were greatly reduced in virulence primarily due to impaired growth in planta. Western blot analysis showed that the protein was secreted and not associated with the fungal cell wall. When purified MSP1 protein was applied to wounded leaf tissue, no apparent phytotoxic effects were noted. This is the first report to the authors' knowledge that directly implicates a snodprot1 protein as a virulence factor.

Magnaporthe oryzae isolates causing gray leaf spot of perennial ryegrass possess a functional copy of the AVR1-CO39 avirulence gene

SUMMARY Gray leaf spot of perennial ryegrass (Lolium perenne) is a severe foliar disease caused by the ascomycete fungus Magnaporthe oryzae (formerly known as Magnaporthe grisea). Control of gray leaf spot is completely dependent on the use of fungicides because currently available perennial ryegrass cultivars lack genetic resistance to this disease. M. oryzae isolates from perennial ryegrass (prg) were unable to cause disease on rice cultivars CO39 and 51583, and instead triggered a hypersensitive response. Southern hybridization analysis of DNA from over 50 gray leaf spot isolates revealed that all of them contain sequences corresponding to AVR1-CO39, a host specificity gene that confers avirulence to rice cultivar CO39, which carries the corresponding resistance gene Pi-CO39(t). There was also an almost complete lack of restriction site polymorphism at the avirulence locus. Cloning and sequencing of the AVR1-CO39 gene (AVR1-CO39(Lp)) from 16 different gray leaf spot isolates revealed just two point mutations, both of which were located upstream of the predicted open reading frame. When an AVR1-CO39(Lp) gene copy was transferred into ML33, a rice pathogenic isolate that is highly virulent to rice cultivar CO39, the transformants were unable to cause disease on CO39 but retained their virulence to 51583, a rice cultivar that lacks Pi-CO39(t). These data demonstrate that the AVR1-CO39 gene in the gray leaf spot pathogens is functional, and suggest that interaction of AVR1-CO39(Lp) and Pi-CO39(t) is responsible, at least in part, for the host specificity expressed on CO39. This indicates that it may be possible to use the Pi-CO39(t) resistance gene as part of a transgenic strategy to complement the current deficiency of gray leaf spot resistance in prg. Furthermore, our data indicate that, if Pi-CO39(t) can function in prg, the resistance provided should be broadly effective against a large proportion of the gray leaf spot pathogen population.

'PACLIMS': a component LIM system for high-throughput functional genomic analysis

BACKGROUND

Recent advances in sequencing techniques leading to cost reduction have resulted in the generation of a growing number of sequenced eukaryotic genomes. Computational tools greatly assist in defining open reading frames and assigning tentative annotations. However, gene functions cannot be asserted without biological support through, among other things, mutational analysis. In taking a genome-wide approach to functionally annotate an entire organism, in this application the approximately 11,000 predicted genes in the rice blast fungus (Magnaporthe grisea), an effective platform for tracking and storing both the biological materials created and the data produced across several participating institutions was required.

RESULTS

The platform designed, named PACLIMS, was built to support our high throughput pipeline for generating 50,000 random insertion mutants of Magnaporthe grisea. To be a useful tool for materials and data tracking and storage, PACLIMS was designed to be simple to use, modifiable to accommodate refinement of research protocols, and cost-efficient. Data entry into PACLIMS was simplified through the use of barcodes and scanners, thus reducing the potential human error, time constraints, and labor. This platform was designed in concert with our experimental protocol so that it leads the researchers through each step of the process from mutant generation through phenotypic assays, thus ensuring that every mutant produced is handled in an identical manner and all necessary data is captured.

CONCLUSION

Many sequenced eukaryotes have reached the point where computational analyses are no longer sufficient and require biological support for their predicted genes. Consequently, there is an increasing need for platforms that support high throughput genome-wide mutational analyses. While PACLIMS was designed specifically for this project, the source and ideas present in its implementation can be used as a model for other high throughput mutational endeavors.

Agrobacterium tumefaciens-mediated transformation of Beauveria bassiana using an herbicide resistance gene as a selection marker

Beauveria bassiana has been investigated for use in the biological control of several insects in agricultural practice. To understand the molecular basis of virulence and host specificity and to improve the entomopathogenicity of B. bassiana, we have developed a simple, highly efficient and reliable Agrobacterium-mediated transformation method for B. bassiana using a phosphinothricin acetyltransferase (bar) gene as a selectable marker. Most transformants contained single copies of T-DNA and the T-DNA inserts were stably inherited after five generations. With this highly efficient transformation method for B. bassiana, we also obtained two putative T-DNA-tagged mutants that may have altered growth habits or virulence. Thus, the described protocol could provide a useful tool to manipulate the genetic make-up and to tag genes that may be important for virulence or growth and development of B. bassiana.

Development of a transformation system for Crinipellis perniciosa, the causal agent of witches' broom in cocoa plants

Protoplasts of the pathogenic plant fungus, Crinipellis perniciosa, were transformed to hygromycin B resistance using the pAN7-1 plasmid, which contains the Escherichia coli hph gene under the control of Aspergillus nidulans regulatory sequences. The pAN7-1 plasmid was introduced by PEG/CaCl(2) treatment. Transformation frequencies of 1.6-2.5 transformants/microg of DNA were achieved. About 54% of the transformants were abortive and 40 analyzed transformants were mitotically stable and showed different hygromycin B resistance levels. The presence of the hph gene was checked by PCR in five transformants and the integration of multiple plasmid copies into different genome sites was observed by Southern analysis. This is the first report of a C. perniciosa transformation system and represents an important step for further research into genetic manipulation of this fungal plant pathogen.

Disruption of the alternative oxidase gene in Magnaporthe grisea and its impact on host infection

Plants and numerous fungi including Magnaporthe grisea protect mitochondria from interference by respiration inhibitors by expressing alternative oxidase, the enzymatic core of alternative respiration. The alternative oxidase gene AOXMg of M. grisea was disrupted. Several lines of evidence suggested that the disruption of AOXMg was sufficient to completely curb the expression of alternative respiration. In the infection of barley leaves, several AOXMg-minus and, thus, alternative respiration-deficient mutants of M. grisea retained their pathogenicity without significant impairment of virulence. However, differences between the wild-type strain and an AOXMg-minus mutant were apparent under oxidative stress conditions generated by the treatment of infected barley leaves with the commercial respiration inhibitor azoxystrobin. Symptom development was effectively suppressed on leaves infected with the alternative respiration-deficient mutant, while lesions on leaves infected with the wild-type strain continued to develop at much higher inhibitor doses. However, respective lesions rarely developed to the stage of full maturity. The results did not conform to a previous model implying that expression of alternative respiration is silenced during pathogenesis by the presence of constitutive plant antioxidants. Rather, alternative respiration provided protection from azoxystrobin during both saprophytic and infectious stages of the pathogen. The nature of similar oxidative stress conditions in the ecology of M. grisea remains an open question.

MagnaportheDB: a federated solution for integrating physical and genetic map data with BAC end derived sequences for the rice blast fungus Magnaporthe grisea

We have created a federated database for genome studies of Magnaporthe grisea, the causal agent of rice blast disease, by integrating end sequence data from BAC clones, genetic marker data and BAC contig assembly data. A library of 9216 BAC clones providing >25-fold coverage of the entire genome was end sequenced and fingerprinted by HindIII digestion. The Image/FPC software package was then used to generate an assembly of 188 contigs covering >95% of the genome. The database contains the results of this assembly integrated with hybridization data of genetic markers to the BAC library. AceDB was used for the core database engine and a MySQL relational database, populated with numerical representations of BAC clones within FPC contigs, was used to create appropriately scaled images. The database is being used to facilitate sequencing efforts. The database also allows researchers mapping known genes or other sequences of interest, rapid and easy access to the fundamental organization of the M.grisea genome. This database, MagnaportheDB, can be accessed on the web at http://www.cals.ncsu.edu/fungal_genomics/mgdatabase/int.htm.

Chromosome walking to the AVR1-CO39 avirulence gene of Magnaporthe grisea: discrepancy between the physical and genetic maps

The avrCO39 gene conferring avirulence toward rice cultivar CO39 was previously mapped to chromosome 1 of Magnaporthe grisea between cosegregating markers CH5-120H and 1.2H and marker 5-10-F. In the present study, this region of the chromosome was physically mapped using RecA-mediated Achilles' cleavage. Cleavage of genomic DNA sequences within CH5-120H and 5-10-F liberated a 610-kb restriction fragment, representing the physical distance between these markers. Chromosome walking was initiated from both markers but was curtailed due to the presence of repetitive DNA sequences and the absence of overlapping clones in cosmid libraries representing several genome equivalents. These obstacles were overcome by directly subcloning the target region after release by Achilles' cleavage and a contig spanning avrCO39 was thus assembled. Transformation of two cosmids into a virulent recipient strain conferred a cultivar-specific avirulence phenotype thus confirming the cloning of avrCO39. Meiotic crossover points were unevenly distributed across this chromosomal region and were clustered around the avrCO39 locus. A 14-fold variation in the relationship between genetic and physical distance was measured over the avrCO39 chromosomal region. Thus the poor correlation of physical to genetic distance previously observed in M. grisea appears to be manifested over relatively short distances.

Interactions between spore morphogenetic mutations affect cell types, sporulation, and pathogenesis in Magnaporthe grisea

We have previously defined four single-gene mutations, con1, con2, con4, and con7, that control various stages of spore morphogenesis in the rice blast fungus. To delineate the developmental pathway of spore morphogenesis, we investigated the interactions among these morphogenetic genes by generating strains with double mutations via transformation-mediated gene disruption. Plasmids containing portions of the inactivated CON4 and CON7 genes were introduced into strains harboring single mutation to produce double mutants. Interaction between con1 and con4 resulted in reduced vegetative growth and suppression of sporulation. Interaction between con1 and con7 suppressed spore production but not vegetative growth. The con2/con4 and con4/con7 double mutants produced hybrid spore types with characteristic features of both parental mutants. The con2/con7 mutant produced con2 type spores, indicating that con2 is epistatic to con7 in spore morphogenesis. The epistatic relationship, however, reversed when traits related to pathogenesis were considered. Double mutants harboring the con7 mutation could not form appressoria or colonize plant tissue, indicating that con7 is epistatic to con2 and con4 in appressorium formation and pathogenesis. Thus, morphogenetic genes interact at multiple levels leading to different epistatic relationships in the pathways of spore morphogenesis, appressorium formation, and pathogenesis.

G protein alpha subunit genes control growth, development, and pathogenicity of Magnaporthe grisea

Three G protein alpha subunit genes have been cloned and characterized from Magnaporthe grisea: magA is very similar to CPG-2 of Cryphonectria parasitica; magB is virtually identical to CPG-1 of Cryphonectria parasitica, to gna1 of Neurospora crassa, and to fadA of Emericella nidulans; and magC is most similar to gna2 of Neurospora crassa. Homologous recombination resulting in targeted deletion of magA had no effect on vegetative growth, conidiation, or appressorium formation. Deletion of magC reduced conidiation, but did not affect vegetative growth or appressorium formation. However, disruption of magB significantly reduced vegetative growth, conidiation, and appressorium formation. magB- transformants, unlike magA- and magC- transformants, exhibited a reduced ability to infect and colonize susceptible rice leaves. G protein alpha subunit genes are required for M. grisea mating. magB- transformants failed to form perithecia, whereas magA- and magC- transformants did not produce mature asci. These results suggest that G protein alpha subunit genes are involved in signal transduction pathways in M. grisea that control vegetative growth, conidiation, conidium attachment, appressorium formation, mating, and pathogenicity.

The melanin biosynthesis genes of Alternaria alternata can restore pathogenicity of the melanin-deficient mutants of Magnaporthe grisea

The phytopathogenic fungi Magnaporthe grisea and Alternaria alternata produce melanin via the polyketide biosynthesis, and both fungi form melanized colonies. However, the site of melanin deposition and the role of melanin in pathogenicity differ between these two fungi. M. grisea accumulates melanin in appressoria, and their melanization is essential for host penetration. On the other hand, A. alternata produces colorless appressoria, and melanin is not relevant to host penetration. We examined whether the melanin biosynthesis genes of A. alternata could complement the melanin-deficient mutations of M. grisea. Melanin-deficient, nonpathogenic mutants of M. grisea, albino (Alb-), rosy (Rsy-), and buff (Buf-), were successfully transformed with a cosmid clone pMRB1 that carries melanin biosynthesis genes ALM, BRM1, and BRM2 of A. alternata. This transformation restored the melanin synthesis of the Alb- and Buf- mutants, but not that of the Rsy- mutant. The melanin-restored transformants regained mycelial melanization, appressorium melanization, and pathogenicity to rice. Further, transformation of Alb- and Buf- mutants with subcloned ALM and BRM2 genes, respectively, also produced melanin-restored transformants. These results indicate that the Alternaria genes ALM and BRM2 can restore pathogenicity to the mutants Alb- and Buf-, respectively, due to their function during appressorium development in M. grisea.

New cosmid vectors for library construction, chromosome walking and restriction mapping in filamentous fungi

New cosmid vectors were constructed for the ascomycete fungus, Magnaporthe grisea and the basidiomycete fungus, Ustilago maydis. These vectors are capable of transforming M. grisea at frequencies of up to 5 transformants/micrograms linear DNA and U. maydis at up to 25 transformants/microgram circular DNA for integrative transformation. In addition, 2800 transformants/microgram DNA are possible when using an autonomously replicating vector. Since the promoters used in these vectors function in other ascomycete and basidiomycete fungi, we anticipate that these vectors will be widely applicable.

Genetic transformation of the vascular wilt fungusVerticillium dahliae

To facilitate genetic analysis of pathogenicity of Verticillium dahliae, a vascular wilt pathogen, a DNA-mediated transformation system has been developed. Resistance to hygromycin B was obtained by transforming spheroplasts with the cosmid vector pAN7-2. Transformation efficiencies ranged between 3 and 5 transformants/μg vector DNA. The transforming DNA was integrated into the V. dahliae genome, in single and multiple copies and in tandem array. In several multicopy transformants, minor alterations in the integrated DNA sequences were evident following extensive vegetative growth in the absence of hygromycin B. Electrophoretic karyotype analysis also provided direct evidence of chromosome rearrangements in two transformants. The availability of a transformation system for V. dahliae will facilitate the cloning and characterization of genes that are important for pathogenicity and development. Key words: Verticillium wilt, fungal transformation, electrophoretic karyotype, hygromycin B resistance, chromosome rearrangement.

A cosmid with a HyR marker for fungal library construction and screening

The construction of a double-cos-site cosmid vector, pMOcosX, for use in making filamentous fungal genomic DNA libraries, is described. The vector has features that allow for selection of clones introduced into fungi by transformation and for efficient chromosome walking experiments. These features include (i) two cos sites allowing for easy construction of libraries without requiring size selection of insert DNA; (ii) an XhoI site for insertion of Sau3AI or MboI partially digested genomic DNA inserts that allows usage of a half-site fill-in method which minimizes the possibility of producing clones containing chimeric inserts; (iii) a bacterial hygromycin phosphotransferase-encoding gene fused to a modified cpc-1 promoter of Neurospora crassa for direct selection of cosmid clones upon introduction into fungal cells; and (iv) T7 and T3 bacteriophage promoters and EcoRI, NotI and BamHI restriction sites flanking the cloning site that allow for synthesis of, or isolation of, end-specific probes for chromosome walking. The combination of features in this vector allows for the easy construction and use of high-quality fungal DNA libraries from small amounts of genomic DNA.

Genome organization of Magnaporthe grisea: genetic map, electrophoretic karyotype, and occurrence of repeated DNAs

A genetic map of Magnaporthe grisea (anamorph=Pyricularia oryzae and P. grisea), the causal agent of rice blast disease, was generated from segregation data utilizing 97 RFLP markers, two isoenzyme loci and the mating type locus among progeny of a cross between parental strains Guy 11 and 2539. Of the seven chromosomes of M. Grisea, three were resolved by contour-clamped homogeneous electric field (CHEF) electrophoresis, while the remaining four migrated as two doublet bands. By utilizing differences between CHEF mobilities of unresolved chromosomes from the parental strains, Southern analysis with selected markers allowed the chromosomal assignment of all linkage groups. A small translocation involving 1 marker was found in the parental strains used to produce the segregating population from which the map was constructed. Nine classes of repetitive DNA elements were found in the genome of a fungal isolate pathogenic to rice. These occurred only a few times or not at all in the genomes of isolates showing reduced virulence on rice. One repetitive DNA was shown to have structural similarity to the Alu sequences found in primates, a sequence similarity to the copia-like elements of Drosophila, and peptide similarity to transposable elements found in Drosophila, other fungi, and higher plants.

Genetic organization of a repeated DNA sequence family in the rice blast fungus

The fungal rice pathogen Magnaporthe grisea contains repetitive DNA sequences called MGR. We have used a DNA probe, MGR586, derived from these sequences and crosses between rice-pathogenic and non-rice-pathogenic laboratory strains of M. grisea to rapidly map genes in this organism. The rice-pathogenic strain contained 57 EcoRI restriction fragments that hybridize to the MGR586 probe; the other five non-rice-pathogenic parent strains contained a single MGR586 sequence. Genetic analysis of MGR segregation detected eight linkage groups and allowed the mapping of three pigmentation genes (Alb1, Rsy1, and Buf1), the mating type locus (Mat1), the nucleolar organizer (Rdn1), the Smol gene, and two restriction fragment length polymorphisms linked to Smol. Our results indicate that the MGR586 loci are randomly distributed about the M. grisea genome and permit the construction of a well-marked linkage map useful for future studies on genome organization and genetic analysis in M. grisea.

Transformation frequencies are enhanced and vector DNA is targeted during retransformation of Leptosphaeria maculans, a fungal plant pathogen

Leptosphaeria maculans, a fungal pathogen of Brassica spp., was successfully transformed with the vector pAN8-1, encoding phleomycin resistance. Protoplasts of a vigorous Phleor transformant were then retransformed using the partially homologous vector, pAN7-1 which encodes hygromycin B resistance. Retransformation of this strain to hygromycin resistance occurred at frequencies that were consistently twofold higher than with the original recipient strain. Linearised pAN7-1 DNA transformed phleomycin-resistant protoplasts at higher frequencies still. All the transformants that were tested retained a phleomycin-resistant phenotype (20/20). Molecular analysis of five transformants generated with circular pAN7-1 DNA indicated that in four cases the pAN7-1 vector had integrated into pAN8-1 sequences. These results suggest that transformation frequencies in L. maculans are limited by the ability of vector DNA to integrate into the genome. Hence, construction of strains with target sites for integration may prove to be a generally useful method for improving transformation frequencies of poorly characterised filamentous fungi, particularly when using heterologous vectors. This would greatly facilitate the identification of genes by transfer of gene libraries and the standardisation of chromosomal location effects in studies of expression of nested promoter deletions.

Altered expression of the steroid bioconverting pathway in pAN 7-1 transformants of Cochliobolus lunatus

The filamentous fungus C. lunatus converts progesterone mainly to its 11 beta-hydroxy derivative. C. lunatus transformed with the plasmid pAN 7-1, which contains the E. coli hph gene expressed under the control of the A. nidulans gpd and trpC expression signals, lacks this activity, but exhibits acetyl side chain degradation of progesterone through the reaction scheme progesterone----20 beta-hydroxy-progesterone----delta 4-androstene-3,17-dione---- testolactone + testosterone. The main part of this metabolic pathway is not expressed in the non-transformed strain. It was determined that the site-specific integration of the plasmid into the genome directly influences the expression of genes involved in the bioconversion of steroids.