Transient transformation of the rust fungus Puccinia graminis f. sp. tritici

Progress on Molecular Genetics and Manipulation of Rust Fungi

Among the thousands of rust species described, many are known for their devastating effects on their hosts, which include major agriculture crops and trees. Hence, for over a century, these basidiomycete pathogenic fungi have been researched and experimented with. However, due to their biotrophic nature, they are challenging organisms to work with and, needing their hosts for propagation, represent pathosystems that are not easily experimentally accessible. Indeed, efforts to perform genetics have been few and far apart for the rust fungi, though one study performed in the 1940s was famously instrumental in formulating the gene-for-gene hypothesis describing pathogen-host interactions. By taking full advantage of the molecular genetic tools developed in the 1980s, research on many plant pathogenic microbes thrived, yet similar work on the rusts remained very challenging though not without some successes. However, the genomics era brought real breakthrough research for the biotrophic fungi and with innovative experimentation and the use of heterologous systems, molecular genetic analyses over the last 2 decades have significantly advanced our insight into the function of many rust fungus genes and their role in the interaction with their hosts. This has allowed optimizing efforts for resistance breeding and the design and testing of various novel strategies to reduce the devastating diseases they cause.

Stable and transient transformation, and a promoter assay in the selective lignin-degrading fungus, Ceriporiopsis subvermispora

A genetic transformation system was developed for the selective white rot basidiomycete Ceriporiopsis subvermispora using a modified protocol with polyethylene glycol and CaCl₂ treatment of the protoplasts and plasmids harboring recombinant hygromycin phosphotransferase (hph) driven by a homologous promoter. During repeated transfer on fresh potato dextrose agar plates containing 100 µg/ml hygromycin B, most transformants lost drug resistance, while the remaining isolates showed stable resistance over five transfers. No drug-resistant colonies appeared in control experiments without DNA or using a promoter-less derivative of the plasmid, indicating that a transient expression of the recombinant hph was driven by the promoter sequence in these unstable drug-resistant transformants. Southern blot analysis of the stable transformants revealed random integration of the plasmid DNA fragment in the chromosome at different copy numbers. This transformation system yielding mostly transient transformants was successfully used for promoter assay experiments, and only a 141-bp fragment was found to be essential for the basic promoter function of glyceraldehyde dehydrogenase gene (gpd) in this fungus. Subsequent mutational analyses suggested that a TATAA sequence is important for the basic promoter function of gpd gene. The promoter assay system will enable the functional analysis of gene expression control sequences quickly and easily, mostly in the absence of undesirable effects from differences in copy number and chromosomal position of an integrated reporter gene among stable transformants.

Transformation by growth onto agro-infiltrated tissues (TGAT), a simple and efficient alternative for transient transformation of the cucurbit powdery mildew pathogen Podosphaera xanthii

A major limitation of molecular studies in powdery mildew fungi (Erysiphales) is their genetic intractability. This is because they are obligate biotrophs. In these parasites, biotrophy is determined by the presence of haustoria, which are specialized structures of parasitism that play an essential role in the acquisition of nutrients and the deliverance of effectors. Podosphaera xanthii is the main causal agent of cucurbit powdery mildew and a major limitation for crop productivity. In a previous study using P. xanthii conidia, we showed, for the first time, the transformation of powdery mildew fungi by Agrobacterium tumefaciens. In this work, we hypothesized that the haustorium could also act as a natural route for the acquisition of DNA. To test our hypothesis, melon cotyledons were agro-infiltrated with A. tumefaciens that contained diverse transfer DNA (T-DNA) constructs harbouring different marker genes under the control of fungal promoters and, after elimination of the bacterium, the cotyledons were subsequently inoculated with P. xanthii conidia. Our results conclusively demonstrated the transfer of different T-DNAs from A. tumefaciens to P. xanthii, including two fungicide resistance markers (hph and tub2), a reporter gene (gfp) and a translational fusion (cfp-PxEC2). These results were further supported by the co-localization of translational fluorescent fusions of A. tumefaciens VirD2 and P. xanthii Rab5 proteins into small vesicles of haustorial and hyphal cells, suggesting endocytosis as the mechanism for T-DNA uptake, presumably by the haustorium. From our perspective, transformation by growth onto agro-infiltrated tissues (TGAT) is the easiest and most reliable method for the transient transformation of powdery mildew fungi.

Transient transformation of Podosphaera xanthii by electroporation of conidia

BACKGROUND

Powdery mildew diseases are a major phytosanitary issue causing important yield and economic losses in agronomic, horticultural and ornamental crops. Powdery mildew fungi are obligate biotrophic parasites unable to grow on culture media, a fact that has significantly limited their genetic manipulation. In this work, we report a protocol based on the electroporation of fungal conidia, for the transient transformation of Podosphaera fusca (synonym Podosphaera xanthii), the main causal agent of cucurbit powdery mildew.

RESULTS

To introduce DNA into P. xanthii conidia, we applied two square-wave pulses of 1.7 kV for 1 ms with an interval of 5 s. We tested these conditions with several plasmids bearing as selective markers hygromycin B resistance (hph), carbendazim resistance (TUB2) or GFP (gfp) under control of endogenous regulatory elements from Aspergillus nidulans, Neurospora crassa or P. xanthii to drive their expression. An in planta selection procedure using the MBC fungicide carbendazim permitted the propagation of transformants onto zucchini cotyledons and avoided the phytotoxicity associated with hygromycin B.

CONCLUSION

This is the first report on the transformation of P. xanthii and the transformation of powdery mildew fungi using electroporation. Although the transformants are transient, this represents a feasible method for the genetic manipulation of this important group of plant pathogens.

Transient transformation of Podosphaera xanthii by electroporation of conidia

Background

Powdery mildew diseases are a major phytosanitary issue causing important yield and economic losses in agronomic, horticultural and ornamental crops. Powdery mildew fungi are obligate biotrophic parasites unable to grow on culture media, a fact that has significantly limited their genetic manipulation. In this work, we report a protocol based on the electroporation of fungal conidia, for the transient transformation of Podosphaera fusca (synonym Podosphaera xanthii), the main causal agent of cucurbit powdery mildew.

Results

To introduce DNA into P. xanthii conidia, we applied two square-wave pulses of 1.7 kV for 1 ms with an interval of 5 s. We tested these conditions with several plasmids bearing as selective markers hygromycin B resistance (hph), carbendazim resistance (TUB2) or GFP (gfp) under control of endogenous regulatory elements from Aspergillus nidulans, Neurospora crassa or P. xanthii to drive their expression. An in planta selection procedure using the MBC fungicide carbendazim permitted the propagation of transformants onto zucchini cotyledons and avoided the phytotoxicity associated with hygromycin B.

Conclusion

This is the first report on the transformation of P. xanthii and the transformation of powdery mildew fungi using electroporation. Although the transformants are transient, this represents a feasible method for the genetic manipulation of this important group of plant pathogens.

Development of resources for the analysis of gene function in Pucciniomycotina red yeasts

The Pucciniomycotina is an important subphylum of basidiomycete fungi but with limited tools to analyze gene functions. Transformation protocols were established for a Sporobolomyces species (strain IAM 13481), the first Pucciniomycotina species with a completed draft genome sequence, to enable assessment of gene function through phenotypic characterization of mutant strains. Transformation markers were the URA3 and URA5 genes that enable selection and counter-selection based on uracil auxotrophy and resistance to 5-fluoroorotic acid. The wild type copies of these genes were cloned into plasmids that were used for transformation of Sporobolomyces sp. by both biolistic and Agrobacterium-mediated approaches. These resources have been deposited to be available from the Fungal Genetics Stock Center. To show that these techniques could be used to elucidate gene functions, the LEU1 gene was targeted for specific homologous replacement, and also demonstrating that this gene is required for the biosynthesis of leucine in basidiomycete fungi. T-DNA insertional mutants were isolated and further characterized, revealing insertions in genes that encode the homologs of Chs7, Erg3, Kre6, Kex1, Pik1, Sad1, Ssu1 and Tlg1. Phenotypic analysis of these mutants reveals both conserved and divergent functions compared with other fungi. Some of these strains exhibit reduced resistance to detergents, the antifungal agent fluconazole or sodium sulfite, or lower recovery from heat stress. While there are current experimental limitations for Sporobolomyces sp. such as the lack of Mendelian genetics for conventional mating, these findings demonstrate the facile nature of at least one Pucciniomycotina species for genetic manipulation and the potential to develop these organisms into new models for understanding gene function and evolution in the fungi.

Poplar leaf rusts: model pathogens for a model treeThis minireview is one of a selection of papers published in the Special Issue on Poplar Research in Canada

With the availability of the entire genome of the model tree Populus trichocarpa Torr. & A. Gray and the current genome sequencing project of its rust pathogen Melampsora larici-populina Kleb., rust–poplar interaction research has entered the genomic era. Recent genomics research on poplars has attempted to connect the genetic localizations of loci for qualitative and quantitative disease resistance with putative genes encoding resistance or signalling proteins. The interactions between these putative resistance genes and rust effectors remain unknown. Genomic resources developed for Melampsora spp. promise to contribute to our understanding of the molecular basis of pathogenicity by facilitating the isolation of pathogenicity genes. A multifaceted approach for the identification of such genes that relies largely on trimming and sequence data analysis has been developed. The strategy takes advantage of the resources available and combines EST libraries, bioinformatics data mining for extracellularly expressed secreted proteins, intra- and inter-specific comparative genomics, and testing for the presence of positive selection. It has resulted in the discovery of several putative candidate genes. In silico evidence for candidate genes will be further validated by robust experimental evidence through functional analyses.

Expression of the fluorescence markers DsRed and GFP fused to a nuclear localization signal in the arbuscular mycorrhizal fungus Glomus intraradices

Here, arbuscular mycorrhizal (AM) fungi were monitored in vivo introducing the fluorescent reporters DsRed and GFP (green fluorescent protein) in Glomus intraradices using a biolistic approach and Agrobacterium tumefaciens-mediated transformation. Both reporter genes were fused to the nuclear localization signal of the Aspergillus nidulans transcription factor StuA to target fluorescence to nuclei. Expression of DsRed was driven by two Glomus mosseae promoters highly expressed during early symbiosis, GmPMA1 and GmFOX2, while expression of GFP was driven by the A. nidulans gpd promoter. All promoters worked in G. intraradices as well as in A. nidulans. Red and green fluorescence was localized to nuclei of G. intraradices spores and hyphae 3 d after bombardment. However, expression was transient. The efficiency of the Agrobacterium-mediated transformation was very low. These results indicate that the biolistic method allows the expression of foreign DNA into G. intraradices with high frequency, but it is insufficient to render stable transformants. DsRed vs GFP is a more appropriate living reporter to be used in G. intraradices because of the lower autofluorescence in the red channel but targeted to the nucleus both marker genes can be visualized. This is the first report of fluorescent marker expression in an AM fungus driven by arbuscular mycorrhizal promoters.

Generation of a wheat leaf rust, Puccinia triticina, EST database from stage-specific cDNA libraries

Thirteen cDNA libraries constructed from small amounts of leaf rust mRNA using optimized methods served as the source for the generation of 25 558 high-quality DNA sequence reads. Five life-cycle stages were sampled: resting urediniospores, urediniospores germinated over water or plant extract, compatible, interactive stages during appressorium or haustorium formation just before sporulation, and an incompatible interaction. mRNA populations were subjected to treatments such as full-length cDNA production, subtractive and normalizing hybridizations, and size selection methods combined with PCR amplification. Pathogen and host sequences from interactive libraries were differentiated in silico using cereal and fungal sequences, codon usage analyses, and by means of a partial prototype cDNA microarray hybridized with genomic DNAs. This yielded a non-redundant unigene set of 9760 putative fungal sequences consisting of 6616 singlets and 3144 contigs, representing 4.7 Mbp. At an E-value 10(-5), 3670 unigenes (38%) matched sequences in various databases and collections but only 694 unigenes (7%) were similar to genes with known functions. In total, 296 unigenes were identified as most probably wheat and ten as rRNA sequences. Annotation rates were low for germinated urediniospores (4%) and appressoria (2%). Gene sets obtained from the various life-cycle stages appear to be remarkably different, suggesting drastic reprogramming of the transcriptome during these major differentiation processes. Redundancy within contigs yielded information about possible expression levels of certain genes among stages. Many sequences were similar to genes from other rusts such as Uromyces and Melampsora species; some of these genes have been implicated in pathogenicity and virulence.

Complementation of Ustilago maydis MAPK mutants by a wheat leaf rust, Puccinia triticina homolog: potential for functional analyses of rust genes

From a large expressed sequence tag (EST) database representing several developmental stages of Puccinia triticina, we discovered a mitogen-activated protein kinase (MAPK) with homology to kinases with known pathogenic functions in other fungi. This PtMAPK1 is similar to the Ustilago maydis MAPK, Ubc3/Kpp2, but has a longer N-terminal extension of 43 amino acids (aa) with identities to U. maydis Kpp6, a homolog of Ubc3/Kpp2 with a 170-aa N-terminal extension. Ubc3/Kpp2 is involved in mating and subsequent pathogenic development, whereas Kpp6 functions during invasive growth in corn tissue. PtMAPK1, expressed from a Ustilago sp.-specific promoter, was able to complement a ubc3/kpp2 deletion mutant and restore mating. It also substantially increased virulence on corn, measured as tumor formation, of a kpp6 deletion mutant. Moreover, this construct restored to near-full pathogenicity a ubc3/kpp2 kpp6 nonpathogenic double deletion mutant. Complementation of the ubc3/kpp2 mutant with the complete PtMAPK gene and verification of expression by reverse-transcription polymerase chain reaction indicated that the rust promoter is recognized in U. maydis. Phylogenetically, these basidiomycete plant pathogens are related, which was reflected in comparison of P. triticina ESTs to U. maydis gene sequences. The U. maydis heterologous expression system allows functional analysis of rust genes, currently frustrated by the lack of efficient transformation and selection procedures.

Cloning of the glyceraldehyde-3-phosphate dehydrogenase gene from Pseudozyma flocculosa and functionality of its promoter in two Pseudozyma species

Pseudozyma flocculosa is a yeast-like epiphyte recently classified as a basidiomycete related to the Ustilaginales. In this study, we report the cloning of its gene coding for a putative glyceraldehyde-3-phosphate dehydrogenase (GPD). This gene was selected on the premise that its transcripts are abundant during the growth phase of P. flocculosa. The complete sequence of this gene was found to contain two introns in the coding region and one in the 3'-untranslated region. This gene was present in a single copy in the genome of P. flocculosa. By comparing its deduced amino acid sequence with various sequences from basidiomycetous and ascomycetous fungi, we observed a stronger homology with the former group as predicted by the new classification of P. flocculosa. The promoter region lacked a typical TATA or CAAT box but contained a CT-rich region including the transcription start site. Although the GPD promoter showed a stronger affinity within P. flocculosa, it remained active across species as shown by expressing the green fluorescent protein in Pseudozyma antarctica. The cloning of this gene and its promoter brings new and versatile options to the limited genetic tools currently available for the study of the recently defined Pseudozyma genus.

The Pseudozyma flocculosa actin promoter allows the strong expression of a recombinant protein in the Pseudozyma species

Fungi belonging to the recently classified genus Pseudozyma possess some unique properties such as biocontrol activity, production of rare antimicrobial glycolipids and production of recombinant proteins. In this work, we report the first cloning of a promoter endogenous to the multi-faceted yeast-like Pseudozyma flocculosa, that of the actin gene. The promoter region lacked typical TATA or CAAT box but displayed three putative GC box and two CT-rich regions. As in other related basidiomycetes, only one copy of the actin gene was present in the genome of P. flocculosa. The activity of the actin promoter was compared to that of the HSP70 promoter from Ustilago maydis in two Pseudozyma species. In P. flocculosa, the actin promoter allowed the expression of a very high amount of GFP protein (27.8 mg g(-1) total protein) compared to those obtained with the HSP70 promoter in liquid culture. By contrast, the levels of GFP expression obtained in liquid culture were similar with the actin or the HSP70 promoter in Pseudozyma antarctica. A similar pattern of GFP expression was observed in solid culture. The cloning of this new promoter offers a unique genetic tool to further exploit and study the unusual properties of fungi from the Pseudozyma genus.

Cereal rust fungi genomics and the pursuit of virulence and avirulence factors

Rust diseases cause significant reductions annually in yield of cereal crops worldwide. Traditional monoculture cropping systems apply significant selection pressure on the pathogen to cause rapid shifts in pathotypes. Plant breeders strive to stay ahead of the evolving pathogens by releasing new crop genotypes with new rust resistance genes or gene combinations. Owing to the limited number of known resistance genes and the lack of molecular understanding of the plant-pathogen interaction, rusts remain challenging organisms to study, both at organismal and molecular levels. This review discusses recent progress by a number of laboratories towards better understanding the molecular component of rust disease resistance.

Survey and expression analysis of five new chitin synthase genes in the biotrophic rust fungus Puccinia graminis

We have isolated and characterised the first set of chitin synthase genes from a rust fungus, a large group of economically highly important, obligately biotrophic plant pathogens. Puccinia graminis was used as a model organism for the rust fungi which are not well investigated on the molecular level today. One of the major structural components of most fungal cell walls is the chitin polymer which is synthesised by a family of enzymes called chitin synthases. In P. graminis, we have isolated five new chitin synthase genes from four different classes, chsII, chsIIIa, chsIIIb, chsIV, and chsV. The genes contain a high number of introns, unusual for other known fungal chitin synthases. The dinucleic stage of the fungus seems to contain two slightly different genes or alleles for four isoforms. One isoform, chsIIIa, seems to be expressed only in the youngest stages of fungal growth. Analysis of the derived proteins shows that together with other basidiomycete CHS, the pgtCHS form separate subgroups in the phylogenetic tree. This set of five rust chitin synthase genes, with some unusual features compared to known fungal chitin synthases, allows new insights into chitin synthase classification, and may help in the development of novel functional fungicides.

Shotgun identification of proteins from uredospores of the bean rust Uromyces appendiculatus

We are interested in learning more about the proteome of Uromyces appendiculatus, the fungus that causes common bean rust. Knowledge of the proteins that differentiate life-cycle stages and distinguish infectious bodies such as uredospores, germlings, appressoria, and haustoria may be used to define host-pathogen interactions or serve as targets for chemical inhibition of the fungus. We have used 2-D nanoflowLC-MS/MS to identify more than 400 proteins from asexual uredospores. A majority of the proteins appear to have roles in protein folding or protein catabolism. We present a model by which an abundance of heat shock proteins and translation elongation factors may enhance a spore's ability to survive environmental stresses and rapidly initiate protein production upon germination.

Development of an expression vector for Metarhizium anisopliae based on the tef-1alpha homologous promoter

The high-conserved translation elongation factor 1 alpha (tef-1alpha) gene from the enthomopathogenic fungus Metarhizium anisopliae was characterized to select the promoter region. A 640-bp DNA fragment upstream to the start codon was employed to drive the expression of the reporter protein sGFP or a dominant selectable marker, the gene bar (resistance to ammonium glufosinate). Transformants carrying this homologous promoter system showed no difference in virulence bioassays against the cattle tick Boophilus microplus comparing to the M. anisopliae wild-type strain. Moreover, GFP fluorescence was detected during tick infection bioassay.

Transient expression and insertional mutagenesis of Puccinia triticina using biolistics

The fungal genus Puccinia contains more than 4,000 species. Puccinia triticina, causal agent of wheat leaf rust, is an economically significant, biotrophic basidiomycete. Little is known about the molecular biology of this group, and tools for understanding gene function have not yet been established. A set of parameters was established for the transient transformation of urediniospores. The expression of three heterologous promoters (actin, elongation factor 1-alpha, and Hss1, Heat Shock 70 protein), derived from Puccinia graminis, was evaluated along with the potential for insertional mutagenesis. The UidA (GUS) gene was used as a marker for transient expression. When transferred into P. triticina urediniospores, transient expression was observed across four helium pressures using one size of gold and three sizes of tungsten microprojectiles. Each of the three promoters displayed strong transient expression in germinated urediniospores; however, higher numbers of GUS-positive urediniospores were observed when either the actin or Hss1 promoters were used. Possible concomitant insertional mutagenesis of several avirulence genes was selected in wheat cultivars harboring the cognate resistance genes. Using a linearized cloning plasmid, stable integration into the genome was achieved as demonstrated by PCR and sequencing analysis.

Stem rust of small grains and grasses caused by Puccinia graminis

SUMMARY Stem rust has been a serious disease of wheat, barley, oat and rye, as well as various important grasses including timothy, tall fescue and perennial ryegrass. The stem rust fungus, Puccinia graminis, is functionally an obligate biotroph. Although the fungus can be cultured with difficulty on artificial media, cultures grow slowly and upon subculturing they develop abnormal ploidy levels and lose their ability to infect host plants [Bushnell and Bosacker (1982) Can. J. Bot. 60, 1827-1836]. P. graminis is a typical heteroecious rust fungus with the full complement of five distinct spore stages that occur during asexual reproduction on its gramineous hosts and sexual reproduction that begins in the resting spore stage and culminates on the alternate host, barberry (Berberis spp.). There appears to be little polymorphism for resistance/susceptibility in Berberis species, but complex polymorphisms of resistance/susceptibility and matching virulence/avirulence exist in gene-for-gene relationships between small grain species and the forms of P. graminis that infect them.

TAXONOMY

Puccinia graminis is a rust fungus in the phylum Basidiomycota, class Urediniomycetes, order Uredinales, and family Pucciniaceae, which contains 17 genera and approximately 4121 species, of which the majority are in the genus Puccinia[Kirk et al. (2001) Ainsworth and Bisby's Dictionary of the Fungi. Wallingford, UK: CAB International]. Various subdivisions of P. graminis into subspecies, varieties and formae speciales have been proposed based on spore size and host range. Crossing studies and DNA sequence comparisons support the separation of at least two subspecies, but not the proposed separation based on spore size.

HOST RANGE

The host range of P. graminis is very broad compared with that of most Puccinia spp.; it includes at least 365 species of cereals and grasses in 54 genera [Anikster (1984) The Cereal Rusts. Orlando, FL: Academic Press, pp. 115-130]. Wheat stem rust, P. graminis f. sp. tritici, was shown to infect 74 species in 34 genera in artificial inoculations of seedlings, but only 28 of those species belonging to eight genera were known to be natural hosts of the fungus. Other formae speciales of P. graminis have narrower host ranges than P. graminis f. sp. tritici. Disease symptoms: Infections in cereals or grasses occur mainly on stems and leaf sheaths, but occasionally they may be found on leaf blades and glumes as well. The first macroscopic symptom is usually a small chlorotic fleck, which appears a few days after infection. About 8-10 days after infection, a pustule several millimetres long and a few millimetres wide is formed by rupture of the host epidermis from pressure of a mass of brick-red urediniospores produced in the infection. These uredinial pustules are generally linear or diamond shaped and may enlarge up to 10 mm long. The powdery masses of urediniospores appear similar to rust spots on a weathered iron surface. With age, the infection ceases production of brick-red urediniospores and produces a layer of black teliospores in their place, causing the stems of heavily infected plants to appear blackened late in the season.

Establishing compatibility between plants and obligate biotrophic pathogens

The apparent under-representation of the term 'plant disease susceptibility' as opposed to 'plant disease resistance' in the current scientific literature might indicate that 'compatibility' has not gained the same appreciation as 'resistance' in the past. However, these seemingly contrary phenomena are intimately linked, and progress in understanding one process inherently contributes to our comprehension of the other. Recent progress in analyzing plant-biotroph compatibility includes the molecular isolation and functional characterization of haustorium-specific cDNAs that encode presumptive hexose- and amino-acid-transporter proteins for proton-driven nutrient uptake. Accumulating evidence from cytological, pharmacological, phytopathological and molecular studies indicates that pathogens mediate the suppression of host defenses in a range of plant-biotroph interactions. Arabidopsis thaliana mutants that are resistant to powdery or downy mildew but that do not exhibit constitutively activated defense could be affected in host-compatibility factors.

In planta induced genes of Puccinia triticina

SUMMARY Wheat leaf rust disease, caused by the biotrophic fungus Puccinia triticina, is a result of complex interactions requiring the coordinated activities of the two organisms involved. In an effort to understand the molecular basis of wheat-rust interactions, we isolated and characterized cDNA corresponding to in planta induced fungal genes (PIGs) from susceptible wheat leaves infected with P. triticina by using suppression subtractive hybridization to construct a cDNA library. 350 clones were sequenced, of which 104 were unique. Forty-four cDNA clones encode ribosomal proteins, comprising the single largest category of clones isolated. Twenty-five of these ribosomal protein genes are likely to be of fungal origin, as was suggested by sequence homology. Hybridization of 56 selected non-ribosomal protein clones to rust germling cDNA or genomic DNA probes showed that at least 44 were of fungal origin, demonstrating that the library was highly enriched for fungal cDNA. Differential expression analysis identified 26 non-ribosomal protein genes that were induced in rust-infected leaves. At least 21 of the induced genes were from the rust fungus, indicating that the majority of the induced genes were rust PIGs that are likely to play a role in parasitism. Some of the induced genes share homology to known PIGs or virulence genes in other fungi, suggesting similarities in parasitism among different fungi. Eight clones correspond to novel PIGs that have not been reported in any organism. This paper reports, for the first time, the isolation of P. triticina PIGs and discusses the use of total rust genomic DNA to identify the source of genes.

Transformation of the edible mushroom Pleurotus ostreatus by particle bombardment

Uracil auxotroph of Pleurotus ostreatus was transformed to prototrophy by means of particle bombardment. Five transformants were obtained under three conditions differing in the two parameters of target distance and helium pressure. The transformation frequency was one transformant per microg of DNA. In the transformants, plasmid DNAs were integrated into the genomic DNA and stably maintained. This is the first report on transformation of P. ostreatus by particle bombardment.

Infection structures of biotrophic and hemibiotrophic fungal plant pathogens

Summary Biotrophic plant pathogenic fungi are one of the major causes of crop losses. The infection processes they exhibit are typified by infected host plant cells remaining alive for several days. This requires the development of specialized infection structures such as haustoria which are produced by obligate biotrophs, and intracellular hyphae which are produced by many hemibiotrophs. These infection hyphae are surrounded by the host plant plasma membrane, and in the case of haustoria the extrahaustorial membrane differs biochemically and structurally from the normal membrane. An interfacial matrix separates haustoria and intracellular hyphae from the invaginated membrane and this seems to be characteristic of biotrophic interactions. There is clear evidence for molecular differentiation of the haustorial plasma membrane in powdery mildews and rusts in comparison with the other fungal membranes. Relatively few pathogenicity genes related to biotrophy, and the switch from biotrophy to necrotrophy in hemibiotrophs, have been identified.

New (and used) approaches to the study of fungal pathogenicity

The fungi are the most economically important plant pathogens and continue to be the focus of extensive research with a wide variety of methodologies. Enhancements in microscopy techniques have increased our ability to visualize the intimate interaction of fungi and their host plants. Improving methods allow pharmacological inhibition and genetic dissection of the determinants of fungal pathogenicity in a gene-by-gene approach. Identification and analysis of genes differentially transcribed in ways pertinent to pathogenicity continues to be a frequent research approach. Genome-wide analysis is gaining favor in biological research and fungal plant pathogens are no exception. Several industrial research groups are exploring fungal plant pathogenesis based on genomic sequence data and genome-wide mutagenesis. In March 2001 the first publicly available complete genome of a filamentous fungus (Neurospora crassa) was released. N. crassa is of course a saprophyte and there is no complete sequence available for a plant pathogenic fungus in public databases. However, freely accessible entire genome sequences for both plant pathogenic fungi and their hosts are on the horizon. Sequence availability promises to revolutionize the rate at which data relevant to disease processes will be accrued. In this review we describe approaches currently applied to the study of plant pathogenic fungi and explore developments of potential future benefit with existing technologies not yet applied to this group of important organisms.

Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?

Plant cells engage in mutualistic and parasitic endosymbioses with a wide variety of microorganisms, ranging from Gram-negative (Rhizobium, Nostoc) and Gram-positive bacteria (Frankia), to oomycetes (Phytophthora), Chytridiomycetes, Zygomycetes (arbuscular mycorrhizal fungi) and true fungi (Erysiphe, ascomycete; Puccinia, basidiomycete). Endosymbiosis is characterised by the 'symbiosome', a compartment within host cells in which the symbiotic microorganism is either partially or completely enclosed by a host-derived membrane. The analysis of plant mutants indicates that the genetic requirements for the interaction with rhizobia and arbuscular mycorrhiza fungi are partially overlapping. The extent to which plants use similar or identical developmental programs for the intracellular accommodation of different microorganisms is, however, not clear. For example, plant cells actively weaken their cell wall to facilitate bacterial colonisation, whereas penetration by fungal symbionts appears not to be assisted in this manner. Moreover, different transport requirements are imposed on the symbiotic interface of different interactions indicating that additional system-specific components are likely to exist.