Fungal colonization and host defense reactions in Ulmus americana callus cultures inoculated with Ophiostoma novo-ulmi

The host-pathogen interaction leading to Dutch elm disease was analyzed using histo- and cyto-chemical tests in an in vitro system. Friable and hard susceptible Ulmus americana callus cultures were inoculated with the highly aggressive pathogen Ophiostoma novo-ulmi. Inoculated callus tissues were compared with water-treated callus tissues and studied with light microscopy (LM), transmission-electron microscopy (TEM), and scanning-electron microscopy (SEM). New aspects of this interaction are described. These include the histological observation, for the first time in plant callus cultures, of suberin with its typical lamellar structure in TEM and the intracellular presence of O. novo-ulmi. Expression of the phenylalanine ammonia lyase gene, monitored by real-time quantitative polymerase chain reaction, was correlated with the accumulation of suberin, phenols, and lignin in infected callus cultures. This study validates the potential use of the in vitro system for genomic analyses aimed at identifying genes expressed during the interaction in the Dutch elm disease pathosystem.

Use of insertional mutagenesis to tag putative parasitic fitness genes in the Dutch elm disease fungus Ophiostoma novo-ulmi subsp. novo-ulmi

We used insertional mutagenesis to produce genetically tagged mutants of the Dutch elm disease fungus Ophiostoma novo-ulmi subsp. novo-ulmi. We first optimized transformation of O. novo-ulmi protoplasts by the restriction enzyme mediated integration method. A concentration of 80 U of HindIII with 108 fungal protoplasts and 5 microg of plasmid DNA was the most efficient for generating a high number of O. novo-ulmi mutants carrying a single insertion in their genome. Mycelium- and yeast-like growth kinetics of 24 O. novo-ulmi mutants were evaluated in vitro. Flanking sequences were successfully recovered in 8% of the transformants analyzed. Some mutant phenotypes appeared to result from gene disruption events, whereas others likely involved modifications of noncoding regions. Several nuclear loci that control vegetative growth and could potentially impact parasitic fitness were successfully tagged.

Microbial thiosulphate reaction arrays: the interactive roles of Fe(III), O2 and microbial strain on disproportionation and oxidation pathways

In this work, we experimentally evaluate pH and SO4(2-) dynamics associated with abiotic and microbial S2O3(2-) oxidation under varying [O2], [Fe(III)] and microbial strain/consortia (two pure strains, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, their consortia, and two enrichments from an acidic environmental system, Moose Lake 2002 and Moose Lake 2003). Results of the batch experiments demonstrate highly active microbial processing of S2O3(2-) while abiotic controls under identical experimental conditions remain static with no pH decrease. When abiotic controls were manually titrated with acid to achieve similar pH decreases to those occurring in the microbial treatments, different S pathways were involved. In particular, disproportionation is a substantial component of initial microbial S2O3(2-) processing, and is accelerated by the presence of Fe(III), indicating that recycling of S through intermediate oxidation states is likely to be widespread in acidic mine environments where high [Fe(III)] is common. Furthermore, the microbially mediated S reaction pathways were dependent on both environmental conditions and microbial strain/consortia, indicating that microbial community structure also plays a key role. Collectively, these results highlight the importance of microbial activity, their poor representation by abiotic S models, the likelihood that Fe(III), rather than O2, is a key control on microbial S processing in acid environments and the need to identify the microbial community/strain involved.

Fungal diversity, dominance, and community structure in the rhizosphere of clonal Picea mariana plants throughout nursery production chronosequences

Fungal diversity in the rhizosphere of healthy and diseased clonal black spruce (Picea mariana) plants was analyzed with regard to nursery production chronosequences. The four key production stages were sampled: mother plants (MP), 8-week-old cuttings (B + 0), second-year cuttings (B + 1), and third-year cuttings (B + 2). A total of 45 fungal taxa were isolated and identified based on cultural, phenotypic, and molecular characters. Members of phylum Ascomycota dominated, followed by Basidiomycota and Zygomycota. Diagnosis characters and distance analysis of the internal transcribed spacer rDNA sequences allowed the identification of 39 ascomycetous taxa. Many belong to the order Hypocreales, families Hypocreaceae and Nectriaceae, which contain many clusters of potentially pathogenic taxa (Cylindrocladium, Fusarium, and Neonectria) and are also ecologically associated with antagonistic taxa (Chaetomium, Hypocrea, Microsphaeropsis, Penicillium, Paecilomyces, Verticillium, Trichoderma, and Sporothrix). This is also the first report of a Cylindrocladium canadense association with disease symptoms and relation with Pestalotiopsis, Fusarium, Exserochilum, Rhizoctonia, and Xenochalara fungal consortia. Both production chronosequence and plant health considerably influenced fungal taxa assemblages. Unweighted pair-group arithmetic average clustering showed that isolates from MP, B + 0, and B + 1 plant rhizospheres clustered together within healthy or diseased health classes, whereas isolates from healthy and diseased B + 2 plants clustered together. Canonical correspondence analysis revealed substantial alteration in community assemblages with regard to plant health and yielded a principal axis direction that regrouped taxa associated with diseased plant rhizosphere soil, whereas the opposite axis direction was associated with healthy plants. Two diversity indices were defined and applied to assess the fungal taxa contribution (Tc) and persistence (Pi) throughout the production.

Characterization of three DNA transposons in the Dutch elm disease fungi and evidence of repeat-induced point (RIP) mutations

Transposable elements (TEs) are fundamental components of eukaryotic genomes and can contribute in various ways to genome plasticity and evolution. We describe here the first three DNA transposons in the Dutch elm disease (DED) pathogens Ophiostoma ulmi and O. novo-ulmi, named OPHIO1, OPHIO2 and OPHIO3. We demonstrate that OPHIO transposons, which show high homology to Fot1/pogo TEs within the Tc1/mariner superfamily, have different distribution patterns and specificity in the DED fungi and that interspecific hybrids could act as genetic bridges for transmission of TEs between closely related fungal species. OPHIO3 was found to have undergone repeat-induced point mutations (RIP). We have also developed a complementary method to Margolin's ratios based on the computation of cumulative transition scores (CTS) in order to visualize rapidly RIP signatures on individual DNA strands of OPHIO transposons and TEs found in other ascomycete fungi.

Genetic Diversity in Poplar Leaf Rust (Melampsora medusae f. sp. deltoidae) in the Zones of Host Sympatry and Allopatry

ABSTRACT Poplar leaf rust caused by Melampsora medusae f. sp. deltoidae is a widespread disease in North America, where epidemics occur within zones of sympatry and allopatry of telial hosts (Populus spp.) and aecial hosts (Larix spp.). To test the hypothesis that epidemics originate in the zone of sympatry where the rust can complete its life cycle, populations in sympatry and allopatry were analyzed with single-strand conformational polymorphism for codominant detection of alleles directly from uredinia. More alleles were detected in rust populations in the zone of host sympatry than in allopatry. Almost all alleles found in the zone of allopatry were a subset of the allelic diversity present in the zone of host sympatry. Distance analyses clustered populations according to geographic origin, but not sampling year or type of stand (plantation or natural stands). Large differences in allelic and genotypic frequency were observed between years in allopatry but not in sympatry, suggesting new colonizations in allopatric populations. Our results point to a dynamic and complex pattern of inoculum dissemination in polar leaf rust. The hypothesis most consistent with our results is that populations in sympatry represent a source of inoculum for epidemics, with some annual recolonization in allopatry, possibly via intermediate population jumps.

Variability of nuclear SSU-rDNA group introns within Septoria species: incongruence with host sequence phylogenies

We report structural features and distribution patterns of 26 different group I introns located at three distinct nucleotide positions in nuclear small subunit ribosomal DNA (SSU-rDNA) of 10 Septoria and 4 other anamorphic species related to the teleomorphic genus Mycosphaerella. Secondary structure and sequence characteristics assigned the introns to the common IC1 and IE groups. Intron distribution patterns and phylogenetic relationships strongly suggested that some horizontal transfer events have occurred among the closely related fungal species sampled. To test this hypothesis, we used a comparative approach of intron- and rDNA-based phylogenies through MP- and ML-based topology tests. Our results showed two statistically well-supported major incongruences between the intron and the equivalent internal transcribed spacer (ITS) tree comparisons made. Such absence of a co-evolutive history between group I introns and host sequences is discussed relatively to the intron structures, the mechanisms of intron movement, and the biology of the Mycosphaerella pathogenic fungi.

A spontaneous albino mutant of Ceratocystis resinifera results from a point mutation in the polyketide synthase gene, PKS1

We characterized a spontaneous albino mutant of Ceratocystis resinifera. Compared with the wild-type progenitor strain, the albino mutant had a reduced linear growth on culture medium, but its growth on lodgepole pine sapwood was unaffected. The albino mutant did not produce any coloured pigment on agar media or wood. However, upon exposure to exogenous scytalone, an intermediate metabolite of the melanin pathway, the production of a brownish melanin was restored. This suggests that the albino phenotype resulted from a mutation affecting the melanin synthesis pathway, upstream of the scytalone synthesis step. Melanin production was restored in the mutant by transforming it with a wild-type copy of the Ceratocystis resinifera polyketide synthase gene, PKS1. The complemented transformants produced melanin, indicating that the PKS1 gene was defective in the albino mutant. Sequence analysis revealed that the PKS1 allele found in the albino contained a single point mutation that resulted in an amino acid change from serine to proline at the 3' end of the beta-ketoacyl synthase motif.

Attributes and congruence of three molecular data sets: Inferring phylogenies among Septoria-related species from woody perennial plants

To improve our understanding of phylogenetic relationships within the anamorphic genus Septoria, three molecular data sets representing 2,417 bp of nuclear and mitochondrial genes were evaluated. Separate gene analyses and combined analyses were performed using first, the maximum parsimony criterion and second, a Bayesian framework. The homogeneity of data partitions was evaluated via a combination of homogeneity partition tests and tree topology incongruence tests before conducting combined analyses. A last incongruence re-evaluation using partitioned Bremer support was performed on the combined tree, which corroborated the previous estimates. After each separate data set attributes were examined, simple explanations were advocated as the causes of the significant incongruences detected. The analysis of multiple gene partitions showed unprecedented phylogenetic resolution within the genus Septoria that supported the results from previously published single gene phylogenies. Specifically, we have delimited distinct but closely related species representing monophyletic groups that frequently correlated with their respective host families. Conversely, the occurrence of well-supported groups including closely related but distinct molecular taxa sampled on unrelated host-plants allowed us to reject, in these particular cases, the co-evolutionary concept expected between a parasite and its host and to discuss alternative evolutionary models recently proposed for these pathogens.

Bioprotection of Spruce Logs Against Sapstain Using an Albino Strain of Ceratocystis resinifera

ABSTRACT We recovered a spontaneous albino strain from ascospores of Ceratocystis resinifera, a sapstain fungus that grows deeply and rapidly in freshly felled conifer trees. This albino strain, named Kasper, was tested for its ability to prevent discoloration of spruce sapwood caused by wild-type sapstain fungi and compared with Cartapip 97, a commercially available biological control agent of sapstain in lodgepole pine and red pine logs. In a laboratory trial, Kasper reduced sapstain of white spruce logs as much as 94.4% and was more efficient than Cartapip 97. In field trials conducted in an area north of Québec City, Kasper reduced sapstain of black spruce as much as 80%. In three of four field trials, Kasper was significantly more efficient than Cartapip 97 in reducing sapstain development. The exception was encountered in a 2003 trial conducted in a sawmill yard where Kasper did not reduce sapstain. In a field trial conducted in western Canada, at Aleza Lake forest near Prince George, Kasper almost totally prevented the development of sapstain, even after 24 weeks. These results suggest albino strains derived from C. resinifera might be an additional source of potential biocontrol agents against sapstain.

Specific and sensitive PCR-based detection of Septoria musiva, S. populicola and S. populi, the causes of leaf spot and stem canker on poplars

The development of a PCR assay for the detection of the poplar pathogenic fungi Septoria musiva (teleomorph Mycosphaerella populorum), S. populicola (M. populicola) and S. populi (M. populi) is described. Three pairs of species-specific PCR primers were designed using interspecific polymorphisms in the internal transcribed spacer (ITS) of nuclear ribosomal RNA gene (rDNA) repeats. The specificity of the three primer pairs was successfully tested on a collection of 40 S. musiva, 39 S. populicola and six S. populi isolates. Using stringent PCR conditions, no cross-reaction was observed with any of the isolates tested. The specificity of the PCR assay was further confirmed with DNA extracted from 12 additional Septoria species and 17 other fungal species obtained from stems or leaves of poplars. Specific amplification of the fragments for S. musiva and S. populicola was sensitive relatively to the technique used, detecting as low as 1 pg template DNA, and 10 pg of DNA of the target species in a background of 1 ng of DNA of the other species. Moreover, using DNA purified directly from disrupted conidia, it was possible to detect with a probability of 90%, using one unique PCR assay, the DNA equivalent of 166 conidia per microl of S. musiva and 156 conidia per microl of S. populicola. The procedures developed in this work can thus be applied for rapid and accurate detection and identification of Septoria species from poplars.

Genetic Structure of Mycosphaerella populorum (Anamorph Septoria musiva) Populations in North-Central and Northeastern North America

ABSTRACT In order to characterize the genetic variation of the poplar pathogen Mycosphaerella populorum (anamorph Septoria musiva), we have studied seven North American populations using the polymerase chain reaction random amplified polymorphic DNA (RAPD) technique. The fungal populations were sampled in 2001 and 2002 by obtaining 352 isolates from cankers and leaf spots in hybrid poplar plantations and adjacent eastern cottonwood (Populus deltoides). A total of 21 polymorphic RAPD markers were obtained with the six RAPD primers used. A fine-level scale analysis of the genetic structure within the populations revealed that subpopulations sampled on P. deltoides and on hybrid trees were not significantly differentiated. In contrast, analyses performed on the entire data set showed high levels of haplotypic diversity and moderate to high genetic differentiation, with 20% of the expected genetic diversity found at the interpopulation level. Moreover, a high and significant correlation between genetic and geographic distances among populations was found, suggesting isolation by distance of the sampled populations. Although the occurrence of the sexual stage of this fungus remained unclear in field populations, five of the six populations were at gametic equilibrium for RAPD loci, suggesting the occurrence of recombination episodes in Septoria musiva populations. Overall, S. musiva appears to consist of differentiated subpopulations, with both asexual and sexual recombination contributing to the local level of genetic structure.

External symptoms and histopathological changes following inoculation of elms putatively resistant to Dutch elm disease with genetically close strains of Ophiostoma

To better characterize the host–pathogen interaction leading to Dutch elm disease, pathogenicity tests were carried out under controlled conditions. Putative resistant hybrid clones 2213 and 2245 from the same Ulmus parvifolia Jacq. × Ulmus americana L. cross and putative resistant U. americana clone 503, as well as saplings of U. americana grown from seeds, were inoculated with strains of Ophiostoma ulmi (Buism.) Nannf. or Ophiostoma novo-ulmi Brasier, including strains H327 and AST27, which carry different alleles at the Pat1 pathogenicity locus and display different levels of aggressiveness. The occurrence of wilted leaves and xylem streaks in inoculated elms indicated that the three clones tested were in fact susceptible to Dutch elm disease, although clones 2213 and 2245 were less susceptible than other elm material tested. In addition to the usual histopathological changes induced during the development of Dutch elm disease on clones 2213 and 2245, such as the formation of alveolar structures, tyloses, gels, and barrier zones, microscopic observations also revealed the presence of cells exhibiting a yellow autofluorescence under blue illumination around xylem vessels invaded by the pathogen. This may represent a new defence reaction against Dutch elm disease. The more aggressive H327 strain induced different levels of xylem responses than the less aggressive AST27 strain.Key words: Dutch elm disease, vascular wilt, histopathology.

Genetic Variability and Structure of Canadian Populations of the Sapstain Fungus Ceratocystis resinifera

ABSTRACT Genomic DNA was extracted from 129 isolates of Ceratocystis resinifera, a species belonging to the C. coerulescens complex, and 19 polymorphic random amplified polymorphic DNA markers were used to study the population genetic structure of this fungus. The analysis suggested a moderate value for genetic diversity (H(S) = 0.209). However, when monomorphic markers and rare alleles, representing 89 markers, also were included in the calculation, the genetic diversity of Canadian populations of C. resinifera appeared to be much lower (H(S) = 0.045). This could be explained by two hypotheses: (i) recent introduction of this species into North America and (ii) clonal reproduction (by selfing). No specialization by C. resinifera for coniferous tree species was observed based on genetic differentiation index between isolates sampled from Pinus and Picea spp. and on phylogenetic analysis using Dice coefficient of association. In spite of a low genetic diversity, a very high genetic differentiation was observed among the nine geographical populations studied (F(ST) = 20.8%). The genetic differences were especially striking when populations from Eastern Canada were compared with populations from Western Canada (phiST = 0.27%; P < 0.001), suggesting that a geographic reproductive barrier occurs in Central Canada. This barrier may be the consequence of a weak migration of insect vectors of C. resinifera due to reduced presence of hosts in the Canadian Great Plains, where extensive agriculture occurs. However, results from pairwise F(ST) matrix and phylogeny of haplotypes suggest that the barrier is not totally impenetrable because some gene flow occurred from the west and from the east in the Big River (Saskatchewan) population located in the middle of the Great Plains.

Reproductive and therapeutic cloning, germline therapy, and purchase of gametes and embryos: comments on Canadian legislation governing reproduction technologies

In Canada, the Assisted Human Reproduction Act received royal assent on 29 March 2004. The approach proposed by the federal government responds to Canadians' strong desire for an enforceable legislative framework in the field of reproduction technologies through criminal law. As a result of the widening gap between the rapid pace of technological change and governing legislation, a distinct need was perceived to create a regulatory framework to guide decisions regarding reproductive technologies. In this article the three main topics covered in the new legislation are commented on: cloning, germline therapy, and purchase of gametes and embryos. Some important issues also covered in the new legislation, such as privacy and access to information, data protection, identity of donors, and inspection, will not be addressed.

Molecular profiling of rhizosphere microbial communities associated with healthy and diseased black spruce (Picea mariana) seedlings grown in a nursery

Bacterial and fungal populations associated with the rhizosphere of healthy black spruce (Picea mariana) seedlings and seedlings with symptoms of root rot were characterized by cloned rRNA gene sequence analysis. Triplicate bacterial and fungal rRNA gene libraries were constructed, and 600 clones were analyzed by amplified ribosomal DNA restriction analysis and grouped into operational taxonomical units (OTUs). A total of 84 different bacterial and 31 different fungal OTUs were obtained and sequenced. Phylogenetic analyses indicated that the different OTUs belonged to a wide range of bacterial and fungal taxa. For both groups, pairwise comparisons revealed that there was greater similarity between replicate libraries from each treatment than between libraries from different treatments. Significant differences between pooled triplicate samples from libraries of genes from healthy seedlings and pooled triplicate samples from libraries of genes from diseased seedlings were also obtained for both bacteria and fungi, clearly indicating that the rhizosphere-associated bacterial and fungal communities of healthy and diseased P. mariana seedlings were different. The communities associated with healthy and diseased seedlings also showed distinct ecological parameters as indicated by the calculated diversity, dominance, and evenness indices. Among the main differences observed at the community level, there was a higher proportion of Acidobacteria, Gammaproteobacteria, and Homobasidiomycetes clones associated with healthy seedlings, while the diseased-seedling rhizosphere harbored a higher proportion of Actinobacteria, Sordariomycetes, and environmental clones. The methodological approach described in this study appears promising for targeting potential rhizosphere-competent biological control agents against root rot diseases occurring in conifer nurseries.

First Report of Shining Willow as a Host Plant for Septoria musiva

During the summer of 2001, leaf spots resembling those caused by Septoria musiva Peck. were observed on shining willow (Salix lucida Mühl. subsp. lucida) at Leclerville, Québec, Canada (46°34'19″N,71°59'35″W). Affected leaves had brown, necrotic leaf spots (>5mm in diameter) surrounded by a darker brown halo. Conidia were cylindrical, straight to curved with 1 to 4 septa, 28 to 54 × 3.5 to 4 μm, and were produced in pycnidia located on the abaxial surface in the center of the leaf spots. The causal agent of this disease was successfully isolated by germinating the conidia on corn meal agar that was supplemented with streptomycin (50mg/ml) and chloramphenicol (300mg/ml) and followed with the transfer of the germinated conidia to potato dextrose agar. Leaf symptoms and morphology matched those of S. musiva, the cause of leaf spot and stem canker of hybrid poplars in North America (2,4). The internal transcribed spacers and the 5.8S portion of the rDNA were amplified using PCR with the ITS1 (5'-TCC GTA GGT GAA CCT GCG G-3') and ITS2 (5'-GCT GCG TTC TTC ATC GAT GC-3') primer pair on total genomic DNA extracted from a pure culture of the pathogen. The rDNA sequence obtained (GenBank Accession No. AY555277) had 100% identity at 506 base positions with the ITS1, 5.8S, and ITS2 of three S. musiva isolates from Québec and one from Wisconsin (GenBank Accession Nos. AY549464 to AY549467). To test for pathogenicity, excised leaf disks from plants propagated by softwood cuttings of the source plant and from one hybrid poplar clone (Populus maximowiczii × P. xjackii) were inoculated with 3 μl of a suspension of ground mycelium or sterile water (control). Disks were placed in a 24-well tissue culture plate with 1 ml of distilled water per well and incubated in a growth room maintained at 22°C with a 16-h photoperiod. After 1 month, symptoms were similar to those previously observed. Isolates collected from shining willow or hybrid poplar were able to induce S. musiva leaf spot symptoms on leaf disks excised from shining willow or the hybrid poplar clone. From symptomatic leaf disks, S. musiva was consistently reisolated. To our knowledge, this is the first report of S. musiva on a member of the genus Salix. S. didyma, S. salicicola, and S. salicina have been reported from leaves of species of Salix (1,3). Only a vague morphological description of S. didyma was found (3). Moreover, conidia of S. salicicola (20 to 50 × 2.5 to 3.5 μm) and S. salicina (40 to 60 μm long, unspecified width) overlap dimensions of S. musiva conidia (1). There is a need to reexamine the relationships between these species of Septoria. Evidently, the complete host range of S. musiva is not yet known. References: (1) L. Lanier et al. Mycologie et Pathologie Forestières. Masson. Paris, 1978. (2) M. E. Ostry. Eur. J. For. Pathol. 17:158, 1987. (3) P. A. Saccardo. Sylloge fungurum omnium hucusque cognitorum. Patavii: Sumptibus Auctoris, 1882. (4) L. J. Spielman et al. Plant Dis. 70:968, 1986.

Real time RT-PCR quantification and Northern analysis of cerato-ulmin ( CU) gene transcription in different strains of the phytopathogens Ophiostoma ulmi and O. novo-ulmi

Cerato-ulmin is a surface protein that belongs to the class of fungal proteins known as hydrophobins. This class II hydrophobin is produced throughout the life cycle and in all developmental stages of Ophiostoma novo-ulmi and O. ulmi; the aggressive and non-aggressive pathogens responsible for Dutch elm disease. Since yeast/mycelial transitions are often important to pathogenesis in dimorphic fungi such as Ophiostoma, we have examined the levels and abundance of cu mRNA in the yeast and mycelial stages of this fungus. The fungus contains one copy of the cu gene per haploid genome, located on chromosome IV. Our studies have been done using phosphoimager-based Northern analysis and real-time quantitative RT-PCR (qRT-PCR) to measure levels of cu mRNA. These measurements were made in both yeast-like and mycelial stages of the pathogen. Two wild-type, aggressive, strains of O. novo-ulmi (VA30 and H327) and one wild type non-aggressive strain of O. ulmi (H5) were analysed. As controls, we have utilized two types of mutants that we had previously generated, the null cu mutants THEK5-8 and THEK5-8-1, and a cu over-expression mutant, H5-tf16. Data generated by both Northern hybridization and real-time qRT-PCR analyses demonstrate that there is no cu mRNA transcription in the null mutants. The Northern analysis clearly showed that the over-expressing mutant H5-tf16 produces much more cu mRNA than the non-aggressive or aggressive strains. The quantitative data generated using qRT-PCR demonstrated that mycelium generally had 20-60% more cu mRNA than the yeast form. The non-aggressive strain of O. ulmi (H5) produces one-tenth as much cu mRNA as the aggressive strains (VA30 and H327). When transformed with additional copies of the cu gene, this same non-aggressive strain (H5-tf16) expressed about 20 times more cu mRNA than the wild type H5 strain. These data were consistently generated in multiple real-time qRT-PCR experiments with different RNA preparations, clearly demonstrating that the quantitative abundance values obtained were reproducible. Our study represents the first report on the use of real-time qRT-PCR to compare and quantify gene transcription in different growth phases of a fungal pathogen.