Subversion or coersion? Pathogenic deteminants in fungal phytopathogens

Recognition of Davidsoniella virescens on Fagus sylvatica Wood in Poland and Assessment of Its Pathogenicity

Davidsoniella virescens is so far only known in North America. However, recently in southern Poland, blackish growth consisting of fungal mycelia and sporulation structures was found on the wood of Fagus sylvatica. As a result of isolation, 17 cultures of this fungus were obtained. All cultures produced an intense sweet odor. This fungus, both in situ and in vitro, abundantly produced perithecia with long necks and asexual stage. Particularly characteristic was the production of variable endoconidia in two types of phialophores differing mainly in the width of the collarette. The nucleotide sequences for five gene fragments of representative cultures were used in phylogenetic analyses: 18S; the internal transcribed spacer regions ITS1 and ITS2, including the 5.8S gene (ITS); 28S region of the ribosomal RNA (rRNA), β-tubulin 2 (TUB2) and translation elongation factor 1-α (TEF1). Based on morphological and phylogenetic analyses, the fungus on European beech in Poland was identified as Davidsoniella virescens. The optimal temperature for radial colony growth was 20 °C. However, the differences between colony diameter at 25 °C compared to that at the optimal temperature were not statistically significant. Six D. virescens isolates were used for pathogenicity assay. They were inoculated into wounds on stems of two-year-old seedlings of Fagus sylvatica and Acer saccharum (36 seedlings of each tree species). Final evaluation was performed 4 months after inoculation. No external symptoms were observed in any A. saccharum seedling, neither in the crown nor on the stem. However, 13.9% of F. sylvatica seedlings showed wilting symptoms throughout the entire crown within 3-6 weeks after inoculation. Moreover, after 4 months on the stems of 30.6% beech seedlings, necrotic lesions with a length of 1.3 to 7.2 cm were formed, without any symptoms of wilting. The most noticeable internal symptom was the discoloration of the wood, which was observed in all inoculated seedlings of both tree species. All D. virescens isolates caused greater wood discoloration in F. sylvatica than in A. saccharum. Most of the differences found in the extent of discoloration between host plants were statistically significant. The discoloration caused by all D. virescens isolates in F. sylvatica was significantly greater than in the control. However, none of the isolates tested on A. saccharum caused significantly greater wood discoloration compared to the control. Pathogenicity tests showed that the D. virescens isolates identified in southern Poland may pose a greater threat to native European beech than to foreign sugar maple.

Sclerotinia sclerotiorum: An Evaluation of Virulence Theories

Oxalic acid production in Sclerotinia sclerotiorum has long been associated with virulence. Research involving UV-induced, genetically undefined mutants that concomitantly lost oxalate accumulation, sclerotial formation, and pathogenicity supported the conclusion that oxalate is an essential pathogenicity determinant of S. sclerotiorum. However, recent investigations showed that genetically defined mutants that lost oxalic acid production but accumulated fumaric acid could cause disease on many plants and substantiated the conclusion that acidic pH, not oxalic acid per se, is the necessary condition for disease development. Critical evaluation of available evidence showed that the UV-induced mutants harbored previously unrecognized confounding genetic defects in saprophytic growth and pH responsiveness, warranting reevaluation of the conclusions about virulence based on the UV-induced mutants. Furthermore, analyses of the evidence suggested a hypothesis for the existence of an unrecognized regulator responsive to acidic pH. Identifying the unknown pH regulator would offer a new avenue for investigating pH sensing/regulation in S. sclerotiorum and novel targets for intervention in disease control strategies.

pH dependency of sclerotial development and pathogenicity revealed by using genetically defined oxalate-minus mutants of Sclerotinia sclerotiorum

The devastating plant pathogen Sclerotinia sclerotiorum produces copious (up to 50 mM) amounts of oxalic acid, which, for over a quarter century, has been claimed as the pathogenicity determinant based on UV-induced mutants that concomitantly lost oxalate production and pathogenicity. Such a claim was made without fulfilling the molecular Koch's postulates because the UV mutants are genetically undefined and harbour a developmental defect in sclerotial production. Here, we generated oxalate-minus mutants of S. sclerotiorum using two independent mutagenesis techniques, and tested the resulting mutants for growth at different pHs and for pathogenicity on four host plants. The oxalate-minus mutants accumulated fumaric acid, produced functional sclerotia and have reduced ability to acidify the environment. The oxalate-minus mutants retained pathogenicity on plants, but their virulence varied depending on the pH and buffering capacity of host tissue. Acidifying the host tissue enhanced virulence of the oxalate-minus mutants, whereas supplementing with oxalate did not. These results suggest that it is low pH, not oxalic acid itself, that establishes the optimum conditions for growth, reproduction, pathogenicity and virulence expression of S. sclerotiorum. Exonerating oxalic acid as the primary pathogenicity determinant will stimulate research into identifying additional candidates as pathogenicity factors towards better understanding and managing Sclerotinia diseases.

The Pathogen-Host Interactions database (PHI-base): additions and future developments

Rapidly evolving pathogens cause a diverse array of diseases and epidemics that threaten crop yield, food security as well as human, animal and ecosystem health. To combat infection greater comparative knowledge is required on the pathogenic process in multiple species. The Pathogen-Host Interactions database (PHI-base) catalogues experimentally verified pathogenicity, virulence and effector genes from bacterial, fungal and protist pathogens. Mutant phenotypes are associated with gene information. The included pathogens infect a wide range of hosts including humans, animals, plants, insects, fish and other fungi. The current version, PHI-base 3.6, available at http://www.phi-base.org, stores information on 2875 genes, 4102 interactions, 110 host species, 160 pathogenic species (103 plant, 3 fungal and 54 animal infecting species) and 181 diseases drawn from 1243 references. Phenotypic and gene function information has been obtained by manual curation of the peer-reviewed literature. A controlled vocabulary consisting of nine high-level phenotype terms permits comparisons and data analysis across the taxonomic space. PHI-base phenotypes were mapped via their associated gene information to reference genomes available in Ensembl Genomes. Virulence genes and hotspots can be visualized directly in genome browsers. Future plans for PHI-base include development of tools facilitating community-led curation and inclusion of the corresponding host target(s).

Genomic analysis of the necrotrophic fungal pathogens Sclerotinia sclerotiorum and Botrytis cinerea

Sclerotinia sclerotiorum and Botrytis cinerea are closely related necrotrophic plant pathogenic fungi notable for their wide host ranges and environmental persistence. These attributes have made these species models for understanding the complexity of necrotrophic, broad host-range pathogenicity. Despite their similarities, the two species differ in mating behaviour and the ability to produce asexual spores. We have sequenced the genomes of one strain of S. sclerotiorum and two strains of B. cinerea. The comparative analysis of these genomes relative to one another and to other sequenced fungal genomes is provided here. Their 38-39 Mb genomes include 11,860-14,270 predicted genes, which share 83% amino acid identity on average between the two species. We have mapped the S. sclerotiorum assembly to 16 chromosomes and found large-scale co-linearity with the B. cinerea genomes. Seven percent of the S. sclerotiorum genome comprises transposable elements compared to <1% of B. cinerea. The arsenal of genes associated with necrotrophic processes is similar between the species, including genes involved in plant cell wall degradation and oxalic acid production. Analysis of secondary metabolism gene clusters revealed an expansion in number and diversity of B. cinerea-specific secondary metabolites relative to S. sclerotiorum. The potential diversity in secondary metabolism might be involved in adaptation to specific ecological niches. Comparative genome analysis revealed the basis of differing sexual mating compatibility systems between S. sclerotiorum and B. cinerea. The organization of the mating-type loci differs, and their structures provide evidence for the evolution of heterothallism from homothallism. These data shed light on the evolutionary and mechanistic bases of the genetically complex traits of necrotrophic pathogenicity and sexual mating. This resource should facilitate the functional studies designed to better understand what makes these fungi such successful and persistent pathogens of agronomic crops.