Conifer root and butt rot caused by Heterobasidion annosum (Fr.) Bref. s.l

Modelling disease spread and control in networks: implications for plant sciences

Networks are ubiquitous in natural, technological and social systems. They are of increasing relevance for improved understanding and control of infectious diseases of plants, animals and humans, given the interconnectedness of today's world. Recent modelling work on disease development in complex networks shows: the relative rapidity of pathogen spread in scale-free compared with random networks, unless there is high local clustering; the theoretical absence of an epidemic threshold in scale-free networks of infinite size, which implies that diseases with low infection rates can spread in them, but the emergence of a threshold when realistic features are added to networks (e.g. finite size, household structure or deactivation of links); and the influence on epidemic dynamics of asymmetrical interactions. Models suggest that control of pathogens spreading in scale-free networks should focus on highly connected individuals rather than on mass random immunization. A growing number of empirical applications of network theory in human medicine and animal disease ecology confirm the potential of the approach, and suggest that network thinking could also benefit plant epidemiology and forest pathology, particularly in human-modified pathosystems linked by commercial transport of plant and disease propagules. Potential consequences for the study and management of plant and tree diseases are discussed.

Suppression of plant defence response by a mycorrhiza helper bacterium

The aim of the present study was to determine whether the mycorrhiza helper bacterium Streptomyces sp. AcH 505 could serve as a biocontrol agent against Heterobasidion root and butt rot. Bacterial influence on mycelial growth of Heterobasidion sp. isolates, on the colonization of wood discs and Norway spruce (Picea abies) roots was determined. The effect of AcH 505 on plant photosynthesis, peroxidase activity and gene expression, and needle infections were investigated. AcH 505 was antagonistic to 11 of 12 tested fungal Heterobasidion isolates. The antagonism resulted in a suppression of fungal colonization of Norway spruce roots and wood discs. Mycelial growth rate of the 12th strain, Heterobasidion abietinum 331 was not affected by AcH 505, and colonization of roots by this fungal strain was promoted by AcH 505. Bacterial inoculation led to decreased peroxidase activities and gene expression levels in roots. AcH 505 promotes plant root colonization by Heterobasidion strains that are tolerant to antifungal metabolites produced by the bacterium. This may result from unknown bacterial factors that suppress the plant defence response.

Identification and analysis of differentially expressed cDNAs during nonself-competitive interaction between Phlebiopsis gigantea and Heterobasidion parviporum

The molecular factors regulating interspecific interaction between the saprotrophic biocontrol fungus Phlebiopsis gigantea and the conifer pathogen Heterobasidion parviporum were investigated. We constructed cDNA libraries and used expressed sequence tag analysis for the identification and characterization of genes expressed during the self and nonself-hyphal interaction. cDNA clones from either the pathogen or biocontrol agent were arrayed on nylon membrane filters and differentially screened with cDNA probes made from mycelia forming the barrage zone during nonself-interactions, mycelia growing outside the barrage zones or monocultures. BlastX analysis of the differentially expressed clones led to the identification of genes with diverse functions, including those with potential as virulence factors, such as hydrophobins. Because of the high sequence conservation (r2 = 0.81) between P. gigantea and H. parviporum, a selected number of genes from either fungus were used to monitor the expression profile under varying interaction conditions by virtual northern blot. The results are discussed with respect to the potential role of the induced genes during the nonself-competitive interaction for space and nutrients between P. gigantea and H. parviporum.

A macroarray expression analysis of novel cDNAs vital for growth initiation and primary metabolism during development of Heterobasidion parviporum conidiospores

The pathogen Heterobasidion parviporum causes significant losses to forest industries in Europe and North America. The fungus is spread by basidiospores on fresh stumps where it differentiates into a specialized infection hyphae to colonize its host. This differentiation is driven by recognition and its strategic success lies in its ability to do this rapidly and efficiently. To investigate gene expression pattern during the spore germination stages, mRNA of germinated and ungerminated conidiospores of H. parviporum harvested at distinct developmental time points (18, 36, 72 and 120 h) corresponding to periods of isotropic/germ tube emergence, polarized apical, early and late mycelial lateral branching growth stages was hybridized to macroarrays containing 338 cDNAs from H. parviporum. The results of the statistical analysis identified a total of 24, 39, 38 and 30 genes that were differentially upregulated at 18, 36, 72 and 120 h, respectively, relative to time 0. The number of the downregulated genes was 4, 6, 8 and 13 genes respectively. During isotropic, polarized and mycelial growth stages, majority of the differentially expressed genes belonged to functional categories metabolism (21-32%) and protein formation (21-30%). Real-time polymerase chain reaction (real-time-PCR) data essentially confirmed the macroarray analyses. The real-time-PCR result showed that transcript levels of genes involved in glucose metabolism (phosphoglucomutase), amino acid metabolism (arginase, delta-1-pyrroline-5-carboxylate reductase, sulfur metabolism-negative regulator, imidazoleglycerol phosphate dehydratase) and protein synthesis (40S ribosomal protein S15) were significantly increased during polarized growth (36 h) stage but decreased at early and late stages of mycelial growth (72-120 h). An understanding of the various molecular and physiological processes during the development of H. parviporum spores is an important step towards the goal of identifying novel antifungal strategies.