Gene expression profiling of a compatible interaction between Douglas-Fir and the root rot fungal pathogen Phellinus sulphurascens

Cryptic Speciation in Western North America and Eastern Eurasia of the Pathogens Responsible for Laminated Root Rot

Coniferiporia sulphurascens is a facultative fungal pathogen that causes laminated root rot (LRR) in commercially important coniferous species worldwide. This fungus spreads primarily by way of vegetative mycelium transferring at points of contact between infected and healthy roots. Successful intervention to control LRR requires a better understanding of the population structure and genetic variability of C. sulphurascens. In this study, we investigated the population genetic structure and origin of C. sulphurascens populations in western North America and eastern Eurasia collected from multiple coniferous hosts. By analyzing the small and large mitochondrial ribosomal RNA subunit genes combined with six nuclear loci (internal transcribed spacer region, actin, RNA polymerase II largest subunit, RNA polymerase II second-largest subunit, laccase-like multicopper oxidase, and translation elongation factor 1-α), we observed that none of the alleles among the loci were shared between North American (NA) and Eurasian C. sulphurascens populations. In total, 55 multilocus genotypes (MLGs) were retrieved in C. sulphurascens isolates occurring in these two continental regions. Of these, 41 MLGs were observed among 58 isolates collected from widespread locations in British Columbia (Canada) and the northwestern United States, while 14 MLGs were observed among 16 isolates sampled in Siberia and Japan. Our data showed that the levels of genetic differentiation between the NA and Eurasian populations are much greater than the populations from within each continental region; the two continental populations formed clearly divergent phylogenetic clades or lineages since they were separated approximately 7.5 million years ago. Moreover, the Eurasian population could be the source of the NA population. Our study indicates the existence of cryptic diversity in this pathogen species, and strongly suggests that the NA and Eurasian populations represent two lineages, which have progressively diverged from each other in allopatry.

Gene expression profiling of candidate virulence factors in the laminated root rot pathogen Phellinus sulphurascens

BACKGROUND

Phellinus sulphurascens is a fungal pathogen that causes laminar root rot in conifers, one of the most damaging root diseases in western North America. Despite its importance as a forest pathogen, this fungus is still poorly studied at the genomic level. An understanding of the molecular events involved in establishment of the disease should help to develop new methods for control of this disease.

RESULTS

We generated over 4600 expressed sequence tags from two cDNA libraries constructed using either mycelia grown on cellophane sheets and exposed to Douglas-fir roots or tissues from P. sulphurascens-infected Douglas-fir roots. A total of 890 unique genes were identified from the two libraries, and functional classification of 636 of these genes was possible using the Functional Catalogue (FunCat) annotation scheme. cDNAs were identified that encoded 79 potential virulence factors, including numerous genes implicated in virulence in a variety of phytopathogenic fungi. Many of these putative virulence factors were also among 82 genes identified as encoding putatively secreted proteins. The expression patterns of 86 selected fungal genes over 7 days of infection of Douglas-fir were examined using real-time PCR, and those significantly up-regulated included rhamnogalacturonan acetylesterase, 1,4-benzoquinone reductase, a cyclophilin, a glucoamylase, 3 hydrophobins, a lipase, a serine carboxypeptidase, a putative Ran-binding protein, and two unknown putatively secreted proteins called 1 J04 and 2 J12. Significantly down-regulated genes included a manganese-superoxide dismutase, two metalloproteases, and an unknown putatively secreted protein called Ps0058.

CONCLUSIONS

This first collection of Phellinus sulphurascens EST sequences and its annotation provide an important resource for future research aimed at understanding key virulence factors of this forest pathogen. We examined the expression patterns of numerous fungal genes with potential roles in virulence, and found a collection of functionally diverse genes that are significantly up- or down-regulated during infection of Douglas-fir seedling roots by P. sulphurascens.