Abstract
BackgroundThe highly aggressive pathogenic fungus Ophiostoma novo-ulmi continues to be a serious threat to the American elm (Ulmus americana) in North America. Extensive studies have been conducted in North America to understand the mechanisms of virulence of this introduced pathogen and its evolving population structure, with a view to identifying potential strategies for the control of Dutch elm disease. As part of a larger study to examine the genomes of economically important Ophiostoma spp. and the genetic basis of virulence, we have constructed an expressed sequence tag (EST) library using total RNA extracted from the yeast-like growth phase of O. novo-ulmi (isolate H327).ResultsA total of 4,386 readable EST sequences were annotated by determining their closest matches to known or theoretical sequences in public databases by BLASTX analysis. Searches matched 2,093 sequences to entries found in Genbank, including 1,761 matches with known proteins and 332 matches with unknown (hypothetical/predicted) proteins. Known proteins included a collection of 880 unique transcripts which were categorized to obtain a functional profile of the transcriptome and to evaluate physiological function. These assignments yielded 20 primary functional categories (FunCat), the largest including Metabolism (FunCat 01, 20.28% of total), Sub-cellular localization (70, 10.23%), Protein synthesis (12, 10.14%), Transcription (11, 8.27%), Biogenesis of cellular components (42, 8.15%), Cellular transport, facilitation and routes (20, 6.08%), Classification unresolved (98, 5.80%), Cell rescue, defence and virulence (32, 5.31%) and the unclassified category, or known sequences of unknown metabolic function (99, 7.5%). A list of specific transcripts of interest was compiled to initiate an evaluation of their impact upon strain virulence in subsequent studies.ConclusionsThis is the first large-scale study of the O. novo-ulmi transcriptome. The expression profile obtained from the yeast-like growth phase of this species will facilitate a multigenic approach to gene expression studies to assess their role in the determination of pathogenicity for this species. The identification and evaluation of gene targets in such studies will be a prerequisite to the development of biological control strategies for this pathogen.
Highlights
The highly aggressive pathogenic fungus Ophiostoma novo-ulmi continues to be a serious threat to the American elm (Ulmus americana) in North America
The disease in North America can be attributed to two separate introduction events: the early epidemic caused by the non-aggressive sub-group O. ulmi and the later, more severe epidemic, caused by the highly pathogenic aggressive sub-group of O. novo-ulmi, which continues to threaten elm populations of Western Canada
A much greater diversity of vc types has been documented in the Eurasian aggressive (EAN) race of O. novoulmi, as compared to populations of the North American aggressive (NAN) race [6]; the EAN and NAN subpopulations of O. novo-ulmi have since been redesignated as subspecies novo-ulmi and americana, respectively [8]
Summary
The highly aggressive pathogenic fungus Ophiostoma novo-ulmi continues to be a serious threat to the American elm (Ulmus americana) in North America. The elm is a popular choice in northern climates because of its resistance to extremes of weather and harsh urban growing conditions, while its abundant crown foliage is large enough to span a city street [1] Populations of this urban tree have been decimated by Dutch elm disease. The disease in North America can be attributed to two separate introduction events: the early epidemic caused by the non-aggressive sub-group O. ulmi and the later, more severe epidemic, caused by the highly pathogenic aggressive sub-group of O. novo-ulmi, which continues to threaten elm populations of Western Canada. The comparatively low diversity of vc groups observed for the subspecies americana is atypical of an established pathogen epidemic, rapidly expanding pathogen populations have previously been reported to exhibit low genetic diversity [10]
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