Abstract
Heterobasidion annosum sensu lato is a serious pathogen causing root and stem rot to conifers in the northern hemisphere and rendering the timber defective for sawing and pulping. In this study we applied next-generation sequencing to i) identify transcriptional responses unique to Heterobasidion-inoculated Norway spruce and ii) investigate the H. annosum transcripts to identify putative virulence factors. To address these objectives we wounded or inoculated 30-year-old Norway spruce clones with H. annosum and 454-sequenced the transcriptome of the interaction at 0, 5 and 15 days post inoculation. The 491860 high-quality reads were de novo assembled and the relative expression was analysed. Overall, very few H. annosum transcripts were represented in our dataset. Three delta-12 fatty acid desaturase transcripts and one Clavaminate synthase-like transcript, both associated with virulence in other pathosystems, were found among the significantly induced transcripts. The analysis of the Norway spruce transcriptional responses produced a handful of differentially expressed transcripts. Most of these transcripts originated from genes known to respond to H. annosum. However, three genes that had not previously been reported to respond to H. annosum showed specific induction to inoculation: an oxophytodienoic acid–reductase (OPR), a beta–glucosidase and a germin-like protein (GLP2) gene. Even in a small data set like ours, five novel highly expressed Norway spruce transcripts without significant alignment to any previously annotated protein in Genbank but present in the P. abies (v1.0) gene catalogue were identified. Their expression pattern suggests a role in defence. Therefore a more complete survey of the transcriptional responses in the interactions between Norway spruce and its major pathogen H. annosum would probably provide a better understanding of gymnosperm defence than accumulated until now.
Highlights
Norway spruce [Picea abies (L.) Karst.] is a dominating tree species in much of northern Eurasia
In an earlier study of the transcriptional responses of genes in the phenylpropanoid biosynthetic pathway in the Norway spruce genotypes we showed that those genes were highly induced in response to wounding and infection by H. annosum s.s. [1]
Our objectives were to i) identify transcriptional responses unique to Heterobasidion-inoculated Norway spruce and ii) investigate the H. annosum s.s. transcripts and compare them to the expression patterns predicted from H. irregulare in Pine bark [14] to identify common virulence factors in H. annosum s.l
Summary
Norway spruce [Picea abies (L.) Karst.] is a dominating tree species in much of northern Eurasia. It relies on both induced and constitutive defences to restrict the spread of invading fungi and insects. The induced transcriptional responses to pathogenic fungi are similar to those induced by wounding in Norway spruce [1, 4, 6,7,8] Both responses involve expression of genes encoding pathogenesis related- (PR) proteins and components of well-described defence signal transduction pathways but the expression levels are generally significantly higher after pathogen infection
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