Abstract
The Heterobasidion annosum s.l species complex comprises the most damaging forest pathogens to Norway spruce. We revisited previously identified Quantitative Trait Loci (QTLs) related to Heterobasidion-resistance in Norway spruce to identify candidate genes associated with these QTLs. We identified 329 candidate genes associated with the resistance QTLs using a gene-based composite map for Pinaceae. To evaluate the transcriptional responses of these candidate genes to H. parviporum, we inoculated Norway spruce plants and sequenced the transcriptome of the interaction at 3 and 7 days post inoculation. Out of 298 expressed candidate genes 124 were differentially expressed between inoculation and wounding control treatment. Interestingly, PaNAC04 and two of its paralogs in the subgroup III-3 of the NAC family transcription factors were found to be associated with one of the QTLs and was also highly induced in response to H. parviporum. These genes are possibly involved in the regulation of biosynthesis of flavonoid compounds. Furthermore, several of the differentially expressed candidate genes were associated with the phenylpropanoid pathway including a phenylalanine ammonia-lyase, a cinnamoyl-CoA reductase, a caffeoyl-CoA O-methyltransferase and a PgMYB11-like transcription factor gene. Combining transcriptome and genetic linkage analyses can help identifying candidate genes for functional studies and molecular breeding in non-model species.
Highlights
The Heterobasidion annosum s.l species complex comprises the most damaging forest pathogens to Norway spruce
One of the marker candidates, PaLAR3 was identified in the first Quantitative Trait Locus (QTL) analysis for resistance against H. parviporum (Fr.) Niemelä & Korhonen[14,15], which is a member of the H. annosum s.l. complex and lives almost exclusively in Norway spruce[19]
The QTL analysis in Norway spruce for resistance to H. parviporum identified 13 QTLs linked with four traits related to host resistance[14]: lesion length at the inoculation site (LL), exclusion of the pathogen from the host after initial infection (E), infection prevention from establishing at all (IP) and fungal spread within the sapwood (SWG)
Summary
The Heterobasidion annosum s.l species complex comprises the most damaging forest pathogens to Norway spruce. PaNAC04 and two of its paralogs in the subgroup III-3 of the NAC family transcription factors were found to be associated with one of the QTLs and was highly induced in response to H. parviporum These genes are possibly involved in the regulation of biosynthesis of flavonoid compounds. One of the marker candidates, PaLAR3 was identified in the first Quantitative Trait Locus (QTL) analysis for resistance against H. parviporum (Fr.) Niemelä & Korhonen[14,15], which is a member of the H. annosum s.l. complex and lives almost exclusively in Norway spruce[19]. We used the composite map of the Pinaceae family and gene expression patterns in Norway spruce after challenging it with H. parviporum; we could identify candidate resistance genes for future validation and functional analyses
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