Abstract
BackgroundNorway spruce trees in subalpine forests frequently face infections by the needle rust fungus Chrysomyxa rhododendri, which causes significant growth decline and increased mortality of young trees. Yet, it is unknown whether trees actively respond to fungal attack by activating molecular defence responses and/or respective gene expression.ResultsHere, we report results from an infection experiment, in which the transcriptomes (via RNA-Seq analysis) and phenolic profiles (via UHPLC-MS) of control and infected trees were compared over a period of 39 days. Gene expression between infected and uninfected ramets significantly differed after 21 days of infection and revealed already known, but also novel candidate genes involved in spruce molecular defence against pathogens.ConclusionsCombined RNA-Seq and biochemical data suggest that Norway spruce response to infection by C. rhododendri is restricted locally and primarily activated between 9 and 21 days after infestation, involving a potential isolation of the fungus by a hypersensitive response (HR) associated with an activation of phenolic pathways. Identified key regulatory genes represent a solid basis for further specific analyses in spruce varieties with varying susceptibility, to better characterise resistant clones and to elucidate the resistance mechanism.
Highlights
Norway spruce trees in subalpine forests frequently face infections by the needle rust fungus Chrysomyxa rhododendri, which causes significant growth decline and increased mortality of young trees
Studying gene expression in trees was so far restricted to the use of microarrays, which suffer from a number of technological weaknesses and only allowed for interrogating the expression pattern of already annotated genes [95]
Such draw backs have recently been overcome by completion of numerous reference genomes, such as for P. abies [74] and emerging next-generation-sequencing technologies (NGS), such as RNA-Seq which allows for the analysis of differential gene expression across the entire genome [48, 61, 89]
Summary
Norway spruce trees in subalpine forests frequently face infections by the needle rust fungus Chrysomyxa rhododendri, which causes significant growth decline and increased mortality of young trees. It is unknown whether trees actively respond to fungal attack by activating molecular defence responses and/or respective gene expression. Studying gene expression in trees was so far restricted to the use of microarrays, which suffer from a number of technological weaknesses (e.g. cross-hybridization of related sequences) and only allowed for interrogating the expression pattern of already annotated genes [95] Such draw backs have recently been overcome by completion of numerous reference genomes, such as for P. abies [74] and emerging next-generation-sequencing technologies (NGS), such as RNA-Seq which allows for the analysis of differential gene expression across the entire genome [48, 61, 89]. RNA-Seq has nowadays become a standard tool for investigating differences in gene expression under contrasting abiotic and biotic treatments, and may be used to compare the expression of fungus-infected and non-infected individuals [91]
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