Norway spruce (Picea abies) is an economically and ecologically
important tree species that grows across northern and central Europe.
Treating Norway spruce with jasmonate has long-lasting beneficial
effects on tree resistance to damaging pests, such as the European
spruce bark beetle Ips typographus and its fungal associates. The
potential involvement of (epi)genetic mechanisms in this long-lasting
jasmonate-induced resistance (IR) has gained much recent interest, but
remains largely unknown. In this study, we treated 2-year-old spruce
seedlings with methyl jasmonate (MeJA) and challenged them with the I.
typographus vectored necrotrophic fungus Grosmannia penicillata. MeJA
treatment reduced the extent of necrotic lesions in the bark and thus
elicited IR to the fungus. The transcriptional response of spruce bark
to MeJA treatment was analyzed over a 4-week time course using mRNA-seq.
This analysis provided evidence that MeJA treatment induced a transient
upregulation of jasmonic acid, salicylic acid and ethylene biosynthesis
and downstream signaling genes. Additionally, genes encoding components
of the RNA-directed DNA methylation pathway showed long-term repression,
suggesting a possible role of DNA demethylation in the maintenance of
MeJA-IR. These results provide new clues about the potential mechanisms
underpinning long-term MeJA-IR in Norway spruce.