Investigating the mechanisms of long-lasting within generation jasmonate induced resistance

Jasmonates, such as jasmonic acid (JA) or methyl jasmonate (MeJA), elicit short-term enhanced resistance in plants against chewing herbivores and necrotrophic pathogens. In Norway spruce (Picea abies), this jasmonate induced resistance (IR) can last for many months, whereas the long-term within-generation impact of jasmonates on biotic stress resistance in Arabidopsis thaliana (Arabidopsis) remains unclear. Furthermore, little is known about the mechanisms controlling long-term jasmonate-IR. This PhD study addressed these knowledge gaps by exploring the phenotypes and immunological and (epi)genetic mechanisms of long-term jasmonate-IR in Arabidopsis and Norway spruce. Two-week-old Arabidopsis seedlings were treated with JA and then challenged three weeks later with different pests and pathogens. Long-term JA-IR against the generalist herbivore Spodoptera littoralis was comparable to short-term JA-IR against this pest. Whereas, plants from JA-treated seedlings displayed enhanced susceptibility to the necrotrophic fungus Plectosphaerella cucumerina which contrasted the short-term IR against this pathogen. Transcriptome analysis revealed that the long-term response to JA is characterised by priming and/or prolonged upregulation of MYC-dependent anti-herbivore defences, but repression of anti-pathogen defences. Bioassays with DNA (de)methylation mutants and a genome-wide methylome analysis suggested that long-term JA-IR against herbivores is based on ROS1-dependent hypomethylation of transposable elements, particularly those from the ATREP2 family. Spruce experiments were performed with two-year-old seedlings that were treated with MeJA and challenged four weeks later. MeJA elicited IR against the necrotrophic pathogen Grosmannia penicillata. A transcriptome analysis suggested that establishment and maintenance of MeJA-IR in spruce involves RNA-directed DNA methylation and defence hormone signalling. The analysis also revealed distinct temporal patterns of global gene expression in response to MeJA treatment. This thesis has advanced our understanding of the (epi)genetic and immunological basis of long-term jasmonate-IR in Arabidopsis and Norway spruce. The results have implications for the exploitation of IR in agriculture and forestry