In their struggle for life, plants can employ sophisticated strategies to defend themselves against potentially harmful pathogens and insects. One mechanism by which plants can increase their level of resistance is by intensifying the responsiveness of their immune system upon recognition of selected signals from their environment. This so-called priming of defence can provide long-lasting resistance, which is based on a faster and/or stronger defence reaction upon pathogen or pest attack (Figure 1). Priming can target various layers of induced defence that are active during different stages of the plant-attacker interaction. Although it has been known for several years that priming can enhance the effectiveness of different plant defence responses, the underpinning molecular mechanisms have remained poorly understood. The main aim of the work described in this thesis was to explore the mechanisms controlling the establishment and long-term maintenance of defence priming. Chapter describes the early signalling events leading to broad-spectrum defence priming after treatment with the chemical agent BABA, whilst the subsequent experimental chapters focus on long-term maintenance of defence priming (Figure 1). Chapter 2 describes the isolation and characterization of the Impaired in J1.ABA induced Immunity (ibi 1) mutant, which was selected for loss of BABA-induced resistance against the oomycete Hyaloperonospora arabidopsidis. Map-based cloning of fBll revealed that this gene encodes a pathogen-inducible aspartyl tRNA synthetase, which mediates esterification of L-aspartic acid to its cognate tRNA. The results in Chapter 2 suggest that IBI I exhibits an additional non-canonical function as a receptor of BABA and a native regulator of basal resistance. Importantly, t.he work described in this Chapter provides evidence that the stress response triggered by relatively high concentrations of BABA is under separate genetic control than BABA-induced priming of broad-spectrum defence. This outcome provides exciting scope to exploit IBI I-dependent resistance in crops, particularly against plant diseases that are difficult to control by fungicides or single resistance genes. Recently, studies have emerged suggesting that long-lasting priming phenomena are controlled by epigenetic regulatory mechanisms. Priming of defence genes has been associated with modifications of histone proteins at defence-related gene promoters, which may facilitate access of the transcriptional machinery to gene promoters. Chapter 3 focuses on the durability of BABA-induced resistance in Arabidopsis. Treatment of 5 day-old Arabidopsis seedlings resulted in protection against H arabidopsidis and Pseudomonas syringae that lasted up to 4 weeks after BABA treatment. This long-lasting component is under the control of the NON-EXPRESS OR OF PR GENES 1 (NPRl) protein and is associated with priming of SA-inducible genes. Analysis of mutants in chromatin remodelling processes revealed that long-lasting induced resistance by BABA requires regulation by post-translational modification of Histone 3 Lysine 9 (H3K9) and H2A.Z occupancy. Conclusive evidence for an epigenetic basis of defence priming came very recently from independent laboratories across the world. Chapter 4 describes one of these studies and shows that defence priming can be transmitted to following generations from diseased Arabidopsis plants after fitness-reducing levels of disease by the bacterial pathogen P. syringae pv. tomato DC3000 (PstDC3000). Compared to progeny from control-inoculated plants (Cl), progeny from diseased plants (PI) expressed significantly higher levels of basal resistance to the oomycete pathogen H arabidopsidis and PstDC3000. In addition, the findings in this chapter demonstrate that PI plants are primed to respond to exogenously applied SA, but are repressed in their responsiveness of lA-inducible genes, which correlated with enhanced susceptibility to the necrotrophic fungus Alternaria brassicicola. Interestingly, the transgenerational SAR phenotype of PI plants is associated with permissive and repressive chromatin modifications at SA- and lA-inducible gene promoters, respectively. Finally, phenotypic analysis of the drmldrm2cmt3 mutant, which is impaired in non-CpG methylation, suggests an important role for this plant-specific form of DNA methylation in the transmission oftransgenerational SAR. Chapter 5 serves as an Addendum to the work described in the previous chapter, and describes further analysis of mutants in RNA-directed DNA methylation (RdDM) for their ability to express transgenerational SAR upon repeated infection by PstDC3000. The results indicate that transgenerational SAR is regulated by the RdDM pathway and likely transmitted through hypomethylation of genomic DNA at CpHpG sites. How all the various molecular and epigenetic mechanisms of priming relate to each other remains unknown and will require further research, which is a critical first step towards large-scale exploitation of the phenomenon in sustainable agriculture. In Chapter 6, I describe how long-lasting induced resistance can be applied to protect the crop Solanum lycopersicum (tomato). The costs (plant growth reduction) and benefits (effectiveness and durability of the induced resistance) of different resistance-inducing treatments have been studied. Seed treatment with jasmonic acid (lA) or f3-aminobutyric acid (BABA) rendered long-lasting protection that was not associated with major costs. However, the consistency of the induced resistance was rather low and was cultivar-dependent. Also, the incubation of seeds for prolonged periods with lA provided long-lasting protection against the necrotrophic pathogen B. cinerea with no adverse effects on seed germination or plant growth. Treatment of tomato seedlings with lA and BABA resulted in a more pronounced resistance response that lasted up to 5 and 6 weeks after the treatment, respectively. However, the disease protection upon these application methods was associated with residual fitness costs. Finally, the possibility of transgenerational induced resistance in tomato was investigated. The results in Chapter 3 set the basis to integrate long-lasting induced resistance in conventional strategies of tomato protection (Chapter 6). Food security is one of the most challenging issues faced by humanity in this century, and is likely to become further aggravated by climate change that can render agricultural lands less suitable for crop production. Consequently, there is a pressing need to improve the efficiency of sustainable food production, including intensification of durable crop protection strategies. Integration of long-lasting induced resistance into existing disease management schemes would allow lower energy costs to reach similar levels of disease protection. My Ph.D project has uncovered different regulatory mechanisms of long-lasting indu~ed resistance based on priming of defence. Future research will be necessary to narrow down the mechanisms by which genetics and epigenetics mediate priming of defence. I hope that these new insights will help to optimise the efficiency of robust and durable induced resistance in plants.