Vascular wilt pathogens, which comprise bacteria, fungi and oomycetes, are among the most destructive plant pathogens that affect annual crops as well as woody perennials, thus not only impacting world food and feed production but also natural ecosystems. Vascular wilt pathogens colonize the xylem vessels of their host plants and interfere with the normal transportation of water and nutrients from the roots to upper parts of the plant, thus causing wilting symptoms. The structure and composition of xylem vessels has a significant impact on the colonization of host plants by these pathogens. Presently, genetic resistance is the most preferred control strategy against this group of plant pathogens. Verticillium wilt disease, which is caused by the vascular fungal pathogen Verticillium spp., is among the major diseases in various horticultural crops in tropical, subtropical, and temperate agro-ecological regions. The genus Verticilllium comprises of three major plant pathogenic species; V. dahliae, V. albo-atrum,and V. longisporum. While V. dahliae and V. albo-atrum arecharacterized with the ability to infect broad host range, V. longisporum has relatively limited host range infecting mainly crucifers family. V. dahliae and V. albo-atrum isolates are categorized into race 1 and race 2 based on their ability to infect tomato plants containing a Ve1 resistance gene. On tomato, while race 1 isolates are contained by Ve1 resistance gene, race 2 isolates overcome Ve1-mediated resistance. Chapter 1is the introduction to the thesis that describes xylem defence responses that are directed against vascular wilt pathogens. Plants recognize xylem-invading vascular wilt pathogens by using extracellular or intracellular receptors. Pathogen recognition activates innate immune responses that include physical and chemical defense responses in the xylem vessels and the surrounding parenchyma cells. While physical defense responses often halt pathogen movement between vessels, chemical defense responses can eliminate the pathogen or inhibit its growth, thereby leading to resistance. In order to identify novel sources of Verticillium wilt resistance, a collection of activation-tagged Arabidopsis mutants was screened for plants that displayed enhanced Verticillium wilt resistance. Chapter 2 describes four mutants (A1 to A4) that showed enhanced resistance to not only V. dahliae, but also to V. albo-atrum, and the Brassicaceae pathogen V. longisporum. Further characterization of resistance in these mutants against other vascular wilt pathogens, Ralstonia solanacearum and Fusarium oxysporum f. sp. Raphani, and the foliar pathogens such Botrytis cinerea, Plectosphaerella cucumerina, Alternaria brassicicola, and Pseudomonas syringae pv. tomato, is presented in this chapter. Except for mutant A2, that showed enhanced resistance to R. solanacearum, and mutants A1 and A3, that showed enhanced susceptibility to P. syringae, all the mutants responded similar as wild-type plants to these pathogens. In chapter 2, we furthermore describe the cloning and functional characterization of the gene encoding the AT-hook DNA-binding protein AHL19 that is responsible for the enhanced resistance of the A1 mutant to Verticillium wilt disease. The Arabidopsis genome contains 29 AHL proteins (Fujimoto et al., 2004)some of which have been implicated in various biological processes including plant development (Lim et al., 2007; Xiao et al., 2009)and defense (Kim et al., 2007Kim et al., 2007; Lu et al., 2010). AHL19 provides Verticillium wilt resistance upon over-expression, whereas knock-out enhances susceptibility, indicating that AHL19 positively regulates Verticillium wilt resistance. AHL19 not only regulates Verticillium wilt resistance, but also affects plant development, as AHL19 over-expressing plants showed larger leaf size, delayed maturity, and low seed production (Yadeta et al., 2011). Chapter 3describes the cloning and functional characterization of EVR1 (for Enhanced Verticillium Resistance 1), the gene that is responsible for the enhanced Verticillium wilt resistance in mutant A2. Mutant A2 furthermore confers resistance to the bacterial vascular wilt pathogen R. solanacearum (Yadeta et al., 2011). While EVR1 over-expression enhances Arabidopsis resistance to three vascular wilt pathogens: V. dahliae, R. solanacearum, and F. oxysporum, knock-out enhances susceptibility to V. dahliae and R. solanacearum. Furthermore, EVR1 appears to regulate drought stress resistance. EVR1 is a single copy gene that encodes a protein of unknown function, and EVR1 homologs are only found in Brassicaceae species thus far. Interestingly, over-expression of the B. oleraceae EVR1 homolog in Arabidopsis conferred Verticillium wilt resistance. Moreover, over-expression of Arabidopsis and B. oleraceae EVR1 and BoEVR1 in the Solanaceous species N. benthamiana enhanced Verticillium wilt resistance. This suggests that the Brassicaceae-specific EVR1 gene can be used to engineer Verticillium wilt resistance in other plant families. Whereas chapters 2 and 3 focus on the identification of novel sources of Verticillium wilt resistance by screening a collection of Arabidopsis gain-of-function mutants, Chapter 4 describes the identification of novel Verticillium wilt resistance in wild tomato accessions. Six wild accessions were identified that displayed enhanced resistance to race 2 isolates. Surprisingly, however, these accessions did not show enhanced resistance to race 1 isolates. Using virus-induced gene silencing, the resistance signalling leading to race 2 resistance in the wild accessions was investigated, showing that the resistance signalling in the wild accessions is distinct from the signalling pathway employed by the resistance protein Ve1. Finally in chapter 5, the highlights of this thesis are discussed and placed in a broader perspective.