108 CHAPTER 5 General Discussion
SCOPE 109 General discussion For a successful parasitic interaction, pathogens and insects have to cope with a plethora of plant defence mechanisms. Some of these defences are pre-existing, while others are inducible by the attacking organism. As was outlined in the General Introduction of this thesis (Figure 1; Chapter 1), induced defence in plants is based on sequentially activated defence layers that are active at different stages of the parasitic interaction. In general terms, these layers of the plant immune system can be separated between pre-invasive defence barriers, such as PAMP-induced closure of stomata (Melotto et al., 2006), early-acting post-invasive defence, such as localised deposition of ROS and callose (Luna et al., 2011), and late-acting post-invasive defences that are under control by de novo produced defence hormones, such as salicylic acid (SA; Heil and Ton, 2008). The overarching objective of the work presented in this thesis was to study the contribution of post-invasive defence barriers to <strong>basal</strong> <strong>resistance</strong> (Chapters 2 and 3) and their impact on the interactions with plant-beneficial microbes, such as rhizosphere-colonizing Pseudomonas putida bacteria (Chapter 4). NATURAL OCCURRING VARIATION IN BASAL RESISTANCE The vast majority of studies on natural variation in plant defence have focussed on ETI (De Meaux and Mitchell-Olds, 2003; Holub, 2007; Van Poecke et al., 2007), which is likely due to the robustness and reproducibility of the ETI phenotype. There are also numerous studies about natural variation in <strong>basal</strong> <strong>resistance</strong> against pathogens and herbivores, many of which are based on the genetic model plant species Arabidopsis (Koornneef et al., 2004). However, relatively few of these have linked this variation to actual <strong>resistance</strong> mechanisms. The natural variation in <strong>basal</strong> <strong>resistance</strong> of Arabidopsis to insects often originates from differences in pre-existing pools of glucosinolates (Kliebenstein et al., 2001; Koornneef et al., 2004). Glucosinolates enable a rapid production of biocidal isothiocyanates after herbivore attack and could therefore, be viewed as a constitutively primed defence mechanism. Natural variation in <strong>basal</strong> <strong>resistance</strong> of Arabidopsis against pathogens, on the other hand, seems to stem from more diverse mechanisms than from glucosinolates. For instance, Denby et al., (2004) reported that natural variation in <strong>basal</strong> <strong>resistance</strong> against the necrotroph Botrytis cinerea correlates with responsiveness of pathogen- and acifluorfen-induced camalexin, an indole-derived phytoalexin. Further genetic dissection of this <strong>basal</strong> <strong>resistance</strong> in a mapping population of recombinant inbred lines (RILs) revealed multiple small-to-medium-effect quantitative trait loci (QTLs), but it remained unclear to what extent these loci influence the responsiveness of camalexin induction itself. Similarly, Llorente et al. (2005) used a RIL population to dissect natural variation in <strong>basal</strong> <strong>resistance</strong> against the necrotrophic fungus Plectosphaerella cucumerina, which identified three different QTLs. The most influential