ABSTRACT
Induced resistance mechanisms are of major importance in the defence of plants against pathogens, as discussed in Chapter 9. Why are these effective in some plants against some pathogens, but not in others? The answer is complex and presumably reflects how and when the induction occurs. In non-host plants, induction of the innate immune response by non-specific elicitors from microbes may be sufficient for resistance. In host plants, the pathogen may have evolved to evade these innate responses or to trigger them later in infection so that they are ineffective, and specific resistance genes for recognising specific elicitors from the pathogen may be required for resistance. These may be either one or a few genes whose individual effects can be easily detected (gene-for-gene or vertical resistance) or numerous genes with small additive effects (quantitative or horizontal resistance) (Figure 10.1). It is the nature of these resistance genes that we focus on in this chapter.
