ABSTRACT
Plants are, in general, resistant or immune to the majority of pathogens in the environment as they exhibit nonhost resistance: i.e., they are not an appropriate host for the pathogen that attacks them. Thus, resistance is the rule and susceptibility is the exception in biotic interactions. This resistance is based on a complex network of constitutive and inducible defense reactions. Only a certain number of host-pathogen pairs exist in which the pathogen has evolved the capability to resist or prevent a specific plant response (host compatibility). Defined plant cultivars have developed resistance against distinct pathogen races (cultivar resistance; see Refs. 1-3). In these well-studied host-pathogen interactions resistance is based on the presence of corresponding pathogen avirulence genes (Avr genes) and plant resistance genes (R genes) according to Flor’s genefor-gene hypothesis (4). During this process, defined products (ligands) of the Avr gene are secreted into the apoplast or the cytoplasm of the plant cell, where they interact with R gene products that often function as receptors (Fig. 1). These receptors are located intracellularly or on the plasma membrane.
