Although some of the resistance strategies rely on simple physical or
chemical barriers, modern concepts of plant immunity emphasize the role and
evolution of protein receptors in the plant cell. These immune receptors, made up of
multidomain proteins, are the key elements in the recognition of pathogen elicitors /
effectors, leading to the susceptibility or resistance of plants. Numerous pairs of plant R
proteins and corresponding pathogenic Avr proteins have been identified as well as
cellular proteins which mediate R/Avr interactions, and the molecular analysis of these
interactions has led to the formulation of models on how R gene products recognize
pathogens. Data from several R/Avr systems indicate that specific domains within R
proteins determine recognition specificity. However, recent evidence suggests that R
proteins have recruited cell recognition cofactors that mediate interactions between Avr
proteins and R proteins. Overall, to explain this direct or indirect interaction, at least
four models are currently widely approved. This chapter highlights the current trends in
understanding host–pathogen interactions through a variety of models.
Keywords: Avirulence (Avr) protein, Compatible/incompatible reaction, Decoy
model, Gene-for-gene model, Guard model, Helper NLRs, Integrated decoy
model, NLR-IDs, R-Avr recognition, Recognition cofactors, Resistance (R)
protein, Resistance gene, Receptor-ligand model.
