Disease resistance genes: form and function.

Plants present numerous barriers to potential pathogens including structural hindrances such as waxy cuticles. Furthermore, plants, like all multicellular organisms, have evolved multiple layers of defences based on the recognition of pathogens via germline encoded receptor-like proteins. The genes encoding these receptor-like molecules confer recognition of specific pathogens or pathogen isolates and are often highly variable, resulting in cultivar or ecotypespecific differences in resistance to pathogens. In addition, many pathogens need to utilize host cellular mechanisms for their own purposes, and produce proteins that interact with host proteins to do so. However, plants also possess diversity in these factors and incompatibility between host and pathogen proteins can lead to a lack of susceptibility. Resistance to pathogens is often determined by variation at single genetic loci encoding either factors mediating active recognition of, or susceptibility to the pathogen. The genes encoded at these variable loci are broadly known as disease resistance (R) genes and encode multiple classes of R proteins. Recent advances in molecular genetics have lead to insights into the mechanisms by which plants either prevent pathogens from infecting them in the first place, or actively recognize and eliminate pathogens.