In their constant struggle for survival, plants have developed a wide range of defence mechanisms to protect themselves against the attack of pathogens. While some of these resistance strategies rely on simple physical or chemical barriers, more sophisticated biochemical mechanisms based on gene-for-gene interactions between plants and their infectious agents have been reported.
Plant disease resistance genes (R-genes) play a key role in recognizing proteins expressed by specific avirulence (Avr) genes of pathogens. R-genes originate from a phylogenetically ancient form of immunity that is common to plants and animals. However, the rapid evolution of plant immunity systems has led to enormous gene diversification. Although little is known about these agriculturally important genes, some fundamental genomic features have already been described. It has been recently shown that proteins encoded by resistance genes display modular domain structures and require several dynamic interactions between specific domains to perform their function. Some of these domains also seem necessary for proper interaction with Avr proteins and in the formation of signalling complexes that activate an innate immune response which arrests the proliferation of the invading pathogen.
R-genes can be functionally grouped in at least six distinct classes based on the presence of specific domains:
-The CNL class comprises resistance genes encoding proteins with at least a coiled-coil domain, a nucleotide binding site and a leucine-rich repeat (CC-NB-LRR)
-The TNL class includes those with a Toll-interleukin receptor-like domain, a nucleotide binding site and a leucine-rich repeat (TIR-NB-LRR)
-The RLP class, acronym for receptor-like protein, groups those with a receptor serine-threonine kinase-like domain, and an extracellular leucine- rich repeat (ser/thr-LRR)
-The RLK class contains those with a kinase domain, and an extracellular leucine-rich repeat (Kin-LRR)