Structure, function, and evolution of disease resistance genes in rice.

The recent cloning and characterization of several rice genes with resistance to pathogens represent a breakthrough in our understanding of the molecular basis of disease resistance and also provide a starting point for dissecting the resistance pathway in rice. The first resistance gene cloned in rice was Xa21, a gene introgressed from the wild rice Oryza longistaminata. It encodes a putative receptor-like kinase consisting of leucine-rich repeats (LRRs) in the extracellular domain and serine/threonine kinase in the intracellular domain. Sequence analysis of seven members of the gene family at the locus suggests that duplication, recombination, and transposition have occurred during the evolution of this gene family. Experiments with a truncated member indicate that the LRR domain determines race-specific recognition and is subject to adaptive evolution. To identify additional components in the Xa21-mediated resistance pathway, both the yeast two-hybrid screen and mutagenesis approaches are being used. Several defense-related genes were found to interact with the Xa21 protein in yeast when the kinase domain was used in the screen. Using diepoxybutane and fast-neutron mutagenesis, we recovered 31 mutants that have changed from resistant to fully susceptible (10) or partially susceptible (21) to nine races of the bacterial blight pathogen in the Philippines. All fully susceptible mutants showed changes at the Xa21 locus as detected by polymerase chain reaction and Southern hybridization. For the partially susceptible mutants, no detectable changes were found at the Xa21 locus, suggesting that these mutations occur at other loci controlling the Xa21-mediated defense pathway.

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