Rice Genetics IV - IRRI books - International Rice Research Institute

Structure, function, and evolution ofdisease resistance genes in riceGuo-Liang Wang, Hei Leung, and P. RonaldThe recent cloning and characterization of several rice genes with resistanceto pathogens represent a breakthrough in our understanding of the molecularbasis of disease resistance and also provide a starting point for dissectingthe resistance pathway in rice. The first resistance gene cloned in ricewas Xa21, a gene introgressed from the wild rice Oryza longistaminata. Itencodes a putative receptor-like kinase consisting of leucine-rich repeats(LRRs) in the extracellular domain and serine/threonine kinase in the intracellulardomain. Sequence analysis of seven members of the gene family atthe locus suggests that duplication, recombination, and transposition haveoccurred during the evolution of this gene family. Experiments with a truncatedmember indicate that the LRR domain determines race-specific recognitionand is subject to adaptive evolution. To identify additional componentsin the Xa21-mediated resistance pathway, both the yeast two-hybrid screenand mutagenesis approaches are being used. Several defense-related geneswere found to interact with the Xa21 protein in yeast when the kinase domainwas used in the screen. Using diepoxybutane and fast-neutron mutagenesis,we recovered 31 mutants that have changed from resistant tofully susceptible (10) or partially susceptible (21) to nine races of the bacterialblight pathogen in the Philippines. All fully susceptible mutants showedchanges at the Xa21 locus as detected by polymerase chain reaction andSouthern hybridization. For the partially susceptible mutants, no detectablechanges were found at the Xa21 locus, suggesting that these mutationsoccur at other loci controlling the Xa21-mediated defense pathway.The use of resistant cultivars is the most preferred method for disease control in cropplants. This method relies mainly on the incorporation of resistance genes from wildspecies or traditional cultivars into improved cultivars. In recent years, the successfulcloning of about 25 disease resistance genes has dramatically advanced our understandingof the molecular basis of disease resistance and provided new ways to engineerresistant crop plants (reviewed by Staskawicz et al 1995, Baker et al 1997, EllisStructure, function, and evolution of disease resistance . . . 335