DISSECTING DISEASE RESISTANCE PATHWAY IN RICE BY FORWARD AND REVERSE GENETICS

¹ The International Rice Research Institute, P.O. Box 3127, Makati Central Post Office, 1271 Makati City, Philippines

² Department of Plant Pathology, Kansas State University, Manhattan, Kansas, 66506, USA

³ Department of Plant Pathology, Ohio State University, Columbus, Ohio, 43210, USA

We have established a large collection of chemical and irradiation-induced mutants of IR64 to provide the biological variation needed for gene discovery and dissection of metabolic pathways. As of now, we have approximately 40,000 M3 lines derived from 11,000 M2 families generated by mutagenesis with diepoxybutane, fast neutron, and gamma ray. About 6% of the mutant lines showed segregation of defects in vegetative growth, chlorophyll synthesis, panicle development and fertility. To identify genes involved in host defense, we screened for mutants with altered disease response (gain and loss of function) and analyzed mutant DNA with candidate genes presumably involved in defense response. Based on M3 segregation data, we have identified nine mutants with loss of resistance to blast and one mutant with loss of resistance to bacterial blight. Eight mutants showed a gain of resistance to blast but a large proportion (60%) of them are morphologically abnormal. We isolated 16 lesion mimic mutants, one of which showed enhanced resistance to bacterial blight. To develop a reverse genetics system, several strategies are being tested to detect molecular changes using PCR and hybridization assays. We have analyzed DNA from susceptible mutants using PCR primers and RFLP probes of candidate resistance genes. Several susceptible mutants showed a loss of DNA fragments or altered RFLP patterns. Distinct categories of mutations are being intercrossed to produce double mutants to determine their functional relationships in the defense pathway. Candidate genes with functions assigned by reverse genetics are being used to examine allelic diversity in elite germplasm and breeding lines.