ISOLATION AND CHARACTERIZATION OF RICE MUTANTS SUSCEPTIBLE TO BACTERIAL BLIGHT AND BLAST

¹ International Rice Research Institute, P.O. Box 933, Manila, Philippines

² Institute of Molecular Agrobiology, 1 Research Link, National University of Singapore, Singapore 117604

³ Department of Plant Pathology, University of California, Davis, CA, USA

To identify genes essential for resistance against multiple diseases, 4,500 IRBB21 mutants induced by diepoxybutane and fast neutrons were screened for a change from resistance to susceptibility to bacterial blight and blast. We recovered 31 mutants that have changed from resistant to fully susceptible (10) or partially susceptible (21) to 9 races of the bacterial blight pathogen in the Philippines. For the partially susceptible mutants, no detectable changes were found at the Xa21 locus, suggesting that the mutations may be in the Xa21 mediated defense pathway. Southern and PCR analyses of the molecular changes at the Xa21 locus revealed two classes of fully susceptible mutants: those with a loss of both the kinase domain and leucine rich repeats, and those with rearrangements in the kinase domain. Based on the presence or absence of a marker flanking Xa21, we estimated that some deletions are >100 kb. Three of these deletion mutants are susceptible to 2 out of 7 blast fungus pathotypes tested. Genetic tests are underway to determine whether Xa21 confers dual resistance to bacterial blight and blast or a closely linked gene is involved in blast resistance. Backcrosses and intercrosses have been made with the mutants to determine the inheritance of and epistatic relationship between these mutations.