Production and characterization of gamma ray-induced rice mutants with broad-spectrum resistance to the bacterial blight pathogen Xanthomonas oryzae pv. oryzae.

An initial bulk M-2 population produced through gamma irradiation of mature seeds of rice variety NSIC Rc144 was subjected to a series of induced mass screening for potentially novel resistance to bacterial blight, one of the most serious diseases of rice. Lines initially found to be highly resistant to race 3 of the pathogen Xanthomonas oryzae pv. oryzae (Xoo) were further evaluated in the same manner using other Xoo races. Selection and generation advance ultimately led to identification of two highly resistant M-7 lines. DNA fingerprinting using 63 genome-wide SSR markers revealed 100% similar banding patterns in the two mutant lines, indicating that these are sister lines whose origin can be traced to a single M-2 plant DNA fingerprinting also revealed 98% similarity of the mutant lines with NSIC Rc144 indicating that the resistance is due to mutation and not due to genetic admixture or seed impurity. These lines were advanced and further evaluated using 14 isolates representing the 10 races of Xoo found in the Philippines. Two seasons of induced screening revealed an identical reaction pattern in these lines across a wide range of Xoo races. Such pattern is unique compared to known patterns elicited by four other Xoo-resistant isolines, suggesting possible novel resistance. F-2 segregation data on reaction to bacterial blight suggest a single-gene recessive mutation in these lines (p=0.4 for 3:1 test of fitness). These mutants are now being used as resistance donors in the breeding program while further molecular characterization is being pursued.