Understanding the Molecular Mechanisms of Disease Resistance Using Rice Mutants

The Pi-ta gene in rice has been used to prevent the rice blast disease worldwide for several decades. In the US, Pi-ta was introgressed from a landrace indica variety, Tetep, into several tropical japonica cultivars including Katy. Pi-ta is predicted to encode a cytoplasmic receptor that directly binds to the elicitor produced by the pathogen avirulence gene AVR-Pita for initiating resistance. Katy, expressing resistance conditioned by Pi-ta, Pi-ta2, and Pi-ks to the races of M. oryzae, IB1, IB45, IB49, IB54, IC17, IH1, IE1, and IG1, was treated with mutagens, fast neutrons and ethyl methyl sulfate (EMS). Six mutants with enhanced resistance or susceptibility were identified by screening M2 seedlings derived from 15,000 M1 plants. Among them, M562, induced by EMS, is a lesion mimic mutant (named as LMM1) that produces spontaneous hypersensitive cell death. This rapid cell death was quickly induced using detached leaves at and near the site of inoculation by the virulent race IE1k and more slowly induced when inoculated with the avirulent isolate IB49. Similar hypersensitive cell death was observed after detached leaves were inoculated with the fungus Rhizoctonia solani, the causal agent of rice sheath blight disease. Hypersensitive cell death is known to be a form of the defense response. Thus, we suggest that LMM1 has enhanced resistance to both rice blast and sheath blight pathogens. Although the Pi-ta gene in rice provides resistance to the races, IB1, IB45, IB49, IC17, IH1, IE1, IB54, and IG1, the mutant M2354 was observed to be susceptible to all races except IB54 although there was no change in the Pi-ta DNA sequence. Expression of Pi-ta in M2354 was also similar to that of the parent examined by qRT-PCR. Thus, mutations in M2354 likely occurred at a new locus named as Ptr(t). Another four lines were determined to be near isogenic lines at a 9 megabase genomic region spanning the Pi-ta locus of Katy. Progress on characterizing these six genetic stocks is presented. (author)