Characterization And Gene Mappingofa Domiant Spotted-Leaf Mutant HM113 In Rice (Oryza Sativa L.)

Rice spotted-leaf mutants produce spontaneous cell death lesions similar to the symptom caused by pathogen infection-induced hypersensitive response(HR) and show resistance to rice blast and bacterial blight pathogens. Thus, characterizations of spotted-leaf mutants could provide insights on the mechanisms of programmed cell death(PCD) and disease resistance. In this study, we report the agronomic trait performance, physiological / biochemical characteristics, disease resistance to bacterial blight pathogen(Xanthomonas oryzae pv. oryzae, Xoo), genetic control and gene mapping of the mutant HM113, which was generated by ethyl methanesulfonate(EMS) mutagenesis of the cultivar IR64. Our study provides the basis for gene isolation, functional analysis and investigation into the mechanism underlying the mutant HM113-mediated bacterial blight resistance. The results are summerised as follows:1. Under natural conditions, the brown spots appeared on the older leaves of the mutant about 20 days after sowing, and then spread gradually to the whole leaf and leaf sheath. Compared with the wild type IR64, the mutant’s agronomic traits including plant height, number of filled grain/panicle, seed-setting rate and 1000-grain weight were reduced significantly. In addition, the initiation of lesions was induced by natural sun-light.2. The photosynthetic pigment contents and net photosynthetic rate in the mutant HM113 were significantly lower than those in the wild type IR64. Histochemical analysis showed that cell death occurred and ROS accumulation was detected in and around the lesions in the mutant HM113.Activities of CAT and SOD were significantly lower and activity of POD was significantly higher in the mutant than in IR64, however, no difference in APX activity between the two genotypes was found. Furthermore, the SPcontentwas significantly lower than that in IR64, however, MDA content was similar to that in IR64.3. The mutant HM113 exhibited a certain degree of enhanced disease resistance to multiple races of Xanthomonas oryzae pv. Oryzae(Xoo) including HB17, PXO71, JS97-2, PXO112, Zhe173, PXO339, PXO347 and PXO349. Meanwhile the enhanced resistance was apparently associated with the presence of spotted leaf phenotype. RT-PCR analysis indicated that expression level of defense-related genes such as AOS2, LOX, PAL4, PR10 and PR1 b were apparently up-regulated in HM113. Our results revealed that significant up-regulation in expression of multiple defense-related genes were correlated with the enhancement of disease resistance in the mutant HM113.4. All F1 plants derived from crosses of HM113/ 02428, HM113/ Moroberekan, HM113/ Nekken 1 and HM113/ CPSLO17 showed spotted leaves with a dosage effect. In the F2 populations, segregation of spotted-leaf plants and normal-leaf plants fit a 3:1 ratio, indicating that the the brow lesion phenotype was controlled by a single dominant nuclear gene, tentatively termed as splHM113, which was located on the long arm of chromosome 7 in a region covering 308 kb flanked by RM21605 and RM418.