Development of blast resistant rice variety derived from crossing between MR219 and Pongsu Seribu 2 through marker-assisted backcross breeding

Rice blast caused by fungus Magnaporthe oryzae is a major rice disease due to significant yield losses worldwide as well as in Malaysia. Cultivating blast-resistant rice varieties is the most effective, economical and practical approach to prevent blast disease. Marker-assisted backcross breeding contribute an effective and vital role in incorporating blast resistant genes into blast susceptible high yielding rice varieties. In this study two blast resistant genes (putative Pi-b and Pi-kh) were identified from the Malaysian rice variety Pongsu Seribu 2 and revealed that it contain Nucleotide Binding Site-Leucine Rich Repeat (NBS-LRR) domain. The blast resistant genes (Pi-b and Pikh) were introgressed from Pongsu Seribu 2 variety using marker-assisted backcross breeding strategy into a high-yielding blast susceptible rice variety MR219. Therefore crosses were made between the MR219 used as recurrent parent and Pongsu Seribu 2 as donor parent to transfer blast resistant genes into MR219 without losing their actual quality and yield sustainability. Eleven SSR molecular markers linked to rice blast resistant genes were found polymorphic between the two parental varieties and used to find the potential relation with blast resistance in the present developed backcross population. The polymorphic markers were used in the further subsequent generation for confirmation of blast resistant genes. Out of the 11 markers, only 2 markers RM208 (located on chromosome 2 linked to Pi-b gene) and RM206 (located on chromosome 11 linked to Pi-kh gene) conferring blast resistance were confirmed and used in F1,BC1F1, BC2F1 and BC2F2 generations providing resistance against most virulent Malaysian rice blast fungus M. oryzae pathotype P7.2. 300-SSR markers were screened, out of them 72 markers were found to be polymorphic between the parental lines and used for background recovery of the recurrent parent (MR219) in each backcross population. The inheritance patterns and identification of microsatellite markers linked to the rice blast resistance were observed in BC2F1 and BC2F2 generations. The recurrent parent MR219 showed susceptibility with lesion 5 to 7 score, and donor parent PS2 showed resistivity with lesion 0 to 2 while challenging to pathotype P7.2 fungus inoculum under control conditions. In BC2F1 generation, 320 plants were inoculated with pathotype P7.2 and 154 plants showed the resistance mechanism while another 166 plants showed susceptible reaction to blast. Chi-square test (χ2) for the single-gene model was applied for testing goodness of fit of observed frequencies. The two linked markers RM208 (χ2= 1.5130; p= 0.2188) and RM206 (χ2= 0.6130; p= 0.4338) for blast resistance to pathotype P7.2 showed good fit with expected test cross ratio (1:1) for single-gene model analysis. The markers RM208 and RM206 found suitable for marker-assisted selection of Pi-b and Pi-kh blast resistance genes conferring resistance against the blast pathotype P7.2. Phenotypically BC2F2 population segregated into 3:1 ratio. The genotypic segregation of the BC2F2 population segregated into 1:2:1 ratio. The background selection analysis for the recovery of MR219 variety among the best improved lines ranged from 73 to 94% in BC1F1, 79.4 to 96.1 in the BC2F1 and 94 to 97.5% in BC2F2 generations. The average proportions of the recurrent parent genome in the selected 15 improved lines of BC2F2 were 96.17%, explaining that very close phenotypic resemblance to the recurrent parent MR219. The 15 homozygous lines carrying blast resistant genes with similar background to MR219 were selected for the development of improved blast resistant rice variety. The agro-morphological traits of the improved lines and recurrent parent showed no significant difference between those lines. In conclusion, from the present rice breeding program, 15 homozygous advanced blast resistant rice lines were developed with a high potential to be released as a new variety for commercial cultivation.

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