Molecular marker validation for bacterial leaf blight resistant genes Xa21 and Xa4 in Ld 99-12- 38 / IRBB 60 genetic background

Bacterial leaf blight (BLB) disease is an emerging issue in rice cultivation of Sri Lanka. Xanthomonas oryzae pv. oryzae is causal organism. There is no chemical control method practice in the world to control the disease. Use of resistance varieties is most appropriate mode of control. Bacterial blight resistance Xa21 and Xa4 are dominant genes responsible for durable resistance of rice varieties for the disease. The advanced rice line, Ld 99-12-38 is farmer demanding, non lodging type, high yielding rice line and is susceptible to BLB. Therefore, an attempt was taken to improve it resistant to BLB through markerassisted backcross breeding (MAB) by crossing rice line with resistant variety IRBB60, and advanced plants up to BC3F2 level. At the beginning selected two molecular markers namely pTA248 (Xa21) and PM1+MP2 (Xa4) were tested for their polymorphism with parental materials. To identify BLB resistance Xa21 and Xa4 alleles, markers were validated in thirty five BC3F2 progenies derived from a cross between Ld 99-12- 38/IRBB60. As the results selected two molecular markers were polymorphic with parental materials with the expected fragment sizes. Out of the tested 35 progenies, eight were confirmed with possessing both Xa21 and Xa4 with resistant to the disease, twenty one lines carried only the resistant allele Xa4. A paired t- test revealed that there is a significant difference in the resistance level exerted by progeny carrying resistance alleles of two genes than with one (p = 0.0064). In addition, six progeny lines were heterozygote for Xa4 locus; one progeny line was heterozygous at both loci. No lines were identified carrying only the Xa21. Therefore it can be concluded that resistant genes Xa4 and Xa21 has been pyramided to rice lines derived from cross between Ld 99-12-38/IRBB60 in BC 3 F 2 progenies with similar morphological characters of Ld 99-12-38 to withstand BLB. The rice line Ld 99-12-38 does not carry any resistance genes regarding BLB and IRBB 60 consist of Xa21 and Xa4 alleles. The selected markers pTA248 and MP1+MP2 can be recommended to in marker- assisted selection of Xa21 and Xa4 genes in crosses involving the tested genetic background.