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Abstract Bacterial leaf blight (BLB) is one of the major rice diseases in Malaysia. This disease causes substantial yield loss as high as 70%. Development of rice varieties which inherited BLB resistant traits is a crucial approach to promote and sustain rice industry in Malaysia. Hence, this study aims were to enhance BLB disease resistant characters of high yielding commercial variety MR219 through backcross breeding approach with supporting tool of marker-assisted selection (MAS). Broad spectrum BLB resistance gene, Xa7 from donor parent IRBB7 were introgressed into the susceptible MR219 (recurrent parent) using two flanking markers ID7 and ID15. At BC3F4, we managed to generate 19 introgressed lines with homozygous Xa7 gene and showed resistant characteristics as donor parent when it was challenged with Xanthomonas oryzae pv. oryzae through artificial inoculation. Recurrent parent MR219 and control variety, MR263 were found to be severely infected by the disease. The improved lines exhibited similar morphological and yield performance characters as to the elite variety, MR219. Two lines, PB-2-107 and PB-2-34 were chosen to be potential lines because of their outstanding performances compared to parent, MR219. This study demonstrates a success story of MAS application in development of improved disease resistance lines of rice against BLB disease.

Utilization of resistance (R) genes to breed resistant cultivars is one of the most effective and economical approach to control rice bacterial blight (BB). Xa7, a dominant, broad-spectrum and durable BB-resistant gene, is an ideal gene resource to improve the resistance of rice varieties to bacterial blight, and this well-known gene with important breeding value has been cloned in our recent study. The isolation of Xa7 will facilitate its application in rice breeding by molecular maker-assisted selection (MAS). In this study, based on the specific sequences in the promoter of Xa7, a functional marker, named as MX7, was developed, which can effectively distinguish the dominant BB-resistant Xa7, the recessive BB-susceptible xa7 and the null allele from different rice varieties. Since MX7 is a dominant marker, it can't tell homozygous from heterozygous, a co-dominant marker closely linked to Xa7, named as M6, was developed simultaneously. After verified by amplification in numerous rice varieties and sequence alignment in RICE 3 K database, it is proved that marker M6 is co-segregated with the Xa7 locus. In addition, the effectiveness and accuracy of the two markers were further validated by two F2 populations. Finally, the designed makers were effectively applied in MAS breeding to improve the BB-resistance of a susceptible variety. This study not only provides reliable functional markers for the identification of Xa7 gene in different rice materials, but also will contribute to the application of Xa7 gene in marker-assisted selection to breed rice varieties with durable disease resistance. [ABSTRACT FROM AUTHOR]