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KMR-3R is a stable restorer line with medium-bold grain type and is the male parent of the popular public-bred Indian rice hybrid, KRH2. As both KMR-3R and KRH2 are highly susceptible to bacterial blight (BB) and possess undesirable bold-grain type, we crossed KMR-3R with a high-yielding, BB resistant, fine-grain-type variety, Improved Samba Mahsuri, possessing the major BB resistant gene, Xa21. The F 1 s were backcrossed to KMR-3R, and the BC 1 F 1 plants were subjected to marker-assisted selection (MAS) for Xa21 and two major fertility restorer genes, Rf3 and Rf4. ÔTripleÕ positive BC 1 F 1 plants were selfed and their progeny were subjected to MAS for Xa21 coupled with phenotype-based visual selection for agromorphological and grainquality traits. At BC 1 F 5 generation, three backcross derived lines (BK9, BK49 and BK61) possessing higher yield than KMR-3R, tall plant stature along with fine-grain type were identified and crossed with IR598025A, the female parent of KRH2. The hybrids were completely fertile, possessed long-slender grain type, exhibited resistance to BB and displayed a yield advantage of 24% over KRH2 under BB infection.

DRR17A is a stable wild-abortive cytoplasmic male sterile line with medium-slender grain type. DRR17A and its maintainer line DRR17B are highly susceptible to two of the major rice diseases, bacterial blight (BB) and blast. To improve DRR17B for resistance against BB and blast, we have introgressed a major dominant gene each conferring resistance against BB (Xa21) and blast (Pi54) into CH Balachiranjeevi, S. Bhaskar Naik, V. Abhilash and R. M. Sundaram have contributed equally to this work.Electronic supplementary material The online version of this article (

Xa27 is one of the important R-genes, effective against bacterial blight disease of rice caused by Xanthomonas oryzae pv. oryzae (Xoo). Using natural population of Oryza, we analyzed the sequence variation in the functionally important domains of Xa27 across the Oryza species. DNA sequences of Xa27 alleles from 27 rice accessions revealed higher nucleotide diversity among the reported R-genes of rice. Sequence polymorphism analysis revealed synonymous and non-synonymous mutations in addition to a number of InDels in non-coding regions of the gene. High sequence variation was observed in the promoter region including the 5'UTR with 'π' value 0.00916 and 'θ w ' = 0.01785. Comparative analysis of the identified Xa27 alleles with that of IRBB27 and IR24 indicated the operation of both positive selection (Ka/Ks > 1) and neutral selection (Ka/Ks ≈ 0). The genetic distances of alleles of the gene from Oryza nivara were nearer to IRBB27 as compared to IR24. We also found the presence of conserved and null UPT (upregulated by transcriptional activator) box in the isolated alleles. Considerable amino acid polymorphism was localized in the trans-membrane domain for which the functional significance is yet to be elucidated. However, the absence of functional UPT box in all the alleles except IRBB27 suggests the maintenance of single resistant allele throughout the natural population.

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