IR 58025A is a very popular wild-abortive cytoplasmic male sterile (WA-CMS) line of rice and is extensively used for hybrid rice breeding. However, IR 58025A and many hybrids derived from it possess mild aroma (undesirable in some parts of India) and are highly susceptible to bacterial blight (BB) and blast diseases. To improve IR 58025A for BB and blast resistance, we have introgressed a major dominant gene conferring resistance against BB (i.e. Xa21) and blast (i.e. Pi54) into IR 58025B, the maintainer line of IR 58025A. An introgression line of Samba Mahsuri (i.e. SM2154) possessing Xa21 and Pi54 genes in homozygous condition and fine-grain type was used as donor parent, and backcross breeding strategy was adopted for targeted introgression of the resistance genes. PCR-based molecular markers tightly linked to Xa21 and Pi54 were used for selection of BB-and blast-resistant lines, while closely linked markers were used for identification of backcross-derived plants devoid of Rf4 and aroma. At BC 2 F 5 , four backcross-derived lines possessing resistance against BB and blast, devoid of aroma, high yield, short plant stature, long-slender grain type and with recurrent parent genome recovery ranging from 88.8% to 98.6% were selected and advanced for further evaluation. The improved versions of IR 58025B, viz. behaved as perfect maintainers when testcrossed with WA-CMS lines. Agronomically superior lines of improved IR 58025B are being converted to CMS line through backcrossing for developing high-yielding and biotic stress-resistant rice hybrids.

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 (

Bacterial blight (BB) disease reduces the yield of rice varieties and hybrids considerably in many tropical rice growing countries like India. The present study highlights the development of durable BB resistance into the background of an elite maintainer of rice, DRR17B, by incorporating two major dominant genes, Xa21 and Xa33 through marker-assisted backcross breeding (MABB). Through two sets of backcrosses, the two BB resistance genes were transferred separately to DRR17B. In this process, at each stage of backcrossing, foreground selection was carried out for the target resistance genes and for non-fertility restorer alleles concerning the major fertility restorer genes Rf3 and Rf4, using gene-specific PCR-based markers, while background selection was done using a set of 61 and 64 parental polymorphic SSR markers respectively. Backcross derived lines possessing either Xa21 or Xa33 along with maximum genome recovery of DRR17B were identified at BC3F1 generation and selfed to develop BC3F2s. Plants harboring Xa21 or Xa33 in homozygous condition were identified among BC3F2s and were intercrossed with each other to combine both the genes. The intercross F1 plants (ICF1) were selfed and the intercross F2(ICF2) plants possessing both Xa21 and Xa33 in homozygous condition were identified with the help of markers. They were then advanced further by selfing until ICF4 generation. Selected ICF4 lines were evaluated for their resistance against BB with eight virulent isolates and for key agro-morphological traits. Six promising two-gene pyramiding lines of DRR17B with high level of BB resistance and agro-morphological attributes similar or superior to DRR17B with complete maintenance ability have been identified. These lines with elevated level of durable resistance may be handy tool for BB resistance breeding.

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