Pyramiding of Four Broad Spectrum Bacterial Blight Resistance Genes in Cross Breeds of Basmati Rice

Pyramiding of major resistance (<i>R</i>) genes through marker-assisted selection (MAS) is a useful way to attain durable and broad-spectrum resistance against <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> pathogen, the causal agent of bacterial blight (BB) disease in rice (<i>Oryza sativa</i> L.). The present study was designed to pyramid four broad spectrum BB-<i>R</i> genes (<i>Xa</i>4, <i>xa</i>5, <i>xa</i>13 and <i>Xa</i>21) in the background of Basmati-385, an indica rice cultivar with much sought-after qualitative and quantitative grain traits. The cultivar, however, is susceptible to BB and was therefore, crossed with IRBB59 which possesses <i>R</i> genes <i>xa</i>5, <i>xa</i>13 and <i>Xa</i>21, to attain broad and durable resistance. A total of 19 F₁ plants were obtained, some of which were backcrossed with Basmati-385 and large number of BC₁F₁ plants were obtained. In BC₁F₂ generation, 31 phenotypically superior genotypes having morphological features of Basmati-385, were selected and advanced up to BC₁F₆ population. Sequence-tagged site (STS)-based MAS was carried out and phenotypic selection was made in each successive generation. In BC₁F₆ population, potentially homozygous recombinant inbred lines (RILs) from each line were selected and evaluated on the bases of STS evaluation and resistance to local <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> (<i>Xoo</i>) isolates. Line 23 was found pyramided with all four BB-<i>R</i> genes i.e., <i>Xa</i>4, <i>xa</i>5, <i>xa</i>13 and <i>Xa</i>21. Five genotypes including line 8, line 16, line 21, line 27 and line 28 were identified as pyramided with three <i>R</i> genes, <i>Xa</i>4, <i>xa</i>5 and <i>xa</i>13. Pathological study showed that rice lines pyramided with quadruplet or triplet <i>R</i> genes showed the highest level of resistance compared to doublet or singlet <i>R</i> genes. Thus, line 23 with quadruplet, and lines 8, 16, 21, 27, and 28 with triplet <i>R</i> genes, are recommended for replicated yield and resistance trials before release as new rice varieties. Further, traditional breeding coupled with MAS, is a solid way to attain highly effective BB-resistant rice lines with no yield cost.