Molecular breeding for the development of blast and bacterial blight resistance in rice cv. IR50

The objective of this study was to improve IR50, an elite Indica rice line, by molecular breeding approach involving marker-aided selection (MAS) and genetic transformation for resistance against blast (BL) and bacterial blight (BB). On the basis of BL pathogen population dynamics and lineage exclusion assays in southern India, the major BL resistance gene Piz-5 can exclude most Pyricularia grisea Sacc. {= P. oryzae Cavara [teleomorph: Magnaporthe grisea (Hebert) Barr]} lineages. Resistant CO39 near-isogenic line (NIL) C101A51 carrying Piz-5 was used as the donor parent and IR50 as the recurrent parent in backcrossing up to four generations. BC4F1 plants were finally selfed to produce BC4F2 seeds. A polymerase chain reaction (PCR)-based sequence-tagged site (STS) marker RG64 was used to identify Piz-5 in the segregating population and the resultant resistant progenies were obtained through phenotypic assays and MAS. To have further resistance to bacterial blight in this cultivar, these blast-resistant isolines were transformed with Xa21, which is known to confer resistance to all races of Xanthomonas oryzae pv. Oryzae [ = X. campestris pv. Oryza (Ishiyama 1922) Dye 1978] (Xoo). Stable integration and inheritance of the Xa21 gene were demonstrated by PCR and Southern blot analysis from three independent transformants. Bioassay data showed that transgenic IR50 is resistant to pathogens, M. grisea and Xoo. Apart from that, pedigree analysis and phenotypic studies with IR50 revealed the endogenous presence of the Xa4 gene, which may show an increased level of resistance to the BB pathogen along with the transformed Xa21 gene. This is the first report documenting the stacking of two major genes (Piz-5 + Xa21) in rice using molecular breeding through MAS and transformation.