Broadening gene pool of rice for resistance to biotic stresses through wide hybridization

Variability in the cultivated germplasm for economic traits such as resistance to rice tungro virus, sheath blight, yellow stem borer, drought and salt tolerance is limited. This necessitated search for the genes in secondary and tertiary gene pool of genus Oryza. Fortunately, wild species are an important reservoir of useful genes for resistance to major disease, pests and tolerance to abiotic stresses. Wide hybridization in Oryza is normally difficult to achieve because many wild species of genus Oryza are difficult to cross with cultivated rice because of difference in chromosome number or genetic constitution. Fertilization may occur, but the embryo is aborted. Embryo rescue is used to maintain the hybrid embryos and F1s through several cycles of back crosses until fertility is restored. Interspecific hybridization has been attempted by a number of workers and resulting hybrids and progenies have been used for taxonomic and phylogenic analysis with limited efforts to transfer desirable traits from wild species to cultivated rice. Transfer of grassy stunt virus resistance from one accession of O. nivara has been achieved successfully. Another species belonging to the AA genome, O. longistaminata has been exploited for transfer bacterial blight resistance gene to cultivated rice. Wild species with genomes non-homologous to the AA genome of O. sativa such as O. officinalis (CC), O. australiensis (EE) and O. minuta (BBCC) possessing resistance to brown planthooper, white backed planthopper, bacterial blight and blast have been used to transfer these desirable alien traits to cultivated rice. Therefore, wide hybridization is one of the key components in programme aiming at transferring alien genes from diverse sources surmounting sexual barriers. Advances in embryo rescue, anther culture, chromosome engineering and genetics have facilitated in the transfer of genes and in precise monitoring and characterization of alien introgression from different genomes of Oryza into cultivated rice. Integrating conventional breeding with advanced methods of alien introgression offers great potent to develop disease and insect resistant varieties.