ABSTRACT

Rice (Oryza sativa. L) is the most important food crop in the world and feeds more than half of the global population. The productivity of rice is being affected by a number of bacterial, fungal and viral diseases. Among them bacterial blight, caused by Xanthomonas oryzae pv. oryzae, is one of the oldest and most serious diseases of rice. As the pathogen is difficult to manage, the development of host-plant resistance is considered the most effective and economical means to control BB. However, this approach is difficult through conventional breeding due to masking effect of genes such as Xa21, which confers resistance to many BB races. It is impossible to distinguish between plants having Xa21 alone and those having Xa21 and other genes.  Marker aided selection (MAS) allows the identification of plants with multiple resistance genes. Now major genes conferring disease resistance in rice have been mapped with linked markers facilitating MAS. Many high yielding rice cultivars have been developed and widely grown by farmers since the green revolution of 1970s. However, there is a concern about the limited genetic diversity among them. Therefore, there is a need to look for diverse germplasm sources for further genetic improvement of rice.

Present investigation was a) Marker assisted selection for bacterial blight resistance genes in two segregating backcross populations of rice. b) Phenotypic evaluation of O. sativa X O. glaberrima derivatives for resistance to bacterial blight disease. c) Assessing genetic diversity among O. sativa X O. glaberrima derivativesandvarieties of Oryza sativa L. using molecular markers. After screening all the plants of two populations for four genes with the help of molecular markers linked to the genes, five plants from one population and two plants from one population were found to have these genes. These plants will be selected and tested under field conditions to bacterial blight disease reaction. When these plants show better performance towards disease resistance to BB in the field will be selected as parents for further backcross generations in MAS breeding program. In the phenotypic evaluation of O. sativa X O. glaberrima derivatives for resistance to bacterial blight disease all of them showed susceptibility towards BB disease reaction. Further genetic diversity among O. sativa X O. glaberrima derivativesandvarieties of Oryza sativa L. using molecular markers were assessed. The 23 selected SSR primers amplified 86 alleles across the 27 genotypes with varying degree of polymorphism. Considerable genetic similarity between the genotypes as evident from the dice coefficient value of 0.35 – 1.00, is also reflected in the clustering pattern of O. sativa X O. glaberrima derivatives and O. sativa varieties. In UPGMA clustering analysis all the O. sativa X O. glaberrima derivatives form one cluster and O. sativa varieties formed another cluster. Moroberekan, which is a land race and widely cultivated upland rice of Africa fall close to the O. sativa X O. glaberrima derivatives indicating that it is having some of the wild traits of O. glaberrima. Azucena, which is a tropical Japonica variety, formed a separate cluster. Among the O. sativa varieties all the IRRI varieties formed one group and Co39 and Mahsuri (MRF) formed another group indicating they were closely related to each other.