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. Yield losses as high as 80% have been reported in some cases. The pathogen normally enters the host through natural openings like stomata or hydathodes, invades into xylem tissues and infects the whole plant. There is no chemical control, as the disease is vascular in nature. Knowledge of the pathogen population structure is a prerequisite for deployment of efficient genes/gene combinations. As control of the pathogen is difficult, the development of host-plant resistance is considered as 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 resistance to BB in rice have been mapped with linked markers facilitating MAS.

The present investigation was undertaken with an aim to a) DNA fingerprint the bacterial blight isolates collected from Maruteru and b) Marker aided selection for bacterial blight resistance genes in different populations.

To study the variation within the bacterium, intensified collections of infected leaf samples were made from Maruteru (West Godavari district, Andhra Pradesh) a hotspot for bacterial blight. Collections were made in the form of a W walk at the place showing highest incidence of the disease. The area was divided into five hills and three samples were collected from each hill. A total of twenty-five samples were collected and DNA was extracted from the respective 25 Xoo isolates in lab. ISIII2based primers namely JEL1 and JEL2 were used for DNA fingerprinting of these isolates to gain an insight into the variation present in different races of pathogen. In addition pathotyping studies were also conducted with single gene, two gene, three gene and  four gene combinations as near isogenic lines (in IR24 background) to test their virulence spectrum. This will give us not only the knowledge of the existing pathogen population, but also provide information of the different haplotypes.

In UPGMA, clustering analysis of the 25 BB isolates at 95% level of similarity, 17 lineages could be distinguished. Pathotyping studies revealed that no single gene used in the study showed complete resistance to all the isolates. Among the two gene, three gene and four gene combinations used in this study the four gene combination was the only one to impart resistance to all the 25 isolates used.

From the molecular screening, of all the plants of the population with the respective STS markers for the detection of the four genes, nine plants from the population were found to have the desired genes of interest and had close resemblance with the recurrent parent (25B).