Development and Identification of Novel Rice Blast Resistant Sources and Their Characterization Using Molecular Markers

To develop and characterize introgression lines for leaf and neck blast resistance, 326 introgression lines were developed using various accessions of six different AA genome wild species in the genetic background of elite Indian varieties like PRl14 and Pusa 44 and were screened for blast resistance. Stringent phenotyping coupled with genotyping using gene based markers led to the identification of four resistant introgression lines, which showed promising resistance and do not possess any of the tested genes. Furthermore, multi-location screening confirmed the field resistance of the four introgression lines to both leaf and neck blast. Molecular characterization of these introgression lines using genome-wide simple sequence repeat markers revealed the presence of small percentage of wild Oryza genome introgrssion. So these lines can be used for mapping and identification of novel leaf and neck blast resistance genes. Thus, these four introgression lines can be considered as new genetic resources for blast resistance.

Genetic Improvement of Rice for Bacterial Blight Resistance:Present Status and Future Prospects

The production and productivity of rice has been challenged due to biotic and abiotic factors.Bacterial blight(BB)disease,caused by Xanthomonas oryzae pv.oryzae,is one of the important biotic stress factors,which reduces rice production by 20%-50%.The deployment of host plant resistance is the most preferred strategy for management of BB disease,and breeding disease resistant varieties remains a very economical and effective option.However,it is difficult to develop rice varieties with durable broad-spectrum resistance against BB using conventional approaches alone.Modern biotechnological tools,particularly the deployment of molecular markers,have facilitated the cloning,characterization and introgression of BB resistance genes into elite varieties.At least 46 BB resistance genes have been identified and mapped from diverse sources till date.Among these,11 genes have been cloned and characterized.Marker-assisted breeding remains the most efficient approach to improve BB resistance by introducing two or more resistance genes into target varieties.Among the identified genes,xa5,xa13 and Xa21 are being widely used in marker-assisted breeding and more than 70 rice varieties or hybrid rice parental lines have been improved for their BB resistance alone or in combination with genes/QTLs conferring tolerance to other stress.We review the developments related to identification and utilization of various resistance genes to develop BB resistant rice varieties through marker-assisted breeding.

A Single-Tube, Functional Marker-Based Multiplex PCR Assay for Simultaneous Detection of Major Bacterial Blight Resistance Genes Xa21, xa13 and xa5 in Rice

In marker-assisted breeding for bacterial blight(BB) resistance in rice, three major resistance genes, viz., Xa21, xa13 and xa5, are routinely deployed either singly or in combinations. As efficient and functional markers are yet to be developed for xa13 and xa5, we have developed simple PCR-based functional markers for both the genes. For xa13, we designed a functional PCR-based marker, xa13-prom targeting the In Del polymorphism in the promoter of candidate gene Os8N3 located on chromosome 8 of rice. With respect to xa5, a multiplex-PCR based functional marker system, named xa5 FM, consisting of two sets of primer pairs targeting the 2-bp functional nucleotide polymorphism in the exon II of the gene TFIIA5(candidate for xa5), has been developed. Both xa13-prom and xa5 FM can differentiate the resistant and susceptible alleles for xa13 and xa5, respectively, in a co-dominant fashion. Using these two functional markers along with the already reported functional PCR-based marker for Xa21(p TA248), we designed a single-tube multiplex PCR based assay for simultaneous detection of all the three major resistance genes and demonstrated the utility of the multiplex marker system in a segregating population.