USE OF DIVERSE GERMPLASM FOR GENETIC IMPROVEMENT OF RICE
Location: Dale Bumpers National Rice Research Center
Title: Developing a Bengal / O. nivara Advanced Backcross Mapping Population to Identify Sheath Blight QTL
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2007
Publication Date: March 1, 2008
Citation: Prasad, B., Eizenga, G.C. 2008. Developing a Bengal/O. nivara advanced backcross mapping population to identify sheath blight QTL. In: Proceedings of the 32nd Rice Technical Working Group Meetings, February 18-21, 2008, San Diego, CA. 2008. CDROM.
Rice sheath blight disease, caused by the soilborne necrotrophic fungus Rhizoctonia solani Kühn, is one of the most important diseases of cultivated rice (Oryza sativa L.). Wild relatives of rice (Oryza spp.) are a valuable source of genes for biotic and abiotic stress tolerance, and may contain novel resistance genes or QTLs for sheath blight that could be used to enhance resistance to this very important disease in commercial rice. To identify possible resistant sources for sheath blight disease, three greenhouse/growth chamber-based screening methods were evaluated with 73 Oryza genotypes. There are significant limitations to screening the wild Oryza spp. under field conditions with the most important being that many of the wild Oryza spp. have a seed shattering trait. For the micro-chamber method, 4-week-old seedlings were inoculated with a potato dextrose agar plug containing mycelia, covered with a 2-liter soft drink bottle, and rated one week after inoculation. The detached-leaf method involved placing an agar plug containing mycelia on the abaxial surface of a leaf section that was cut from a 5-week-old plant and placed on moist filter paper in a petri dish under constant light, then evaluated after 72 hours. For the toothpick inoculation method, toothpicks colonized with mycelia were placed in the leaf collar region of plants at the panicle initiation stage, plants were placed in a growth chamber, and disease symptoms evaluated after seven days. The micro-chamber method gave a more uniform and reproducible response, and was better correlated with the disease reactions of the reference cultivars grown under field conditions. The micro-chamber and detached leaf data were subjected to a cluster analysis, and seven Oryza spp. accessions were identified as moderately resistant. O. nivara (IRGC 100898) was identified as one of the most resistant accessions.
In order to incorporate the resistance identified in O. nivara (IRGC 100898), the advanced backcross (ABC) method was selected for developing a mapping population from which sheath blight QTL could be identified and subsequently the most resistant lines made available as germplasm to rice breeders. The ABC method is an attractive procedure especially when the donor parent is very poorly adapted like the Oryza spp. Also, when the recurrent parent is adapted, the ABC method can greatly reduce the number of undesirable segments being transferred from the wild parent into the advanced progenies. To develop this ABC population, a single plant selection of O. nivara (IRGC 100898) was crossed as the male (donor) parent with a single plant selection of Bengal, a medium grain rice cultivar that is moderately susceptible to sheath blight. The resulting F1 plant was confirmed by phenotype. Subsequently the F1 plant was crossed as the male parent to Bengal and 104 BC1F1 seed were produced. From these 104 BC1F1 plants, 54 were selected based on fertility, phenotype, and genotype as verified with 24 SSR markers. The selected BC1F1 were used as male parents in crosses with Bengal and approximately 1,000 BC2F1 seed were produced. These BC2F1 seeds were grown in the greenhouse and the plants were allowed to self, generating BC2F2 families. To discover sheath blight QTL, a set of 279 BC2F2 families from 44 BC1F1 plants were selected based on fertility. Presently, these families are being phenotyped for sheath blight using the micro-chamber method and genotyped with 200 SSR markers distributed throughout the twelve rice chromosomes. This research was funded in part by USDA/CSREES-NRI RiceCAP.