Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/1/2006
Publication Date: 2/15/2006
Citation: McClung, A.M., Yan, W., Jia, Y., Lee, F.N., Marchetti, M.A., Fjellstrom, R.G. 2006. Genotypic and phenotypic assessment of the NSGC Core Collection of rice for resistance to Pyricularia grisea. Rice Technical Working Group Meeting Proceedings, February 29-March 1, 2006. Houston, Texas. 2006 CDROM. Interpretive Summary:
Technical Abstract: Currently, the USDA ARS National Small Grains Collection (NSGC) of rice consists of over 18,000 accessions that have been collected from 115 countries. Although, the first accessions were entered into the rice collection in 1904, over 40% of the accessions were introduced during the 1970’s, predominantly from Asia. Data on 27 traits are curated on the Germplasm Resource Information Network website (www.ars-grin.gov/npgs) for the accessions. However, only 2% of the collection has been evaluated for resistance to blast disease caused by Pyricularia grisea. Because it is difficult to collect meaningful data over such a large set of material, a “core” subset of the collection has been developed. The NSGC core collection is comprised of some 1800 accessions that represent about 10% of the whole collection. Over the last several years, progress has been made in developing molecular markers that are closely linked to major Pi- blast resistance genes. These have been successfully used as selectable markers in US cultivar development programs. The objective of this study was to characterize the rice core collection for resistance to blast disease using phenotypic and genotypic assessment. Some 1600 accessions of the core collection were evaluated for resistance to blast using a mixture of races in inoculated blast nurseries and for their reaction to seven individual races of blast that are found in the US: IB33, IB1, IB49, IH1, IG1, IC17, and IE1k. In addition, the accessions were scored for molecular markers that are associated with major blast resistance genes: Pi-ta2, Pi-b, Pi-z, and Pi-k. 1160 accessions were evaluated for their reaction to all seven races of blast. Two accessions, T442-57 (PI 406577) from Thailand and Blakka Tere Thelma (PI 369804) from Suriname were observed to be resistant to all races of blast. These cultivars likely have multiple resistant genes, some of which are unknown in US germplasm. In addition, 1% of the collection was found to have resistance to race IB33, for which there is no known resistance in the US. Genotypic analysis of the accessions, indicated that over 30% were heterozygous at one or more loci, suggesting that these are mixtures or landraces. Twelve percent of the core collection was observed to possess the Pi-ta2 allele which provides broad spectrum resistance to many races of blast found in the US. However, over 60% of the accessions having Pi-ta2 were susceptible to one or more of these races. This suggests that there may be variability at this locus for resistance to specific races of blast. In addition, 11% of the accessions which possessed the Pi-ta2 gene, but lacked other Pi-genes, were resistant to IE-1k which generally causes a susceptible reaction when Pi-ta2 is present. This indicates that other novel resistance genes may be present in this germplasm. For the accessions that were genotyped using markers for the Pi-z and Pi-b genes, about 10% of the collection had resistant genotypes. However, 10 or more marker alleles were identified for these genes indicating wide genetic diversity at these loci. Using both molecular markers and phenotypic reactions to blast inoculations has demonstrated that the core collection offers new sources of resistance to blast disease that can be exploited in US breeding programs. Molecular markers are useful for determining if multiple known genes are present in the resistant germplasm or if new genes or alleles should be expected. This information is valuable for breeders to know how to best utilize the germplasm in crossing programs.