Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2008
Publication Date: 1/1/2009
Citation: Sharma, A., McClung, A.M., Pinson, S.R., Kepiro, J.L., Shank, A.R., Tabien, R.E., Fjellstrom, R.G. 2009. Genetic mapping of sheath blight resistance to QTLs within tropical Japonica rice cultivars. Crop Science. 49:256-264. Interpretive Summary: Almost all rice varieties grown in the U.S. are severely susceptible to sheath blight, a fungal disease causing widespread disease damage and crop loss in the U.S. Previous studies of sheath blight (SB) resistance originating from Asian rice varieties have shown that SB resistance is complexly inherited by several genes, with each of these genes being variably affected by the environment. However, Asian varieties with SB resistance are, in general, poorly adapted for cultivation in the U.S. Since fungicide application continues as the most common measure to control this disease, using sources of SB resistance in U.S. adapted varieties is of high value. This research study focused on the identification of DNA markers associated with the noteworthy SB resistance found in the well-adapted U.S. variety, Pecos. Pecos was crossed to a SB susceptible variety, Rosemont, and 279 offspring lines derived from this cross were genetically analyzed for the inheritance of SB resistance. Data from 142 DNA markers uniformly spread out over the 12 chromosomes of rice were recorded and analyzed for their associations with SB resistance recorded from replicated field plot trials grown in two years. The SB resistance in Pecos was seen to be highly correlated with DNA markers associated with tall plant height, which is often an undesirable trait in rice. However, two different chromosomal regions of the rice genome not associated with plant height were identified as significantly and repeatedly contributing to the SB resistance found in Pecos. DNA markers for these two genetic regions are now available for their use by rice breeders and plant pathologists to help select for increased SB resistance in U.S. rice.
Technical Abstract: Nearly all commercial varieties of rice, Oryza sativa L., are severely susceptible to sheath blight (SB), a devastating fungal disease causing significant losses in grain yield and quality in the U.S. and elsewhere. SB resistance has been reported to be horizontal and quantitative. There are limited sources of genetic resistance that are adapted to U.S. growing conditions. Application of preventive fungicides is the most common SB control measure, which greatly increases production costs and presents an environmental risk. The identification of genes associated with SB resistance would facilitate the development of cultivars having durable resistance. A mapping population consisting of 279 F2:3 progeny rows derived from a cross between two tropical japonica U.S. rice cultivars, Rosemont (semi-dwarf and susceptible to SB) and Pecos (tall and resistant to SB) was used in this study. A genetic linkage map was constructed using 142 simple sequence repeat (SSR) markers, spanning ~1465 cM of the 12 rice chromosomes with an average marker distance of 10.4 cM. All 279 F2:3 families were evaluated for disease symptoms and two agronomic traits, plant height (PH) and heading date (HD) in replicated field trials during 2002 and 2003. Correlation analysis between PH and HD with SB ratings showed both plant traits were significantly correlated with SB resistance. Two QTL mapping approaches, interval mapping (IM) and composite interval mapping (CIM), were used to identify QTLs for SB resistance, PH, and HD. Approximately four significant QTLs (LOD greater than or equal to 3.6, P lesser than or equal to 0.05) were identified for SB resistance, with individual effects ranging from 5.6% to 33.4% of the total phenotypic variation. PH appears to have a direct influence on SB resistance, with QTLs for these traits being co-located on chromosome 1. Results across years were consistent indicating the stability of the identified QTLs and their potential usefulness for improving rice SB resistance using marker-assisted selection (MAS).