Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/1/2006
Publication Date: 2/15/2006
Citation: Sharma, A., Kepiro, J.L., Fjellstrom, R.G., Pinson, S.R., Shank, A.R., McClung, A.M., Tabien, R.E. 2006. Mapping sheath blight resistance QTL(s) in tropical japonica rice. Rice Technical Working Group Meeting Proceedings, February 29-March 1, 2006, Houston, Texas. 2006 CDROM.
Technical Abstract: Sheath blight (SB), caused by the fungus Rhizoctonia solani Kühn, is a destructive disease of rice (Oryza sativa L.) causing severe loss in grain yield and quality each year in the U.S. and elsewhere. Resistance has been reported to be horizontal and quantitative, and does not follow the gene-for-gene model. There are limited sources of genetic resistance that are adapted to the U.S. No commercial long grain cultivar of rice with an acceptable level of SB resistance is currently available. Widespread application of preventive fungicides is the most common control measure, greatly increasing production cost and environmental hazards. Breeders are seeking to develop genetically-resistant cultivars as a durable solution to control SB. Therefore, the identification of genes significantly controlling SB resistance is important. To facilitate the identification of QTL(s) for SB resistance among US-adapted germplasm, a mapping population was developed by crossing two tropical japonica rice cultivars, Rosemont (semi-dwarf and susceptible to SB) and Pecos (tall and tolerant to SB). The population was advanced to F2-derived F3 families and phenotypic evaluation for SB disease was performed on 279 F3 family rows during the years 2002 and 2003 at Beaumont. In both years the field inoculated screening experiment was replicated twice and three disease scores were recorded during the growing season in each replication. The disease scores were given based on a disease response rating of 1 to 9, where score of 1 indicates very resistant and 9 very susceptible disease reactions. The three time-sequential scores from each plots were averaged. To avoid the confounding impact of escapes, which are common during disease evaluations, later analysis will focus on the maximum plot average obtained from the four replications. Leaf tissues were collected for DNA isolation from all F3 progeny rows representing the original F2 plants. Genetic analysis of this mapping population was initiated in May 2005. For high throughput SSR marker genotyping, ABI 3100 Genetic Analyzer and Li-Cor 4200 genetic analysis systems are being used. Both systems allow multiplexing of differently labeled primer pairs, facilitating generation of ~400 data points per day. Genotypic data for 127 polymorphic SSR markers have been generated on the 279 lines. Preliminary mapping analysis using JoinMap 3.0 revealed that the linear order of 127 SSR markers is in good agreement with IRMI-2003 map available at http://www.gramene.org/. The average inter-marker distance is less than 20 cM with a few exceptions covering all 12 chromosomes of rice genome. QTL analysis using MapQTL 5.0 thus far shows highly significant SB-QTLs on rice chromosomes 1 and 3. Additional QTL analysis being performed on the mapping population will be presented and discussed.