|Prasad, Bishwajit -|
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: January 15, 2010
Publication Date: February 22, 2010
Citation: Eizenga, G.C., Prasad, B. 2010. RiceCAP: Sheath blight QTLs identified in two Bengal/O. nivara advanced backcross populations. In: Proceedings 33rd Rice Technical Working Group Meetings, February 22-25, 2010, Biloxi, MS. CDROM Technical Abstract: Rice sheath blight disease, caused by Rhizoctonia solani, is one of the most important fungal diseases worldwide. Wild relatives of rice may contain novel genes for biotic/abiotic stress resistance lost during domestication. We identified seven moderately resistant accessions from a collection of 67 wild Oryza species accessions. To identify QTLs and ultimately genes related to sheath blight resistance, we developed two mapping populations with two of these accessions as donor parents using the advanced backcross method. This method has proven to be effective for simultaneously identifying QTLs and improving germplasm, especially when the donor parent is not adapted. Bengal (PI 561735), a popular medium grain variety in the southern USA, which is moderately susceptible to sheath blight disease, was used as the recurrent parent and the donor parents were two wild O. nivara accessions (IRGC100898, IRGC104705). The “Wild-1” population with IRGC100898 is composed of 252 BC2F2 families and was genotyped with 129 SSR markers distributed throughout the 12 rice chromosomes. The same SSRs are being used to genotype the “Wild-2” population with IRGC104705. Products of the PCR reactions were visualized on an ABI Prism 3730 and GeneMapper 4.0 to identify the alleles. Sheath blight disease was evaluated under inoculated conditions in the greenhouse using the micro-chamber method and in replicated field trials over two years using the standard field inoculation method. Plant height, days to heading, and plant type which are known to confound sheath blight ratings, were collected from the field trials. The linkage map was created using JoinMap 4.0 and QTLs identified using multiple interval mapping as performed by QGene 4.2.0. The preliminary linkage map for the Wild-1 population covered a 1742 cM distance with an average interval size of 13.8 cM between two markers and the heterozygous O. nivara genome segments in the BC2F1 plants varied from 5.7 to 60% with an average proportion of 24.5%. The correlation coefficients between each pair of traits revealed plant height had a significant negative correlation with plant type (rated as 1=upright, 3, 5, 7, 9=spreading) and field sheath blight ratings. Heading date had a significant negative correlation with greenhouse and field sheath blight ratings. Most importantly, sheath blight ratings in the greenhouse had a significant positive correlation with sheath blight ratings taken in the field. Preliminary QTL mapping results identified at least three sheath blight QTL (SB-QTL) located on chromosomes 2, 3 and 6; qSB2, qSB3 and qSB6, respectively, were detected using both screening methods. The SB-QTL, qSB2 and qSB6, were attributed to the O. nivara parent with 3.8 to 8.9% of the variance explained and 0.42 to 1.7 of the additive effects. One QTL (qSB2) previously had not been reported, suggesting a potential novel SB-QTL. Three heading date QTL, all attributed to the O. nivara parent, were identified on chromosomes 3, 6 and 7 with the LOD peaks ranging from 2.9 to 7.3, delaying heading by 1.1 to 4.6 days. Two SB-QTLs (qSB3, qSB6) were located in the same region as days to heading QTL, suggesting this morphological trait influences sheath blight response. The QTL identified on chromosome 6 (qSB6, qHD6) coincided with previously identified QTL for sheath blight and days to heading. The fact that these QTL were found in approximately the same region as a previously reported QTL from O. sativa cultivars indicates QTL are conserved across the Oryza genus. One QTL for plant height detected on chromosome 7 with a LOD peak of 2.9, was attributed to the O. nivara parent with an additive effect of increasing height by 21.2 cm. Two QTLs were detected for plant type on chromosomes 1 and 9. The QTL on chromosome 9, attributed to the O. nivara parent, increased tiller angle by nearly a class (1.5 score), with a LOD score of 11.7 and was in the region of previously reported QTL for tiller angle. The other QTL for tiller angle was attributed to the Bengal parent. QTL mapping results from the Wild-2 population will be analyzed once the genotyping is complete and compared with these results as a validation of the SB-QTL. Through additional backcrossing of lines containing the putative SB-QTLs from O. nivara with Bengal and subsequent genotypic selection, we are developing germplasm with improved sheath blight resistance.