Searching For Sheath Blight Resistant Germplasm And Identification Of Resistant Alleles By Association Mapping

Sheath blight (ShB), caused by the fungal pathogen Rhizoctonia solani, is one of the most important and widely distributed diseases capable of infesting numerous crops including rice (Oryza sativa L.). Resistant germplasm with wide variation is essential for controlling this disease via breeding efforts, and genetic background helps design breeding strategies. We used the micro-chamber method to evaluatethe USD A rice core collection, including1794accessions from114countries for ShB resistance. Seventy-two molecular markers were used for genotyping. Compared with Jasmine85which has been recognized as one of the most resistant germplasm accessions,52accessions were significantly more resistant at the5%level of probability, and of these17were more resistant at the1%level of probability. The resistant accessions originated from26countries in nine geographic regions, and are diversified for13phenotypic characteristics. The resistant accessions all belonged to the cultivated species O. sativa. Genetic analysis using72molecular markers revealed that45resistant accessions (87%) were indica type that were further classified in two groups. Three accessions were identified as A US, two as aromatic and one each as temperate japonica and tropical japonica. Breeders could use these findings to choose ShB resistant accessions for cultivar improvement.Global attention has focused on examining individual mapping populations for quantitative trait loci (QTLs) for ShB resistance, but to date no study has taken advantage of association mapping to examine hundreds of lines for potentially novel QTLs. Our objective was to identify ShB QTLs via association mapping in rice using217entries of the USDA rice mini-core collection, which were phenotyped with a micro-chamber screening methodand genotyped with155genome-wide markers. Structural analysis divided the mapping panel into five groups, and model comparison revealed that PCA5with a genomic control was the best model for association mapping of ShB. Ten marker loci on seven chromosomes were significantly associated with response to ShB. Among multiple alleles in each identified locus, the allele with the greatest resistance was named the resistant allele. Among217entries, entry GSOR310389had the most resistant alleles, eight out of ten. The number of resistant alleles contained in an entry was highly and significantly correlated with the decrease of ShB rating (r=-0.535) or the increase of ShB resistance. The majority of the resistant entries that contained a large number of the resistant alleles belonged to indica, which is consistent with the general observation that most ShB resistant rice accessions are of indica origin. These findings demonstrate the potential to improve breeding efficiency by using marker-assisted selection to pyramid resistant alleles from various loci in a cultivar for enhanced ShB resistance.