Association Mapping of Early-Season Cold Tolerance in Sorghum [Sorghum Bicolor (L.) Moench].

Cold temperature is one of the most important abiotic stresses affecting sorghum production in temperate regions. It reduces seed germination, seedling emergence and vigor thus limiting the production of the crop both temporally and spatially. The objectives of this study were (1) to identify new sources of early-season cold tolerance in sorghum and (2) to identify molecular markers associated with early-season cold tolerance in sorghum using association studies. One hundred and thirty-six sorghum accessions from colder regions of the world were phenotyped for seedling emergence, vigor ratings, height and dry weight through early planting in two field environments. The accessions were genotyped using 67 simple sequence repeats (SSR) markers representing all ten linkage groups of sorghum, of which 50 highly polymorphic markers were used in the analysis. Association mapping was performed using general linear model (GLM) and mixed linear model (MLM). Significant genotypic variability for response to the stress was observed. Some accessions showed either superior or comparable level of tolerance to that of the cold tolerant check Shan Qui Red (SQR). STRUCTURE, neighbor-joining tree and principal component analyses identified four subpopulations in the entire population and the tolerant sources were distributed in all subpopulations. Six marker-trait associations were identified, of which three markers (Xtxp34,Xtxp88 and Xtxp319) were associated with seedling emergence, two (Xtxp211 and Xtxp304) with seedling dry weight and one (Xtxp20) with seedling height. The markers were detected on chromosomes previously found to harbor QTLs associated with cold tolerance in sorghum. Once validated these markers may serve as genomic tools in marker-assisted breeding and screening larger pool of genotypes to identify additional sources of cold tolerance in sorghum