| Single segment substitution lines(SSSLs) which carries a single chromosome segment of donor genome in the genetic background of a recurrent genotype are genetically stable inbred lines. A set of SSSLs covering the whole genome of the recurrent parent are good materials for genetic study.In a previous study, a chilling tolerance maize inbred line Huangzaosi(HZ4) and an inbred line CML343 was used to construct a set of SSSLs. In this study, molecular marker selection was conducted on the progenies of backcross or self of the SSSLs, and the number of substitution chromosome segments in the SSSLs and the average percentage of background genome was also assessed. The set of SSSLs will be useful to explore the mechanisms and genetic basis of stress resistance in maize.In maize, seedling stage is a critical period and sensitive to low temperature, chilling affects seed germination, seedling emergence, growth vigor of seedlings and limits yield potential. A set of testcrossing association population was used to dissect the genetic basis of chilling tolerance through Genome-wide association analysis(GWAS) in this study. Several traits under stress condition, water content of shoot, leaf roll degree, ratio of root to shoot and soluble sugar content were used as chilling tolerance indices. The significant loci identified in this study would provide useful information for genetic improvement of chilling tolerance in maize. The main results are as follows,1. Totally 59 SSSLs(BC7F4 or BC8F4) were selected after one generation of backcross and two times of selfing. About 79.1% genomic region of HZ4 is covered by the introgression segments which harbored by the 59 lines. The average of background recoverage is more than 80%.2. A total of 338 testcrosses were used to conduct genome-wide association study(GWAS) on chilling tolerance at seeding stage. In total, 3, 12, 4 and 9 significant loci(P<9.0×10-6) were identified for the trait of LRD(leaf roll degree), RRS(fresh ratio from root to shoot), SSC(soluble sugar content) and WCS(water content of shoot), respectively. Individual loci explained 5.53%~9.06% of phenotypic variation.3. The association panel can be divided into three subpopulations(55, 119 and 164 belong to SS, NSS and TST subpopulations, respectively). Chilling tolerance in testcrosses derived from temperate area(SS and NSS) was better than that from tropical and subtropical area(TST). Within the three subpopulation, 2, 8 and 1 subpopulation specific significant loci(P<1.8×10-6)were identifiedrespectively, individual loci explained 0.86%~13.94% of phenotypic variation, and three of them were related to more than two traits.4. Totally 24(with the entire population) and 14(subpopulation specific) candidate genes were predicted within the 50 kb regions up and down stream of the significant loci for these four cold tolerance related traits. Based on the information of gene annotation in the B73 reference genome, these genes can be classified into four group, transcription factor and signal transduction genes, transporter genes, genes involved in metabolism pathway of stress tolerance and saccharometabolism.5. Gene expression of 13 candidate genes under cold condition were analyzed. The expression of most of the genes were influenced by chilling stress. In general these genes were induced or stable in the hybrids or in only one of the parents by chilling stress.Results in this study revealed that chilling resistance at seeding stage is a quantitative trait and controlled by polygenes. Through GWAS with testcrossing population a number of significant heterotic loci and candidate genes were identified, which would be useful for the utility of heterosis in maize breeding. Meanwhile, the construction of SSSLs would provide materials for fine–mapping and cloning of chilling tolerance genes. |
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