Among all cereals, rice is highly sensitive to cold stress, especially at the germination stage, which adversely impacts its germination ability, seed vigor, crop stand establishment, and, ultimately, grain yield. The dissection of novel quantitative trait loci (QTLs) or genes conferring a low-temperature germination (LTG) ability can significantly accelerate cold-tolerant rice breeding to ensure the wide application of rice cultivation through the direct seeding method. In this study, we identified 11 QTLs for LTG using 144 recombinant inbred lines (RILs) derived from a cross between a cold-tolerant variety, Lijiangxintuanheigu (LTH), and a cold-sensitive variety, Shennong265 (SN265). By resequencing two parents and RIL lines, a high-density bin map, including 2,828 bin markers, was constructed using 123,859 single-nucleotide polymorphisms (SNPs) between two parents. The total genetic distance corresponding to all 12 chromosome linkage maps was 2,840.12 cm. Adjacent markers were marked by an average genetic distance of 1.01 cm, corresponding to a 128.80 kb physical distance. Eight and three QTL alleles had positive effects inherited from LTH and SN265, respectively. Moreover, a pleiotropic QTL was identified for a higher number of erected panicles and a higher grain number on Chr-9 near the previously cloned DEP1 gene. Among the LTG QTLs, qLTG3 and qLTG7b were also located at relatively small genetic intervals that define two known LTG genes, qLTG3-1 and OsSAP16. Sequencing comparisons between the two parents demonstrated that LTH possesses qLTG3-1 and OsSAP16 genes, and SN-265 owns the DEP1 gene. These comparison results strengthen the accuracy and mapping resolution power of the bin map and population. Later, fine mapping was done for qLTG6 at 45.80 kb through four key homozygous recombinant lines derived from a population with 1569 segregating plants. Finally, LOC_Os06g01320 was identified as the most possible candidate gene for qLTG6, which contains a missense mutation and a 32-bp deletion/insertion at the promoter between the two parents. LTH was observed to have lower expression levels in comparison with SN265 and was commonly detected at low temperatures. In conclusion, these results strengthen our understanding of the impacts of cold temperature stress on seed vigor and germination abilities and help improve the mechanisms of rice breeding programs to breed cold-tolerant varieties.

Cold damage often hinders rice growth, affects its morphogenesis and leads finally to serious yield loss. Identifying favorable genes tolerant to cold, and breeding elite rice varieties with cold tolerance is an effective way to solve this problem. In this study, two sets of reciprocal introgression lines and a set of recombinant inbred line derived from xian variety Minghui 63 and geng variety 02428 were evaluated for cold tolerance in climate chamber using cold tolerance-related traits such as wilting degree and survival rate after recovered growth. Combined with the available Bin genotype data generated by previous re-sequencing, 13 QTLs for cold tolerance at seedling stage were identified on chromosomes 1, 4, 5, 7, and 12, which explained 0.36% to 13.63% of phenotypic variation. No QTL was stably detected in different genetic backgrounds, whereas five QTLs were simultaneously detected for both wilting degree and survival rate on chromosomes 1, 5, 7, and 12. Among them, QTLs in the regions of 12,732,139-13,202,097 bp and 14,445,778-14,585,009 bp on chromosome 1, and 14,658,891-15,684,510 bp on chromosome 5 had high LOD values above 25 and were considered as reliable main-effect QTLs. Combined with gene annotation, gene expression profile and phenotyping data of cold tolerance of 3K germplasms, the previously cloned gene OsWKRY45 for cold tolerance was regarded as the responsible gene for the QTL in the region of 14,658,891-15,684,510 bp on chromosome 5, while the two candidate genes, LOC_Os01g25540 and LOC_Os01g25560 in the two regions on chromosome 1, were the new candidate genes for both wilting degree and survival rate. Haplotype analysis indicated that Hap2, Hap4, and Hap9 of LOC_Os01g25540 and Hap1, Hap8, and Hap10 of LOC_Os01g25560 were favorable haplotypes for cold tolerance. The results provide the novel germplasms and favorable genes for molecular improvement of cold tolerance in rice.