Rice salt tolerance at the germination stage directly affects the germination rate and seedling establishment of rice directly seeded in saline soils, which in turn affects yield. In this study, we determined the relative germination potential (RGP) and relative germination index (RGI) under 200 mM salt stress and control conditions using 295 japonica rice accessions. Statistical analysis showed extensive phenotypic variations under salt stress conditions. Twenty-one varieties with an RGP ≥ 80% and an RGI ≥ 80% were screened. Based on genotypic data including, 788,396 single-nucleotide polymorphisms (SNPs), 40 quantitative trait loci (QTL) were localized on rice chromosomes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12, which were shown to be significantly associated with rice salt tolerance at the germination stage, including 20 for RGP and 20 for RGI, using genome-wide association analysis. Six QTL with ≥ 3 consecutive significant SNP loci and localized in the same LD interval were selected for further analysis. Four rice genes (LOC_Os01g04920, LOC_Os10g38350, LOC_Os10g38470, and LOC_Os10g38489) were selected as important candidates for salt tolerance based on haplotype analysis and functional annotation. The findings could facilitate the development of valuable rice varieties for direct seeding in salinized soil and improve japonica rice salt tolerance at the germination stage through molecular breeding.

Grain length (GL) is one of the crucial determinants of rice yield and quality. However, there is still a shortage of knowledge on the major genes controlling the inheritance of GL in japonica rice, which severely limits the improvement of japonica rice yields. Here, we systemically measured the GL of 667 F₂ and 1570 BC₃F₃ individuals derived from two cultivated rice cultivars, Pin20 and Songjing15, in order to identify the major genomic regions associated with GL. A novel major QTL, qGL9.1, was mapped on chromosome 9, which is associated with the GL, using whole-genome re-sequencing with bulked segregant analysis. Local QTL linkage analysis with F₂ and fine mapping with the recombinant plant revealed a 93-kb core region on qGL9.1 encoding 15 protein-coding genes. Only the expression level of LOC_Os09g26970 was significantly different between the two parents at different stages of grain development. Moreover, haplotype analysis revealed that the alleles of Pin20 contribute to the optimal GL (9.36 mm) and GL/W (3.31), suggesting that Pin20 is a cultivated species carrying the optimal GL variation of LOC_Os09g26970. Furthermore, a functional-type mutation (16398989-bp, G>A) located on an exon of LOC_Os09g26970 could be used as a molecular marker to distinguish between long and short grains. Our experiments identified LOC_Os09g26970 as a novel gene associated with GL in japonica rice. This result is expected to further the exploration of the genetic mechanism of rice GL and improve GL in rice japonica varieties by marker-assisted selection.