Soybean seed hardness is a key trait that influences
planting,
nutritional quality, and postharvest processing, but its genetic and
molecular mechanisms remain to be clarified. We used meta-analysis
to detect 17 meta-quantitative trait locus (QTLs) for soybean seed
hardness. We then identified a hard-seeded chromosome segment substitution
line, R75, with fragments introduced from hard-seeded wild germplasm
in four of the meta-QTL intervals. Observations of the seed coat ultrastructure
revealed thicker palisade tissue in R75 than in its soft-seeded recurrent
parent. Transcriptomics and proteomics of R75 and its recurrent parent
revealed multiple candidate genes associated with seed hardness. Fifty-seven
were located on homozygous introduced fragments, 26 in meta-QTL intervals,
and one in both (Glyma.02G268600). Five initial candidates
were selected for KASP marker development on the basis of their predicted
functions and nonsynonymous SNPs. The selection efficiency of the
markers was as high as 90% for nonhard lines and 43% for hard lines
in the chromosome segment substitution line (CSSL) population.