Trypsin inhibitors (TIs) in soybean seeds reduce the availability of processed soybean foods and animal feed. This study aimed to evaluate the trypsin inhibitor activity (TIA) in 999 Korean soybean accessions and conduct molecular characterization of soybean accessions with low TIA. TIA was evaluated using colorimetric analysis through a substrate–enzyme reaction. The average TIA of the 999 soybean accessions was 90.31%, ranging from 43.70% to 99.51%. Kunitz trypsin inhibitor 3 (KTi-3) gene target sequencing analysis was performed on seven soybean germplasm accessions (IT105782, IT170889, IT273590, IT274513, IT274515, IT276197, and IT022891) showing less than 60% TIA. Four soybean accessions (IT274513, IT274515, IT276197, and IT022891) exhibited the same mutations (a G-to-T transversion and an AG deletion) in the KTi-3 gene (Glyma.08g341500) in PI542044. In addition, whole-genome re-sequencing was performed on three soybean accessions with no mutations in the KTi-3 gene. Compared with the reference soybean genome, an in-frame insertion and five missense mutations were identified in the coding sequencing of the KTi-1 gene (Glyma.01g09500) in IT105782. A RT-qPCR analysis showed that the mRNA expression level of KTi-1 was reduced by approximately 17% in IT105782 during seed development. In this study, we identified a previously unreported mutation in the KTi-1 gene and developed a KASP marker using this new KTi-1 variant.

Salt stress is a major form of abiotic stress that disrupts soybean germination and early seedling establishment. In this study, physiological, biochemical, and transcriptomic analyses—including germination index, antioxidant enzyme activity, and RNA-seq profiling—were conducted during soybean germination to elucidate early responses to salt stress and biostimulant treatment. A preliminary screening of six biostimulants (nanoparticle zinc oxide (NP-ZnO), nanoparticle silicon dioxide (NP-SiO₂), silicon dioxide (SiO₂), glucose, humic acid, and fulvic acid) revealed NP-SiO₂ as the most effective in promoting germination under salt stress. Under 150 mM NaCl, NP-SiO₂ increased the germination rate and length of the radicle compared with the control, also enhancing peroxidase and ascorbate peroxidase activities while reducing malondialdehyde accumulation, suggesting alleviation of oxidative stress. RNA sequencing revealed extensive transcriptional reprogramming under salt stress, identifying 4579 differentially expressed genes (DEGs) compared with non-stress conditions, while NP-SiO₂ treatment reduced this number to 2734, indicating that NP-SiO₂ mitigated the transcriptional disturbance caused by salt stress and stabilized gene expression networks. Cluster analysis showed that growth- and hormone-related genes suppressed by salt stress were restored under NP-SiO₂ treatment, whereas stress-responsive genes that were induced by salt were attenuated. Hormone-related DEG analysis revealed that NP-SiO₂ down-regulated the overactivation in the abscisic acid, jasmonic acid, and salicylic acid pathways while partially restoring gibberellin, auxin, cytokinin, and brassinosteroid signaling. Overall, NP-SiO₂ at 100 mg/L mitigated salt-induced oxidative stress and promoted early soybean growth by fine-tuning physiological and transcriptional responses, representing a promising nano-based biostimulant for enhancing salt tolerance in plants.

Sucrose is a desirable component of processed soybean foods and animal feed, and thus, its content is used as an important characteristic for assessing the quality of soybean seeds. However, few studies have focused on the quantitative trait loci (QTLs) associated with sucrose regulation in soybean seeds. This study aims to measure the sucrose content of 1014 soybean accessions and identify genes related to high sucrose levels using QTL analysis. Colorimetric analysis based on the enzymatic reaction of invertase (INV) and glucose oxidase (GOD) was employed to test the germplasms. A total of six high-sucrose genetic resources (IT186230, IT195321, IT263138, IT263276, IT263286, and IT276521) and two low-sucrose genetic resources (IT025668 and IT274054) were identified. Two F_2:3 populations, IT186230 × IT025668 and Ilmi × IT186230, were then established from these germplasms. QTL analysis identified four QTLs (qSUC6.1, qSUC11.1, qSUC15.1, and qSUC17.1), explaining 7.3–27.6% of the phenotypic variation in the sugar content. Twenty candidate genes were found at the four QTLs. Notably, Glyma.17G152300, located in the qSUC17.1 QTL region, exhibited a 17-fold higher gene expression in the high-sucrose germplasm IT186230 compared to the control germplasm Ilmi, confirming its role as a major gene regulating the sucrose content in soybean. These results may assist in marker-assisted selection for breeding programs that aim to develop soybean lines with a higher sucrose content.