Summary: Soybean (Glycine max) is a major contributor to the world oilseed production. Its seed oil content has been increased through soybean domestication and improvement. However, the genes underlying the selection are largely unknown. The present contribution analyzed the expression patterns of genes in the seed oil quantitative trait loci with strong selective sweep signals, then used association, functional study and population genetics to reveal a sucrose efflux transporter gene, GmSWEET39, controlling soybean seed oil content and under selection. GmSWEET39 is highly expressed in soybean seeds and encodes a plasma membrane‐localized protein. Its expression level is positively correlated with soybean seed oil content. The variation in its promoter and coding sequence leads to different natural alleles of this gene. The GmSWEET39 allelic effects on total oil content were confirmed in the seeds of soybean recombinant inbred lines, transgenic Arabidopsis, and transgenic soybean hairy roots. The frequencies of its superior alleles increased from wild soybean to cultivated soybean, and are much higher in released soybean cultivars. The findings herein suggest that the sequence variation in GmSWEET39 affects its relative expression and oil content in soybean seeds, and GmSWEET39 has been selected to increase seed oil content during soybean domestication and improvement.

Soybean is one of the most important oil crops in the world. Revealing the molecular basis and exploring key candidate genes for seed oil synthesis has great significance for soybean improvement. In this study, we found that oil accumulation rates and gene expression levels changed dynamically during soybean seed development. The expression levels of genes in metabolic pathways such as carbon fixation, photosynthesis, glycolysis, and fatty acid biosynthesis were significantly up-regulated during the rapid accumulation of oil in developing soybean seeds. Through weighted correlation network analysis, we identified six co-expression modules associated with soybean seed oil content and the pink module was the most positively correlated (r = 0.83, p = 7 × 10⁻⁴) network. Through the integration of differential expression and co-expression analysis, we predicted 124 candidate genes potentially affecting soybean seed oil content, including seven genes in lipid metabolism pathway, two genes involved in glycolysis, one gene in sucrose metabolism, and 12 genes belonged to transcription factors as well as other categories. Among these, three genes (GmABI3b, GmNFYA and GmFAD2-1B) have been shown to control oil and fatty acid content in soybean seeds, and other newly identified candidate genes would broaden our knowledge to understand the molecular basis for oil accumulation in soybean seeds.

Key message: Four major quantitative trait loci for 100-seed weight were identified in a soybean RIL population under five environments, and the most likely candidate genes underlying these loci were identified. Seed weight is an important target of soybean breeding. However, the genes underlying the major quantitative trait loci (QTL) controlling seed weight remain largely unknown. In this study, a soybean population of 300 recombinant inbred lines (RILs) derived from a cross between PI595843 (PI) and WH was used to map the QTL and identify candidate genes for seed weight. The RIL population was genotyped through whole genome resequencing, and phenotyped for 100-seed weight under five environments. A total of 38 QTL were detected, and four major QTL, each explained at least 10% of the variation in 100-seed weight, were identified. Six candidate genes within these four major QTL regions were identified by analyses of their tissue expression patterns, gene annotations, and differential gene expression levels in soybean seeds during four developmental stages between two parental lines. Further sequence variation analyses revealed a C to T substitution in the first exon of the Glyma.19G143300, resulting in an amino acid change between PI and WH, and thus leading to a different predicted kinase domain, which might affect its protein function. Glyma.19G143300 is highly expressed in soybean seeds and encodes a leucine-rich repeat receptor-like protein kinase (LRR-RLK). Its predicted protein has typical domains of LRR-RLK family, and phylogenetic analyses reveled its similarity with the known LRR-RLK protein XIAO (LOC_Os04g48760), which is involved in controlling seed size. The major QTL and candidate genes identified in this study provide useful information for molecular breeding of new soybean cultivars with desirable seed weight.

Although seed weight has increased following domestication from wild soybean (Glycine soja) to cultivated soybean (Glycine max), the genetic basis underlying this change is unclear. Using mapping populations derived from chromosome segment substitution lines of wild soybean, we identified SW16.1 as the causative gene underlying a major quantitative trait locus controlling seed weight. SW16.1 encodes a nucleus‐localized LIM domain‐containing protein. Importantly, the GsSW16.1 allele from wild soybean accession N24852 had a negative effect on seed weight, whereas the GmSW16.1 allele from cultivar NN1138‐2 had a positive effect. Gene expression network analysis, reverse‐transcription quantitative polymerase chain reaction, and promoter‐luciferase reporter transient expression assays suggested that SW16.1 regulates the transcription of MT4, a positive regulator of seed weight. The natural variations in SW16.1 and other known seed weight genes were analyzed in soybean germplasm. The SW16.1 polymorphism was associated with seed weight in 247 soybean accessions, showing much higher frequency of positive‐effect alleles in cultivated soybean than in wild soybean. Interestingly, gene allele matrix analysis of the known seed weight genes revealed that G. max has lost 38.5% of the G. soja alleles and that most of the lost alleles had negative effects on seed weight. Our results suggest that eliminating negative alleles from G. soja led to a higher frequency of positive alleles and changed genetic backgrounds in G. max, which contributed to larger seeds in cultivated soybean after domestication from wild soybean. Our findings provide new insights regarding soybean domestication and should assist current soybean breeding programs.

Soybean is an important source of oil, protein, and feed. However, its yield is far below that of major cereal crops. The green revolution increased the yield of cereal crops partially through high‐density planting of lodging‐resistant semi‐dwarf varieties, but required more nitrogen fertilizers, posing an environmental threat. Genes that can improve nitrogen use efficiency need to be integrated into semi‐dwarf varieties to avoid the overuse of fertilizers without the loss of dwarfism. Unlike cereal crops, soybean can assimilate atmospheric nitrogen through symbiotic bacteria. Here, we created new alleles of GmGID1‐2 ( Glycine max GIBBERELLIN INSENSITIVE DWARF 1‐2) using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR‐associated nuclease 9 (Cas9) editing, which improved soybean architecture, yield, seed oil content, and nitrogen fixation, by regulation of important pathways and known genes related to branching, lipid metabolism, and nodule symbiosis. GmGID1‐2 knockout reduced plant height, and increased stem diameter and strength, number of branches, nodes on the primary stem, pods, and seeds per plant, leading to an increase in seed weight per plant and yield in soybean. The nodule number, nodule weight, nitrogenase activity, and nitrogen content were also improved in GmGID1‐2 knockout soybean lines, which is novel compared with the semi‐dwarf genes in cereal crops. No loss‐of‐function allele for GmGID1‐2 was identified in soybean germplasm and the edited GmGID1‐2s are superior to the natural alleles, suggesting the GmGID1‐2 knockout mutants generated in this study are valuable genetic resources to further improve soybean yield and seed oil content in future breeding programs. This study illustrates the pleiotropic functions of the GID1 knockout alleles with positive effects on plant architecture, yield, and nitrogen fixation in soybean, which provides a promising strategy toward sustainable agriculture.

The kinesin (KIN) gene family is a subgroup of motor proteins. It plays a critical role in plant development and responses to environmental stresses. However, their function in soybean salt tolerance has yet to be clearly defined. This study employed bioinformatics approaches and identified 139 kinesin family members in the soybean genome. These 139 genes were classified into 10 subgroups, unevenly distributed across the chromosomes. The promoter regions of GmKIN genes harbored several stress-responsive elements, and segmental duplication was the primary driver of the expansion of the GmKIN gene family. Based on publicly available RNA-seq data, we studied the response patterns of 139 GmKIN genes to salt stress and found that 20 KIN genes in soybeans were upregulated after salt stress, with GmKIN114, GmKIN102, GmKIN109, and GmKIN99 showing more than a threefold increase in their expression under salt stress. Using quantitative fluorescence PCR, transgenic yeast, and a transgenic hairy root system, we preliminarily validated the salt tolerance functions of the four KIN genes in soybeans. This study probed into the GmKIN gene family in soybean, offering valuable insights into the functional roles of these genes in stress adaptation.