Salt and drought stresses are major factors limiting soybean (Glycine max [L.] Merr.) growth and development; thus, improving soybean stress tolerance is critical. In this study, both salt stress and drought stress induced mRNA levels of CONSTANS-like 1a (GmCOL1a) and stabilized the GmCOL1a protein. Transgenic 35S:GmCOL1a soybean plants exhibited enhanced salt and drought tolerance, with higher relative water content in leaves, greater proline content, lower malondialdehyde (MDA) content, and less reactive oxygen species (ROS) production compared with wild-type plants; the GmCOL1a knockout co-9 mutant showed opposite phenotypes. In addition, GmCOL1a promoted the expression of genes related to salt tolerance, effectively reducing the Na⁺/K⁺ ratio in soybean plants, especially in stems and leaves of 35S:GmCOL1a soybean. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis identified two potential direct targets of GmCOL1a, late embryogenesis abundant (GmLEA) and Δ¹-pyrroline-5-carboxylate synthetase (GmP5CS) genes, which were verified by chromatin immunoprecipitation quantitative polymerase chain reaction (ChIP-qPCR), electrophoretic mobility shift assay (EMSA), and transient transcriptional activation assays. GmCOL1a bound directly to the Myc(bHLH)-binding and Che-binding motifs of GmLEA and GmP5CS promoters to stimulate mRNA expression. Analysis of transgenic hairy-root GmP5CS:GmP5CS soybean plants in wild type, co-9, and 35S:GmCOL1a backgrounds further revealed that GmCOL1a enhances salt and drought tolerance by promoting GmP5CS protein accumulation in transgenic soybean hairy roots. Therefore, we demonstrate that GmCOL1a plays an important role in tolerance to abiotic stress in soybean.

Soybean yield is a complex quantitative trait, which is greatly affected by environmental conditions. The main objective of this study is not only to identify specific traits contributing to yield in different latitudes, which can be further used in breeding, but also to identify the outperforming varieties, as this can help to select new lines with these traits. One hundred and seventy-three soybean genotypes were tested in three different ecological environments, including Harbin, Changchun, and Shenyang in China during 2015–2016 cropping seasons. The evaluation on the different agronomic and physiological traits indicated that the soybean varieties with higher plant height, more nodes of main stem, branches, pods, grains, and 100-grain weight, or longer growth periods may have higher yield. Pods, grains and 100-grain weight can be used as direct selection criteria for yield increase, and likewise the other traits such as plant height, nodes of main stem, branches, growth periods indirectly affected yield by affecting the three traits above. The effect of genotype × environment (G × E) interaction on different agronomic traits was significant. The representativeness and discriminability for grains yield per plant was the most significant in Harbin, which could be used to screen varieties with high yield and wider adaptability. Genotype “Suinong 1” was considered stable with higher value of grain yield per plant than other genotypes used in this study. As the yield of certain soybean cultivars may be significantly reduced if they are grown in a region as little as 2°N beyond its normal cultivation latitudes, therefore, the identification and analysis on the stable and widely adaptive soybean genotypes would be very important, and it would provide the significant reference accordance of soybean variety selection for the soybean breeders.

The flowering time and plant height of soybean are important agronomic characters, which control the adaptability and yield of soybean. R2R3 MYB transcription factor plays an important regulatory role in plant growth and development. In this study, soybean GmGAMYB gene of R2R3-MYB type was induced by long-days (LDs). GmGAMYB showed higher transcriptional levels in the flowers, leaves and pods of soybean. Overexpression of GmGAMYB in transgenic soybean showed earlier flowering time and maturity in LDs and short-days (SDs). GmGAMYB interacted with GmGBP1 and might promote flowering time by up-regulating the expression of GmFULc gene in soybean. Moreover, the expression level of GmGAMYB was also induced by gibberellins (GAs) and the plant height of GmGAMYB-ox plants was significantly increased, which was caused by the enlargement of internode cell in stem. Furthermore, GmGAMYB overexpression led to increased GA sensitivity in the hypocotyl of soybean seedlings compared with WT. GmGAMYB may be a positive regulator of GA response of promoting plant height by up-regulating the expression of GmGA20ox gene in soybean. Together, our studies preliminarily showed that the partial functions of GmGAMYB in regulating flowering time and GA pathway.

Background: As a photoperiod-sensitive and self-pollinated species, the growth periods traits play important roles in the adaptability and yield of soybean. To examine the genetic architecture of soybean growth periods, we performed a genome-wide association study (GWAS) using a panel of 278 soybean accessions and 34,710 single nucleotide polymorphisms (SNPs) with minor allele frequencies (MAF) higher than 0.04 detected by the specific-locus amplified fragment sequencing (SLAF-seq) with a 6.14-fold average sequencing depth. GWAS was conducted by a compressed mixed linear model (CMLM) involving in both relative kinship and population structure. Results: GWAS revealed that 37 significant SNP peaks associated with soybean flowering time or other growth periods related traits including full bloom, beginning pod, full pod, beginning seed, and full seed in two or more environments at -log₁₀(P) > 3.75 or -log₁₀(P) > 4.44 were distributed on 14 chromosomes, including chromosome 1, 2, 3, 5, 6, 9, 11, 12, 13, 14, 15, 17, 18, 19. Fourteen SNPs were novel loci and 23 SNPs were located within known QTLs or 75 kb near the known SNPs. Five candidate genes (Glyma.05G101800, Glyma.11G140100, Glyma.11G142900, Glyma.19G099700, Glyma.19G100900) in a 90 kb genomic region of each side of four significant SNPs (Gm5_27111367, Gm11_10629613, Gm11_10950924, Gm19_34768458) based on the average LD decay were homologs of Arabidopsis flowering time genes of AT5G48385.1, AT3G46510.1, AT5G59780.3, AT1G28050.1, and AT3G26790.1. These genes encoding FRI (FRIGIDA), PUB13 (plant U-box 13), MYB59, CONSTANS, and FUS3 proteins respectively might play important roles in controlling soybean growth periods. Conclusions: This study identified putative SNP markers associated with soybean growth period traits, which could be used for the marker-assisted selection of soybean growth period traits. Furthermore, the possible candidate genes involved in the control of soybean flowering time were predicted.

Photoperiod and sucrose levels play a key role in the control of flowering. GmRAV reflected a diurnal rhythm with the highest expression at 4 h after the beginning of a dark period in soybean leaves, and was highly up-regulated under short-day (SD) conditions, despite of not following a diurnal pattern under long-day (LD) conditions. GmRAV-i (GmRAV-inhibition) transgenic soybean exhibited early flowering phenotype. Two of the FT Arabidopsis homologs, GmFT2a and GmFT5a, were highly expressed in the leaves of soybeans with inhibition (-i) of GmRAV under SD conditions. Moreover, the transcript levels of the two FT homologs in GmRAV-i soybeans were more sensitive to SD conditions than LD conditions compared to the WT plant. GmRAV-i soybeans and Arabidopsis rav mutants showed more sensitive hypocotyl elongation responses when compared with wild-type seedlings, and GmRAV-ox overevpressed in tobacco revealed no sensitive changes in hypocotyl length. These indicated that GmRAV was a novel negative regulator of SD-mediated flowering and hypocotyl elongation. Although sucrose has been suggested to promote flowering induction in many plant species, high concentration of sucrose (4% [w/v]) applied into media defer flowering time in Arabidopsis wild-type and rav mutant. This delayed flowering stage might be caused by reduction of LEAFY expression. Furthermore, Arabidopsis rav mutants and GmRAV-i soybean plants were less sensitive to sucrose by the inhibition assays of hypocotyls and roots growth. In contrast, transgenic GmRAV overexpressing (-ox) tobacco plants displayed more sensitivity to sucrose. In conclusion, GmRAV was inferred to have a fundamental function in photoperiod, darkness, and sucrose signaling responses to regulate plant development and flowering induction.

Photoperiod strictly controls vegetative and reproductive growth stages in soybean (Glycine max). A soybean GmRAV (Related to ABI3/VP1) transcription factor containing both AP2 and B3 domains was shown to be a key component of this process. We identified six polymorphisms in the GmRAV promoter that showed significant association with flowering time and maturity of soybean in one or multiple environments. Soybean varieties with minor polymorphism exhibited a longer growth period contributing to soybean adaptation to lower latitudes. The cis-acting element GT1CONSENSUS motif of the GmRAV promoter controlled the growth period, and the major allele in this motif shortened duration of late reproductive stages by reducing GmRAV expression levels. Three GmRAV-overexpressing (GmRAV-ox) transgenic lines displayed later flowering time and maturity, shorter height and fewer numbers of leaves compared with control plants, whereas transgenic inhibition of GmRAV expression resulted in earlier flowering time and maturity and increased plant height. Combining DNA affinity purification sequencing and RNA sequencing analyses revealed 154 putative target genes directly bound and transcriptionally regulated by GmRAV. Two GmRAV binding motifs [C(A/G)AACAA(G/T)A(C/T)A(G/T)] and [C(T/A)A(C)C(T/G)CTG] were identified, and acting downstream of E3E4, GmRAV repressed GmFT5a transcriptional activity through binding a CAACA motif, thereby delaying soybean growth and extending both vegetative and reproductive phases.