Malate dehydrogenase (MDH) is a widely expressed enzyme that plays a key role in plant growth, development, and stress responses. However, information on MDH genes in the soybean genome is limited. Seventeen members of the soybean MDH family were identified by genome-wide analysis, and the genes were analyzed for the presence of conserved protein motifs. The genes were divided into five clusters according to their phylogenetic relationships. The intracellular localizations of six GmMDHs were determined by confocal microscopy of Arabidopsis mesophyll protoplasts. Transcripts of GmMDHs were significantly increased by abiotic stress (drought, salt, and alkalinity) and hormone treatments, as shown by an analysis of cis-regulatory elements and quantitative real-time polymerase chain reaction (qRT-PCR). The GmMDHs displayed unique expression patterns in various soybean tissues. Notably, the expression levels of a chloroplast isoform (GmMDH2) were unusually high under salt stress, presumably indicating a critical role in soybean responses to salinity. Expression of GmMDH2 in Escherichia coli showed that the recombinant enzyme has nicotinamide adenine dinucleotide phosphate (NADP)-dependent MDH activity. The redox states of the NADP (reduced form) (NADPH) pool and antioxidant activities were shown to be modulated by GmMDH2 gene overexpression, which in turn reduced reactive oxygen species (ROS) formation in transgenic soybean, significantly enhancing the salt stress resistance. Gene-based association analysis showed that variations in GmMDH2 were strongly linked to seedling salt tolerance. A polymorphism potentially associated with salt tolerance was discovered in the promoter region of GmMDH2. These findings not only improve our understanding of the stress response mechanism by identifying and characterizing the MDH gene family throughout the soybean genome but they also identified a potential candidate gene for the future enhancement of salt tolerance in soybean.

Plant DnaJA proteins act as molecular chaperones in response to environmental stressors. The purpose of this study was to characterize the function and regulatory mechanisms of DnaJA genes in soybean. Gene expression profiles in various soybean tissues at various stages of development indicated that GmDnaJAs function in the coordination of stress and plant hormone responses. GmDnaJA6 was identified as a candidate regulator of saline and alkaline stress resistance and GmDnaJA6 overexpression lines showed increased soybean saline and alkaline tolerance. DnaJ interacted with Hsp70, and GmHsp70 increased the saline and alkaline tolerance of plants with chimeric soybean hairy roots.

Drought stress is an important factor affecting soybean yield. Improving drought tolerance of soybean varieties can increase yield and yield stability when the stress occurs. Identifying QTL related to drought tolerance using molecular marker-assisted selection is able to facilitate the development of drought-tolerant soybean varieties. In this study, we used a high-yielding and drought-sensitive cultivar ‘Zhonghuang 35’ and a drought-tolerant cultivar ‘Jindou 21’ to establish F_6:9 recombinant inbred lines. We constructed a high-density genetic map using specific locus amplified fragment sequencing (SLAF-Seq) technology. The genetic map contained 8078 SLAF markers distributing across 20 soybean chromosomes with a total genetic distance of 3780.98 cM and an average genetic distance of 0.59 cM between adjacent markers. Two treatments (irrigation and drought) were used in the field tests, the Additive-Inclusive Composite Interval Mapping (ICIM-ADD) was used to call QTL, and plant height and seed weight per plant were used as the indicators of drought tolerance. We identified a total of 23 QTL related to drought tolerance. Among them, seven QTL (qPH2, qPH6, qPH7, qPH17, qPH19-1, qPH19-2, and qPH19-3) on chromosomes 2, 6, 7, 17, and 19 were related to plant height, and five QTL (qSWPP2, qSWPP6, qSWPP13, qSWPP17, and qSWPP19) on chromosomes 2, 6, 13, 17, and 19 were related to seed weight and could be considered as the major QTL. In addition, three common QTL (qPH6/qSWPP6, qPH17/qSWPP17, and qPH19-3/qSWPP19) for both plant height and seed weight per plant were located in the same genomic regions on the same chromosomes. Three (qPH2, qPH17, and qPH19-2) and four novel QTL (qSWPP2, qSWPP13, qSWPP17, and qSWPP19) were identified for plant height and seed weight per plant, respectively. Two pairs of QTL (qPH2/qSWPP2 and qPH17/qSWPP17) were also common for both plant height and seed weight per plant. These QTL and closely linked SLAF markers could be used to accelerate breeding for drought tolerant cultivars via MAS.