Genome-wide characterization of soybean malate dehydrogenase genes reveals a positive role of GmMDH2 in salt stress response

Malate dehydrogenase (MDH) is a widely expressed enzyme that plays a key role in plant growth, development, and the stress response. 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 presence of conserved protein motifs was analyzed. The genes were divided into five clusters according to their phylogenetic relationships. The intracellular localizations of six GmMDHs were determined by confocal microscopy on Arabidopsis mesophyll protoplasts. Transcripts of GmMDHs were significantly increased by abiotic stress (drought, salt, and alkalinity) and hormone treatments, as shown by analysis of cis-regulatory elements and quantitative real-time polymerase chain reaction (qRT-PCR). GmMDHs displayed unique expression patterns in diverse soybean tissues. It is noteworthy that under salt stress, the expression levels of a chloroplast isoform (GmMDH2) were unusually high, presumably indicating a critical role in soybean responses to salinity. Expression of GmMDH2 in Escherichia coli showed that the recombinant enzyme had NADP-dependent MDH activity. The redox states of the nicotinamide adenine dinucleotide phosphate (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 possibly 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 it also identified a potential candidate gene for the future enchancement of salt tolerance in the soybean.