Genome-Wide Identification of the OPR Gene Family in Soybean and Its Expression Pattern Under Salt Stress

12-oxo-phytodienoic acid reductase (OPR) is a core component of the jasmonic acid (JA) biosynthetic pathway and participates in JA synthesis by catalyzing the reduction in the precursor 12-oxo-phytodienoic acid (OPDA), as well as broadly regulating plant development, stress response, and hormone signaling networks. This study analyzed the OPR gene family using 28 soybean genomes. A total of 15 OPR gene family members in soybean were identified, including 14 core genes and one variable gene. Analysis of gene duplication types showed that whole-genome duplication (WGD)/segmental duplication was the main mode of duplication in GmOPRs. The phylogenetic tree constructed from multiple species showed that the OPRs in subgroup VII were functionally important OPR genes and that the OPRs underwent Leguminosae and Cruciferae divergence, and large-scale duplication occurred in Leguminosae. Analysis of natural selection pressures on 28 soybean accessions indicated that the overall evolutionary pressures on GmOPRs were dominated by purifying selection, but there were also potential positive selection signals. Analysis of cis-acting elements revealed a large number of light- and hormone-responsive cis-acting elements in the GmOPRs. Some specific cis-acting elements were only present in a few genes or accessions. The protein interaction network consisted of 12 GmOPR proteins, 4 allene oxide synthase (AOS) proteins, and 6 allene oxide cyclase (AOC) proteins, where AOCs interact with GmOPRs and AOSs. Tissue transcriptome expression profiling showed that GmOPR3, GmOPR7, and GmOPR15 were specifically expressed in roots, whereas GmOPR2, GmOPR10, and GmOPR14 were specifically expressed in leaves, suggesting that these genes play an important role in the growth and development of the tissues. Moreover, GmOPRs usually responded to salt stress, and GmOPR3, GmOPR8, GmOPR9, GmOPR10, and GmOPR11 were significantly up-regulated in roots and leaves under salt stress. This suggests that these genes may be involved in biological processes such as osmoregulation, ion homeostasis, and scavenging of reactive oxygen species, thus helping soybeans to resist salt stress. This study comprehensively analyzed the OPR gene family in soybean based on the 28 soybean accessions and clarified the salt stress response pattern, which provides a new and more effective and reliable way to analyze the soybean gene family.