Soybean α-Amylase Gene Family: Structure and Expression in Response to Abiotic Stresses

α-Amylases are involved in starch breakdown, thereby influencing plant development. Information on the α-amylase genes in soybean is limited. Here, we identified five soybean α-amylase genes from subfamilies AtAMY1 (GmaAMY5), AtAMY2 (GmaAMY4), AtAMY3 (GmaAMY1, GmaAMY3), and AMY6 (GmaAMY2). In silico analysis indicated that all five genes were actively expressed in leaves, flowers, and pods but weakly in roots. GmaAMY1–GmaAMY5 mRNAs were predicted to be targets of miRNAs associated with stress response, organ development, and nitrogen fixation. Putative GmaAMY1–GmaAMY5 proteins contained α-amylase-specific catalytic domain, signatures, and active sites. Short-term abiotic stresses (100 mM NaCl, 2.5–20% PEG, and 4 °C cold) applied to the cv. Doka affected both GmaAMY1–GmaAMY5 expression and the content of starch and soluble sugars in leaves. GmaAMY1 gene expression increased in response to NaCl and PEG, GmaAMY2 in response to PEG, and GmaAMY5 in response to NaCl. Salt stress suppressed the expression of the GmaAMY2–GmaAMY4 genes. The mRNA levels of all five genes increased after 2 h of cold exposure. Under salinity stress, there was inverse correlation of starch content with GmaAMY4 expression (r = −0.5135, p = 0.0293) and overall GmaAMY1–GmaAMY5 expression (r = −0.6318, p = 0.0049), suggesting a possible role of GmaAMY genes in protecting soybean from salinity by maintaining the starch/soluble sugars balance. Our results may aid in the breeding of stress-tolerant soybean varieties.