| Summary: | SWEET (Sugars will eventually be exported transporter) proteins are a group of sugar transporters that are involved in sugar efflux, phloem loading, reproductive development, plant senescence, and stress responses. In this study, 23 SWEET transporter members were identified in the <i>Medicago polymorpha</i> genome, heterogeneously distributed on seven chromosomes. These <i>MpSWEET</i> genes were divided into four subfamilies, which showed similar gene structure and motif composition within the same subfamily. Seventeen <i>MpSWEET</i> genes encode seven transmembrane helices (TMHs), and all MpSWEET proteins possess conserved membrane domains and putative serine phosphorylation sites. Four and three pairs of <i>MpSWEET</i> genes were predicted to be segmentally and tandemly duplicated, respectively, which may have contributed to their evolution of <i>M. polymorpha</i>. The results of microarray and RNA-Seq data showed that some <i>MpSWEET</i> genes were specifically expressed in disparate developmental stages (including seedling stage, early flowering stage, and late flowering stage) or tissues such as flower and large pod. Based on protein network interaction and expression patterns of <i>MpSWEET</i> genes, six <i>MpSWEET</i> genes were selected for further quantitative real-time PCR validation in different stress treatments. qRT-PCR results showed that <i>MpSWEET05</i>, <i>MpSWEET07</i>, <i>MpSWEET12</i>, <i>MpSWEET15</i>, and <i>MpSWEET21</i> were significantly upregulated for at least two of the three abiotic stress treatments. These findings provide new insights into the complex transcriptional regulation of <i>MpSWEET</i> genes, which facilitates future research to elucidate the function of <i>MpSWEET</i> genes in <i>M. polymorpha</i> and other legume crops.
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