Genome-Wide Identification of the SWEET Gene Family and Functional Analysis of BraSWEET10 in Winter B. rapa (Brassica rapa L.) Under Low-Temperature Stress

Sugars will eventually be exported transporter (SWEET), a class of glucose transport proteins, is crucial in plants for glucose transport by redistribution of sugars and regulates growth, development, and stress tolerance. Although the SWEET family has been studied in many plants, little is known about its function in winter <i>B. rapa</i> (<i>Brassica rapa</i> L.). Bioinformatics approaches were adopted to identify the SWEET gene (<i>BraSWEETs</i>) family in <i>B. rapa</i> to investigate its role during overwintering. From the whole-genome data, 31 <i>BraSWEET</i> genes were identified. Gene expansion was realized by tandem and fragment duplication, and the 31 genes were classified into four branches by phylogenetic analysis. As indicated by exon-intron structure, cis-acting elements, MEME (Multiple EM for Motif Elicitation) motifs, and protein structure, <i>BraSWEETs</i> were evolutionarily conserved. According to the heat map, 23 <i>BraSWEET</i> genes were differentially expressed during overwintering, revealing their potential functions in response to low-temperature stress and involvement in the overwintering memory-formation mechanism. <i>BraSWEET10</i> is mainly associated with plant reproductive growth and may be crucial in the formation of overwintering memory in <i>B. rapa</i>. The <i>BraSWEET10</i> gene was cloned into <i>B. rapa</i> (Longyou-7, L7). The BraSWEET10 protein contained seven transmembrane structural domains. Real-time fluorescence quantitative PCR (qRT-PCR) showed that the <i>BraSWEET10</i> gene responded to low-temperature stress. BraSWEET10 was localized to the cell membrane. The root length of overexpressing transgenic <i>A. thaliana</i> was significantly higher than that of wild-type (WT) <i>A. thaliana</i> under low temperatures. Our findings suggest that this gene may be important for the adaptation of winter <i>B. rapa</i> to low-temperature stress. Overall, the findings are expected to contribute to understanding the evolutionary links of the BraSWEET family and lay the foundation for future studies on the functional characteristics of <i>BraSWEET</i> genes.