Genome-Wide Identification of the Two-Pore Potassium Channel Gene Family in Sugar Beet (Beta vulgaris L.) and Functional Analysis of BvTPK2 in Salt Tolerance

Tandem-pore potassium (TPK/KCO) channels play important roles in plant salt tolerance and K+ homeostasis. However, the TPK/KCO gene family has not been systematically characterized in sugar beet (Beta vulgaris L.), the world’s second-largest sugar crop, and its functions in salt-stress responses remain largely unknown. In this study, six TPK/KCO family members were identified from the sugar beet genome and systematically analyzed by bioinformatics and expression analyses. The BvTPK genes were unevenly distributed across three chromosomes, designated BvTPK1–BvTPK6, and contained characteristic Ion_trans_2 and PRK10537 domains. Phylogenetic analysis classified TPK/KCO proteins into three major groups, with BvTPK1, BvTPK2, BvTPK3, and BvTPK6 in Group II, BvTPK5 in Group I, and BvTPK4 in Group III. The promoter regions of BvTPKs contained numerous cis-regulatory elements associated with light, hormone, and stress responses. Transcriptomic and RT–qPCR analyses revealed that several BvTPK genes responded to salt stress, with BvTPK2 showing strong salt-induced expression in leaves. Subcellular localization confirmed that BvTPK2 was localized to the tonoplast. Moreover, heterologous overexpression of BvTPK2 enhanced seed germination and survival, improved plant growth and fresh and dry weights, and maintained better photosynthetic performance in Arabidopsis under salt stress, indicating that BvTPK2 positively contributes to salt tolerance. The overexpressing lines also exhibited reduced Na+ accumulation, enhanced K+ retention, and lower Na+/K+ ratios, indicating improved ionic homeostasis under salt stress. Collectively, these findings provide a characterization of the TPK/KCO gene family in sugar beet and identify BvTPK2 as a key candidate gene associated with salt tolerance.