Overexpression of Lotus NnSWEET4a Alters Sugar Homeostasis and Induces Salt Hypersensitivity in Arabidopsis

SWEET (Sugars Will Eventually be Exported Transporter) proteins constitute a conserved family of sugar transporters that play pivotal roles in carbohydrate allocation and stress responses. In this study, we systematically identified 14 SWEET homologs in the genome of sacred lotus (Nelumbo nucifera) and validated their transport activity for both hexoses and sucrose. Subsequent analysis revealed that stress-responsive elements are the most enriched promoter sequences of NnSWEET genes. Quantitative expression profiling of the members found that NnSWEET4a was strongly upregulated under salt stress. NnSWEET4a was localized at the plasma membrane; its expression conferred salt sensitivity in both yeast and Arabidopsis thaliana. Transgenic Arabidopsis lines overexpressing NnSWEET4a exhibited substantial downregulation of the SOS3–SOS2–SOS1 signaling module and concomitant alterations in cellular sugar homeostasis. Further analysis revealed that exogenous sugar application aggravated salt sensitivity and SOS pathway inhibition in NnSWEET4a transgenic plants, and NnSWEET15-overexpressing Arabidopsis recapitulated identical phenotypic and molecular responses, including salt sensitivity and repression of SOS genes. These findings indicate that NnSWEET4a impairs salt tolerance through disruption of sugar homeostasis. The results establish a mechanistic framework for future investigations into SWEET-dependent regulation of sugar homeostasis and salt stress adaptation.