Unraveling the Mechanism of 1-Deoxynojirimycin (DNJ) Accumulation: The Role of SWEET3 in Mulberry Chloroplasts

Transporters are vital for metabolite accumulation in plants. 1-Deoxynojirimycin (DNJ), a medicinal compound, is mainly derived from mulberry leaves; yet, its supply remains limited. Here, we visualized DNJ localization using a fluorescent probe and found that DNJ predominantly accumulates in the chloroplasts of mulberry leaves but not in callus tissue. The sugar transporter gene SWEET3, highly expressed in leaves, was identified to mediate DNJ transport. Molecular docking revealed that acidic residues E68 and D126 are key DNJ-binding sites and their mutation impaired transport activity. Transient overexpression or RNA interference of SWEET3 in mulberry leaves significantly increased or decreased DNJ levels, respectively, while stable overexpression in hairy roots enhanced DNJ accumulation. Our findings demonstrate that SWEET3 is essential for DNJ transport and accumulation in mulberry and provide a molecular basis for developing high-DNJ varieties through transporter-targeted genetic improvement.