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.