Tissue-specific and intracellular localization of indican synthase from Polygonum tinctorium

The plant Polygonum tinctorium produces the secondary metabolite indican (indoxyl-β-D-glucoside), a precursor of the blue dye indigo. P. tinctorium synthesizes indican through the actions of the UDP-glucosyltransferase (UGT), indican synthase. Herein, we partially purified an indican synthase from the leaves and subsequently performed peptide mass fingerprinting analysis. Consequently, we identified a fragment that was homologous to a UDP-glucosyltransferase 72B (UGT72B) family member. We named it PtIgs (P. tinctoriumindoxyl-β-D-glucoside synthase) and obtained the full-length cDNA using rapid amplification of the cDNA ends. The primary structure of PtIGS, which PtIgs encoded, showed high identity with indican synthases (ItUGT1 and ItUGT2) from Indigofera tinctoria (Inoue et al., 2017). Moreover, in expression analyses of P. tinctorium, PtIGS mRNA was virtually found only in the leaves, was most highly expressed in the 1st leaves, and decreased with leaf age. Because PtIGS expression tended to reflect indican contents and synthesis activities, we concluded that PtIGS functions as an indican synthase in plant cells. To examine intracellular localization of PtIGS, crude leaf extracts were separated into cytosol and microsome fractions, and found PtIGS in the cytosol and in microsome fractions. Furthermore, microsomal PtIGS was soluble in the presence of detergents and urea and was strongly associated with membranes. Finally, we confirmed endoplasmic reticulum (ER) membrane localization of PtIGS using ultracentrifugation with a sucrose density gradient. These data suggest that PtIGS interacts with some kind of proteins on ER membranes to certainly carry out a delivery of substrate.