An indigo-producing plant, Polygonum tinctorium, possesses a flavin-containing monooxygenase capable of oxidizing indole

Polygonum tinctorium (P. tinctorium) is an indigo plant that is cultivated for a specific metabolite that it produces i.e., indoxyl β-D-glucoside (indican). In this study, flavin-containing monooxygenase (PtFMO) from P. tinctorium was cloned. When recombinant PtFMO was expressed in E. coli in the presence of tryptophan, indigo production was observed. Furthermore, we measured the activity of PtFMO using the membrane fraction from E. coli and found that it could produce indigo using indole as a substrate. The co-expression of PtFMO with indoxyl β-D-glucoside synthase (PtIGS), which catalyzes the glucosylation of indoxyl, brought about the formation of indican in E. coli. The results showed that indican was synthesized by sequential reactions of PtFMO and PtIGS. In three-week-old P. tinctorium specimens, the first leaves demonstrated higher levels of PtFMO expression than the subsequent leaves. This result coincided with that of our prior study on PtIGS expression level. Our study provides evidence that PtFMO might contribute to indican biosynthesis.