Fig. 2

A proposed biosynthetic network of C-glycosylated apigenin in rice. a UDP-arabinose (Ara) is generated from glucose-1-phosphate (Glc-1-P) via UDP-glucose (Glc) and UDP-xylose (Xyl). Grey arrows represent the native metabolism in E. coli, while the bold red arrows represent an introduced heterologous UDP-Ara pathway. Glc-6-P glucose-6-phosphate, Gal galactose, GlcA glucuronate, UGPase UDP-glucose pyrophosphorylase, Uge UDP-glucose 4-epimerase, Ugd UDP-glucose 6-dehydrogenase, Uxs UDP-xylose synthase/UDP-glucuronic acid decarboxylase, Uxe UDP-xylose 4-epimerase. b A proposed C-glycosylated apigenin pathway starting from naringenin (Nar). Purple arrows represent 2-hydroxylation by flavanone 2-hydroxylases (F2H) and equilibrium of ring-open/closed 2-hydroxynaringenin (2-OHNar). Black arrows indicate two-step glycosylation reactions in (iso)schaftoside biosynthesis (Sch). Glycosylation reactions forming apigenin 6,8-di-C-arabinoside (Api-di-C-Ara) are indicated in sky blue arrows. After the formation of C-glycosylated intermediates, dehydration reactions (green arrows) occur spontaneously in acidic solvent or by dehydratases (DH), producing a mixture of 6-C- or 8-C-isomers. Api apigenin, 2OHNar 2-hydroxylnaringenin, Vit vitexin, Isovit isovitexin