Anthocyanins are major color pigments in plants. Their biosynthetic pathways are well established, and the majority of these biosynthetic enzymes have been identified in model plants such asArabidopsis, maize, and petunia. One exception inArabidopsis is UDP-glucose:flavonoid 3-O-glucosyltransferase (UF3GT). This enzyme is known as Bronze-1 (Bz1 ) in maize, where it converts anthocyanidins to anthocyanins. Phylogenetic sequence analysis of theArabidopsis thaliana UDP-glycosyltransferase (UGT) family previously indicated that UGT78D1, UGT78D2, and UGT78D3 cluster together with UF3GTs from other species. Here, we report thatUGT78D2 encodes a cytosolic UGT that is functionally consistent with maize Bz-1. Biochemically, UGT78D2 catalyzes the glucosylation of both flavonols and anthocyanidins at the 3-OH position. A T-DNA-insertedugt78d2 mutant accumulates very little anthocyanin and lacks 3-O-glucosylated quercetin. Expression analysis indicated thatUGT78D2, in opposite toBANYULS, is highly expressed in anthocyanin-accumulating seedlings but repressed in condensed tannin-accumulating seed coats. This suggests that the reciprocal regulation of these two genes is important in directing the metabolic flux to either anthocyanins or condensed tannins. Consistent with this, the ectopic expression of UGT78D2 produces purple-colored seed coats due to the accumulation of anthocyanins. Taken together, our data indicate thatUGT78D2 encodes an enzyme equivalent to maize Bz1, and that the reciprocal regulation of UGT78D2 and BANYULS is critical for the regulation of metabolic flux of anthocyanidins inArabidopsis.

Arabidopsis, flavonoid, metabolic flux, anthocyanin, UF3GT, UCT78D2

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
3	bDGlcp	103.81	75.68	78.01	71.20	78.38	62.50
7	aLRhap	99.77	71.63	72.04	73.55	71.20	18.13
	xXQuercetin


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
3	bDGlcp	5.32	3.48	3.42	3.33	3.22	3.56-3.71
7	aLRhap	5.56	4.02	3.82	3.47	3.58	1.24
	xXQuercetin


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
3	bDGlcp	103.81/5.32	75.68/3.48	78.01/3.42	71.20/3.33	78.38/3.22	62.50/3.56-3.71
7	aLRhap	99.77/5.56	71.63/4.02	72.04/3.82	73.55/3.47	71.20/3.58	18.13/1.24
	xXQuercetin

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

C[C@@H]1O[C@@H](Oc2cc(O)c3c(=O)c(O[C@@H]4O[C@H](CO)[C@@H](O)[C@H](O)[C@H]4O)c(-c4ccc(O)c(O)c4)oc3c2)[C@H](O)[C@H](O)[C@H]1O

610.521 g/mol (C₂₇H₃₀O₁₆)