Anthocyanin biosynthesis (maize and Arabidopsis genes)

In maize there are a number of regulatory genes that mediate transcriptional activation of the anthocyanin biosynthesis pathway genes (see Selinger and Chandler (1999) for a discussion of these regulatory loci). For example, R has been shown to regulate 3 enzymes involved in anthocyanin biosynthesis; chalcone synthase [EC 2.3.1.74] encoded by C2, dihydroflavonol 4-reductase (dihydroquercitin reductase) (DFR) [EC 1.1.1.219] encoded by A1, and flavonol 3-O-glucosyltransferase (UFGT or 3GT) [EC 2.4.1.91] encoded by bronze1 (Bz1). Regulation of the anthocyanin pathway in maize requires two classes of transcription factors; one class (B and R) contains a bHLH motif, and the other (C1 and P1) contains a Myb domain. To activate the genes of the anthocyanin pathway, a protein from each class must be expressed; neither alone is sufficient for induction (Lesnick and Chandler, 1998; and references cited therein). Cell lines of maize engineered to express the C1 and R accumulate two cyanidin derivatives that are similar to the predominant anthocyanin found in differentiated tissues. In contrast, expression of P causes accumulation of 3-deoxy flavonoids (Grotewold et al, 1998).

In Arabidopsis dihydroflavonol 4-reductase (DFR) [EC 1.1.1.219] is encoded by the tt3 locus. Flavonol synthase (FLS) may be encoded by tt6, chalcone synthase (CHS) [EC 2.3.1.74] by tt4, and flavonoid 3'-hydroxylase (F3'H) [EC 1.14.13.21] by tt7 (where tt refers to transparent testa mutants) (Pelletier et al, 1997).

The A2 locus in maize may encode a dioxygenase; thus anthocyanidin synthase (ANS) may actually represent a leucoanthocyanidin dioxygenase (LDOX). The LDOX of Arabidopsis responsible for converting leucopelargonidin to pelargonidin and leucocyanidin to cyanidin, has recently been cloned (Pelletier et al, 1997). UDP-flavonol 3-O-glucosyltransferase (UFGT or 3GT) [EC 2.4.1.91] would then further elaborate pelargonidin and cyanidin to pelargonidin-3-glucoside and cyanidin-3-glucoside, respectively.

The Bronze2 (Bz2) gene in maize encodes a glutathione S-transferase [EC 2.5.1.18] that performs the last genetically defined step in anthocyanin biosynthesis -- tagging cyanidin-3-glucoside with glutathione, allowing for transport to the vacuole via a tonoplast Mg-ATP-requiring GS-X pump (Marrs and Walbot, 1997; Lu et al, 1998; Alfenito et al, 1998). The equivalent locus to Bz2 in petunia is Anthocyanin9 (An9), although the maize Bz2 gene encodes a type III GST while the petunia An9 locus encodes a type I GST (Alfenito et al, 1998).

The 3-deoxyanthocyanidin phytoalexins (apigeninidin and leuteolinidin) accumulated by Sorghum bicolor in response to inoculation with the fungus Cochliobolus heterostrophus, are thought to be derived from naringenin. Fungal inoculation leads to repression of the transcription of the genes encoding F3H, DFR and ANS, leading to inhibition of light-induced accumulation of anthocyanin. In contrast, PAL and CHS are induced. This presumably diverts metabolic flux away from anthocyanin synthesis towards naringenin and 3-deoxyanthocyanidin synthesis to meet immediate biochemical needs for plant defense (Lo and Nicholson, 1998).

The flavan 3,4-diols produced by the action of DFR are not only the precursors of anthocyanins (whose synthesis is mediated by ANS and 3GT), but also catechins and condensed tannins, synthesized by flavan 3,4-diol reductase (FDR) and condensing and polymerizing enzymes. Consequently, antisense-DFR plants of birdsfoot trefoil exhibit reduced condensed tannin levels (Robbins et al, 1998).