Bulbous Plants: Biotechnology, pp. 230-232  (2014)
edited by Kishan Gopal Ramawat, Jean-Michel Mérillon

Genetic Control of Anthocyanin Synthesis in Dahlia (Dahlia variabilis)
Sho Ohno, Ayumi Deguchi and Munetaka Hosokawa

The range of flower color in dahlia is enormous. According to the Royal Horticultural Society, dahlia colors may be classified into 11 groups: (white, yellow, orange, bronze, flame: red or dark red, pink, lilac, lavender or mauve, purple: wines or violets blends, bicolored, and variegated). Including cultivars that do not fall into this classification, such as black cultivars, it can be said that dahlia is one of the most diversified floricultural species in the world.

The dahlia flower pigments have been studied for a long time, and three major compounds contribute to flower color variation. The first is anthocyanin, which is well known as a purple or red pigment. In dahlia, cyanin (a glycoside of cyanidin) and pelargonin (a glycoside of pelargonidin) accumulate in petals. These anthocyanins are modified with malonylation and finally accumulate as malonyl glycosides. For example, pelargonidin 3-(6'-malonylglucoside)-5-glucoside and pelargonidin 3-(6'-malonylglucoside)-5-malonylglucoside accumulate in "Biddenham Strawberry" and cyanidin 3-(6"-malonylglucoside)-5-glucoside and cyanidin 3-(6"-malonylglucoside)-5-malonylglucoside accumulate in "Shandy" (Takeda et al. 1986).

The second compound is flavone, which is observed in cultivars of almost all colors except for black or the white areas of petals in bicolor cultivars. In dahlia, two flavones, apigenin and luteolin, have been detected and these too are modified with glycosylation. Apigenin 4', 7-glucoside and luteolin 7-diglucoside have been detected in "Dandy" (Nordström and Swain 1953). The last compound is butein (6-deoxychalcone), a species of chalcone named after Butea frondosa. Butein is not widely distributed and is detected in limited genera including dahlia (Price 1939), Cosmos sulphureus (Geissman 1942), and Coreopsis  grandiflora (Geissman 1943). In dahlia, butein is accumulated as butein 4'-malonylsophoroside and butein 4'-malonylglucoside (Harborne et al. 1990). In addition to these three principal compounds, aurone and flavonol are believed to be present. Combinations of these pigments account for the wide range of flower colors. The major colors and their pigment compositions are shown in Table 1.

General regulation of flavonoid synthesis pathway

All pigments in dahlia flowers are synthesized by the flavonoid biosynthetic pathway, the best known secondary metabolite pathway in plants. Anthocyanidin, the aglycone of anthocyanin, is formed in the anthocyanin synthesis pathway by the condensation of three molecules of malonyl-CoA with one molecule of 4-coumaroyl-CoA. The enzymes involved in this pathway are chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), and anthocyanidin synthase (ANS). Generally, anthocyanidins are modified by glucosyltransferase (3GT, 5GT), malonyl transferase (MT), and/or acyl transferase (AT) and are accumulated as anthocyanin (Grotewold 2006, Tanaka et al. 2008). Flavones are synthesized from flavanone by flavone synthase (FNS). In the anthocyanidin and flavone synthetic pathway, cyanidin and luteolin are synthesized when flavonoid 3-hydroxylase (F3'H) is active, whereas pelargonidin and apigenin are synthesized when F3'H is inactive. Butein is synthesized with three molecules of malonyl-CoA and one molecule of 4-cournaroyl-CoA, the same substrate as that of tetrahydroxychalcone (chalcone), is converted to trihydroxychalcone (isoliquiritigenin) by CHS and chalcone reductase (CHR) (Bomati et al. 2005) and then hydroxylated by chalcone 3-hydroxylase (CH3H). The simplified flavonoid synthetic pathway in dahlia is shown in Fig. 2.

Some genes encoding the abovementioned enzymes involved in the anthocyanin synthesis pathway are regulated by transcription factors such as basic helix-loop-helix (bHLH), R2R3-MYB, and WD4O repeats (WDR); these three proteins function as transcription factors by forming complexes or acting alone (Koes et al. 2005, Hichri et al. 2011). Thus not only structural genes in the flavonoid synthetic pathway but also transcription factors can be targets for molecular breeding.

Fig. 2. The flavonoid synthesis pathway in dahlia. The simplified biosynthesis pathway of three major compounds: anthocyanidin, flavone, and butein in dahlia. Abbreviations: ANS, anthocyanidin synthase; CH3H, chalcone 3-hydroxylase; CHI, chalcone isomerase; CHR, chalcone reductase; CHS, chalcone synthase; DFR, dihydroflavonol 4-reductase; F3H, flavanone 3-hydroxytase; F3'H, flavonoid 3'-hydroxylase; FNS, flavone synthase.