The Plant Cell, Vol. 7, 1071-1083, July 1995 O 1995 American Society of Plant Physiologists Genetics and Biochemistry of Ant hocyanin Biosynthesis Timothy A. Holton’ and Edwina C. Cornish Florigene Proprietary Ltd., 16 Gipps Street, Collingwood, Victoria 3066, Australia INTRODUCTION Flavonoids represent a large class of secondary plant metab- centrate on the more recent developments in gene isolation olites, of which anthocyaninsare the most conspicuous class, and characterization.A review of the genetics of flavonoid bio- dueto the wide range of colors resulting from their synthesis. synthesis in other species was recently covered by Forkmann Anthocyanins are important to many diverse functions within (1993). plants. Synthesis of anthocyanins in petals is undoubtedly in- The characterization of genetically defined mutations has tended to attract pollinators, whereas anthocyanin synthesis enabled the order of many reactions in anthocyanin synthesis in seeds and fruits may aid in seed dispersal. Anthocyanins and their modification to be elucidated. Some reactions have and other flavonoids can also be important as feeding deter- been postulated only on the basis of genetic studies and have rents and as protection against damage from UV irradiation. not yet been demonstrated in vitro. Chemico-genetic studies The existence of such a diverse range of functions and types have been very important in determining the enzymatic steps of anthocyanins raises questions about how these compounds involved in anthocyanin biosynthesis and modification. The are synthesized and how their synthesis is regulated. generation of transposon-tagged mutations and the subse- The study of the genetics of anthocyanin synthesis began quent cloning of the transposons provided a relatively last century with Mendel’s work on flower color in peas. Since straightforward means of isolating many genes from maize that time, there have been periods of intensive study into the (Wienand et al., 1990) and snapdragon (Martin et al., 1991). genetics and biochemistry of pigment production in a num- However, a number of genes in the pathway have not been ber of different species. In the early studies, genetic loci were amenable to transposon tagging. correlated with easily observable color changes. After the struc- Anthocyanin biosynthetic genes have been isolated using tures of anthocyanins and other flavonoids were determined, a range of methodologies, including protein purification, trans- it was possible to correlate single genes with particular struc- poson tagging, differential screening, and polymerase chain tural alterations of anthocyanins or with the presence or reaction (PCR) amplification. Functions of isolated anthocya- absence of particular flavonoids. Mutations in anthocyanin nin genes can be confirmed by restriction fragment length genes have been studied for many years because they are polymorphism (RFLP) mapping, complementation,or expres- easily identified and because they generally have no deleteri- sion in heterologoussystems. Reverse genetics has also been ous effect on plant growth and development. In most cases, used recently to identify gene function; this requires a well- mutations affecting different steps of the anthocyanin biosyn- defined pathway to correlate phenotype with gene function. thesis pathway were isolated and characterized well before Once a gene has been isolated from one species, it is usually their function was identified or the corresponding gene was a straightforward task to isolate the homologous gene from isolated. More recently, many genes involved in the biosyn- other species by using the original clone as a molecular probe. thesis of anthocyanin pigments have been isolated and characterized using recombinant DNA technologies. Three species have been particularly important for elucidat- ing the anthocyanin biosynthetic pathway and for isolating ANTHOCYANIN BIOSYNTHETIC PATHWAY genes controlling the biosynthesis of flavonoids: maize (Zea mays), snapdragon (Anfirrhinum majus), and petunia (Wtunia The anthocyaninbiosynthetic pathway is well established (MOI hybrida).Petunia has more recently become the organism of et al., 1989; Forkmann, 1991). A generalized anthocyanin choice for isolating flavonoid biosynthetic genes and study- biosynthetic pathway is shown in Figure 1. Although the bio- ing their interactions and regulation. At least 35 genes are synthetic pathways in maize, snapdragon, and petunia share known to affect flower color in petuniawiering and de Vlaming, a majority of common reactions, there are some important 1984). Because this field of research has been reviewedfairly differences between the types of anthocyanins produced by extensively in the past (Dooner et al., 1991; van Tunen and each species. One major difference is that petunia does not MOI, 1991; Gerats and Martin, 1992), in this review we con- normally produce pelargonidin pigments, whereas snapdra- ‘To whom correspondence should be addressed. gon and- maize are incapable bf producing delphinidin 1072 The Plant Cell 3 x Malonyl-COA p-Coumaroy I-COA CHS OH O hydroxychalcone ÓH Ò Naringenin OHKaempferol O Homo&OH /-- -0H Dihydrokaempferol 7-H 'OH / OH O \ OU A MyricetG Quercetin - Ty.OH OH 0 Dihydroquercetin OH O Dihydromyricetin 1DFR 1DFR II ÒH ÒH OH OH OH OH Leucopelargonidin Leucocyanidin Leucodelphinidin ANS I 3GT i .OH OH OH OH Pelargonidin-3-glucoside Cyanidin-3-glucoside Delphinidin-3-glucoside Figure 1. Anthocyanin and Flavonol Biosynthetic Pathway. Anthocyanin Biosynthesis 1073 pigments. The extent of modificationof the anthocyanins also by flavonoid 3'-hydroxylase (F3'H) to produce dihydroquerce- varies among the three species. The reasons for these differ- tin (DHQ) or by flavonoid 3;5'-hydroxylase (F33'H) to produce ences are discussed in the following outline of the enzymes dihydromyricetin (DHM). F33'H can also convert DHQ to DHM. and genes involved in anthocyanin biosynthesis. At least three enzymes are required for converting the color- The precursors for the synthesis of all flavonoids, including less dihydroflavonols (DHK, DHQ, and DHM) to anthocyanins. anthocyanins, are malonyl-COA and p-coumaroyl-COA.Chal- The first of these enzymatic conversions is the reduction of cone synthase (CHS) catalyzes the stepwise condensation of dihydroflavonols to flavan9,4-cis-diols (leucoanthocyanidins) three acetate units from malonyl-COA with p-coumaroyl-COA by dihydroflavonol4-reductase(DFR). Further oxidation, dehy- to yield tetrahydroxychalcone.Chalcone isomerase (CHI) then dration, and glycosylation of the different leucoanthocyanidins catalyzes the stereospecific isornerization of the yellow-colored produce the corresponding brick-redpelargonidin, red cyani- tetrahydroxychalcone to the colorless naringenin. Naringenin din, and blue delphinidin pigments. Anthocyanidin3-glucosides is converted to dihydrokaempferol (DHK) by flavanone may be rnodified further in many species by glycosylation, 3-hydroxylase(F3H). DHK can subsequently be hydroxylated methylation, and acylation, as illustrated in Figure 2. There .OH PH "-0H "-0H Oclc "mHOHOClC OH Cyanidin-Sglucoside Delphinidin-Sglucoside PH oH Cyanidin-3-rutinoside oH Delphinidin-3-rutinoide 1. OH PH "-0. "-0. O-Glc-Rha OClc-ORh9H Glc-Ò Cyanidin-3-(pcoumaroyl)- "'-0 Delphinidin-b(p-coumaroyl)- ru tinoside-Sglucoside ru tinoside-5glucoside Mtf,MtZ IMt 1,MtZ Mf\ I,MfZ I PCH O-Glc-Rha I Glc-Ò Glc-O HowoHPeonidin->(pcoumacoyI> Glc-o Petunidin->(pcoumaroyl)- Malvidin-S(p-coumaroyl)- rutinoside-5-glucoside rutinoside-5-glucoside rutinoside-Sglucoside Figure 2. Genetic Control of Anthocyanin Modifications in Petunia. Genetic loci controlling each enzymatic step of the pathway are shown in italics. No genetic locus encoding the 5GT gene has yet been identified. Glc, glucose; Rha, rhamnose. 1074 The Plant Cell are both species and variety differences in the extent of modifi- Two genes encode CHS in maize: c2 is involved in anthocya- cation and the types of glycosides and acyl groups attached. nin biosynthesis in seed (Dooner, 1983; Wienand et al., 1986), and whp controls CHS activity in pollen (Coe et al., 1981). The maize c2 gene was isolated followingtransposon tagging using STRUCTURAL GENES the En (Spm) transposable element (Wienand et al., 1986). Precursorfeeding studies and enzyme activity measurements in different c2 mutant lines indicatedthat c2 might encode CHS. Many genes encoding anthocyanin biosynthetic enzymes have A cDNA clone corresponding to the c2 locus was sequenced been characterized and cloned frorn maize, snapdragon, and and shown to have a high sequence similarity with the pars- petunia. Table 1 summarizes the genetic loci and structural ley CHS clone. A second CHS gene, whp, has subsequently genes isolated from each of these species. been isolated from maize (Franken et al., 1991). The nivea locus controls CHS enzyme activity in flowers of snapdragon (Spribille and Forkmann, 1982). A CHS gene from Chalcone Synthase snapdragon was isolated by hybridization to the parsley CHS clone (Sommer and Saedler, 1986). The snapdragon genome The first flavonoid biosynthetic gene isolated was the CHS gene contains only one CHS gene. from parsley (Kreuzaler et al., 1983). A cornbinationof differen- tia1 screening and hybrid-arrested and hybrid-selected translation was used to identify a cDNA clone homologous to Chalcone lsomerase CHS mRNA. The parsley CHS clone was also used as a mo- lecular probe to isolate clones of two different CHS genes from The accumulationof chalcone in plant tissues is