ANRV274-PP57-29 ARI 29 March 2006 12:31 The Genetics and Biochemistry of Floral Pigments Erich Grotewold Department of Plant Cellular and Molecular Biology, Plant Biotechnology Center, Ohio State University, Columbus, Ohio 43210; email: [email protected] Annu. Rev. Plant Biol. Key Words 2006. 57:761–80 anthocyanins, betalains, carotenoids, pollination, flower color The Annual Review of Plant Biology is online at plant.annualreviews.org Abstract doi: 10.1146/ Three major groups of pigments, the betalains, the carotenoids, and annurev.arplant.57.032905.105248 the anthocyanins, are responsible for the attractive natural display Copyright c 2006 by of flower colors. Because of the broad distribution of anthocyanins Access provided by Iowa State University on 02/04/20. For personal use only. Annual Reviews. All rights (synthesized as part of the flavonoid pathway) among the flowering Annu. Rev. Plant Biol. 2006.57:761-780. Downloaded from www.annualreviews.org reserved plants, their biosynthesis and regulation are best understood. How- First published online as a ever, over the past few years, significant progress has been made in Review in Advance on February 7, 2006 understanding the synthesis and participation of carotenoids (de- rived from isoprenoids) and betalains (derived from tyrosine) in 1543-5008/06/0602- 0761$20.00 flower pigmentation. These three families of pigments play impor- tant ecological functions, for example in the attraction of pollinating animals. Anthocyanins in particular have also been the target of nu- merous biotechnological efforts with the objective of creating new, or altering the properties of existing, coloring compounds. The fo- cus of this review is to examine the biosynthesis, regulation, and contribution to flower coloration of these three groups of pigments. 761 ANRV274-PP57-29 ARI 29 March 2006 12:31 “tulipomania” for which records are preserved Contents in many classical paintings (47). Florigene’s Moonseries of genetically engineered car- INTRODUCTION................. 762 nations (http://www.florigene.com/), mar- BETALAINS ....................... 762 keted in the United States, Australia, Canada, Biosynthesis ...................... 762 Japan, and some European countries, provide Occurrence and the Mutual the first genetically engineered commercial Exclusion with the flowers. Anthocyanins ................. 765 Three types of chemically distinct pig- CAROTENOIDS ................... 766 ments, betalains, carotenoids, and antho- Functions ........................ 766 cyanins are responsible for the colors of flow- Biosynthesis ...................... 766 ers (Figure 1). Of the three, the anthocyanins ANTHOCYANINS ................. 768 have been studied the most in the context of Biosynthesis ...................... 768 flower pigmentation, reflecting their broader Transport and Storage ............ 771 distribution among the angiosperms. There Regulation ....................... 772 are several excellent reviews documenting CELLULAR ARCHITECTURE, the complex biochemistry and distribution of pH, AND PIGMENTATION .... 772 flavonoid pigments (e.g., 16, 30, 32, 82) and FLOWER PIGMENTATION AS A their biosynthesis and regulation (e.g., 56, VISUAL CUE ................... 773 70a, 101, 102). Much of the molecular infor- mation available on the regulation and biosyn- thesis of flower pigments derives from stud- ies performed in model systems that include INTRODUCTION maize, Arabidopsis, petunia, and snapdragon. Fruit and flower colors are of paramount However, nonclassical plant models continue importance in the ecology of plants and in to provide unique insights into the ecophys- their ability to attract pollinators and seed- iological regulation and functions of flower dispersing organisms (43). In addition, plants pigmentation. This review describes recent DOPA: dihydroxyphenyl- also play an important aesthetic function by advances in our understanding of the biosyn- alanine providing flowers with a broad spectrum of thesis, storage, and regulation of the different colors. Not surprisingly, ornamentals were flower pigments. among the first plants to be hybridized to alter specific color traits, and fruit and flower color have contributed to elucidating fundamen- BETALAINS tal genetic principles. Today, the market for Access provided by Iowa State University on 02/04/20. For personal use only. Biosynthesis Annu. Rev. Plant Biol. 2006.57:761-780. Downloaded from www.annualreviews.org ornamental plants and cut flowers is rapidly expanding and totals over $70 billion in an- Betalains are water-soluble, nitrogen- nual sales (5). Although increasing posthar- containing compounds synthesized from vest life, altering scent, and modifying flower tyrosine by the condensation of betalamic shape are areas where progress is being ac- acid (Figure 2), a central intermediate in the tively pursued, much of the novelty in the formation of all betalains, with a derivative cut flower industry continues to be targeted of dihydroxyphenylalanine (DOPA). This toward the generation of new colors (87). reaction results in the formation of the red The high stakes associated with the devel- to violet betacyanins, such as those found opment of new floral traits are best exempli- in red beets or in the flowers of portulaca fied by the passions awaked in the seventeenth (Figure 1a). The condensation of betalamic century by the unique pigmentation patterns acid with an amino acid (e.g., Ser, Val, of the “broken tulips,” which led to the Leu, Iso, and Phe) or amino acid derivative 762 Grotewold ANRV274-PP57-29 ARI 29 March 2006 12:31 Figure 1 Flower displaying the three major types of pigments and the corresponding structures. (a) Portulaca (Portulaca grandiflora) flowers accumulating primarily the betalain pigment, betanin (R1 = R2 = H). (b) Marigold (Tagetes patula) flowers accumulating the carotenoid pigment, lutein. (c) Petunia (Petunia hybrida) flower accumulating an anthocyanidin, cyanidin. Pictures were kindly provided by the Missouri Botanical PlantFinder and F. Quattrocchio. (e.g., 3-methoxytyramine) (Figure 2) The conversion of tyrosine to DOPA results in the formation of the yellow to (Figure 2) is carried out by a tyrosinase- orange betaxanthins. Betacyanins and be- type phenoloxidase (84), a group of copper- Access provided by Iowa State University on 02/04/20. For personal use only. Annu. Rev. Plant Biol. 2006.57:761-780. Downloaded from www.annualreviews.org taxanthins can be further classified into containing bifunctional enzymes involved in several subclasses, based on the chemical the hydroxylation of phenols to o-diphenols. characteristics of the betalamic acid conjugate In addition to participating in the formation of (86). Recent advances in the separation and the betalamic acid core, the tyrosinase enzyme analysis of betalains, which are unstable also oxidizes DOPA to dopaquinone, con- under the acidic conditions normally used for tributing to the biosynthesis of cyclo-DOPA, Nuclear Magnetic Resonance (NMR) spectra which conjugates with betalamic acid to form analyses, are likely to shed additional light the chromophore of all betacyanins, betani- on the existence of novel conjugates (85). din (86). The formation of betalamic acid As is common for many other phytochem- from DOPA requires the extradiol cleavage icals, light and hormones have a dramatic of the 4,5 bond carried out by a DOPA dioxy- effect on the accumulation of betalains genase, first identified in the basidiomycete (70). fly agaric (Amanita muscaria) (35). The plant www.annualreviews.org • Flower Pigmentation 763 ANRV274-PP57-29 ARI 29 March 2006 12:31 O OH NH2 HO Tyrosine Tyrosinase O HO OH NH2 HO Dopa Dopa 4.5-dioxygenase Dopa decarboxylase OH H2N O Tyrosinase O O NH2 OH O NH2 HO O HO OH OH O Dopaquinone 4,5-seco-Dopa 3-Methoxytyramine O OH O HO H N O O O N H - H N HO O OH OH 2 cyclo-Dopa Betalamic acid 3-Methoxytyramine Figure 2 Schematic representation of HO O the biosynthetic O- HO N+ HO NH+ pathway of some O betalain pigments. The known O O O O enzymes are N N H H indicated in black OH OH OH OH boxes and the compound names Betanidin Betaxanthin are shown. (3-methoxytyraine betaxanthin) enzyme was subsequently cloned by a subtrac- The introduction of the DOPA dioxygenase tive cDNA approach using Portulaca grandi- from Amanita muscaria into Portulaca grandi- Access provided by Iowa State University on 02/04/20. For personal use only. Annu. Rev. Plant Biol. 2006.57:761-780. Downloaded from www.annualreviews.org flora isogenic lines with different color phe- flora petals by particle bombardment resulted notypes (7). The plant enzyme exhibits no in the accumulation of various betalains, and obvious sequence or structural similarity with also of muscaflavin, a pigment normally not the fungal enzymes. Moreover, the plant found in plants (59), which is synthesized by enzyme displays regiospecific extradiol 4,5- the extradiol ring cleavage of the 2,3 bond dioxygenase (7), in contrast to the 2,3- and followed by recyclization into the 6-atom, 4,5 dioxygenase activity of the Amanita mus- N-containing ring muscaflavin. caria enzyme (35). The 4,5-seco-DOPA is sub- The next step in the biosynthesis of be- sequently recyclized, a step likely to occur talains involves the formation of an aldimine spontaneously (86). This different activity of link between betalamic acid and cyclo-DOPA the plant and fungal enzymes permits Amanita (to make betanidin) or an amino acid deriva- muscaria to accumulate muscaflavin, in ad- tive (to make betaxanthin) (Figure 2). No dition to betalain, in the cuticle