[MAIN PAGE][INTRODUCTION][GENERAL PATHWAY ILLUSTRATION]
[PETUNIA][MAIZE][GRAPE][ARABIDOPSIS]
Courtney-Gutterson (1994) summarizes three major "blocks" leading to anthocyanin products. Chalcone synthase commits the products of the phenylpropanoid pathway to flavonoid biosynthesis. As a result, dihydrokaempferol is produced. At this point, a variety of side reactions can convert the flavonoid intermediates into flavones and flavonols, which can interact with anthocyanins to modify flower color. The specific conversion of flavonoid to anthocyanin is accomplished in a series of four steps. The final step is the addition of glucose to the 3'-hydroxyl group of an anthocyanidin. This glycosylation is essential for stable expression of color. These biosynthetic reactions occur in the cytosol, however, anthocyanins accumulate in vacuoles.
The final color an anthocyanin expresses is dependent on one or more of three additional reactions. These include addition of hydroxyl or additional sugar(s) groups to the 3' and or 5' positions or methylation of the B-ring hydroxyls at the 3' and/or 5' positions. Anthocyanins are produced in many plant organs, including seeds, flower organs such as anthers and petal limbs, and pollen.
Several organisms share conserved genes for anthocyanin biosynthesis. These were summarized in a review by Dooner et al. 1991.
Table C1: Enzymes of the anthocyanin biosynthetic pathway for which genes have been defined mutationally and/or isolated molecularly in maize, Antirrhinum, and Petunia (from Dooner et al. 1991).
MAIZE ANTIRRHINUM PETUNIA Enzyme Locus Clone Locus Clone Locus Clone CHS c2 + niv + - + whp + CHI - + - + po + F3H - - inc + an3 + DRF a1 + pal + an6 + *a a2 + canb + - - UF3GT bz1 + - + - - a enzyme between leucoanthocyanidin and anthocyanidin was unknown in 1991, but is now known to be anthocyanidin synthase 3-glucosyltransferase
b can is used as abbreviation for candica
Abbreviation Enzyme Name Compound Produced CHS Chalcone Synthase Yellow Chalcone CHI Chalcone Isomerase Colorless Flavanone F3H Flavanone 3-Hydroxylase Colorless Leucoanthocyanidin DFR Dihydroflavonol 4-reductase Colorless Leucoanthocyanidin *a now known as AS 3-GT anthocyanidin UF3GT UDP:glucosyltransferase anthocyanidin
The regulation of these loci has been determined, with regulation in maize occurring at all levels of anthocyanin biosynthesis and in a subset of the pathway in snapdragon and petunia. Such results have indicated conserved regulatory anthocyanin genes between species and suggest that divergent evolution of the target gene promoters is responsible for the species-specific differences in regulatory networks. Analysis of distribution of flavonoids among plant species at conserved homologous sequences has allowed proposed models in the evolution of flavonoid biosynthesis (Quattrocchio et al. 1993, Quattrocchio et al. 1998).
Specifically, we have constructed the following summary for the plants we focused on in this review of anthocyanin biosynthesis.
Table C2: Enzymes of the anthocyanin biosynthetic pathway for which genes have been defined mutationally and/or isolated molecularly in maize, Arabidopsis, Petunia, and Grape (abbreviations are the same as in Table C1).
Enzyme Petunia Maize Grape Arabidopsis CHS c2 tt4 whp CHI po tt5 FS tt6 FSH an3 tt7 DFR an6 a1 tt3 AS3GT a2 UF3GT bz1
Many factors are thought to be involved in the regulation of anthocyanin biosynthesis (Table C3). Many of the following factors were specifically identified in Arabidopsis, however have application to other plant systems.
| Regulator | Species | References |
| Sugar (+) | A | Tsukaya 1991 |
| Phytohormones | ||
| Cytokinin (+) | A | Deikman and Hammer 1995 |
| Abscisic acid (-) | A | Parcy et al 1997 |
| Abscisic acid (+) | M | Quattrocchio et al. 1993 |
| Gibberellic acid | P | Quattrocchio et al. 1993 |
| Low Temperature (+) | A | Leyva 1995 |
| Blue Light (+) | A | Ahmad and Cashmore 1995 |
| Red Light (+) | A | Neff and Chory 1998 |
| Pathogens | ||
| Bacterial (+) | A | Dong 1991 |
| Viral (+) | A | Lee et al. 1994 |
| Insect Herbivory | ||
| Nutrient Deficiency (+) | A | Dixon and Paiva 1995 |
| pH (+/-) | P | Harborne 1975 |
A= Arabidopsis,
M=Maize, P=Petunia, G=Grape
NOTE: Complete references are found on the web page associated
with each species.
Most of these factors regulate the major enzymes in the biosynthetic pathway. This suggests that the major pathway may be regulated by a broad range of stimuli. Refined regulation may occur with the other enzymes in the pathway. Investigation of the branched enzymes would greatly contribute to the understanding of anthocyanin biosynthetic regulation.
Anthocyanins have long been known to act in flowers as pollinator attractants, and in fruit for animal dispersal. More recently several other funtions have been attributed to anthocyanins such as defense against pathogens, protection against UV or intense white light, and prevention of nutrient stress. Different classes of anthocyanins might be related to the various functions. Sutdies on the quality and quantity of anthocyanins in different organs would also contribute to understaning of anthocyanin functions.
In addition, further studies on differences in anthocyanin production in above-ground and below-ground plant organs should be conducted. Although much work has been completed with floral anthocyanins, they may also play a role in the diversity of colors found in tubers and storage roots.
Evolution of the flavonoid pathway and its regulators is the subject of many recent reviews. Quattrocchio et al. (1994) expand on the idea that a common ancestral gene with alteration among promoters is responsible for different pigmentation patterns among species. It appears that different sets of flavonoid biosynthetic genes appeared at distinct time points during evolution. Compatible modification in regulatory sequences have followed.
Figure 9 in their 1993 article is re-created below.
The following figure is from a recent analysis of conserved anthocyanin sequences in maize and petunia (Quattrocchio et al. 1998). The figure depicts the biosynthetic pathway and its. Enzymes are indicated in capital letters. The boxes on each side indicate regulatory loci and their interaction with structural genes. Question marks indicate that the interaction has not been verified or that the gene has not been cloned. Future research will focus on such unresolved issues and add to our general knowledge of anthocyanin biosynthesis.
Enzyme Abbreviations: CHS:chalcone sythase, CHI: chalcone flavnone isomerase, F3H: flavanone 3-hydroxylase, DFR: dihydroflavonol 4-reductase, AS: anthocyanin synthase, 3-GTflavonoid 3-glucosyltranserase: UDP-glucose , RT: anthocyanin rhamnosyltransferase, A3'MT3'-methyltransferase: anthocyanin , A3'5'MT: anthocyanin 3'5'methyltransferase, GST: glutathione-S-transferase
Genetic loci of P. hybrida: rt: rhamnosylation at three, mt: methylation at three, mf: mehtylation at five, an9: anthocyanin-9, an1: anthocyanin-1, an2: anthocyanin-2, an4: anthocyanin-4, an11: anthocyanin-11
Genetic loci of Z. mays: c2: color-2, a1: anthocyanin-1, a2: anthocyanin-2, bz1: bronze-1, bz2: bronze-2
(By: Merrilee Anderson)
REFERENCES
Courtney-Gutterson, N. 1994. The biologist's palette: genetic engineering of anthocyanin biosynthesis and flower color. In, Genetic Engineering of Plant Secondary Metabolism, Ellis, B.E., G.W. Kuroki, and H.A. Stafford, eds. Plenum Press NY. 93:124.
Dooner, H.K., T.P. Robbins, and R.A. Jorgensen. 1991. Annu. Rev. Genet. 25:173-199.
Quattrocchio, F., J.F. Wing, H.T.C. Leppen, J. N.M. Mol, and R.E. Koes. 1993. Regulatory genes controlling anthocyanin pigmentation are functionally conserved among plant species and have distinct sets of target genes. Plant Cell. 5:1497-1512.
Quattrocchio, F., J.F. Wing, K. van der Woude, J.N.M. Mold, and R. Koes. 1998. Analysis of bHLH and MYB domain proteins: species-specific regulatory
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