Molecular Biotechnology: Principles and Applications - Glick, B., & Pasternak, J. 2002

Molecular Biotechnology of Microbiological Systems
Plant Genetic Engineering: Applications
Modification of Flower Color

Flower growers are constantly striving to develop plant varieties with more attractive blooms and a longer vase life after cutting. Over the years, traditional crossbreeding Methods have produced thousands of new cultivars differing in flower color and shape. However, plant breeding is a tedious and time-consuming process constrained by the Gene pool of a particular species; thus, for example, no one has yet succeeded in breeding a truly blue rose. As an alternative, genetic manipulation of the Enzymes involved in anthocyanin Biosynthesis can be employed to engineer flowers with novel colors. Anthocyanins, which belong to the flavonoid Class of compounds, are the most widespread floral pigments. They are synthesized from The amino acid phenylalanine through a series of enzymatic reactions. The flower color is determined by The chemical properties of their side chains, with cyanidin derivatives responsible for red hues and delphinidin derivatives for blue (Fig. 18.15).

Petunia dihydroflavonol 4-reductase catalyzes The conversion of colorless dihydroquercetin into cyanidin 3-glucoside, a red compound, and colorless dihydromyricetin into blue delphinidin 3-glucoside, but it cannot utilize colorless dihydrokaempferol as a substrate (Fig. 18.15). However, upon transformation of petunia with the maize dihydroflavonol 4-reductase gene, its flowers acquire a brick-red coloration. This unusual color, never observed before in petunias, results from the synthesis of pelargonidin 3-glucoside from dihydrokaempferol in the transgenic plant.

Approximately 70% of the commercial floriculture industry relies on just four crops: roses, carnations, tulips, and chrysanthemums. Consequently, efforts to generate genetically modified plants with altered flower colors have focused primarily on these species. For example, transgenic chrysanthemums have been developed carrying sense and antisense cDNA constructs for chalcone synthase. This enzyme catalyzes the first step in anthocyanin biosynthesis (Fig. 18.15). Researchers hypothesized that both sense and antisense cDNAs would suppress chalcone synthase Gene Expression IN the Transgenic Plants. "Sense suppression," also known as "cosuppression," involves the inhibition of the accumulation of a specific mRNA in the presence of an extra copy of the endogenous gene. The Molecular Basis of this phenomenon is still not fully understood. Conversely, antisense chalcone synthase RNA blocks the Translation of endogenous chalcone synthase mRNA. The sense and antisense constructs, driven by the cauliflower mosaic virus 35S promoter, were inserted into a Ti plasmid-based binary vector and introduced into plant Cells. Out of 133 "sense" transformants and 83 "antisense" transformants, three of each group produced white flowers, indicating the suppression of endogenous chalcone synthase gene expression and, consequently, the inhibition of anthocyanin synthesis. These white-flowered plants were vegetatively propagated by cuttings in the field, and approximately 90–98% of them continued to produce white rather than pink flowers. This work marks a significant milestone in breeding commercially valuable new ornamental varieties with unique coloration.

Fig. 18.15. Anthocyanin biosynthesis. Abbreviations: CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3'H, flavonoid 3'-hydroxylase; F3'5'H, flavonoid 3',5'-hydroxylase; DFR, dihydroflavonol 4-reductase; 3GT, UDP-glucose:flavonoid 3-O-glucosyltransferase. Petunia DFR can catalyze the conversion of dihydroquercetin to cyanidin 3-glucoside and dihydromyricetin to delphinidin 3-glucoside, a blue compound. Maize DFR catalyzes the synthesis of pelargonidin 3-glucoside, a brick-red compound, from dihydrokaempferol.