Table 816

Betalain Distribution in Plant Structures

Plant Structure

Color Produced

Examples

Flowers

Red, yellow, pink, and orange Yellow, red, and purple Red-purple Wide range of colors Yellow and red, among others A range of colors

Aizoaceae and Portulacaceae plants

Prickly pear

Red-beet root

Bougainvillea spp.

Amaranthus spp.

Teloxis spp.

Fruits Roots Bracts Seeds

Leaves and stems

Source: Adapted from Delgado-Vargas et al. (2000).17

3. Distribution

Betalains are restricted to higher plants to the order Caryophyllales; ten betalain-producing families have been identified: Aizoacea, Amaranthaceae, Basellaceae, Cactaceae, Chenopodiaceae, Didieraceae, Holophytaceae, Nyctaginaceae, Phytolac-caceae, and Portulacaceae. Betalains are found in different plant organs, and they are accumulated in cell vacuoles, mainly in epidermal and subepidermal tissues (Table 8.16).17 However, they are sometimes accumulated in plant stalks such as in the underground parts of red beet. Many plants accumulate betalains but only two (B. vulgaris and the prickly pear Opuntia ficus-indica) are approved to be used in food.486 They are also present in the higher fungi Amanita, Hygrocybe, and Hygrosporus.84

4. Biosynthesis: Biochemistry and Molecular Biology a. Biochemistry

Knowledge about betalain biosynthesis started with analysis of their chemical structures, with their use in feeding experiments with isotopically labeled precursors, and with their production in in vitro cell cultures.8387 However, very few enzymes involved in their synthesis have been purified and characterized.84,88 Figure 8.8 sketches the proposed biosynthetic pathway. Betalains arise from arogenate of the shikimate pathway and are considered secondary metabolites; arogenate is converted to tyrosine, an amino acid, via arogenate dehydrogenase. The tyrosine structure with its phenyl group bonded to a lateral n-propyl chain gives place to a C6-C3 unit (Figure 8.8A)4-82

The basic dihydropyridine structure in all betalains is synthesized from two molecules of tyrosine; thus, two molecules of L-5,6-dihydroxyphenylalanine (l-DOPA) are formed.8389 The hydroxylation of tyrosine to l-DOPA was recognized as the first step in the biogenesis of betalains by using radioactive precursors to label {14C}-tyrosine; evaluation was carried out in A. tricolor and B. vulgaris seedlings, which produce amaranthine and betanin, respectively.90 It was suggested that the first enzyme is a phenol-oxidase complex catalyzing both the conversion of tyrosine

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