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FIGURE 2.16 Examples of carotenoid and xanthophyll pigments from plants.

P-carotene is the orange pigment in carrot roots, pumpkin fruits, leaves of deciduous trees, and some flower petals. Zeaxanthin and violaxantin are also found in autumn-colored leaves and flower petals and are responsible for the

FIGURE 2.17 Carotenoid biosynthesis pathway in plants. (From Bartley, G. E. and Scolnik, P. A., The Cell Plant, 7, 1027-1028, 1995. With permission.)

bright yellows that are sometimes seen. Coloration of flowers is very important to the survival success of the plants producing them. The color of the flowers is one of the primary factors involved in attracting pollinators. For more information on the attraction of pollinators, see Section 2.6.6. For humans, P-carotene is important in our diet because of its purported anticancer activity, its use as a food coloring, and as an important source of vitamin A that is synthesized from 0-carotene and other carotenoids. Vitamin A produced by animals is in turn converted to the pigment, retinal. This pigment is one of the essential components in the light receptors of the eye that allow us to see.

Carotenoid pigments can also act in fruit and seed dispersal by attracting animals which in turn spread the seeds. Most fruits also produce odor compounds such as monoterpenes to help attract these organisms, and the sugars produced and stored in the fruits act as a positive reward. In ripening fruits, as in leaves turning color in the autumn, chlorophyll pigments gradually break down in chlo-roplast thylakoid membranes, revealing the carotenoid pigments that were masked by the chlorophyll pigments. During ripening there is also significant synthesis of new carotenoid pigments. In the case of ripening tomatoes and peppers, for example, the unripe fruits are characteristically bright green. As ripening progresses (triggered by the plant hormone, ethylene) various carotenoid pigments appear and, newly synthesized, account for the color of the ripe fruits. Lycopene is the red pigment seen in mature tomato and red pepper fruits. Tomatoes can have both red and yellow fruits depending on the genotype of the parent. In some peppers, we encounter ripe fruits which are green at maturity (bell peppers). Here, chlorophyll pigments do not breakdown during the ripening process. Other ripe peppers may be yellow or red depending again on the genotype of the parents. Similar types of color changes occur in fruits of ripening cucurbits (squash, gourds, pumpkins) and in the fruits of egg plant.

Some plants such as red maple (Acer rubrum) trees produce red-colored flowers and/or leaves, yet they are wind pollinated. It's obvious that these plants do not have to attract pollinators. So, why the color? One interesting theory behind why they spend their energy to do this is that the pigments help to warm the flowers or leaves during early spring or late fall. This extra heat would greatly aid seed development and photosynthetic processes in early spring allowing the plant to get a head start on growth over other plants as well as provide a longer period to produce energy reserves in the fall. Red coloration in many plants is not due to carotenoids, but rather, anthocyanin pigments.

2.4.10 Anthocyanin Biosynthesis

Anthocyanins are flavonoid-type compounds responsible for most of the red, pink, purple, and blue pigments found in roots, stems, leaves, flowers, seeds, and fruits. Examples include the red anthocyanins in red radish, the red leaves of some Norway maple cultivars (e.g., Acer saccharum cv. Schwedleri), the red fruits of some peppers, apples, and Acerola cherry (Malpighia glabra, said also to contain the highest content of vitamin C and ascorbic acid of any fruit), and the red, pink, purple, and blue flowers of Rhododendron, Hibiscus, and Fuchsia to name a few. Anthocyanin pigments occur in the vacuoles of plant cells. They are synthesized from the aromatic amino acid, phenylalanine, in the phenylpro-panoid pathway (Figure 2.18). This is the same pathway that is responsible for the synthesis of tannins, flavonones (Figure 2.19) like naringenin, flavonols, flavonoids, isoflanonoids like genistein and daidzein, lignin, lignans, and coumarin.13

Anthocyanins And Anthocyanidins
FIGURE 2.18 Diagram of the general phenylpropanoid and flavonoid branch pathways. (From Burbulis, I. E., Iacobucci, M., and Shirley, B. W., The Plant Cell, 8, 1013-1025, 1996. With permission.)

The primary enzyme that commits the pathway to biosynthesis of the antho-cyanin pigments is chalcone synthase (CHS). There is a whole gene family of CHS genes within most plants. Some of the genes are expressed in very specific tissues. CHS(A), for example, is only expressed in the petals and stamens of flowers that produce anthocyanins. This and subsequent enzymes in the pathway have been well characterized. In petunia, genetic loci controlling the synthesis of most of these enzymes have been located with the exception of 5GT (5-glucosyl transferase).12 The different colored anthocyanins arise from precursors that

FIGURE 2.19 Survey of several flavan derivatives (below) based on the basic flavan skeleton (above).

include dihydrokaempferol (a precursor of the orange-to-red anthocyanin, pel-argonidin), dihydroquercetin (a precursor of the purplish red anthocyanin, cya-nindin), and dihydromyricetin (a precursor of the bluish purple anthocyanin, delphinidin) (Figure 2.20). All of these anthocyanidins are converted to their glucosides such as pelargonidin-3-glucoside, cyanidin-3-glucoside, and delphini-din-3-glucoside which allows them better solubility in the aqueous solution of the vacuole.

The glucosyl moieties are typically glucose and rhamnose sugars. The color of anthocyanins is affected by the number of hydroxyl and methoxyl groups in the B ring of the anthocyanidin, but apart from structure, color is also affected by the presence of chelating metals such as iron and aluminum, the presence of flavone or flavonol co-pigments, and the vacuolar pH where these pigments are stored (Table 2.3).2 As one example, in Hydrangea flowers, where the vac-uolar pH is acidic, the flower petals appear blue; where it is alkaline, they appear pink. So the vast variety of coloration of many leaves, flowers, and fruits is often the result of several different pigments — chlorophylls, carotenoids, and antho-cyanins.

Anthocyanins serve many diverse functions in plants, including attraction of insect and bird pollinators to flowers and dispersal of seeds and fruits by birds and mammals. In some cases, they are feeding deterrents and, like other flavonoids, can also protect the plant against damage from UV irradiation.12

Anthocyanins have great economic importance in expression of the wide array of flower colors in plants grown as ornamentals. In fact, attempts to obtain blue roses, chrysanthemums and carnations are now possible with transgenic plants. In these plants, synthesis of the blue pigment, delphinidin-3-glucoside, does not normally occur because the 3',5'-hydroxylase is not normally expressed. In the transgenic plants, this gene, obtained from other plants like petunia, is expressed, resulting in the synthesis of the blue delphinidin-3-glucoside antho-cyanoid pigment. One interesting application in the use of naturally occurring

Pelargonidin Synthetic
FIGURE 2.20 Anthocyanin and flavanol biosynthetic pathway. (From Holton, T. A. and Cornish, E. C., The Plant Cell, 7, 1071-1083, 1995. With permission.)

anthocyanin pigments is the one present in the red roots of radish, Raphanus sativus. This water-soluble pigment is extracted from these roots and is currently used to dye Maraschino cherries bright red instead of using a synthetic red dye

TABLE 2.3

Factors Controlling Cyanic Color in Flowers aHydroxylation pattern of the anthocyanidins (i.e., based on pelargonidin, cyanidin, or delphinidin) Pigment concentration

Presence of flavone or flavonol co-pigment (may have blueing effect)

Presence of chelating metal (blueing effect)

Presence of aromatic acyl substituent (blueing effect)

Presence of sugar on B-ring hydroxyl (reddening effect)

Methylation of anthocyanidins (small reddening effect)

Presence of other types of pigment (carotenoids have browning effect)

a In approximate order of importance. There are other minor factors, including pH, physical phenomena, etc.

as was done previously. This process was developed by horticulturists at Oregon State University in Corvallis, OR.

2.4.11 Alkaloid Biosynthesis

So far we have tried to touch upon each of the major categories of products produced by plants in general. We have discussed the biosynthesis of the major cellular components found in the majority of plants including primary storage compounds and key compounds which start the carbon fixation process (chlorophylls). We have used carotenoids to demonstrate the production of terpenoids and anthocyanins to give examples of phenolic compounds. Now we will say a few words about nitrogen-containing compounds which will be represented by the alkaloids.

Most of these products are not considered to be essential to the growth and development of the plant, but some, such as pyrimidine nucleotides and tet-rapyrroles are absolutely essential. This is why we have separated these compounds from the rest of the nitrogen-containing compounds in Figure 2.1.

There are literally thousands of different plant products that have nitrogen in their structures. Perhaps the most diverse of these types of compounds (found in 20 to 30% of vascular plants) are the alkaloids which, like most other nitrogen-containing compounds, are synthesized from amino acids. Alkaloids are especially interesting because they are quite toxic to both herbivores and humans; yet they have some very important medicinal properties for humans. The nitrogen atom in these substances is almost always part of a heterocyclic ring whose origin is found innately in the structure of the amino acids from which they came or are the result of the circularization of the given amino acid. This is the case with aspartic acid which combines with glyceraldehyde-3-phosphate in the production of nicotinic acid (a precursor of the alkaloid, nicotine) in plants such as tobacco. Nicotine is well known as a toxic component of tobacco smoke. There are many categories of alkaloids, including pyrrolidine, tropane, piperidine, pyrrolidine, quinolizidine, isoquinoline, and indole alkaloids. Much of the carbon skeleton of some of these alkaloids is derived from the mevalonic acid pathway, but it is beyond the scope of this chapter to go into the details of the biosynthesis of all types of alkaloids. Figure 2.21 shows the major alkaloid classes and the biosynthetic precursors.

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FIGURE 2.21 Major classes of alkaloids, their chemical structures, their biosynthetic precursors, and well-known examples of each class.

FIGURE 2.21 Major classes of alkaloids, their chemical structures, their biosynthetic precursors, and well-known examples of each class.

An old idea about the function of alkaloids in plants depicted them as waste products of plant metabolism. However, plants are, energetically, very efficient organisms. They simply don't waste their energy in the production of compounds that they don't need — there always seems to be a reason for their production. The predominant activity of alkaloids in plants seems to be the deterrence of herbivores. Many livestock deaths are caused by ingestion of alkaloid-containing plants such as lupines (Lupinus spp.), larkspur (Delphinium spp.), and groundsel (Senecio spp.). They also have been shown to be toxic to insects, bacteria, and fungi. Alkaloids are not solely defensive substances. Some red and yellow-colored alkaloids called betalains, like carotenoids and antho-cyanins, act as attractants in flowers and fruits of plant species such as beets and cacti. It is interesting to note that plant families that contain betalain pigments never contain anthocyanins. Some pyrrolizidine alkaloids also act as attractants by mimicking such compounds as the sexual pheromones normally produced by some insects like butterflies. These compounds trick the insect into visiting the flower and spreading the plant's pollen, but alkaloids in general are toxic. When taken in sufficient quantity, alkaloids are dangerously toxic to humans, but at lower doses, many are helpful — morphine, codeine, atropine, and ephedrine to name a few. Other alkaloids, including nicotine, caffeine, and cocaine find popularity as nonmedicinal stimulants or sedatives, but they too have their toxic effects.

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