How And Where Some Of The Wellknown Plant Metabolites Are Synthesized In Plant Cells

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2.4.1 Lipids, Proteins, and Nucleotides

All living organisms produce three major categories of compounds: (1) lipids, which make up both the plasma membrane and the membranes of all internal compartments and organelles; (2) proteins, which make up both structural units of the cell such as microtubles and all the enzymes of every biochemical process; and (3) nucleic acids and nucleotides, which code for all proteins, act as metabolic energy molecules such as ATP and biochemical regulators such as GTP or cAMP, and in some cases work in conjunction with proteins to produce certain specific activities. Ribosomes, for example, consist of both protein and RNA, the combination of which allows the production of all other proteins. Since all organisms produce these compounds, their synthesis in plants will not be considered in detail. We refer the interested reader to any modern biochemistry or cell biology text.

Lipids are highly hydrophobic compounds produced by a partnership between plastids and the ER. Most lipids have a fatty acid portion made from acetyl-CoA and malonyl-CoA in a reaction whose repetition produces longer molecules. Malonyl-CoA is simply the carboxylated form of acetyl-CoA. In animals, fatty acid biosynthesis takes place in the cytosol, but in plants it occurs in plastids (chloroplasts in green tissue, proplastids in non-green tissue). In higher plants and animals, the predominant fatty acid residues are those of the C16 and C18 species, palmitic, oleic, linoleic, and stearic acids (Figure 2.7). However, there are many different forms of lipids. Membrane lipids such as phospholipids and glycolipids are made from a combination of glycerol, fatty acids, and hydrophilic compounds such as serine, choline, inositol, or various sugars. The many varieties of phospholipids and glycolipids are made from phosphatidate, a phosphorylated sugar derivative which acts as the precursor for the polar heads of these lipids. Vesicles that bud off of the ER or Golgi apparatus carry specific phospholipids to their proper location in the plasma membrane or organelles. Other than the typical lipid cell components, plants

FIGURE 2.7 Illustration of the most common fatty acids in oils derived from plant seeds that are used for non-food purposes.

also have different metabolic pathways that produce waxes (Tables 2.1 and 2.2)

TABLE 2.1

Some Long-Chain Saturated Acids and Alcohols Found Free or Esterified in Plant Waxes

Number of carbons

Acid

Alcohol

24 26

25 30 32 34

Lignoceric acid Cerotic acid Montanic acid Melissic acid Lacceroic acid n-Tetratriacontanoic acid

Lignoceryl (n-tetrocosanol) Ceryl (n-hexacosanol) Octacosyl (n-octacosanol) n-Myricyl (n-triacontanol) n-Lacceryl (n-dotriacontanol) Tetratriacontyl (n-tetratriacontanol)

which make up the protective cuticle of epidermal cells (see Cellulose Biosynthesis) and terpenes which are lipids synthesized from acetyl CoA via the mevalonic acid pathway. Terpenes produced in the terpenoid pathway serve a variety of functions in photosynthesis (see Carotenoid Biosynthesis), hormone controlled development (gibberellins and abscisic acid), and flower coloration and scent (see Section 2.6.6), to name a few. For humans, they are a source for rubber, essential oils (perfumes), and medicinal drugs such as taxol (an anticancer drug). Plants do produce very important storage forms of lipids (fats and oils) as energy reserves in fruits and seeds such as the fats and oils found in avocados, olives, soybeans, sunflower seeds, and peanuts. In some cases, these reserves also may serve as rewards for animals that disperse the plant's seeds. These stored lipids are often found in the cytoplasm of either cotyledon or endosperm cells in organelles known as spherosomes (also called lipid bodies) which, like vesicles, bud off of the ER.

The production of proteins is completely dependent on the presence of nucle-otides because every protein is coded by nucleic acids which are made from nucleotides. In eukaryotic cells, most proteins are initially produced in the cytosol and then transported to their final destination in the cell where they will perform their specific function. Organelles, such as chloroplasts and mitochondria, can also make proteins specific to these organelles. We have already mentioned that proteins may be enzymatic or structural in function, but plants do produce storage forms of proteins, like phytate, to provide a reserve of amino acids and energy especially in the process of seed germination. Some of these storage proteins can be lectins which are highly toxic and serve as herbivore deterrents (see Section 2.6.5), but their ability to bind sugars gives them function in recognition of sym-bionts, pathogens, and species-specific pollen grains as well. The purine and pyrimidine nucleotides that allow the synthesis of nucleic acids (DNA or RNA) are made in the cytoplasm from sugars and aliphatic amino acids. Purine nucle-

TABLE 2.2

Some Common Components of Plant Cuticular Waxes

Compound type Structural formula n-Alkanes CH3(CH2)nCH3

Iso-alkanes CH3CH(CH2)nCH3

Alkenes CH3(CH2)nCH*CH(CH2)mC

Monoketones CH3(CH2)nC(CH2)mCH3

Usual range of chain lengths

C25-C35

C25 C35 C17 C33

C24 C33

ß-Diketones

CH3(CH2)nCCH2C(CH2)mCH C3i-C3

Secondary alcohols

Wax esters

CH3(CH2)nCH(CH2)mCH3 O

CH3(CH2)nC-O(CH2)mCH3

Primary alcohols CH3(CH2)nCH2OH

Normal fatty acids CH3(CH2)nCoH

OH O

ffl-hydroxy acids CH2(CH2)nCOH

C20-C33

C30-C60 C12-C36

C12-C36 C10 C34

otides are made from ribose-5-phosphate, a modified ribose sugar, while pyrimi-dine nucleotides also require glutamine. So, a nucleotide is simply one of several different nitrogen-containing ring compounds linked to a five-carbon sugar (either ribose or deoxyribose) that carries a phosphate group. Nucleotides are also salvaged within the cell from the degradation or breakdown of nucleic acids (usually RNA). Please remember that all biochemical processes are ultimately controlled by the timing of the expression of the genes encoded by DNA.

2.4.2 Cellulose and Cellulose Biosynthesis

Cellulose is the world's most common naturally synthesized polymer. It makes up the majority of all the biomass on the planet and is the primary component of all plant cell walls (Figure 2.8). This homopolymer is made from the glucose molecules produced by photosynthesis and is organized as glucan chains of P-1,4-linked glucose units in which every other glucose unit is rotated 180° with respect to its neightbor.1 The glucan chains in primary walls of growing plant cells aggregate into fibers called cellulose microfibrils. In secondary walls, laid down after cell growth has ceased, the cellulose microfibrils are organized into macrofibrils or bundles.1 Cells which expand more or less equally in all directions have cellulose microfibrils oriented in a random pattern; in contrast, cells which expand by elongation growth (e.g., fibers, pollen tubes, root hairs, and conducting cells of the vascular system) have cellulose microfibrils oriented parallel to each other, lying at right angles to the direction in which the cell elongates. These patterns of orientation of cellulose microfibrils help govern the specific function of a given cell and can be determined microscopically by the use of crossed polarizers and a red filter placed diagonally to the crossed polarizers.

The synthesis of cellulose occurs at the plasma membrane which is located at the interface between the cell wall and the cytoplasm. The monomeric unit that donates glucose units to a growing cellulose chain is UDPG (uridine diphosphate glucose). The glucose in UDPG comes from the hydrolysis of the disaccharide sugar, sucrose, catalyzed by the enzyme, SuSy (sucrose phosphate synthase). An elegant hypothetical model of a cellulose synthase complex in the plasma membrane is provided in the excellent review article on cellulose biosynthesis.1

The cell wall in plants provides structural support for the plant. This structural support is provided not only by cellulose but by other polymers such as hemicellulose and pectic polysaccharides. Like cellulose, these are chains of sugars, but their many varieties differ from cellulose in the kinds of sugars present, how they are linked together, and how many branches they have in their chains. Hemicellulose and pectins are not made at the plasma membrane. Instead, they appear to be made in the secretory system of the ER and Golgi. Then they are transported to the cell wall via vesicles. In woody plants such as vines, trees, and shrubs, the cell walls become lignified through deposition of the polymer, lignin (see Section 2.4.3). Cellulose combined with lignin is the primary plant product involved in support and provides the physical structure that allows such plants as trees to grow very tall. In vascular plants such as grasses, sedges, and scouring rushes (Equisetum spp.) as well as diatoms (one type of algae), the cell walls become infiltrated with amorphous silica gel which, like lignin, provides structural support (see Section 2.4.4). In epidermal cells of plant shoots, the cell walls can also become infiltrated and covered on their outer surfaces with a waxy lipid coating called the cuticle, made up of a polymer called cuticular wax or cutin made and secreted by the ER of epidermal cells combined with a wide variety of saturated and unsaturated acids as well as many forms of alcohols (Tables 2.1 and 2.2). This "water-proofing" of the surface of the shoot (leaves and stems, flowers, and fruits) prevents excess water loss fr T ,vy {■

Composition Hair

FIGURE 2.8 Interpretation of plant cell-wall structure in a fiber cell (A). It shows the structure of the primary wall in B, a cellulose macrofibril in C, a cellulose microfibril in D, a crystalline micelle of cellulose in E, the molecular architecture of repeating, ordered D-glucose units that make up cellulose in F, and two D-glucose residues connected by P-1,4-glucosidic bonds in G. (Modified from Esau, K., Plant Anatomy, 2nd ed., John Wiley & Son, New York, 1965.)

FIGURE 2.8 Interpretation of plant cell-wall structure in a fiber cell (A). It shows the structure of the primary wall in B, a cellulose macrofibril in C, a cellulose microfibril in D, a crystalline micelle of cellulose in E, the molecular architecture of repeating, ordered D-glucose units that make up cellulose in F, and two D-glucose residues connected by P-1,4-glucosidic bonds in G. (Modified from Esau, K., Plant Anatomy, 2nd ed., John Wiley & Son, New York, 1965.)

from the plant and consequent desiccation. In some species, such as those living at high altitudes, the cuticle is very white which helps reflect damaging ultraviolet (UV) light. Cellulose infiltrated with lignin and/or cuticular wax also provides a physical barrier which greatly deters most potential herbivores because of the toughness of the polymers. In contrast, roots do not produce a cuticular wax layer on their outer surfaces, but they do synthesize a wax known as suberin in an interior layer of cells called the endodermis that prevents leakage of ions and metabolites out of the vascular cylinder in the center of the root.

In commerce, cellulose is important in fabric made from cotton or other plant fibers, in softwood fibers (derived from conifers) that make up paper and cardboard, and in purified or modified forms as a matrix used in column and thin layer chromatography to purify compounds such as plant pigments and enzymes (e.g., DEAE cellulose = diethylami-noethyl cellulose). Obviously, it is also a major structural component of wood derived from trees used to make lumber. Figure 2.9 illustrates a cross-section of the trunk of a California redwood tree (Sequoiadendron sempervirens) whose wood (secondary xylem tissues) is mostly composed of cellulose, but which also is lignified (see the following section).

FIGURE 2.9 Photo of Peter Kaufman and Mike Messler examining a cut stump of a —

California redwood (Sequoiadendron sempervirens) tree whose wood (secondary xylem) O

is made primarily of lignified cellulose. Diameter of this tree at cut surface is about 5 m. u

(Photo courtesy of Casey Lu, Humboldt State University, Arcata, CA.) S

2.4.3 Lignin and Lignin Biosynthesis —

FIGURE 2.9 Photo of Peter Kaufman and Mike Messler examining a cut stump of a —

California redwood (Sequoiadendron sempervirens) tree whose wood (secondary xylem) O

is made primarily of lignified cellulose. Diameter of this tree at cut surface is about 5 m. u

(Photo courtesy of Casey Lu, Humboldt State University, Arcata, CA.) S

2.4.3 Lignin and Lignin Biosynthesis —

Lignin is a complex polymer (Figure 2.10) that exists as a 3-dimensional matrix around the polysaccharides of secondary cell walls S found in plant fibers and in the tracheids and vessel elements of secondary xylem (wood). It is composed of varying amounts of the

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How and Why These Compounds are Synthesized by Plants

How and Why These Compounds are Synthesized by Plants

Diagram Alcholunits
FIGURE 2.10 Partial polymeric structure of a lignin molecule made up of phenylpropane (C6-C3) monolignol alcohol units (see Figure 2.11).

aromatic phenylpropanoid subunits (monolignols), para-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol made via the shikimic acid pathway (Figure 2.1). These monolignols are usually synthesized from the amino acid l-phenyla-lanine, although tyrosine can also be used. Subsequent steps in the monolignol biosynthetic pathway are shown in Figure 2.11. These monolignols appear to be made in the ER and Golgi bodies, but the polymerization of lignin itself occurs outside the plasma membrane.

Lignin makes up between 15 and 35% of the dry weight of woody tissue,2 and it acts to provide additional rigidity and compressive strength to cell walls. Because lignin is hydrophobic, it also makes cell walls that become lignified impermeable to water.3 In plants, there are simple histochemical tests available which allow one to test for the presence of lignin in cell walls. They basically involve the use of phloroglucinol/HCl or para-rosaniline HCl. In either case, the cell walls stain deep reddish brown in color. We have used these reactions to demonstrate that, in cereal grasses, bundles of fibers associated with vascular

bundles show excellent lignin staining in mature stems and leaf sheaths that make up stiff straw. In contrast, it is totally absent in strands of fiber-like collenchyma cells associated with vascular bundles in the swollen leaf sheath bases of cereal grass shoots 5 which are sites for upward bending (negative gravitropic curvature) of lodged shoots prostrated by the action of wind, torrential rain, or hail.4 So, while lignin may provide support, help prevent water loss, and even resist herbivores, it is not a benefit to plant tissues that need to grow or to bend.

2.4.4 Biogenic Silica and Silicification

Some plants have developed the ability to absorb inorganic constituents from their environment and use them towards their benefit. Biogenic silica is a polymer of biological origin that is characteristically found in the cell walls of diatoms, scouring rushes (Equisetum spp.) or horsetails, grasses (all members of the Poaceae or grass family), members of the rush family (Juncaceae), and members of the sedge family (Cyperaceae). Silica found in these silica-accumulating plants has its origin from silicates found in soil minerals. It is taken up as monosilicic acid, Si(OH)4 via the roots (or cell membrane in the case of the single-cell diatoms) from which it moves up the plant in the xylem-conducting elements. This upward movement of monosilicic acid with water and other mineral compounds occurs as a result of "transpirational pull" mediated by transpirational loss of water from stomates (pores) located in epidermal tissues of leaves and stems. Once monosilicic acid arrives in stems and leaves where transpiration is occurring, it irreversibly polymerizes as amorphous silica gel, SiO2 • nH2O, mostly in cell walls which are hydrogen bonded to cellulose molecules. However, in grasses, within specialized silica cells located in the epidermis of leaves and floral bracts, it can also polymerize directly in the cytoplasm after breakdown of all cell organelles has occurred.5 Silica secretions can also result in specialized structures such as the needles on nettles.

The annual scouring rush, Equisetum arvense, can produce up to 20% of its dry weight as silica. A classical experiment done z at California Institute of Technology5 showed that these plants, grown in silicon-free hydroponic nutrient solutions became very u weak and appeared collapsed. Additions of silicon, as sodium metasilicate, to the hydroponic nutrient solution at only 80 ppm 1 yielded plants whose shoots were upright and appeared strong and robust. This indicated that silica provided direct support for r the shoot and, hence, is considered an essential element for normal growth and development in these types of plants. So, the UU primary role of silica in the cell wall is to provide support to the shoot in addition to that provided by cellulose and lignin. Aside a from providing support to shoots of grasses, sedges, rushes, and Equisetum spp., amorphous silica gel that gets deposited in outer o cell walls of epidermal tissue of leaves and stems forms very hard and often very sharp structures which can deter attack by 3 predacious animals, insects, and disease-causing fungi. In fact, the mandibles of many insects that attack rice plants (e.g., green a and brown leaf hoppers that transmit tungro virus pathogens) get worn down and rendered ineffective in piercing the leaves of s

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How and Why These Compounds are Synthesized by Plants 55

Coumaryl Alcohol Biosynthesis
FIGURE 2.11 Diagram depicting the biosynthesis of monolignols, p-coumaryl alcohol, coniferyl alcohol, and synapyl alcohol.

the rice plants. Likewise, the teeth of sheep get worn down significantly by eating high silica-containing pasture grasses. Fortunately, these animals can replace their worn-down teeth with new teeth.

It also should be noted that silica is not the only inorganic constituent that plant roots can absorb. Marine algae can absorb calcium in the form of calcium carbonate that they deposit on their surfaces as crusty support compounds, much like biogenic silica, which seem to prevent the plants from getting damaged by crashing waves. Some plants can absorb toxic elements such as selenium (Se) or bromine which help ward off herbivores. For example, Astragulus (loco weed) accumulates Se and incorporates it into certain amino acids and proteins. The plant itself can distinguish if a protein has Se, so there is no toxic effect to the plant. However, the animal metabolism cannot distinguish proteins that contain Se from those that do not and the effect is quite toxic. The fact that Se is toxic to most other plants also allows Astragulus to avoid competition in soils

that contain Se. These soils often occur around uranium deposits, so Astragulus has been used as an indicator species in botanical prospecting. Other plants such as alpine penny-cress (Thalspi caerulescens) will take up elements such as zinc and cadmium making them very useful when planted in polluted areas needing bioremediation.

2.4.5 Starch and Starch Biosynthesis

Starch is the most common storage polysaccharide found in plants and serves as a primary food source for humans, domestic animals, birds, insects, and microbes. Starch is essentially made up of monomers of sugars linked end to end in long chains through a-1,4 linkages along the chain with a-1,6 linkages as branch points. Martin and Smith6 present an excellent review of how starch is synthesized in plants. A scheme from their review article depicting amylopec-tin (branched chains of starch molecules with a-1,4- and a-1,6-linked glucan), starch granule formation, and the biosynthetic pathway for starch biosynthesis is shown in Figure 2.12. The other configuration of the starch molecule is that of amylose which is unbranched. It is composed exclusively of a-1,4-linked glucan. In rice gains, for example, one encounters varying amounts of amylose (straight-chain starch) and amylopectin (branched chain starch), depending on the cultivar. In sticky, short-grain rice (Japonica cultivars), amylopectin predominates. Such rice is better for soups and for eating with chopsticks. In non-sticky, long-grain rice (Indica cultivars), amylose starch predominates. Both forms of starch are made in the chloroplasts of mesophyll and bundle sheath cells from triose phosphate, a product of the Calvin cycle.

A large amount of free sugar in a cell will cause the cytosol to become thick and syrupy. This causes a hypertonic osmotic condition in the cell which will result in excessive water uptake and potential damage. So, one of the primary benefits of producing starch is to make sugars osmotically inactive by making them insoluble within the cell. The starch produced by chloroplasts is, in most species, the primary storage form that is mobilized for translocation to other plant parts during night periods. It often aggregates into starch grains which typically occur as several granules lying between grana membrane stacks inside the chloroplasts. This starch can be hydrolyzed to D-glucose that can be used for ATP synthesis via aerobic respiration to maintain turgor pressure in growing cells via its osmotic effects and for synthesis of cellulose and other polysaccharides in the cell wall. Translocated sugars and starch are also important in the development of storage organs, such as the above rice grains.

Large quantities of starch can be found in storage organs such as tubers and tuberous roots, taproots, stems located above ground, as well as seeds. These tissues are termed "sinks" by physiologists and agronomists. They allow the plant to survive on stored energy for long periods of winter or drought. In potatoes, under warm weather conditions starch typically gets hydrolyzed to sugar used for growth of new shoots. Starch also occurs in the root caps located at the tips of growing roots in the soil. It is stored in specialized colorless plastids in rootcap cells called amyloplasts. The starch-filled amyloplasts are dense,

FIGURE 2.12 Amylopectin structure (A), starch granule form (B), and starch biosynthesis (C). 1. ADPGPPase - adenosine diphosphate glucose pyrophosphorylase 2. SS -starch synthase 3. SBE - starch branching enzyme (From Martin, C. and Smith, A. M., The Plant Cell, 7, 971-985, 1995. With permission.)

heavy bodies which fall downwards in the rootcap cells when a root is placed horizontally (gravistimulated). These serve as gravisensors that trigger signal transduction events resulting in asymmetric growth of the root downward. Why is this so? It has been shown that if the rootcaps are removed from corn roots, the roots will not curve down when placed horizontally and thus will not grow into the soil where nutrients are located. When the rootcaps are replaced, gravisensitivity is restored. The gravitropic curvature response is much lower4 in Arabidopsis mutants that have a lesion in starch biosynthesis that results in poorly formed, small starch grains in the amyloplasts. The starch grains in chloroplasts (Figure 2.13), as in amyloplasts of root caps, can serve as gravisen-sors in prostrated stems of plants. When this starch is depleted artificially by placing the shoots in the dark for 4 to 5 d, the stems no longer respond to gravity; but, when fed sucrose, starch is resynthesized and gravisensitivity is restored.4

Humans eat starch in products such as potatoes, cereal grains, taro, and tapioca. It is also important in beer brewing as a "modified barley substrate" used in secondary fermentations. What happens here is that starch in barley is hydrolyzed to maltose (a disaccharide) and eventually to D-glucose. This hexose is used as a substrate (food) for beer fermenting yeast which, under anaerobic conditions, convert the sugar to ethyl alcohol (ca. 3.5 to 4.5%) and carbon dioxide. Starch is easily visualized in storage organs, such as potatoes, or in swollen joints (pulvini) of cereal grass stems by the use of a simple histochemical test. Fresh sections of plant tissue are placed in a 1% solution of iodine-potassium iodide (1:1) and the resulting stained starch grains appear blue-black in the light microscope (Figure 2.13).

2.4.6 Fructans and Fructan Biosynthesis

Fructans are soluble storage polysaccharides found in the vacuoles of cells of plants which are known to be fructan accumulators. They are made predominantly from the sugar D-fructose (hence the name), but D-glucose molecules may also be present in the chain. Classic examples include temperate zone monocots such as grasses, lilies, onions, irises, and dicots such as dahlias and Jerusalem artichokes. A complete compilation of families of monocots and dicots in which fructans are known to occur is cited in Reference 7. In dahlia and Jerusalem artichoke, the fructan is referred to as inulin, where the polysaccharide is composed mostly or exclusively of 2-1 fructosyl-fructose linkages (a D-glucose molecule is allowed but not necessary). In the temperate zone grasses, the fructan is termed either graminan, which has both 2-1 and 2-6 fructosyl-fructose linkages, or phlein which contain mostly or exclusively 2-6 fructosyl-fructose linkages.7 As with inulin, D-glucose is allowed in the chain but is not necessary in the structure of graminan and phlein-type fructans.

The basic pathway of synthesis of inulin-type fructans in plant cells has been summarized by Edelman and Jefford.8 It involves the following three steps, as summarized by Suzuki7

FIGURE 2.13 Starch grains stained with I2KI in chloroplasts of oat cells located in the graviresponsive swollen leaf sheath pulvini of the shoot. Arrows indicate the direction of the gravity vector; E = epidermis; V = vascular bundle. A, B, and C x 100; D x 200. (Photo courtesy of Casey Lu and Peter Kaufman.)

FIGURE 2.13 Starch grains stained with I2KI in chloroplasts of oat cells located in the graviresponsive swollen leaf sheath pulvini of the shoot. Arrows indicate the direction of the gravity vector; E = epidermis; V = vascular bundle. A, B, and C x 100; D x 200. (Photo courtesy of Casey Lu and Peter Kaufman.)

• Conversion of sucrose to trisaccharide in the cytosol by the enzyme sucrose-sucrose 1-fructosyltransferase (SST)

• Transfer of the terminal fructosyl moiety of this trisaccharide in the cytosol to sucrose in the vacuole by the enzyme B(2-1') fruc-tan:B(2-1c) fructan 1-fructosyltransferase (FFT), which is possibly located on the tonoplast membrane surrounding the vacuole

• Continued transfers by FFT of terminal fructosyl groups from the resulting molecules of trisaccharide (e.g., kestose or isokestose) in the vacuole to the extending fructan chain resulting in the formation of inulin molecules

FIGURE 2.13 (continued)

Fructan's primary role in plants is that of a reserve carbohydrate similar to starch. Temperate, cold-tolerant grasses like oats, barley, wheat, and rye typically contain fructans and sucrose as the primary carbohydrate reserves. Tropical, warm-loving and cold-intolerant grasses such as maize contain starch and sucrose as the primary reserve carbohydrates. It is interesting that in the shoots of temperate zone grasses and in the tubers of the Jerusalem artichoke, fructan synthesis accelerates under low temperature conditions of autumn; then the stored fructans become hydrolyzed through the action of fructan hydrolase in the spring when temperatures warm and shoot and root growth begin. This appears to provide the plant with a source for energy for a head-start on growth in the early spring.

Jerusalem artichoke (Helianthus tuberosus) tubers are frequently eaten by humans as a potato substitute (but not starch substitute). Humans cannot digest the inulin fructan present in these tubers because of the absence of the gene that makes the fructan-specific hydrolase in humans. Furthermore, the ubiquitous intestinal colon bacterium, Escherichia coli, cannot hydrolyze fructan. This would make one think that these tubers would be perfect food for dieters. However, there is recent evidence from Japanese studies that Bifidobacteria, found in intestinal microflora, can digest fructan; in fact, when fructans are eaten, populations of this microbe in the large intestine increase significantly. This being the case, enrichment of the human diet with fructans from plants such as rye, onions, Jerusalem artichoke tubers, and garlic may be beneficial, not because they are hydrolyzed in the small intestine, but because they are hydrolyzed in the large intestine. There is also evidence that fructans from plant sources may also be beneficial in the diets of swine and poultry (see "Fructans in Human and Animal Diets" by Farnworth in Susuki and Chatterton7).

2.4.7 Gum, Mucilage, and Dietary Fiber

There are other forms of polysaccharides that plants produce. Some of these are gums and mucilage which are highly branched heteropolysaccharides (related to hemicellulose and pectic polysaccharides) that contain acidic residues thus making them very hydrophobic, insoluble within the plant cell, and often difficult for animals to digest. One benefit to the plant that produces indigestible polymers is that it reduces the reward for herbivores. In other words, the animal spends its time eating yet gets nothing out of the process. For the plant, these polysaccharides can function as a storage reserve for carbohydrates, but they are also found as part of the matrix that surrounds the walls of some cells. This matrix is called the glycocalyx and is mostly seen on the surfaces of roots where it may serve to protect the plant against microbial invasion. Glycocalyx secretions are not unique to plants. They are also found in bacteria and animals where, as in plants, they act in cell-cell recognition of symbionts or pathogens. Another function of mucilage is seen in carnivorous plants like Sundew (Drosera spp.) where a substance called mucin is produced to catch unwary insects in nutrient-poor environments. Gums are also useful in sealing wounds in leaves and stems. For example, when a cherry tree is injured, it will produce a thick substance called gum arabic that fills in the wound thus preventing infection. This also acts as a human cosmetic.

Cellulose, pectin, lignin, waxes, gums, and mucilages are some of the many types of dietary fiber. Fiber is simply the insoluble polymers of plants and most come from cell walls. Fiber stimulates the gastrointestinal tract and acts as a laxative. Fiber containing pectins reduces blood cholesterol by adsorbing cholesterol molecules. Fiber, in general, appears to inhibit many cancers, especially colon cancer, by binding the carcinogens and preventing them from entering the body while they pass through the system. One problem, however, is that fiber may also adsorb vitamins thus carrying them out of the body before they can be absorbed. So, a balance of fiber in the diet is essential.

2.4.8 Chlorophyll and Chlorophyll Biosynthesis

There are three main locations of pigments within the cell: (1) plastids, (2) vacuole, and (3) the cell wall. The chemistry of the pigments varies with the location. Chlorophyll is a porphyrin that constitutes the primary photoreceptor pigment for the process of photosynthesis in plants. It is produced in the chlo-roplasts and is responsible for the green appearance of leaves and stems, aerial and prop roots, many kinds of floral bracts, and green fruits before they ripen. The chlorophyll molecule is made up of four pyrrole rings, made from alliphatic amino acids, (designated I to IV) that are ligated to form a tetrapyrrole ring with a magnesium atom in its center; ring IV is esterified with a hydrophobic long chain phytol molecule (C20H39) made in the terpenoid pathway.9 For light harvesting, plants use two forms of chlorophyll, a and b. Chlorophyll a is in all plants and is the only chlorophyll at the reaction centers. It has a methyl group at C3, while chlorophyll b, found in most plants, has a formyl group at this position and, like other accessory pigments, functions to absorb the energy from wavelengths of light that differ from chlorophyll a (Figure 2.14). The chemical ihtawhrJj

Wnrtmulh inmj ul lifl'-T

FIGURE 2.14 Absorption of different wavelengths of light by various photosynthetic pigments in plants.

structure of chlorophyll a is shown in Figure 2.15.

The biosynthesis of the chlorophylls is quite complex. It starts with the synthesis of 5-aminolevulinic acid from glutamic acid. The porphyrin ring containing conjugated double bonds is assembled in the chloroplast from eight molecules of 5-aminolevulinic acid. Subsequent steps lead to the formation of protochlorophyllide, addition of the phytyl tail, and insertion of a Mg2+ atom in the center of the tetrapyrrole ring. These are illustrated in the reviedw by vonWettstein et al.9 In the chloroplasts, the chlorophyll pigments are bound to proteins of the photosynthetic membranes (stacks of thylakoid membranes inside chloroplasts called grana stacks). These proteins, called chlorophyll a/b binding proteins, are arranged into large complexes with many other proteins, cyto-chromes, and quinones to form the photosynthetic electron-transport chain whose primary function is to produce the ATP required to fix carbon dioxide. It is the pigment chlorophyll that absorbs the energy of the sun and shuttles resulting free electrons to this all-important series of chemical events.

Chlorophyll absorbs photons of light energy from the sun or from artificial lamps (e.g., incandescent lamps, high pressure lamps, light-emitting diodes) in the red and blue portions of the electromagnetic spectrum with peaks of maximal absorption occurring at 660 and 450 nm, respectively. This is called its absorption spectrum. Absorption spectra are commonly used to characterize pigment types.

FIGURE 2.14 Absorption of different wavelengths of light by various photosynthetic pigments in plants.

Photosynthetic Pigment

FIGURE 2.15 Chemical structure of a chlorophyll a molecule. Note the tetrapyrole ring with a Mg atom in its center at the top and the long chain phytol "tail" at the base. Note also the formyl group substitution at the methyl group in upper right corner of the chlorophyll a molecule; such a substitution gives one the structure of chlorophyll b.

FIGURE 2.15 Chemical structure of a chlorophyll a molecule. Note the tetrapyrole ring with a Mg atom in its center at the top and the long chain phytol "tail" at the base. Note also the formyl group substitution at the methyl group in upper right corner of the chlorophyll a molecule; such a substitution gives one the structure of chlorophyll b.

Maximal rates of photosynthesis (measured by the rate of CO2 uptake or O2 evolution) also occur in the red and blue portions of the electromagnetic spectrum. This is called its action spectrum. When the action spectrum peaks, like those for photosynthesis, match the absorption peaks for a given pigment(s), like those for chlorophylls, one can deduce that this pigment(s) is the one that is essential for absorption of light for the particular process under consideration. Another type of proof is to find plants that lack the pigment of interest and determine which processes are functional. For example, albino mutants and parasitic plants such as Indian pipe (Monotropa spp.), which are devoid of chlorophyll pigments, cannot carry out photosynthesis. Also, please note that not all plants photosynthesize. Parasitic plants feed off the nutrients and sugars provided by their hosts.

2.4.9 Carotenoid Biosynthesis

Plant carotenoids (Figure 2.16) are responsible for the red, orange, and yellow pigments found in fruits and roots such as tomatos, red peppers, pumpkins, and carrots. They can be seen in the petals of many flowers and are the primary pigments responsible for the fall coloration of deciduous trees. Carotenoids are synthesized in the terpenoid pathway as C40 tetraterpenes derived from the condensation of eight isoprene units starting with isopentenyl diphosphate (Figure 2.17).1011 There are two basic types of carotenoids: (1) carotene which contains no oxygen atoms and (2) xanthophyll which does contain oxygen (Figure 2.16). At the center of each carotenoid molecule, the linkage order is reversed, resulting in a molecule which is symmetrical. A set of double bonds in the molecule is responsible for the absorption of light in the visible portion of the spectrum.10 As mentioned above, this has an important impact on the absorption of a wider range of light wavelengths for use in photosynthesis (see Figure 2.14). Consequently, in photosynthetic organisms, carotenoids are an integral structural component of photosynthetic antenna and reaction center complexes, but they also protect against the harmful effects of photooxidation processes.10 So, like chlorophyll, carotenoids are found in the thylakoids of green leaves and stems. In fruits and flowers they are also found in plastids, but these plastids have structural differences and are referred to as chromoplasts to indicate that they contain pigments other than chlorophyll.

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