Adaptive Functions Of Metabolites In Plants

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2.6.1 Sources of Metabolic Energy and Energy Transfer

Without a source of metabolic energy or the ability to transfer the energy obtained from the environment through metabolic pathways, a living organism will die. This is why plants devote so much of their time and energy towards the production of pools of compounds that ultimately store the energy of the sun. Plants have several such sources of metabolic energy derived from stored metabolites or from ATP. The stored metabolites include starch (universal in green vascular plants), fructan [in grass family (Poaceae), lily family (Liliaceae), amaryllis family (Amaryllidaceae), aster family (Asteraceae), and in other families], other polysaccharides (gums and mucilage), and stored lipids and proteins (as in the fats, oils, and protein bodies of seeds and fruits). Each of these polymers may be broken down by specific enzymes when the need for energy arises such as during the night when sunlight is not available or during seed germination. The units of these polymers (sugars, amino acids, or acetyl CoA) then enter the mainstream of the plant's metabolism where they can once again help produce ATP. ATP is produced in the electron transport cascade during photosynthetic photophosphorylation in chloroplasts and oxidative phosphory-lation in mitochondria.

Plants have evolved two major pathways of photosynthetic carbon fixation. In C-3 plants (which are represented by most plant species), the primary product is phosphoglyceric acid (PGA) which is used for synthesis of 4-, 5-, 6-, and 7-carbon sugars in the Calvin cycle. In C-3 plants, typically 30% of the fixed carbon is lost as carbon dioxide through photorespiration (a process which liberates CO2 and constitutes a significant energy drain). In contrast, in C-4 plants (such as sugarcane, corn, and many fast growing weeds) the primary products are both the 3-carbon acid, PGA, as well as the 4-carbon acids, malate and aspartate; the former is produced in chloroplasts in leaf mesophyll tissue, whereas the latter are produced in chloroplasts of vascular bundle sheath cells. What is especially interesting is that there is little or no photorespiration in C-4 plants, so that total carbon fixed is, on average, 30% higher than in C-3 plants. The reason for this difference is that in the process of shuttling carbon from mesophyll to bundle sheath cells, a much higher concentration of carbon dioxide is generated in the bundle sheath cells. It is this elevated CO2 partial pressure in bundle sheath cells that suppresses RUDP (ribulose bisphosphate) oxygenation. This is the first step in photorespiration. The enzyme involved here (RUBISCO) has both carboxylase and oxygenase catalytic activity. So, the higher level of CO2 inhibits this enzyme's oxygenase activity.

2.6.2 Cellular Building Blocks and Structural Support

By cellular building blocks, we are referring primarily to the polysaccharides that make up the cell walls of plants — cellulose, hemicellulose (xyloglucans in dicot flowering plants and arabinoxylans in monocot flowering plants), and pectins (polygalacturonans, based on polymers of galacturonic acid coupled to different sugar moieties such as rhamnose and fucose). As mentioned above, it is these polysaccharides that constitute the majority of all plant biomass on this planet. Animal cells do not have cell walls; each cell is circumscribed by a plasma membrane alone. It is this structural support provided by cell walls of plants along with the additional structural support provided by such processes as lignification and/or silicification of these cell walls that turns the plant into a type of scaffold upon which to hang its photosynthetic tissues (leaves or stems) in the best possible orientation to absorb carbon dioxide and the energy of the sun. Without this support terrestrial plants would not be able to support the weight of their leaves, and consequently the leaves would not get good exposure to the sun for photosynthesis. Interestingly, many aquatic plants such as algae do not have this problem. They are supported by the water itself, and hence, usually do not produce additional support compounds such as lignin. In some cases, these cellular building blocks allow the development of massive plant bodies. This is most dramatically exemplified by giant redwood and sequoia trees. Please remember, however, that the other major contributors to the structure of plants and their cells alike are lipids (especially membrane lipids such as phospholipids) and proteins (such as those in membranes, microtubules, and microfilaments).

2.6.3 Sources of Genetic Information

One finds the genetic information (DNA, RNA) of plants residing in their nuclei, chloroplasts, mitochondria, and ribosomes. All of the proteins of the cell including both structural proteins and enzymes are encoded by these nucleic acids. Most of the proteins synthesized in plant cells are encoded by nuclear DNA; on the other hand, many of the proteins that occur in mitochondria or in chloroplasts are synthesized on ribosomes within these respective organelles. Some of these proteins are structural components of membranes and membrane channels, others are enzymatic. Many proteins that occur in organelles are also coded for by DNA in the nucleus. How do nuclear encoded proteins find their way to the proper cell organelle? "Signal peptides" (specific amino acid sequences also encoded by DNA) occur on these proteins to target the proteins to the membrane(s) of specific organelles, such as per-oxysomes, glyoxysomes, Golgi (dictyosomes), mitochondria, or plastids. Once the protein gets targeted to the proper organelle, it is then transported into the organelle across the membrane(s) enclosing that organelle (this involves different mechanisms for targeting and for transport). In most cases the signal peptide gets cleaved off by a specific peptidase which produces a functional protein that may act as a monomeric enzyme, may become associated with other proteins to form multimeric complexes, or may become a structural component of a membrane. Some proteins exist as glycoproteins or as lipo-proteins which have carbohydrate or lipid components attached. These may also be involved in targeting and/or intercalation of the protein onto the inner or outer surface of a given membrane such as the tonoplast membrane that surrounds the vacuole or the plasmalemma that surrounds the cytoplasm and lies just inside the cell wall.

The point here is that all the information for production, localization, and functionality of every protein is ultimately contained on a strand of DNA. The ability to pass this information onto offspring is one of the key factors that determines if a species of organism will survive in a given environment. The fact that genetic information can change (mutate) over evolutionary time is what allows organisms in general to adapt to ever changing environments. This change (evolution) is always occurring and produces new combinations that may or may not work in that environment, but only the individuals that have the combinations that do work will survive to the next generation. This variation between individuals is very important to the survival of each species of plant (or animal). Thus, plants have evolved various methods of sexual reproduction, such as pollination, that allow the sharing of genetic information between individuals or a given species. This holds the benefit of spreading combinations of enzymatic reactions that work throughout a population.

2.6.4 Catalysts of Metabolic Reactions

By now, it has become quite apparent how important enzymes are in catalyzing metabolic reactions in different compartments of plant cells. It is these proteins, coded for by the plant's genetic information and placed in the proper locations within cells of tissues held in the correct positions by the plant's cellular building blocks, that allow not only the production of but the utilization of the metabolic energy compounds that run the biochemical reactions that control the processes of life. In such reactions, binding of the substrate to the active site of the enzyme to form the enzyme-substrate complex is a prerequisite to catalytic action of the enzyme. Enzymes act to lower the amount of free energy required to make a reaction proceed to the formation of the product of the reaction which is released following separation of the enzyme from the enzyme-substrate complex. Without this interaction of enzyme with substrate, the reaction would proceed very slowly or not at all under normal conditions of temperature and pressure. So, enzymes act as organic catalysts by speeding up the rate of a given metabolic reaction.

Some of these enzymes act to cause hydrolysis of substrates and are called hydrolases, like amylase which hydrolyzes starch, invertase which hydrolyzes sucrose, and fructan hydrolase which hydrolyzes fructan. Other enzymes, called synthases or transferases are involved in synthesis, as for example, cellulose synthase that makes cellulose or callose (depending on concentrations of Mg2+ co-factor and substrate concentration) or starch synthase, one of the enzymes involved in starch synthesis. Still others are involved in cyclicization reactions and are called cyclases. They make linear molecules circular as in the conversion of GPP to cyclic monoterpenes. In photosynthesis, you remember the substrate, RUDP; it is acted upon by a single enzyme (RUBISCO) that has carboxylase as well as oxygenase activity connected with photosynthetic carbon fixation from CO2 and with photorespiration, respectively. There are important enzymes involved in signal transduction processes related to hormone action in plant and animal cells. These include phosphorylases, phosphatases, and many kinds of protein kinases. Then, there are enzymes called dehydrogenases, such as man-nitol dehydrogenase, which catalyzes the formation of d-mannose from manni-tol. Chaperones are a group of enzymes which promote the folding of proteins into their correct (i.e., active) forms, hold proteins which are to be transported to organelles in an unfolded form, and help maintain protein integrity during heat stress and thus prevent denaturation. These are the main classes of enzymes, but the list does go on.

Finally, we need to mention the concept of isozymes. These refer to the same type of enzyme which (1) may exist in different cellular compartments, (2) have different pH optima for the same substrate, or (3) at the molecular level, have different nucleotide sequences for the signal peptides of the enzyme. A good example is invertase (a 0-fructofuranosidase) which hydrolyzes sucrose to d-glucose and d-fructose. There are several known isozymes of invertase: (1) intracellular soluble invertase located in the cell vacuole, and possibly the cyto-sol, with pH optima from slightly alkaline (pH 7.5) to acidic (pH 4.5) and (2) insoluble forms ionically bound to the cell wall with pH optima of 4.0 and 5.3.30,31

2.6.5 Deterrence of Predators and Pathogens via Poisons and Venoms

Plants have evolved a vast array of chemical defenses which effectively deter herbivores and pathogens from attacking them. These have obvious selective and survival value for the plants because plants are almost always confined to one spot and thus can fall easy prey to wandering animals out to steal plant nutrients. This brings up several questions. What is the nature of these chemical defense strategies? How do they work? Which came first, the chemical deterrent evolution in different groups of plants or the predator/pathogen-dictated selective pressure for plants to evolve new chemical defense strategies? How effective are human-designed chemical defense strategies, as in transgenic plants, as compared to the multifaceted strategies plants have evolved and continue to evolve to deter predators or pathogens? These questions are addressed in four excellent references.2,32-34

Here, we will consider some of the more important ways plants defend themselves against attack by insect predators, herbivores, pathogenic fungi, bacteria, and viruses. These methods are based on the classification scheme of Becerra34 as follows:

• Structural defense strategies. These include lignification, silicification, callose formation, and wax deposition. We have alluded to these processes in more detail in the preceding sections of this chapter. The chemical polymers act as a sort of armor and present fungi, bacteria, or virus with a physical barrier through which to penetrate or present insects or herbivores a hard surface through which to chew.

• Chemical defense strategies. These include almost all compounds that, based on their chemical nature, deter attack. There are many fasci nating stories behind the mechanisms of each of these compounds, but let it suffice to say that each of these compounds can interfere (usually in a species-specific manner) with at least one critical biochemical pathway within the attaching organism thus killing or making sick this organism. There are literally thousands of examples of chemical defense including

- Alkaloids (e.g., nitrogen-containing, heterocyclic ring compounds)

- Active oxygen species such as H2O2, O2- (superoxide anion), and OH (hydroxyl radical)

- Proteins, including cell wall glycoproteins (hydroxyproline-rich, proline-rich, and glycine-rich glycoproteins); inhibitory proteins (many are induced and endogenous antiviral proteins, antifungal lipid transfer proteins, antibacterial a-thionins); lectins (which are carbohydrate-binding proteins); antioomycete pathogenesis-related protein, and antifungal defensin proteins; extracellular hydrolases (e.g., cellulases, pectinases, chitinases, ribonucleases, proteases, and lipid acyl hydrolases such as patatins); and ribosome-inactivating proteins such as trichosanthin in the Chinese cucumber plant, Tri-chosanthes kiriliowii)

- Saccharides and polysaccharides such as callose and pectins, effusive gums, mucilage, cardiac glycosides, cyanogenic glyosides and glucosides of organic nitrogen-containing compounds consisting of a sugar moiety linked to a cyanide or nitrite, respectively

- Phenolics and coumarins

- Polyphenolics such as suberins, lignins, and tannins [both hydro-lyzable and condensed (see Figure 2.26)]

- Flavonoids and isoflavonoids, quinones and isoquinones

- Terpenoid/steroid compounds such as cardiac glycosides, leguminous saponins (often glycosylated), gossypiol-related terpenoids, aphid alarm pheromones, brassinosteroids (insect hormone-mimicking compounds), and phytoecdysones (insect molting hormone mimics)

- Cyanide-releasing compounds, which release hydrogen cyanide on ingestion and block electron transport during respiration, and include cyanogenic glycosides and the glucosinolates (mustard oil glycosides which release isothiocyanates)

- Organic acids, including the salts of oxalic acid (as found in aroids such as Dieffenbachia, Symplocarpus, and Monstera, as well as the leaf blades of rhubarb, Rheum spp.), monofluoroacetic acid, and l-DOPA (3,4-dihydroxyphenylalanine)

- Long-chain carbon compounds such as antimicrobial polyacety-lene, antifungal alkenales, anti-mammalian polyacetylene toxins, and fatty acid/lipid-containing waxes, oils, and cutin

The point here is that defense is very diverse and often very complex. It is also important to note that not all toxins act in an acute or

FIGURE 2.26 Chemical structures of condensed (A) and hydrolyzable (B) tannins.

immediate manner. Some act as chronic toxins, having a noticeable affect only after a long period of time.

There is an interesting connection between tannins and their possible role in deterring attack by the chestnut blight pathogen (Endothia parasitica) that has caused the near demise of the American chestnut tree in Eastern North America. It has been shown by Hebard and Kaufman35 that in callus cultures of five clones of chestnut, the callus clones of "resistant" American chestnut trees (Castanea dentata) and of resistant chestnuts (C. crenata and C. mollis-sima), as compared with susceptible C. dentata, had much higher levels of hydrolyzable tannins (galloyl esters and ellagitannins) in the clones from resistant trees than in clones from susceptible trees. Challenging the respective callus cultures with virulent strains of the fungal pathogen showed that calli from susceptible chestnuts were overgrown by the pathogen, while calli from resistant strains were not affected and remained healthy. The conclusion from these findings is that the levels of hydrolyzable tannins in chestnut trees is correlated with resistance to the American chestnut blight fungal pathogen, but the correlation does not prove that condensed tannins are responsible for resistance to the pathogen either in culture or in chestnut trees. A question to ponder is this: What experiment(s) are necessary to show this kind of proof? (Hint: one must find a way to prevent the synthesis of tannins in cells that normally produce them.)

2.6.6 Attraction and Deterrence of Pollinators

As with the example of linalool production in Clarkia breweri plants seen in Section 2.5.2, many species of flowering plants have evolved the ability to produce various compounds that appeal to the visual, olfactory, and taste senses of insects or animals. Since many flowering plants are strictly dependent on a mobile organism to visit its flowers and pass its pollen to another plant of the same species, there is a distinct adaptive advantage to the plant that can attract a pollinator that will visit the same plant species over and over rather than spreading pollen around at random. One must remember that the pollinators too are evolving the ability to distinguish the plant species that provide the best rewards (food) over those that do not. This is in their best interest. So, a system of reward plays a critical role in the plant's pollination success.

Attraction of pollinators to flowers is achieved by several mechanisms. As discussed in Sections 2.4.9 and 2.4.10, coloration is a critical factor for attracting insects and animals that come out during the day. The color of flowers may be due to carotenoids in biomembranes (as in chromoplast membranes), brown phlobaphenes and black melanins in the cell walls, or red, yellow, pink, blue, and deep violet flavonoids, betacyanins, and betaxanthines in the cell vacuole (Table 2.4). Many of these colors are dependent on possible complexes with Fe3+ and Al3+ as well as on pH. Different pollinators are attracted to different colors (Table 2.5). Birds are generally attracted to red. Moths are attracted to white or light yellow flowers because these flowers are more visible at night when the moths are active. Flies prefer greens and browns. Butterflies tend to visit brightly colored flowers — yellow, blue, reddish — while bees prefer yellow and blue. Bees do not usually visit red flowers. This is most likely due to the fact that a bee's spectrum of vision includes very little red. It is shifted towards the ultraviolet range. Consequently, bees preferentially pollinate flowers that produce ultraviolet nectar guides (usually present on petals) that are invisible to the human eye and are the result of the biosynthesis of specific phenolic compounds (certain flavonoids) in specific patterns which are apparently discernible to different insects.

Odiforous substances that attract insects, birds, and mammals to flowers are usually produced as soon as the flower opens and help potential pollinators find the flower during both the day and night. These compounds include monoter-penes (e.g., linalool, limonene, geraniol), sesquiterpenes (e.g., P-ionone and a-(-)-bisabolol), aromatics (e.g., vanillin, eugenol, methyl eugenol), aliphatics (e.g., pentadecane, i-octanol), monoamines (e.g., methylamine, ethylamine, propy-

TABLE 2.4

Chemical Basis of Flower Color in Angiosperms (Flowering Plants)

Color

White, ivory, cream

Yellow

Orange Scarlet Brown

Magenta, crimson Pink

Mauve, violet Blue

Black (purple black) Green

Pigments responsible

Flavones (e.g., luteolin) and/or flavonols (e.g., quercetin) Carotenoid alone Yellow flavonol alone Anthochlor alone Carotenoid + yellow flavenoid Carotenoid alone Pelargonidin + aurone Pure pelargonidin Cyanidin + carotenoid Cyanidin on carotenoid background Pure cyanidin Pure peonidin Pure delphinidin Cyanidin + copigment/metal Delphinidin + copigment/metal Delphinidin at high concentration

Chlorophylls

Examples 95% of white flowered spp.

Majority of yellows Primula, Gossypium Linaria, Oxalis, Dahlia Coreopsis, Rudbeckia Calendula, Lilium Antirrhinum Many, incl. Salvia Tulipa

Cheiranhus, many

Orchidaceae Most reds, incl. Rosa Peony, Rosa rugosa Many incl. Verbena Centaurea

Most blues, Gentiana Black tulip, pansy

Helleborus

TABLE 2.5

Color Preferences of Different Pollinators

Animal

Bats

Bees

Beetles Birds

Butterflies (Lepidoptera) Moths (Heterocera) Flies

Wasps

Flower color preferences

White or drab colors, e.g., greens and pale purples

Yellow and blue intense colors, also white

Dull, cream or greenish color

Vivid scarlets, also bicolors (red-yellow)

Vivid colors, including reds and purples

Reds and purples, white or pink

Dull, brown, purple or green

Browns

Comments Mostly color blind

Can see in UV, but not sensitive to red Poor color sense Sensitive to red

Mostly pollinate at night Checkered pattern may be present lamine, butylamine, amylamine, hexylamine), diamines (e.g., putrescine and cadaverine), and indoles (e.g., indole and skatole). It is the various amines and indoles just listed that have unpleasant odors and attract pollinators such as flies and fungal gnats. Some plants such as Skunk Cabbage and Voodoo Lily actually benefit from photorespiration because, while the plant loses stored energy, it creates heat which better volitilizes the amines allowing them to be released more quickly and with a stronger odor. The other compounds, in general, produce pleasant odors which attract pollinators such as bees, butterflies, moths, and bats. The chemical attractants may be produced in special scent glands (called osmophores) produced by various organs of flowers, by epidermal cells along the upper sides of the petals, or in some cases by glandular hairs on leaves. Excellent discussions of these different pollination attractant syndromes are found in Larcher32 and Harborne.36 As mentioned in Section 2.5.2, it is the mixture of these chemicals produced by flowers that allows insects to distinguish between different species of plants, but there can be one specific scent component that determines which pollinator will pollinate a specific species of plant.

The rewards for pollination in plant flowers usually come in the form of sugar-rich solutions (sucrose, fructose, and glucose are the most common) that are secreted into the nectaries of flowers. They act in much the same way as the sugars, fats, and proteins found in mature fruits act to reward animals to disperse plant seeds (good examples here include squirrels that "forget" where they buried the acorn and birds that spread seeds all over your freshly washed car). Nectaries are located in different locations in different species, but they are almost always located at the junction between two different flower organs such as petals and ovary. The nectar held in the nectaries is not usually just sugar and water. It may also contain pigments such as anthocyanins, scents such as monoterpenes, and in some cases toxins.

Some compounds in flowers that are known to attract certain pollinators can also repel other potential pollinators. Indole, for example, can deter bees from pollinating alfalfa (Medicago sativa) flowers.37 Skatole, monoamines, and the offensive smelling diamines (putrescine and cadaverine) seem to serve similar functions in other flowers. Why would a plant "want" to repel a potential pollinator? The answer may lie in the fact that some insects and animals can cue in on plant attractants (odor for example) and take the reward produced by that plant without dispersing the plants pollen. To repel such a visitor would save the plant's energy in producing rewards. So, flowering plants undergo very distinct selective pressures to produce the specific compounds that will attract the best pollinators living in a specific environment.

2.6.7 Allelopathic Action

Allelopathy refers to plants which give off chemical substances that are injurious to other plants or prevent other plants from becoming established in the vicinity of the plant which gives off the allelopathic chemicals (also called allomones).32 Such chemicals have an obvious advantage to the plant that produces them by preventing the growth of other plant species that may compete for soil nutrients, carbon dioxide, or sunlight. Allelopathic chemicals include short-chain fatty acids, essential oils, phenolic compounds, alkaloids, steroids, and derivatives of coumarin. A classic example is the compound naphthalene glucoside produced by leaves and roots of walnut (Juglans spp.). This compound itself is not allelo-pathic; it must undergo hydrolysis and oxidation by soil microorganisms to produce hydrojuglone, and finally, the active compound, juglone. Juglone prevents the germination of seeds of many, but not all, plant species. This is why it's a bad idea to plant a wild flower garden in the same area as walnut trees. Another good example is the release of carboxyphenolic acids and hydroxycin-namic acids by heath family (Ericaceae) members which grow in such places as Scotland. Scotland was once covered with pine trees, but it was stripped to provide fuel for the growing industrial revolution. Now there is a problem with attempts at reforestation because the heath plants inhibit the association of mycorrhizal fungi with young pine roots. These fungi are essential symbionts for pines (see Section 2.6.8), so the seedlings eventually die. There are many more examples including cases with Calluna and Arctostaphylos (bearberry) that inhibit the growth of grasses (Poaceae family) and herbs; the release of terpenes and water-soluble phenolics by plants which inhabit steppes and arid shrub communities (Parthenium or guayule, Encelia, and Artemisia or sagebrush in the Asteraceae family and members of the Lamiaceae, Myrtaceae, Rutaceae, and Rosaceae families); and the release of orcinol depsides and usnic acid by lichens (plants that have algal and fungal partners living in association mutualistically) which exert an alleopathic effect on conifer seedlings and have an antibiotic effect on fungi which again may be symbionts.

2.6.8 Attraction of Symbionts

Not all plants are capable of getting enough nutrients out of the soils in which they live. Bacteria and fungi are sometimes much better at absorbing and/or producing some of the nutrients that plants require. Consequently, it is often the case that plants will elicit a partnership between themselves and specific bacterial or fungal symbionts. A classic case illustrating this concept is that of the establishment of a mutualistic association between a host plant and nitrogen-fixing bacteria such as Brachyrhizobium species. The bacteria become associated with the roots of the host plants and trigger the formation of nodules which provide the bacteria with a safe place to live as well as some plant nutrients and water while they supply the host plant with reduced nitrogen in the form of NH4+ that is derived from atmospheric nitrogen, N2. The reduced nitrogen is then used by the plant for synthesis of amino acids via amination reactions. At the start of this scenario, flavonoids are synthesized in significant amounts within the root systems of leguminous plants (e.g., the isoflavonoid, daidzein, in soybean, Glycine max; and the flavonoid, luteolin, in clovers, Trifolium spp.). These flavonoids play a key role in the establishment of the infection of host roots by nitrogen-fixing bacteria signaling the bacteria to bind to the plant roots after recognition of specific factors contained in the root glycocalyx (see Section 2.4.7). During the infection process, the flavonoids produced by the host plant upregulate the expression of so-called nod genes in the bacterial cells. These nod genes are required for three key steps in the infection process: (1) synthesis of a lipooligosaccharide molecule that induces root hair curling (root hairs are the sites for entry of the bacteria into the host root system), (2) the formation of an infection thread of bacterial cells in the host root hairs (from this thread the bacteria enter the cortical tissue of the root), and (3) the cell divisions in root cortical cells which give rise to root nodules.

Plants other than legumes can also develop symbiotic relationships with nitrogen-fixing organisms. The deciduous tree alder (Alnus spp.) can produce similar nodules upon infection. Grasses can form associations with soil bacteria, but they do not produce root nodules. Here, the bacteria seem to be anchored to the root surfaces. Fungi are also "recruited" for nutritional help. As mentioned, this is very common in the root system of pine and other trees which require an interaction with mycorrhizal fungi. Each of these examples has its own series of communicational signaling events between the plant and its specific symbiont.

2.6.9 Food for Pollinators, Symbionts, Herbivores, Pathogens, and Decomposers

It would not be right if we did not say something about the adaptive value of plant metabolites to other organisms. As we have emphasized throughout this chapter, it is the plant's ability to fix carbon from CO2 into more complex storage forms of metabolic energy that makes plants crucial to the survival of all other organisms including humans. Quite simply, plants provide organisms with most of the food necessary for their growth and reproduction. Witness the following examples:

• Pollinators foraging in flowers to find food rewards in the form of sugars produced in nectaries located near the sites of insertion of the floral organs.

• Other pollinators, e.g., the blastophaga wasp in fig fruits (synconia) which lay their eggs inside the developing fruit and whose larvae hatch out to use the inside portion of the fleshy fruit as a food source before they metamorphose into adult wasps.

• Symbiotic associations that benefit bacteria or fungi as well as the plant itself (e.g., algal/fungal partners in lichens, nitrogen-fixing bacteria in nodules of leguminous and other plants, nitrogen-fixing blue-green algae such as Nostoc and Anabaena in fronds of ferns such as Azolla spp.).

• Birds that devour whole fruits and regurgitate the flesh to their young while dispersing seeds along the way.

• Cows grazing on grasslands and later providing milk which builds strong bones in human offspring.

• Fungal and bacterial pathogens that invade plant cells and cause all sorts of plant diseases including blights.

• Shelf fungi, edible and toxic mushrooms, slime molds, and soil bacteria that feed off the plants even after they have long ceased to fix carbon.

These are only the very beginning of the diversity that we see due to the food supplied by plants.

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