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Chapter: Plant Biochemistry: Secondary metabolites fulfill specific ecological functions in plants

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Secondary metabolites often protect plants from pathogenic microorganisms and herbivores

Plants, because of their protein and carbohydrate content, are an important food source for many animals, such as insects, snails, and many vertebrates.

Secondary metabolites often protect plants from pathogenic microorganisms and herbivores

 

Plants, because of their protein and carbohydrate content, are an important food source for many animals, such as insects, snails, and many vertebrates. Since plants cannot run away, they have had to evolve strategies that make them indigestible or poisonous to protect them from being eaten. Many plants protect themselves by producing toxic proteins (e.g., amylase, protei-nase inhibitors or lectins), which impair the digestion of herbivores . In response to caterpillar feeding, maize plants mobilize a protease that destroys the caterpillar’s intestine. To secondary metabolites belong alkaloids, isoprenoids , and phenylpropanoids , all of which include natural pesticides that protect plants against herbivores and pathogenic microorganisms. In some plants these natural pesticides amount to 10% of the dry matter.

 

Some defense compounds against herbivores are part of the permanent outfit of plants; they are constitutive. Other defense components are only synthesized by the plant after browsing damage (induced defense). Another example,  is when plants damaged by caterpillars use the synthesis of scents (vol-atile secondary metabolites) to attract parasitic wasps, which lay their eggs in the caterpillars, thus killing them (indirect defense).

 

Microorganisms can be pathogens

 

Certain fungi and bacteria infect plants in order to utilize their resources for their own nutritional requirements. As this often leads to plant dis-eases, these infectants are called pathogens. In order to infect the plants effectively, the pathogenic microbes produce aggressive substances such as enzymes, which degrade the cell walls, or toxins, which damage the plant. An example is fuscicoccin , which is produced by the fungus Fusicoccum amygdalis. The production of compounds for infecting plants requires the presence of specific virulence genes. Plants protect themselves against pathogens by producing defense compounds that are encoded by specific resistance genes. The interaction of the virulence genes and resist-ance genes decides the success of the attack and defense.

 

When a plant is susceptible and the pathogen is aggressive, it leads to a disease, and the pathogen is called virulent. Such is termed a compatible interaction. If, on the other hand, the infecting pathogen is killed or at least its growth is very much retarded, this is an incompatible interaction, and the plant is regarded as resistant. Often just a single gene decides on compat-ibility and resistance between pathogen and host.

 

Plants synthesize phytoalexins in response to microbial infection

 

Defense compounds against microorganisms, especially fungi, are synthe-sized mostly in response to an infection (induced defense). These inducible defense substances, which are produced within hours, are called phytoalex-ins (alekein, Greek, to defend). Phytoalexins comprise a large number of compounds with very different structures such as isoprenoids, flavonoids, and stilbenes, many of which act as antibiotics against a broad spectrum of pathogenic fungi and bacteria. Plant root exudates contain bacteriostatic compounds such as cumaric acid, 3-indol propionic acid and methyl p-hydroxybenzoate that can render a plant resistant against pathogens. Plants produce for defense aggressive oxygen compounds such as superoxide rad-icals (•O2- ) and H2O2, as well as nitrogen monoxide (NO) , and enzymes, such as -glucanases, chitinases, and proteinases, which dam-age the cell walls of bacteria and fungi. Also the emission of volatile metab-olites is induced after pathogen attack, which directly or indirectly can alarm defense reactions in the plant or in plants in the neighborhood. The synthesis of these various defense substances is induced by so-called elici-tors. Elicitors are often proteins excreted by the pathogens to attack plant cells (e.g., cell-degrading enzymes). Moreover, polysaccharide segments of the cell’s own wall, produced by degradative enzymes of the pathogen, function as elicitors. But elicitors can also be fragments from the cell wall of the pathogen, released by defense enzymes of the plant. These various elicitors are bound to specific receptors on the outer surface of the plasma membrane of the plant cell. The binding of the elicitor releases signal cas-cades in which protein kinases  and signal substances such as salicylic acid  and jasmonic acid  participate, and which finally induce the transcription of genes for the synthesis of phyto-alexins, reactive oxygen compounds, and defense enzymes .

Elicitors may also cause an infected cell to die and the surrounding cells to die with it. In other words, the infected cells and those surround-ing it commit suicide. This can be caused, for instance, by the production of phenols of the infected cells to poison not only themselves but also their surrounding cells. This programmed cell death, called a hypersensitive response, serves to protect the plant. The cell walls around the necrotic tis-sue are strengthened by increased biosynthesis of lignin, and in this way the plant barricades itself against further spreading of the infection.

 

Plant defense compounds can also be a risk for humans

 

Substances toxic for animals are, in many cases, also toxic for humans. In crop plants, toxic or inedible secondary metabolites have been eliminated or at least decreased by breeding. This is why cultivated plants usually are more sensitive to pests than wild plants, thus necessitating the use of exter-nal pest control, which is predominantly achieved by the use of chemicals. Attempts to breed more resistant crop plants by crossing them with wild plants, however, may lead to problems, e.g., a newly introduced variety of insect-resistant potato had to be taken off the market because the highly toxic solanine content (an alkaloid) made these pota-toes unsuitable for human consumption. In a new variety of insect-resistant celery cultivated in the United States, the 10-fold increase in the content compounds such as cumaric acid, 3-indol propionic acid and methyl p-hydroxybenzoate that can render a plant resistant against pathogens. Plants produce for defense aggressive oxygen compounds such as superoxide rad-icals (•O2 ) and H2O2, as well as nitrogen monoxide (NO) , and enzymes, such as -glucanases, chitinases, and proteinases, which dam-age the cell walls of bacteria and fungi. Also the emission of volatile metab-olites is induced after pathogen attack, which directly or indirectly can alarm defense reactions in the plant or in plants in the neighborhood. The synthesis of these various defense substances is induced by so-called elici-tors. Elicitors are often proteins excreted by the pathogens to attack plant cells (e.g., cell-degrading enzymes). Moreover, polysaccharide segments of the cell’s own wall, produced by degradative enzymes of the pathogen, function as elicitors. But elicitors can also be fragments from the cell wall of the pathogen, released by defense enzymes of the plant. These various elicitors are bound to specific receptors on the outer surface of the plasma membrane of the plant cell. The binding of the elicitor releases signal cas-cades in which protein kinases  and signal substances such as salicylic acid  and jasmonic acid  participate, and which finally induce the transcription of genes for the synthesis of phyto-alexins, reactive oxygen compounds, and defense enzymes .

Elicitors may also cause an infected cell to die and the surrounding cells to die with it. In other words, the infected cells and those surround-ing it commit suicide. This can be caused, for instance, by the production of phenols of the infected cells to poison not only themselves but also their surrounding cells. This programmed cell death, called a hypersensitive response, serves to protect the plant. The cell walls around the necrotic tis-sue are strengthened by increased biosynthesis of lignin, and in this way the plant barricades itself against further spreading of the infection.

 

 

Plant defense compounds can also be a risk for humans

 

Substances toxic for animals are, in many cases, also toxic for humans. In crop plants, toxic or inedible secondary metabolites have been eliminated or at least decreased by breeding. This is why cultivated plants usually are more sensitive to pests than wild plants, thus necessitating the use of exter-nal pest control, which is predominantly achieved by the use of chemicals. Attempts to breed more resistant crop plants by crossing them with wild plants, however, may lead to problems, e.g., a newly introduced variety of insect-resistant potato had to be taken off the market because the highly toxic solanine content (an alkaloid) made these pota-toes unsuitable for human consumption. In a new variety of insect-resistant celery cultivated in the United States, the 10-fold increase in the content of psoralines  caused severe skin damage to people harvesting the plants. This illustrates that natural pest control is not without risk.

 

A number of plant constituents that are harmful to humans (e.g., pro-teins such as lectins, amylase inhibitors, proteinase inhibitors, and cyano-genic glycosides or glucosinolates) decompose when cooked. But most secondary metabolites are not destroyed in this way. In higher concentrations, many plant secondary metabolites are can-cerogenic. It has been estimated that in industrialized countries more than 99% of all cancerogenic compounds that humans normally consume with their diet are plant secondary metabolites that are natural constituents of the food. However, experience has shown that the human metabolism usu-ally provides sufficient protection against many harmful natural substances particularly at lower concentrations. As will be discussed in the following, plants also contain many compounds which are used as pharmaceuticals to combat illnesses.

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