Sun is the ultimate source of energy for the earth. Plants
convert light energy from the sun into chemical
energy (food) by the process of photosynthesis. All living beings
depend on plants for food and oxygen. Animals would disappear from the Earth if
photosynthesizing plants were to disappear, since animals require for their
nutrition the complex organic compounds that can be synthesized only by plants.
The animal excretions and the animal body after death are also converted by a
process of decay to simple products that can be re-utilized only by plants.
Green plants require carbon dioxide, water, mineral salts, and sunlight for the
formation of carbohydrates. Not only
carbohydrates, but plants also synthesize amino acids, proteins, lipids,
pigments, and other organic components during photosynthesis. Instead of
energy, plants also need chemical compounds to interact with their neighbors,
to attract animals for pollination and to defend themselves from animals that
want to eat them. To perform all these functions, plants produce a great number
of chemicals, known as metabolites.
Metabolism
Metabolism constituents all the chemical transformations occurring in the cells of living organisms and these transformations are essential for life of an organism. Primary metabolism comprises all metabolic pathways that are essential to the plant's survival, generating metabolites that are directly involved in the growth and development of the organism. On the other hand, secondary metabolism produces a large number of specialized compounds that are not essential to the functioning of the plant but are required for the plant to survive in its environment. Secondary plant metabolites are useful for defence purposes. They are also used in communication, signalling and regulation of primary metabolic pathways.
Metabolites
End
product of metabolic processes and intermediates formed during metabolic
processes is called metabolites. Metabolites are of two types 1) primary
metabolites and 2) secondary metabolites.
Primary Metabolites
A primary metabolite is a kind of metabolite that is directly involved in normal growth, development, and reproduction. It usually performs a physiological function in the organism. Some common examples of primary metabolites include: ethanol, lactic acid, and certain amino acids. In higher plants such compounds are often concentrated in seeds and vegetative storage organs and are needed for physiological development because of their role in basic cell metabolism. Primary metabolites obtained from higher plants are mainly used as industrial raw materials, foods, or food additives and include products such as vegetable oils, fatty acids, and carbohydrates.
Secondary
metabolites
Secondary metabolite is not directly involved in growth and development processes, but usually has an important ecological function. A secondary metabolite is typically present in a taxonomically restricted set of organisms or cells (Plants, Fungi, Bacteria etc.). Some common examples of secondary metabolites include: alkaloids, antibiotics, naphthalenes, nucleosides, phenazines, quinolines, terpenoids, flavonoids, peptides and growth factors. Plant growth regulators may be classified as both primary and secondary metabolites due to their role in plant growth and development. Some of them are intermediates between primary and secondary metabolism such as ethanol, gluromic and aseptic acid, lactic acid, isoarobic, vitamins B12etc. Over 2,140,000 secondary metabolites are known and are commonly classified according to their vast diversity in structure, function, and biosynthesis. There are five main classes of secondary metabolites such as terpenoids, steroids, alkaloids, flavonoids and nonribosomal polypeptides.
Terpenoids
Terpenids
are modified class of terpenes with different functional groups and oxidized
methyl group removed at various positions. They are major group of substances derived biosynthetically
from isopentenyl diphosphate. Currently, over 35,000 known terpenoid and
steroid compounds are identified. Terpenoids
may be monoterpenes, sesquiterpenes, diterpenes, sesterpenes and triterpenes
depending upon its carbon units. Basically they consist of five carbon isoprene
units. They play diverse role in the field of foods, drugs, cosmetics, hormones
and vitamins.
Steroids
Steroids have a common tetracyclic
carbon skeleton and are modified terpenoids. Biosynthetically,
they are derived from S-squalene-2,3-epoxide via acetate-mevalonate
pathway. Among the plant steroids, phytosterols are ubiquitous in the plant
kingdom. It is significant that some phytosterols have been reported to possess
hypocholesterolemic activity. Withanolides are a large group of steroidal
lactones with various biological activities. Brassinosteroids are a small group
of plant steroids exhibiting plant growth hormonal activity. Phytoecdysteroids
are polyhydroxylated plant steroids, many of which are known to exhibit
anabolic effects with no undesirable side effects. Steroidal alkaloids are
nitrogen-containing plant steroids with an array of biological activities.
Alkaloids
There are over 12,000 known
compounds of alkaloids, and their basic structures consist of basic amine group
and are derived biosynthetically from amino acids. The name alkaloid (“alkali-like”) was
originally applied to the substances because, like the inorganic alkalis, they
react with acids to form salts. Most alkaloids have one or more of their
nitrogen atoms as part of a ring of atoms, frequently called a cyclic system.
Alkaloid names generally end in the suffix -ine, a reference to
their chemical classification as amines. In their pure form most alkaloids are
colourless, nonvolatile, crystalline solids. They also tend to have a bitter
taste.
Flavonoids
Flavonoids are an important class of natural products having
a polyphenolic structure, widely found in fruits, vegetables and certain
beverages. They have favourable biochemical and antioxidant effects associated
with various diseases such as cancer, Alzheimer's disease (AD),
atherosclerosis, etc. Flavonoids are associated with a broad spectrum of
health-promoting effects and are an indispensable component in a variety of
nutraceutical, pharmaceutical, medicinal and cosmetic applications. This is
because of their antioxidative, anti-inflammatory, anti-mutagenic and
anti-carcinogenic properties coupled with their capacity to modulate key
cellular enzyme functions.
Nonribosomal
polypeptides
These amino acids derived compounds
are biologically synthesized by a multifunctional enzyme complex without direct
RNA transcription. NRPs represent a vast range of
bioactivities and pharmacological properties. For example, NRP natural products
such as antibiotics (e.g. actinomycin, penicillin, cephalosporin, vancomycin),
cytotoxics (e.g. bleomycin), and immunosuppressants (e.g., cyclosporines)
have found beneficial applications in human medicine. NRPSs also synthesize
some pigments such as indigoidine, and some harmful toxins such as HC-toxin and
AM-toxin phytotoxins which cause crop losses in agroforestry.
Role
of secondary metabolites in plant defence
Plants control defense activation to save metabolic energy and avoid self-damage. Many secondary metabolites found in plants have a role in defence against herbivores, pests and pathogens. The role of secondary metabolites in defence may involve deterrence activity, toxicity or acting as precursors to physical defence systems. Many specialist herbivores and pathogens do not merely circumvent the deterrent or toxic effects of secondary metabolites but actually utilize these compounds as either host recognition cues or nutrients. Defense investment is typically titrated through feedback regulation, including both positive and negative feedback loops that are built into early defense signaling. Using secondary metabolites as defense activation readouts may also help plants to optimize synergies between different defenses and to compensate for accidental failures of specific defense pathways. As many secondary metabolites are compartmentalized and/or stored in inactive forms, their decompartmentalization and activation likely also helps plants to recognize tissue damage and other forms of environmental stress. In this case, the metabolites would be used as damage–associated molecular patterns. Herbivores, pathogens, and viruses can interfere with defense hormone signaling and thereby manipulate plants for their own benefit. The high degree of conservation in defense hormone signaling may in fact favor the evolution of biotic manipulation of plant signaling. Secondary metabolites can also be regulators and precursors of primary metabolites, then it becomes conceivable that they may have similar roles in herbivores. Specialist herbivores are known to use secondary metabolites as infochemicals and some also sequester defenses to protect themselves against herbivore natural enemies, in analogy to the use of these chemicals as defense regulators and resistance factors in plants. Herbivores can also use plant secondary metabolites for herbivore-specific functions. Cyanogenic glycosides, for instance, can be used by specialized lepidoptera as defenses and nuptial gifts, and glucosinolates are part of the pheromone blend of flea beetles.
Commercial and economic importance of secondary metabolites
Pharmaceutical applications
Plant
secondary metabolites have great importance in the pharmaceutical, agriculture,
and food industries. Plants have been used for medicinal purposes by humans for
several thousands of years. The development of medicinal uses for plants has
been specific to the plants indigenous to a given region, but has occurred
independently in several civilizations. Earlier, herbal medicines were found
primarily in the forms of teas, powders, poultices and tinctures. In the modern
era, however, advances in analytical chemistry and medicine have allowed
researchers to extract and characterize the active components of the medicinal
plants, and to ultimately use these compounds as pharmaceuticals. Uses of plant
secondary metabolites include anti-malarials, anti-inflammatory, anti-cancer
and as antimicrobials. Cancer has always been a focus of medicinal research,
and plant secondary/specialized metabolites have contributed substantially to
this research. Secondary metabolites have been demonstrated to exhibit several
anti-cancer effects, such as the ability to either kill cancer cells, slow the
growth and development of cancer cells or tumors. Secondary metabolites with
anti-cancer activity include flavonoids, quinones, alkaloids and terpenoids. Paclitaxel
(a diterpene) is a famous plant secondary metabolite. Because of toxicity and
hydrophobicity of paclitaxel, which increase the difficulty of administering
this drug in mammalian systems, a hydrophilic derivative docetaxel has also
been developed. Both Taxol and Taxotere are currently being used to treat lung
cancer, breast cancer, prostate cancer and ovarian cancer. Several plant
secondary metabolites can also function to reduce inflammatory responses.
Inflammation in mammalian systems is a protective response to harmful stimuli,
which can result from burn injury, pathogen infection exposure to toxin,
localized trauma and other physical injuries. Recent research efforts have been
focused on expanding the current pharmacological toolkit of anti-inflammatory
secondary metabolites, as well as understanding the mechanisms by which these
compounds reduce inflammation. Numerous studies have been conducted to
elucidate the anti-malarial effects and relevant modes of action of
sesquiterpene lactone.
Viral
diseases cannot be treated using standard antibiotics, and often result in
complications that may cause death. Viruses which attack the immune system,
such as Human Immunodeficiency Virus which is the causal agent of Acquired Immune
Deficiency Syndrome [AIDS] are especially difficult to combat. Numerous plant
secondary metabolites with antimicrobial activity have also been discovered,
including flavonoids such as catechins from tea plants, tannins, terpenoids and
alkaloids such as reserpine, berberine.
Secondary
metabolites can exert anti-microbial activity through a range of mechanisms,
including altering membrane permeability or by inactivating membrane efflux
pumps, resulting in the accumulation of toxins in the bacteria. Alternatively,
secondary metabolites may also function to inhibit pathogen progression is by
inhibiting quorum sensing and blocking the communication between bacteria
necessary for systemic or large-scale infection. In addition to inhibiting
quorum sensing, they can also suppress the biofilm formation of bacterial infectious
pathogens. In addition to cancers and microbial diseases, secondary metabolites
have also been used to treat a range of human ailments such as psoriasis,
eczema and other skin problems.
Agricultural applications
Secondary
metabolites play important role in regulating plant growth, development, and
responses to environmental stimuli. Several of secondary metabolites have been
employed by the agricultural industry to regulate crop production. They have
been developed for agricultural usage as insecticides, herbicides and
herbivore-repellents. Neonicotinoids are a group of insecticides that are
derived from the alkaloid nicotine, commonly found in tobacco and responsible
for the addictive effect of many tobacco products. This group of chemicals
functions by targeting the central nervous systems of a broad range of insect
pests, including aphids, white flies, thrips and Lepidoptera. Similarly,
isoflavone, a pesticide functions by inhibiting the electron flow of the
mitochondrial respiratory chain, results in insect death. They are also used as
herbicidal agents, capable of killing or inhibiting the growth of weeds or
other competing plant species. Herbicides based on plant growth regulators or
phytohormones exert their effects by mimicking the functions of the hormone in
question, leading to aberrant plant growth and/or plant death. For example,
2,4-dichlorophenoxyacetic acid (2,4-D) functions by mimicking the functions of
plant hormone auxin (indole-3-acetic acid), and thus is categorized as an
auxinic herbicide. Application of 2,4-D impacts several 20 developmental
processes regulated by auxin (meristem cell division, stem elongation, cell
wall loosening), eventually leading to “burn-out” or an “overdrive” of growth
leading to death in sensitive plants.
Besides
being used in the pharmaceutical/biomedical and agricultural industries, plant
secondary metabolites have also been used by humans in a broad range of other
industries. In the food industry, they provide flavor, aroma, and/or color to
many beverages or other food products. Coffee is a high-value food product
where almost all of the flavor and sensory characteristics are provided by
secondary metabolites. The aroma of coffee is the result of numerous volatile
secondary metabolites, while the acidity and bitterness of this beverage are
due to alkaloid caffeine, chlorogenic acids, dicaffeyolquinic acids, and other
phenolic compounds. Secondary metabolites can also be used to provide colors in
either foods or industrial products (textiles). Secondary metabolites employed
for this purpose include indigo, shikonin, juglone, and anthocyanins
General Functions of secondary metabolites
- They act as competitive weapons against other living organisms such as animals, plants, insects, and microorganisms.
- They act as metal transporting agents.
- They play a key role in symbiotic relation with other organisms.
- They also play a very important role in pollination.
- Secondary metabolites also act as agents of communication between organisms.
- The other functions include interference in spore formation and germination.
- They are used for variety of biological activities like antimicrobial and antiparasitic agents, enzyme inhibitors and antitumor agent, immunosuppressive agents, etc.