Introduction

Secondary metabolites (SMs) are natural products that are synthesized mainly by plants, bacteria, fungi, and animals. These metabolites are important for the normal defense mechanism of the body. And are used as medicine, flavorings, and pigments. They are molecules of low molecular weight with different chemical structures.

The name secondary metabolite originates from the initial observation that their production is not essential for the growth and reproduction of organisms. It is now accepted that secondary metabolites play key roles in the survival of the organisms that produce them because they determine interactions within their environment. Nowadays, SMs production is a major research field for organic chemists, molecular biologists, and bioinformaticians.

Secondary metabolites

• These are the organic molecules that are not involved in the normal growth and development of an organism.
• While primary metabolites have an important role in the survival of the species, by playing an active function in the process like photosynthesis and respiration.
• However, the absence of secondary metabolites does not result in direct death, but rather in a long-term impairment of the organism’s survivability.
• These compounds are particularly from a different group of natural products and are produced by plants, bacteria, animals, fungi, and algae.
• Most of these metabolites, such as terpenes, phenolic compounds, and alkaloids are classified based on their biosynthetic origin.
• Also, it is suggested that the genes involved in secondary metabolism provide a ‘‘genetic playing field” that allows mutation and natural selection.
• Another view characterizes secondary metabolism as a vital part of cellular metabolism and biology.

Plant secondary metabolites

• Plants possess the capacity to synthesize different organic molecules called secondary metabolites.
• Unique carbon skeleton structures are basic properties of plant secondary metabolites.
• Secondary metabolites are not essential for a cell to live but play a role in the dealings of the cell with its surroundings, assuring the continued existence of the organism in its ecosystems.
• The formation of secondary metabolites is generally organ, tissue, and cell-specific, and these are low molecular weight compounds.
• They are used to protect plants against stresses, both abiotic (temperature, moisture, shading, injury, or presence of heavy metals) and biotic (bacteria, fungi, nematodes, insects, and animals).
• Also are used particularly as chemicals such as flavors, fragrances, insecticides, dyes, and drugs for their economic value.
• In plants, secondary metabolites can be separated into three major groups (Terpenoids, Polyketides, and Phenypropanoids) based on their biosynthesis origin.

Types of secondary metabolites

A simple classification includes three main groups:

• Terpenes (such as plant volatiles, carotenoids, and sterols),
• Phenolics (such as phenolic acids, lignins, flavonoids, tannins, and lignin)
• Nitrogen-containing compounds (such as alkaloids and glycosylates).

Flavonoids, alkaloids, and their medicinal application

Medicinal applications are antitumor, cholesterol-lowering, immunosuppressant, antiprotozoal, antihelminth, antiviral, and anti-aging activities. Polyene antibiotics like amphotericin B are used as antitumor drugs against leishmaniasis. Other natural products that show antibiotic activity are doxorubicin, neomycin, β-lactams, and rapamycin which are used against cancer.

Macrolide antibiotics, such as erythromycin, and azithromycin, improve pulmonary function. Pigments like prodigiosin have antitumor activity, while violacein has anti-ulcer and antitumor activity.

Immunosuppressants have many alternative effects

• Cyclosporine is useful in the cure of inflammatory diseases such as asthma and muscular dystrophy.
• Rapamycin is exceptionally helpful in inflammatory diseases of stents grafted in balloon angioplasty.
• Tacrolimus and ascomycin help in treating inflammatory skin diseases such as allergic contact dermatitis. Artemisinin, an antimalarial agent, is also used for antitumor activity.

Q&A

What are secondary metabolites?

They are compounds that are not required for the growth or reproduction of an organism but are produced to confer a selective advantage to the organism. For example, they may inhibit the growth of organisms with which they compete and, as such, they often inhibit biologically important processes.

What are secondary metabolites in plants?

These are substances manufactured by plants that make them competitive in their own environment. These small molecules exert a wide range of effects on the plant itself and on other living organisms.

Which of the plant secondary metabolites is used as an antimalarial drug?

Artemisinin and its analogs are naturally occurring most effective antimalarial secondary metabolites.

When are secondary metabolites produced?

Secondary metabolites are produced by microorganisms when one or more of the nutrients in the culture medium is depleted. Are usually have an important ecological function and serve diverse survival functions in nature. Secondary metabolites are formed near the stationary phase of growth and are not involved in growth, reproduction, and development.

Closing summary

• Secondary metabolites are synthesized during the end or near the stationary phase of growth and are not involved in cell growth, development, or reproduction.
• Ecological function, including acting as a protection mechanism(s), by serving as antibiotics and by producing pigments.
• Are very important for the health, nutrition, and economics of human societies.
• Gibberellins, alkaloids, antibiotics, and biopolymers are examples of secondary metabolites.
• Plant SMs are important compounds that add color, taste, and odor to plants and also mediate plant responses to adverse environmental conditions.
• Furthermore, the endogenous levels of SM are also influenced by several environmental stresses such as nutrient deficiencies, wounding, metal ions, ultraviolet (UV) radiation, light, circadian rhythm, seasonality, salinity, drought, and temperature.
• The concentration of SM is also affected by biotic factors (pathogen attack) and thereby mediates plant defense mechanisms.
• In vitro tissue culture may also trigger an increase in the biosynthesis of different SMs via signaling molecules and plant growth regulators.
• This indicated that the endogenous levels of SM can be changed.
• Overall, factors that trigger alterations in SMs fall into four categories.

1. environmental factors
2. Morphogenetic factors
3. Ontogenetic factors
4. Genetic factors.

Among these factors, environmental factors are the key determinants for the fluctuations in plant SMs.

• SMs are one of their essential means of growth and defense, and these metabolites are readily available for discovery.
• They with noteworthy biological activity are considered an alternative to most synthetic drugs and other commercially valuable compounds.

Written By: Pallavi Bhoyar