Definition: Secondary metabolites are organic compounds that are not directly involved in the normal growth,
development or reproduction of an organism. They are usually found in very small amounts but have an effect on
microorganisms, animals and humans. They are originally known to accumulate under defense response; against
abiotic, biotic and insect stress. It is believed that plants had to accumulate new secondary constituents during
evolution for their protection. E.g. Carotenoids, Alkaloids, Saponins, Flavonoids.
Sources of plant secondary metabolites:
Carotenoids - e.g. Carrot
Phytosterols - e.g. Sunflower seeds, soya beans
Saponins - e.g. all types of cabbages, mustard, radish
Terpenes - e.g. menthol in peppermint oils
Flavonoids- e.g. all types of fruits and vegetables
Classification: A simple classification includes three main groups:
- the terpenes (made from mevalonic acid, composed almost entirely of carbon and hydrogen)
- phenolics (made from simple sugars, containing benzene rings, hydrogen, and oxygen), and
- nitrogen-containing compounds (extremely diverse, may also contain sulfur).
- Flavonoids are known to be the important group of secondary metabolites with over 4000 species identified across different plant species. They are significant in terms of their antioxidant properties. So. this study is aimed to highlight the basic nomenclature, biosynthesis and significance of flavonoids among different plant systems.
- First discovered as components of plant pigments by Robert Boyle in the year 1664.
- Present in high concentration in the epidermis of leaves, flowers, skin of fruits and beverages.
- They provide pigmentation and are involved in fertility and germination of pollen.
- UV Protection, and act as signal molecules in plant microbe interactions.
- They inhibit the growth of bacteria and viruses, protect cells against damage due to free radicals, protect against cancer and heart attacks.
- Under certain conditions, such as high availability of transition metals, flavonoids can act as pro oxidants and hence promote the oxidation of other compounds.
- Flavonoids are considered to be important as anti-oxidants especially in stressed plants by inhibiting reactive oxygen species (ROS) once formed.
- Flavonoids with the greatest antioxidant potential have been reported to accumulate in response to high solar irradiation, either in the presence or absence of UV-radiation.
Table 1. List of Flavonoids
- Flavonoids consist of C6-C3-C6 carbon frameworks which are benzo –γ- pyran derivative that can be grouped according to the presence of different functional groups on the rings.
- They are characterized by two aromatic rings connected by a C3 bridge and substitutions of hydroxyl groups or other moieties which influence their biological activity.
Figure 1. Basic structure of a flavonoid
- Based on the degree of oxidation and saturation present in the heterocyclic C-ring the flavonoids may be divided into following classes:
Figure 2. Different categories of Flavonoids
- Different classes of flavonoids with various substituent patterns are shown in below table:
Table 2. Substitution patterns of flavonoids on the A, B and C ring and sources
Accumulation pattern of Secondary metabolites in plants
- Higher plants synthesize wide range of secondary metabolites and their production depends largely on the physiological growth and development of plants.
- Accumulation of phenyl-propanoids has been shown to increase under various stress conditions such as UV-irradiation, wounding, pathogen attack, increased light exposure and nutrient deficiencies.
- These factors play a crucial role in increasing accumulation of secondary metabolites in plants.
- Examples of stress factors responsible for specific secondary metabolites accumulation in plants are shown below in the diagram:
Figure 3. Accumulation of various classes of secondary metabolites in different plants by stress mechanisms
Biosynthesis of Flavonoids
- The flavonoid pathway is a part of the larger phenyl propanoid pathway, which produces a range of other secondary metabolites, such as phenolic acids, lignin, lignans and stilbenes.
- The key flavonoid precursors are: phenyl alanine (from shikimate and arogenate pathways) and malonyl CoA (from TCA cycle).
- The flavonoid biosynthetic pathway can be accessed through the link
Model Plant Systems – Described below are three different plant species to understand the accumulation of flavonoids
Tea (Camellia sinensis)
- Tea, member of Theaceae, is an aromatic beverage most widely consumed all over the world. It is a commercial crop which is of high importance due to the presence of polyphenols and also medicinal properties.
- The major steps involved in Tea biosynthetic pathway can be accessed through the link
- The tea leaves are cultivated and processed in different ways. The difference in their varieties of tea is due to the oxidation process.
Table 3. Different varieties of Tea
- Tea polyphenols may account upto 30 % of the dry weight.
- Flavonoids sepecially the derivatives of catechins are abundant in tea.
- Flavonols such as Quercetin, kamepferol and myricetin are also found in tea.
- The anti-carcinogenic activities of tea polyphenols are generally believed to be related to their antioxidant potential.
Table 4. Flavonoids present in different varieties of Tea
Maize (Zea mays)
- Maize, member of Poaceae, is one of the most important cereal produced mainly in South Africa and is widely distributed throughout the world under climatic conditions.
- In maize, various flavonoids are synthesized and accumulate in both epidermal and mesophyll tissues.
- Anthocyanins are responsible for the various colors of maize kernels and protection of the leaf cells from photo-oxidative damage.
- Flavonols contribute to seed pigmentation mainly as co-pigments with anthocyanins.
- All these compounds protect plants against herbivore or pathogen attack.
- The biosynthetic pathway of maize can be accessed through the link
Petunia ( Petunia hybrida)
- Petunia, member of Solanaceae, exhibits different flower colors due to the presence of anthocyanins.
- The hydrophobic interactions between the aromatic rings of the anthocyanins and co-pigments results in visible colors of petunia flower.
- Petunia flower colors are mainly based on six anthocyanidins (pelargonidin, cyandin, peonidin, delphinidin, petunidin and malvidin).
- All anthocyanins found in these flower species are glycosylated at the 3’ position and may possess a glucose group at the 5’position.
- UV-B light has been shown to specifically induce the accumulation of quercetin derivatives in Petunia.
- All the accumulated compounds in Petunia play major roles including UV protection, modulation of flower color and signalling.
- The biosynthetic pathway of Petunia can be accessed through the link