What are secondary metabolites?

Secondary metabolites are phytochemical compounds not directly involved in basic life processes (growth, cell division, respiration, photosynthesis and reproduction), unlike primary metabolites. Secondary metabolites have specific functions in response to adaptation to an environment. Some of the main roles for plants are :

• Plant protection against pests and pathogens.
• Allelopathy (plant-plant competition).
• Plant-microbe symbiosis in root nodules.
• Color, smell and taste. They can therefore be attractive to pollinators.

Secondary metabolites fall into 3 main categories: terpenes, phenolics and alkaloids (or nitrogen compounds). Secondary metabolites found in plants are widely used in medicine for anxiety and stress-related problems, as well as to alleviate the symptoms of chronic illnesses and prevent various diseases ranging from migraine to cancer.

Terpenes

With over 25,000 compounds, terpenes are the most important class of secondary metabolites. Naturally produced by plants, they are characterized by their volatile nature and strong odor. They give flowers their distinctive fragrance. They also have many other functions, including photosynthesis (as a constituent of chlorophyll), photoprotection (as a carotenoid), pigmentation and antioxidant capacity as a constituent of vitamin A.

Some studies have shown the relationship between light and terpene synthesis (Loveys and Wareing, 1971; Gleizes et al., 1980; Yamaura et al., 1991, Schnarrenberger and Mohr (1970), and Tanaka et al (1989)). In fact, monoterpene and carotenoid synthesis is regulated by the red light receptor phytochrome. Through carotenoids, terpenes are also linked to the absorption of blue light.

Phenolic compounds

Phenols, the ubiquitous and diverse group of secondary metabolites found in plants, comprise over 10,000 compounds widely distributed throughout the plant kingdom. The diversity of phenolic compounds ranges from simple molecules (flavonoids, phenolic acids and phenylpropanoids) to highly polymerized molecules (lignin, tannin and melanin).

Phenolic compounds are synthesized in response to abiotic or biotic stresses. They are often synthesized in response to pathogen or insect attack, high temperatures, excessive intensity or harmful wavelengths. Anthocyanins, for example, are a class of flavonoids visible in the purple/red color of leaf tissue. Present in large quantities in leaves, they act primarily as sunscreens. Thanks to their antioxidant, anticancer, anti-inflammatory and cardioprotective activity, phenolic compounds are beneficial to human health.

The concentration of phenolic compounds in a plant can be modified by the quality of a light spectrum. For example, blue light induces an increase in phenolic compounds such as flavonoids (Azad et al. 2018; Park et al. 2019).

Conclusion

In a controlled environment, we can manipulate the synthesis of secondary metabolites to influence the plant’s natural defense mechanisms or to enhance plant quality. In fact, secondary metabolites are synthesized as a function of the environment. One of these environmental factors is light. However, there is a trade-off between growth and secondary metabolite synthesis.