11 Feb 2022

Secondary Metabolites: Definition, Classes, and Functions (Part-2)

Anjali Singh, MS

As a content and community manager, I leverage my expertise in plant biotechnology, passion for tissue culture, and writing skills to create compelling articles, simplifying intricate scientific concepts, and address your inquiries. As a dedicated science communicator, I strive to spark curiosity and foster a love for science in my audience.

Table of Contents

Overview

Secondary metabolites are compounds naturally produced by plants that are not involved in the primary growth and development of plants. However, they help plants in several processes like reproduction and protection from pests and pathogens.

Based on their molecular structure, the secondary metabolites are classified into five major classes:

• Terpenoids
• Phenolics
• Alkaloids
• Polyketides
• Carbohydrates

In the previous part of this article, we already covered three classes: terpenoids, phenolics, and alkaloids, and the secondary metabolites representing these groups.

This article will teach you about two other classes of secondary metabolites—polyketides and carbohydrates—including what they are and their example secondary metabolites. Further, we’ll also cover why these molecules or substances are produced in plants and how they help if they are not involved in their primary functions.

Classes of Secondary Metabolites (Continued from previous part)

Polyketides

Polyketides are secondary compounds derived from precursor molecules consisting of a chain of alternating ketone(or reduced forms of a ketone) and methylene groups (-CO-CH2-). The molecules of this class have several applications, including agricultural, medicinal, and industrial applications.

The subclasses of Polyketides are: macrolactones, aromatics, polyether, decalin ring containing, and polyenes. Its examples include geldanamycin, doxycycline, erythromycin, and aflatoxin B.

Carbohydrates

The bioactive polysaccharides have beta 1–3, 1–4, or 1–6 branch chains. They are primarily derived from fungi but are also found in many plants including cinnamon, aloe, ginseng, gingers, and lallang.

The compounds possess immuno-inhibitory activity under in vitro conditions. However, they are also involved in the induction of cell differentiation, act as an antimicrobial action against Streptococcus mutans, have an antimetastatic effect, and have a role in the inhibition of angiogenesis.

Functions of Secondary Metabolites in Plants

Secondary metabolites have a wide range of functions in plants, ranging from involving in responses to environmental challenges, defending plants against pathogens and pest attacks, and attracting animals for pollination.

Here’re some well-studied roles of secondary metabolites in plants.

Adaptation to life

Scientists suggest that the expansion in secondary metabolites started with the evolution of surface plants about 500 million years ago. The transition of plants’ lives from ocean to land brought some new challenges and stress, including exposure to UV radiation, drought stress, lack of structural support, and attacks by newly evolved (and evolving) pathogens and herbivores.

All the stress led to the initiation of a new metabolic pathway in plants that produced compounds, which helped them to address the challenges. For example:

• For protection from UV, plants produce phenylpropanoids
• Development of lignin in plants’ vascular system, which helps it to reach and grow to heights not adapted by Oliver plant lineages, such as bryophytes. The molecular also protect plants from damages caused by herbivores and wind.

The attraction of seed dispersers and pollinators

Animals play an important role in the sexual reproduction of plants. Though many plants rely on abiotic agents, such as wind for their pollination, there are others that require insects, bats, or birds for reproduction. The insects or birds while feeding on these plants come in contact with their sexual part, and in this way, they transfer pollen from one plant to the stigma of the other.

However, it’s not that easy for plants. To attract pollinators for fertilization, plants produce colorful flowers and produce flavors and volatile scents. Some plants offer nectar to carry the cooperation with their pollinators.

Similarly, some plants produce fruits having beautiful colors, volatile scents, and flavors to attract animals that help in dispersing their seeds while feeding on the fruits.

Defense against pathogens, herbivores, and other pests

Secondary metabolites are very essential for the protection of plants against biotic attacks, like bacteria, fungi, aphids, caterpillars, and castles. Plants produce phytoalexins and other secondary compounds such as alkaloids, terpenes, isoflavonoids, and polyacetylene to protect themselves from predatory attacks.

The secondary metabolites are toxic to plants or taste bitter to them, leading to reduced feeding of plants by these animals. Some plants produce such compounds that attract the enemy of their predators, thus, protecting them from the attackers.

Plant-to-plant interaction

Secondary metabolites are a means for plants to communicate with each other in response to biotic and abiotic stress. However, it’s still an ongoing research area. Though scientists have found volatile-mediated communication between plants in about 30 species, including shrubs, trees, and herbaceous plants.

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