Plant secondary metabolites, also referred to as secondary plant compounds or natural products, are organic chemicals synthesized by plants that are not directly involved in the plant's fundamental metabolic processes, such as growth, development, and reproduction. In contrast to primary metabolites, which are essential for the plant's basic functions, secondary metabolites serve diverse ecological and adaptive roles.
There exists a wide array of secondary metabolites present in plants, encompassing alkaloids (e.g., caffeine, nicotine), phenolics (e.g., flavonoids, tannins), terpenoids (e.g., essential oils), and glucosinolates. The production of these compounds is frequently influenced by genetic factors and environmental conditions, such as light, temperature, and nutrient availability.
Researchers investigate plant secondary metabolites for various objectives, including their potential applications in pharmaceuticals, agriculture, and industry. Many of these compounds have been harnessed for their medicinal attributes, flavors, fragrances, and even as natural agents for pest control or weed management.
Biological Functions and Ecological Roles of Secondary Metabolites
Defense Mechanisms:
Secondary metabolites, including alkaloids and terpenoids, serve as potent chemical defenses against herbivores and pathogens. These compounds often have toxic or repellent effects, discouraging animals and insects from feeding on or damaging the plant.
Attraction of Pollinators:
Some secondary metabolites, like colorful flavonoids and fragrant terpenoids, play a crucial role in attracting pollinators like bees and butterflies. By doing so, they facilitate pollination and contribute to the reproductive success of the plant.
Allelopathy:
Plants release secondary metabolites into their surroundings, which can inhibit the growth of nearby competing plants through a phenomenon known as allelopathy. This provides the producing plant with a competitive advantage in resource acquisition.
Abiotic Stress Tolerance:
Certain secondary metabolites, such as polyphenols, help plants cope with abiotic stressors, including UV radiation and drought. These compounds act as protective agents, shielding the plant's cellular components from damage.
Signaling Molecules:
Secondary metabolites can also function as signaling molecules within the plant. They regulate various physiological processes, including growth, development, and responses to environmental stimuli.
Significance in Human Health and Industry
Medicinal and Pharmaceutical Uses:
Secondary metabolites, particularly alkaloids and some phenolic compounds, have significant medicinal value. They serve as the basis for many pharmaceutical drugs used for pain relief, treatment of diseases, and management of health conditions.
Food and Flavor Industry:
Terpenoids and flavonoids are essential contributors to the flavor, aroma, and color of various foods and beverages. They are commonly used as natural flavoring agents and colorants in the food and beverage industry.
Cosmetics and Perfumes:
Essential oils and aromatic compounds derived from plant secondary metabolites are key ingredients in cosmetics and perfumes. Their fragrances and properties are highly sought after in the cosmetics industry.
Agriculture and Pest Control:
Researchers are exploring the potential of plant secondary metabolites as natural alternatives to synthetic pesticides and herbicides. These compounds may offer sustainable pest management solutions in agriculture.
Environmental Applications:
Some secondary metabolites, such as tannins, have applications in wastewater treatment due to their ability to precipitate and remove pollutants. Additionally, they are used as natural antioxidants in environmentally friendly products.
Reference of schematic workflow for plant secondary metabolomics-microbiome discovery projects (Pang et al., 2021).
Application of Metabolomics Technology in the Study of Secondary Metabolites
Gene Function Analysis Based on Metabolomics of Secondary Metabolites
The use of metabolomics methods to detect changes in the types and levels of secondary metabolites can provide insights into subtle variations in gene expression levels, thereby inferring the relationship between gene expression and related metabolic pathways. In traditional functional analysis, changes in plant phenotypes are used to infer gene-level changes, but it can be challenging to make such determinations when there are no observable phenotype changes. For example, in Arabidopsis, it has been observed that the phenotypes of nearly all mutants remain unchanged, making it difficult to assess gene expression solely based on phenotype changes. However, gene function analysis through metabolomics methods offers a more straightforward approach.
Research on metabolic pathways and metabolic network regulation based on metabolomics reveals that secondary metabolites are more diverse in type compared to primary metabolites. Consequently, the synthesis pathways of secondary metabolites are far more complex than those of primary metabolites. The metabolic network pathways of some secondary metabolites are well understood. These pathways often start with intermediates from the pentose phosphate pathway, such as erythrose-4-phosphate (E4P), and intermediates from glycolysis, such as phosphoenolpyruvate (PEP). These starting materials undergo chemical reactions and several steps to produce shikimic acid. Shikimic acid is phosphorylated to form 5-phospho-shikimic acid and then reacts with PEP to produce branch acids. Branch acids are used to synthesize phenylalanine, which can further yield cinnamic acid under the catalysis of phenylalanine ammonia-lyase (PAL). Cinnamic acid, along with intermediates such as coumaric acid, ferulic acid, and sinapic acid, can further transform into flavonoids, tannins, anthocyanins, and other polyphenolic compounds.
Functional Analysis of Secondary Metabolites Based on Metabolomics Technology
While secondary metabolites are not essential for the basic life processes of plants, they play a crucial role under environmental stress. Secondary metabolites produced by plants under environmental stress are involved in various physiological processes, including growth, development, and defense. They also contribute to disease resistance, seed breeding, pharmaceutical development, and plant resistance. For example, paclitaxel (Taxol) is a secondary metabolite used as a drug to treat late-stage ovarian cancer. Lignin is a complex organic polymer found between cellulose fibers and is important not only for its structural support but also in the formation of the plant cell wall. Over the past few decades, significant progress has been made in exploring the biological functions of plant secondary metabolites.
Application of Metabolomics Technology in the Study of Secondary Metabolites
Metabolomics technology plays a pivotal role in both quantifying and qualitatively assessing secondary metabolites. It aids in gene function analysis, the exploration of metabolic pathways and their regulatory networks, and the functional scrutiny of secondary metabolites such as paclitaxel, phenolic compounds, and anthocyanins, among others. This technology carries substantial significance in diverse fields, including plant identification, disease resistance, breeding, medical research, and pharmaceutical development.
By employing metabolomics technology to scrutinize the secondary metabolites generated by plants in response to environmental shifts and characterizing the alterations in these metabolites, researchers can unravel the connection between plant adaptations and specific metabolites. This leads to the identification of numerous metabolites linked to diseases, facilitating the recognition of plant disease resistance and the study of plant defense mechanisms. In the realm of agricultural production, amalgamating metabolomics with genetics can yield resilient plant varieties, serving as valuable resources for future efforts in disease-resistant breeding.
Researchers aspire to employ metabolites as indicators for gauging the quantity and quality of agricultural products. Through metabolomics technology, they can pinpoint distinct metabolites associated with heightened yield and superior quality. These biomarkers function as guides for the selection of breeding materials, thereby expediting the development of enhanced crop varieties. In practical production processes, the metabolomics analysis of different wheat resistance varieties has already led to the identification of disease-resistant plants based on markers associated with disease resistance.
Reference
- Pang, Zhiqiang, et al. "Linking plant secondary metabolites and plant microbiomes: a review." Frontiers in Plant Science 12 (2021): 621276.
You may be interested in:
