Secondary Metabolites: Classification, Therapeutic Properties, and Potential Usage in the Food Industry

The molecular map of diverse biological molecules linked with structure, function, signaling, and regulation within a cell can be elucidated using an analytically demanding omic approach. The latest trend of using "metabolomics" technologies has explained the natural phenomenon of opening a new avenue to understand and enhance bioactive compounds' production. Examination of sequenced plant genomes has revealed that a considerable portion of these encodes genes of secondary metabolism. In addition to genetic and molecular tools developed in the current era, the ever-increasing knowledge about plant metabolism's biochemistry has initiated an approach for wisely designed, more productive genetic engineering of plant secondary metabolism for improved defense systems and enhanced biosynthesis of beneficial metabolites. Secondary plant metabolites are natural products synthesized by plants that are not directly involved with their average growth and development but play a vital role in plant defense mechanisms. Plant secondary metabolites are classified into four major classes: terpenoids, phenolic compounds, alkaloids, and sulfur-containing compounds. More than 200,000 secondary metabolites are synthesized by plants having a unique and complex structure. Secondary plant metabolites are well characterized and quantified by omics approaches and therefore used by humans in different sectors such as agriculture, pharmaceuticals, chemical industries, and biofuel. The aim is to establish metabolomics as a comprehensive and dynamic model of diverse biological molecules for biomarkers and drug discovery. In this chapter, we aim to illustrate the role of metabolomic technology, precisely liquid chromatography-mass spectrometry, capillary electrophoresis mass spectrometry, gas chromatography-mass spectrometry, and nuclear magnetic resonance spectroscopy, specifically as a research tool in the production and identification of novel bioactive compounds for drug discovery and to obtain a unified insight of secondary metabolism in plants.

Carnivorous plants are an ideal model system for evaluating the role of secondary metabolites in plant ecology and evolution. Carnivory is a striking example of convergent evolution to attract, capture and digest prey for nutrients to enhance growth and reproduction and has evolved independently at least ten times. Though the roles of many traits in plant carnivory have been well studied, the role of secondary metabolites in the carnivorous habit is considerably less understood. This review provides the first synthesis of research in which secondary plant metabolites have been demonstrated to have a functional role in plant carnivory. From these studies we identify key metabolites for plant carnivory and their functional role, and highlight biochemical similarities across taxa. From this synthesis we provide new research directions for integrating secondary metabolites into understanding of the ecology and evolution of plant carnivory. Carnivorous plants use secondary metabolites to facilitate prey attraction, capture, digestion and assimilation. We found ~170 metabolites for which a functional role in carnivory has been demonstrated. Of these, 26 compounds are present across genera that independently evolved a carnivorous habit, suggesting convergent evolution. Some secondary metabolites have been co-opted from other processes, such as defence or pollinator attraction. Secondary metabolites in carnivorous plants provide a potentially powerful model system for exploring the role of metabolites in plant evolution. They also show promise for elucidating how the generation of novel compounds, as well as the co-option of pre-existing metabolites, provides a strategy for plants to occupy different environments.

Root exudates represent complex mixtures of low-molecular-weight and high-molecular-weight compounds. The former comprise predominantly primary and secondary plant metabolites, the latter mucilage precursors and some proteins. Different collection approaches exist. The traditionally most widely used one (also applied in this study) is soaking roots for several hours in distilled water that have been thoroughly cleaned from soil. Other collection methods comprise hydroponic cultures and rhizoboxes with microsuction devices. The former are more used by molecular biologists in attempts to characterize phenotypes reproducibly, the latter by ecologists in efforts to explore specific regions of the rhizosphere. The classical approach is one that still allows plant to be cultured in soil. This is important because mucilage formation is affected by microbial soil communities and soil physicochemical properties. Primary and secondary plant meta¬bolites provide the majority of low-molecular precursors for mucilage development. Quality and quantity of extractable root-exuded plant metabolites is most probably affected by these parameters which are completely absent in hydroponic cultures. Six model plants were chosen on basis of their status as crop plant and on their tolerance of the uniform culture conditions to which all model plants were subjected to in the only available climate chamber. These included the Brassicaceae Arabidopsis and Rapeseed, the Fabaceae Phaseolus and Pisum, the Solanaceae Tobacco and the grass Maize. In all experi¬ments, the model plants received identical amounts of light, the same water supply and nutrient provision. In attempts to simulate drought stress, one-half of the plants was not deprived of water for two consecutive weeks. One aim was to explore which and to what amounts primary as well as secondary plant metabolites do occur in the root exudates and if they differ from those present in the roots. All six plant species showed similar primary metabolite profiles that, however, varied quanti¬tatively between the plant species. A prominent root exudate metabolite was myo-inositol, a sugar alcohol. Root tissues and root exudates showed different profiles with amino acids showing the most profound differences. The found primary metabolites agree with those reported in the literature. By contrast, secondary metabolites showed characteristic profiles, in which only few com-pounds were common to more than one species. One metabolite that was detected in all species was cinnamic acid. Structure elucidation was focussed especially on those secondary plant metabolites that were pointed out by non-parametric multivariate statistics as substantial contributors to similarity and dissimilarity of root exudates and root tissues. Root exudates were found to contain chalcones, flavanols, isoflavones, cinnamides, a cinnamoyl spermidine, indoles, stilbenes, a hydroxamic acid benzoxazine and a gibberellic acid deri¬vative, amongst others. Notably, no glycosides were detected among the elucidated metabolites and a considerably high proportion of aldehydes was noted. In case of Arabidopsis, an extensive analysis of hydroponically obtained root exudates exists in the literature. Many dimeric structures were reported, most of which could not be detected in the present study. Another explored aspect was the effect of water deficit on root exudation. Primary metabolite patterns changed in a more similar way, sugars such as glucose and sucrose increased and myo-inositol proportions decreased. Amino acid pattern changes, by contrast, were more species-specific. Generally, the amounts of detectable secondary metabolites decreased as shoot: root ratios in the affected plants increased. Only Phaseolus and Maize showed higher shoot: root ratios after water deficit. This suggests a different, more opportunistic strategy to survive stress. Only Pisum exuded a new class of secondary metabolites that was absent in the regularly watered plants. Altogether 24 different root exudate samples were available. Bases on the variability of primary and secondary root exudate metabolites correlations with nutrient supply in leaves was explored by Spearman rank correlation. Interestingly and as once suggested in a previous review, weak correlations between secondary metabolite profiles and leaf nutrients were found. Especially more unsaturated metabolites with vicinal oxygen functions correlated with the uptake of several nutrients, most of them being metal cations. The structural properties of the identified secondary metabolites allows them to act as ligands in coordination complexes in which the nutrient represents the central atom. This chemistry can add to the mobilization and uptake of nutrients by plant roots.

Secondary metabolites (SMs) play vital roles in plant defence mechanisms, adaptation to environmental conditions, and interactions with other organisms. Biotic and abiotic stress factors can significantly influence the production, accumulation, and composition of SMs in plants. Understanding the intricate relationship between stress and SM production is crucial for enhancing plant resilience, agricultural productivity, and the development of novel phytopharmaceuticals. This research provides current knowledge regarding the impact of biotic and also abiotic stress on SMs in plants. Biotic stress factors such as pathogen infection, and herbivore attacks, as well as abiotic stress factors like drought, along with temperature extremes, and also salinity, can profoundly influence the biosynthesis and accumulation of SMs in plants. We discussed the methodology based on secondary sources underlying physiological, biochemical, and molecular mechanisms involved in stress-induced SM synthesis and highlight the potential implications for plant biology, agriculture, and human health. The study also emphasizes the functions of SMs in plants including defence against herbivores, pathogens, and abiotic stresses. The mechanism by which thesecompounds act as allelochemicals and signalling molecules is also discussed.

Secondary metabolites, including alkaloids, flavonoids, and tannins, are crucial for human health, agriculture, and ecosystem functioning. Their synthesis is often species-specific, influenced by both genetic and environmental factors. The increasing demand for these compounds across various industries highlights the need for advancements in plant breeding and biotechnological approaches. Transcription activator-like effector nucleases (TALENs) have emerged as a powerful tool for precise genome editing, offering significant potential for enhancing the synthesis of secondary metabolites in plants. However, while plant genome editing technologies have advanced significantly, the application of TALENs in improving secondary metabolite production and expanding genetic diversity remains underexplored. Therefore, this review aims to provide a comprehensive analysis of TALEN-mediated genome editing in plants, focusing on their role in enhancing secondary metabolite biosynthetic pathways and improving genetic diversity. The mechanisms underlying TALENs are examined, including their ability to target specific genes involved in the synthesis of bioactive compounds, highlighting comparisons with other genome editing tools such as CRISPR/Cas9. This review further highlights key applications in medicinal plants, particularly the modification of pathways responsible for alkaloids, flavonoids, terpenoids, and phenolic compounds. Furthermore, the role of TALENs in inducing genetic variation, improving stress tolerance, and facilitating hybridization in plant breeding programs is highlighted. Recent advances, challenges, and limitations associated with using TALENs for enhancing secondary metabolite production are critically evaluated. In this review, gaps in current research are identified, particularly regarding the integration of TALENs with multi-omics technologies and synthetic biology approaches. The findings suggest that while underutilized, TALENs offer sustainable strategies for producing high-value secondary metabolites in medicinal plants. Future research should focus on optimizing TALEN systems for commercial applications and integrating them with advanced biotechnological platforms to enhance the yield and resilience of medicinal plants.

The vast and versatile pharmacological effects of medicinal plants are basically dependent on their phytochemical constituents. Generally, the phytochemical constituents of plants fall into two categories based on their role in basic metabolic processes, namely primary and secondary metabolites. Primary plant metabolites are involved in basic life functions; therefore, they are more or less similar in all living cells. On the other hand, secondary plant metabolites are products of subsidiary pathways as the shikimic acid pathway. In the course of studying, the medicinal effect of herbals is oriented towards the secondary plant metabolites. Secondary plant metabolites played an important role in alleviating several aliments in the traditional medicine and folk uses. In modern medicine, they provided lead compounds for the production of medications for treating various diseases from migraine up to cancer. Secondary plant metabolites are classified according to their chemical structures into various classes. In this chapter, we will be presenting various classes of secondary plant metabolites, their distribution in different plant families and their important medicinal uses.

Epidemiological studies have revealed an inverse association between the consumption of fruit, vegetables, and herbs and the risk of both cancer and cardiovascular disease. This protective effect is mostly due to secondary metabolites present in plant tissues. During the last decade, it has become increasingly clear that UV-B radiation is an important regulator of plant secondary metabolism. Low, ecologically-relevant UV-B levels trigger distinct changes in the accumulation of, among others, phenolic compounds, carotenoids and glucosinolates. Fundamental understanding of plant UV-B perception and responses opens up new opportunities for crop manipulation. Thus, targeted low dosage UV-B radiation treatments as emerging technology may be used to generate fruit, vegetables, and herbs enriched with secondary plant metabolites for either fresh consumption or as a source for functional foods and nutraceuticals, resulting in increased ingestion of these health-promoting substances. The UV-B induced accumulation of secondary plant metabolites is likely to have evolved as a plant defense response against harmful UV-B radiation. However, UV-B induced secondary metabolites also alter other trophic interactions, for example by altering plant herbivore resistance. Thus, UV-B driven metabolic changes in the plant's secondary metabolism have benefits for both ends of the bio-based food chain, i.e., for plants themselves as well as for humans.

The objective of the presentation is to describe the beneficial roles of plant secondary metabolites in beef production with special attention to mode of delivery. Plant and fungal secondary metabolites are responsible for many well-known problems in beef cattle production, such as acute or chronic intoxication. Fescue toxicosis is an example of the latter. However, there are also beneficial effects of secondary metabolites. Plant extracts (e.g., essential oils) that are rich in phenolic compounds and other secondary metabolites have been explored as alternatives to veterinary prophylactic antibiotics, growth promoting antibiotics, and enteric methane suppressants. An example of extracted botanicals is beta-acid from the hops plant. Beta-acid could be considered a model rumen-active phytochemical because of a similar mechanism of action to ionophores, which allows direct comparison with well-studied, microbially produced drugs. The route of delivery of extracted phytochemicals can also be similar to that of other commercial products. In contrast, other beneficial plant secondary metabolites can be delivered without extraction. Anthelmintic forages have been extensively studied for use in small ruminants. These forages contain concentrations of plant secondary metabolites, such as condensed tannins or sesquiterpene lactones, that reduce parasite load without processing the plant. Similarly, health and production benefits were shown in cattle consuming isoflavones by grazing forage legumes (red clover) or by supplementation with hay or other minimally processed legumes (red clover, white clover, soybean meal). The term, functional feeds, borrows from the idea of functional foods in human nutrition, this is, foods that have a benefit traditionally ascribed to medicine. Functional feeds may have advantages over commercially extracted and processed phytochemicals, not the least of which is the ability of cattlemen to produce their own feed. However, to fully take advantage of beneficial forage plant secondary metabolites more research is needed in animal nutrition, plant sciences, and chemical analysis.

There are numerous secondary plant metabolites found in the crop B. juncea, especially glucosinolates. Isothiocyanates, the by-products of glycosinolate breakdown, are beneficial to human health. A number of studies have also called attention to phenolic compounds and carotenoids, both well known for their anti-oxidant properties. A notable feature is that the profiles and concentrations of secondary plant metabolites vary greatly between varieties and that genetic factors are thought to be the most significant factors. In addition, environmental and agronomic factors have also been noted to change the concentrations of secondary plant metabolites. Secondary plant metabolites are primarily produced for defense purposes. Consequently, the intrinsic quality of Indian mustard, including color, aroma, taste, and medicinal properties, is profoundly influenced by its secondary metabolite profile. The health benefits of glycosinolates and the cancer prevention properties of their breakdown products make them of specific interest. Plant cells that have been injured undergo enzymatic decomposition of glucosinolate by endogenous enzymes such as myrosinase, which releases degradation products such as nitriles, epithionitriles, or isothiocyanates. The main phenolic compounds found in B. juncea are flavonoids and hydroxycinnamic acid derivatives. A diverse secondary metabolite pool is also essential for plant-environment interactions.

Secondary metabolites are gaining an increasing importance in various industries, such as pharmaceuticals, dyes, and food, as is the need for reliable and efficient methods of procuring these compounds. To develop sustainable and cost-effective approaches, a comprehensive understanding of the biosynthetic pathways and the factors influencing secondary metabolite production is essential. These compounds are a unique type of natural product which recognizes the oxidative damage caused by stresses, thereby activating the defence mechanism in plants. Various methods have been developed to enhance the production of secondary metabolites in plants. The elicitor-induced in vitro culture technique is considered an efficient tool for studying and improving the production of secondary metabolites in plants. In the present review, we have documented various biosynthetic pathways and the role of secondary metabolites under diverse environmental stresses. Furthermore, a practical strategy for obtaining consistent and abundant secondary metabolite production via various elicitation agents used in culturing techniques is also mentioned. By elucidating the intricate interplay of regulatory factors, this review paves the way for future advancements in sustainable and efficient production methods for high-value secondary metabolites.

Enhancing secondary metabolite production in plants: Exploring traditional and modern strategies

Multitemporal UAV study of phenolic compounds in slash pine canopies

Chapter 11 - Biotransformation in the production of secondary metabolites

Pegagan (Centella asiatica (L.) Urban) is a herbal plant that contain secondary metabolite compounds like phenol and flavonoid. NaCl salinity is one of abiotic stress that enhanced synthesis of some secondary metabolites in plants. This study was investigated the effect of NaCl salinity stress to phenolic compound, total flavonoid and antioxidant activity of pegagan leaves. Pegagan were treated with five different NaCl concentrations, 0 mM (1), 50 mM (2), 100 mM (3), 150 mM (4) and 200 mM (5) for a week. Morphological leaves were observed for the present of necrotic symptom. Phenolic compound and total flavonoid content were measured using spectrophotometer at wavelength 765 nm and 415 nm. Antioxidant activity was measured based on DPPH method. The result showed that increasing NaCl concentration cause increasing necrotic spot in leaves. Phenolic compound, total flavonoid and antioxidant activity are increased by increasing NaCl concentration. The result indicated that phenol and flavonoid have important role in plant defense mechanism against NaCl toxicity effects.

Natural products are in the form of primary and secondary metabolites and are isolated chemical compounds or substances from living organisms. Terpenes, Phenolic compounds, and Nitrogen-containing compounds are secondary metabolites. The biosyntheses of secondary metabolites are derived from primary metabolism pathways, which consist of a tricarboxylic acid cycle (TCA), methylerythritol phosphate pathway (MEP), mevalonic and shikimic acid pathway. This chapter provides an overview of the diversity of secondary metabolites in plants, their multiple biological functions, and multi-faceted cultural history.