Plant Secondary Metabolites Produced in Response to Abiotic Stresses Has Potential Application in Pharmaceutical Product Development

Abstract

Plant secondary metabolites (PSMs) are vital for human health and constitute the skeletal framework of many pharmaceutical drugs. Indeed, more than 25% of the existing drugs belong to PSMs. One of the continuing challenges for drug discovery and pharmaceutical industries is gaining access to natural products, including medicinal plants. This bottleneck is heightened for endangered species prohibited for large sample collection, even if they show biological hits. While cultivating the pharmaceutically interesting plant species may be a solution, it is not always possible to grow the organism outside its natural habitat. Plants affected by abiotic stress present a potential alternative source for drug discovery. In order to overcome abiotic environmental stressors, plants may mount a defense response by producing a diversity of PSMs to avoid cells and tissue damage. Plants either synthesize new chemicals or increase the concentration (in most instances) of existing chemicals, including the prominent bioactive lead compounds morphine, camptothecin, catharanthine, epicatechin-3-gallate (EGCG), quercetin, resveratrol, and kaempferol. Most PSMs produced under various abiotic stress conditions are plant defense chemicals and are functionally anti-inflammatory and antioxidative. The major PSM groups are terpenoids, followed by alkaloids and phenolic compounds. We have searched the literature on plants affected by abiotic stress (primarily studied in the simulated growth conditions) and their PSMs (including pharmacological activities) from PubMed, Scopus, MEDLINE Ovid, Google Scholar, Databases, and journal websites. We used search keywords: "stress-affected plants," "plant secondary metabolites, "abiotic stress," "climatic influence," "pharmacological activities," "bioactive compounds," "drug discovery," and "medicinal plants" and retrieved published literature between 1973 to 2021. This review provides an overview of variation in bioactive phytochemical production in plants under various abiotic stress and their potential in the biodiscovery of therapeutic drugs. We excluded studies on the effects of biotic stress on PSMs.

Keywords: abiotic stress; climate change; drug discovery; secondary metabolites.

Figure 6

General phenylpropanoid pathway and flavonoid biosynthesis (adapted from [247,248]. Solid arrows represent single enzymatic reaction; dashed arrows represent multiple sequential reactions. Enzymes involved: PAL—phenylalanine ammonia lyase; CHS—chalcone synthase; STS—stilbene synthase; CHR—chalcone reductase.

Figure 1

Representative examples of terpenoid plant secondary metabolites.

Figure 2

Representative examples of different subgroups of flavonoids: a major phenolic group of secondary metabolites.

Figure 3

Representative examples of seven different types of alkaloids produced in plants and their chemical structure.

Figure 4

Examples of widely distributed cyanogenic glycosides in plant kingdom.

Figure 5

Abiotic stresses and their influence on the types of secondary metabolites in plants (adapted from [94,95,96,97]). Abbreviations: UV radiation = ultraviolet radiation; PSMs = plant secondary metabolites; O₃ = ozone; CO₂ = carbon dioxide; Isopr = isoprenoids; MT = monoterpenes; SQT = sesquiterpenes; phe. acids = phenolic acids.

Figure 7

Chemical structure of compounds known to accumulate in plants under various abiotic stress conditions.

Figure 7

Chemical structure of compounds known to accumulate in plants under various abiotic stress conditions.