Here we shall argue that biologically active compounds are a general feature of the plant kingdom because they originally evolved to aid the survival of the earliest plants. Indeed, we shall concentrate on the compounds produced by the more “primitive” plants and conclude with a discussion of the general trends in occurrence and evolution of secondary metabolites.

That plant defense compounds are widespread is more than simply an intriguing phenomenon; it may actually have played a key role in the development (or more correctly, hindrance to development) of human civilizations. Most leaves and tubers of wild plant species are potentially toxic to humans, and only through domestication have concentrations of toxic compounds been reduced in cultivated varieties (Harborne, 1997). Presumably, the first people to domesticate toxin-laden plants such as the potato were faced with desperate food shortages. Artificial selection (breeding) has resulted in weak plant defense in domesticated crops and the need for human intervention to ensure the survival of many domesticated plants (Wink, 2004).

Three principal groups of secondary metabolites are involved in plant defense: nitrogen-containing substances, terpenes, and phenolics (Wink, 2004). The first group includes alkaloids, amines, cyanogenic glycosides, glucosinolates (mustard oils), peptides, and specific antifeedant proteins such as protease inhibitors and amylase inhibitors. Approximately 14,000 have been described to date, the vast majority of which are alkaloids (Wink, 2004). Alkaloids possess a heterocyclic nitrogen that can act as a base and interfere with neuronal signal transduction path- ways, having toxic effects on animal nervous systems. Terpenes are aromatic compounds, the structure of which may include multiple aromatic ring formations (e.g., diterpenes, triterpenes, and sesquiterpenes), and in certain forms can interfere with the properties of cell membranes, making them leaky and thereby nonfunctional. Iridoids are a particularly widespread group of monoterpenes. Phenolic compounds, including flavonoids, bisbibenzyls, and tannins, have a range of sinister properties, such as forming crosslinkages between DNA bases and proteins, and many are thus mutagenic and carcinogenic. Indeed, if there is a common concept linking the disparate types of biologically active compounds, it is the disruption of the structure, and thus function, of the cellular components (membranes, proteins, genetic material) that are crucial to the primary metabolism of organisms that are antagonistic to the plant-it is for this reason that many have general antibiological effects against a range of animals, microorganisms, and other plants.

Incidentally, in order to avoid poisoning themselves with their own secondary metabolites, plants must usually separate the precursors of secondary metabolites in different tissues. When tissues are disrupted during herbivory/disease, the precursors are mixed together or mixed with enzymes that cleave particular parts of the precursor molecules to produce the toxic form (e.g., iridoid glucosides become exposed to the enzyme β-glucosidase, which removes the glucose group to produce a toxic iridoid aglucone; Rank et al., 2004).

Most plants share these basic classes of secondary metabolites, even the most ancient of all: the green algae.

Soure: Giacinto Bagetta, Marco Cosentino, Marie Tiziana Corasaniti, Shinobu Sakurada (2012); Herbal Medicines: Development and Validation of Plant-derived Medicines for Human Health; CRC Press

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