In biochemistry, plant secondary metabolism produces a large number of specialized compounds (estimated 200,000) that do not aid in the growth and development of plants but are required for the plant to survive in its environment. Secondary metabolism is connected to primary metabolism by using building blocks and biosynthetic enzymes derived from primary metabolism. Primary metabolism governs all basic physiological processes that allow a plant to grow and set seeds, by translating the genetic code into proteins, carbohydrates, and amino acids. Specialized compounds from secondary metabolism are essential for communicating with other organisms in mutualistic (e.g. attraction of beneficial organisms such as pollinators) or antagonistic interactions (e.g. deterrent against herbivores and pathogens). They further assist in coping with abiotic stress such as increased UV-radiation. The broad functional spectrum of specialized metabolism is still not fully understood. In any case, a good balance between products of primary and secondary metabolism is best for a plant’s optimal growth and development as well as for its effective coping with often changing environmental conditions.
Well known specialized compounds include alkaloids, polyphenols including flavonoids, and terpenoids. Humans use many of these compounds for culinary, medicinal and nutraceutical purposes.
History
Research into secondary plant metabolism primarily took off in the later half of the 19th century, however, there was still much confusion over what the exact function and usefulness of these compounds were. All that was known was that secondary plant metabolites were "by-products" of the primary metabolism and were not crucial to the plant's survival. Early research only succeeded as far as categorizing the secondary plant metabolites but did not give real insight into the actual function of the secondary plant metabolites. The study of plant metabolites is thought to have started in the early 1800s when Friedrich Willhelm Serturner isolated morphine from opium poppy, and after that new discoveries were made rapidly. In the early half of the 1900s, the main research around secondary plant metabolism was dedicated to the formation of secondary metabolites in plants, and this research was compounded by the use of tracer techniques which made deducing metabolic pathways much easier. However, there was still not much research being conducted into the functions of secondary plant metabolites until around the 1980s. Before then, secondary plant metabolites were thought of as simply waste products.
In the 1970s, however, new research showed that secondary plant metabolites play an indispensable role in the survival of the plant in its environment. One of the most ground breaking ideas of this time argued that plant secondary metabolites evolved in relation to environmental conditions, and this indicated the high gene plasticity of secondary metabolites, but this theory was ignored for about half a century before gaining acceptance. Recently, the research around secondary plant metabolites is focused around the gene level and the genetic diversity of plant metabolites. Biologists are now trying to trace back genes to their origin and re-construct evolutionary pathways.Hartmann, Thomas. "From waste products to ecochemicals: fifty years research of plant secondary metabolism." Phytochemistry 68.22–24 (2007): 2831–2846. Web. 31 Mar 2011.
Primary vs. secondary plant metabolism
Primary metabolism in a plant comprises all metabolic pathways that are essential to the plant's survival. Primary metabolites are compounds that are directly involved in the growth and development of a plant whereas secondary metabolites are compounds produced in other metabolic pathways that, although important, are not essential to the functioning of the plant. However, secondary plant metabolites are useful in the long term, often for defense purposes, and give plants characteristics such as color. Secondary plant metabolites are also used in signalling and regulation of primary metabolic pathways. Plant hormones, which are secondary metabolites, are often used to regulate the metabolic activity within cells and oversee the overall development of the plant. As mentioned above in the History tab, secondary plant metabolites help the plant maintain an intricate balance with the environment, often adapting to match the environmental needs. Plant metabolites that color the plant are a good example of this, as the coloring of a plant can attract pollinators and also defend against attack by animals.
Secondary metabolites
There is no fixed, commonly agreed upon system for classifying secondary metabolites. Based on their biosynthetic origins, plant secondary metabolites can be divided into three major groups: Flavonoids and allied phenolic and polyphenolic compounds, Terpenoids, and Nitrogen-containing alkaloids and sulphur-containing compounds.
Other researchers have classified secondary metabolites into following, more specific types Class Type Number of known metabolites Examples Alkaloids Nitrogen-containing 21000 Cocaine, psilocin, caffeine, nicotine, morphine, berberine, vincristine, reserpine, galantamine, atropine, vincamine, quinidine, ephedrine, quinine Non-protein amino acids (NPAAs) Nitrogen-containing 700 NPAAs are produced by specific plant families such as Leguminosae, Cucurbitaceae, Sapindaceae, Aceraceae and Hippocastanaceae. Examples: Azatyrosine, Canavanine Amines Nitrogen-containing 100 Cyanogenic glycosides Nitrogen-containing 60 Amygdalin, dhurrin, linamarin, lotaustralin, prunasin Glucosinolates Nitrogen-containing 100 Alkamides Nitrogen-containing 150 Lectins, peptides and polypeptides Nitrogen-containing 2000 Concanavalin A Terpenes Without nitrogen >15,000 Azadirachtin, artemisinin, tetrahydrocannabinol Steroids and saponins Without nitrogen NA These are terpenoids with a particular ring structure. Cycloartenol Flavonoids and tannins Without nitrogen 5000 Luteolin, tannic acid Phenylpropanoids, lignins, coumarins and lignans Without nitrogen 2000 Resveratrol Polyacetylenes, fatty acids and waxes Without nitrogen 1500 Polyketides Without nitrogen 750 Carbohydrates and organic acids Without nitrogen 200
Some of the secondary metabolites are discussed below:
Atropine
Atropine is a tropane alkaloid. Alkaloids contain nitrogens, frequently in a ring structure, and are derived from amino acids. Tropane is an organic compound containing nitrogen and it is from tropane that atropine is derived. Atropine is synthesized by a reaction between tropine and tropate, catalyzed by atropinase. Both of the substrates involved in this reaction are derived from amino acids, tropine from pyridine (through several steps) and tropate directly from phenylalanine. Within Atropa belladonna atropine synthesis has been found to take place primarily in the root of the plant.Fred R. West, Jr. and Edward S. Mika. "Synthesis of Atropine by Isolated Roots and Root-Callus Cultures of Belladonna." Botanical Gazette: Vol. 119, No. 1 (Sep., 1957), pp. 50–54 https://www.jstor.org/stable/2473263 The concentration of synthetic sites within the plant is indicative of the nature of secondary metabolites. Typically, secondary metabolites are not necessary for normal functioning of cells within the organism meaning the synthetic sites are not required throughout the organism. As atropine is not a primary metabolite, it does not interact specifically with any part of the organism, allowing it to travel throughout the plant.
Flavonoids
Flavonoids are also known as Vitamin P or citrin. These metabolites are mostly used in plants to produce yellow and other pigments which play a big role in coloring the plants. In addition, Flavonoids are readily ingested by humans and they seem to display important anti-inflammatory, anti-allergic and anti-cancer activities. Flavonoids are also found to be powerful anti-oxidants and researchers are looking into their ability to prevent cancer and cardiovascular diseases. Flavonoids help prevent cancer by inducing certain mechanisms that may help to kill cancer cells, and researches believe that when the body processes extra flavonoid compounds, it triggers specific enzymes that fight carcinogens. Good dietary sources of Flavonoids are all citrus fruits, which contain the specific flavanoids hesperidins, quercitrin, and rutin, berries, tea, dark chocolate and red wine and many of the health benefits attributed to these foods come from the Flavonoids they contain. Flavonoids are synthesized by the phenylpropanoid metabolic pathway where the amino acid phenylalanine is used to produce 4-coumaryol-CoA, and this is then combined with malonyl-CoA to produce chalcones which are backbones of FlavonoidsCrozier, Alan, and Hiroshi Ashihara. Plant Secondary Metabolites: Occurrence, Structure and Role in the Human Diet. Ames, IA: Blackwell Publishing Professional, 2006. Print. Chalcones are aromatic ketones with two phenyl rings that are important in many biological compounds. The closure of chalcones causes the formation of the flavonoid structure. Flavonoids are also closely related to flavones which are actually a sub class of flavonoids, and are the yellow pigments in plants. In addition to flavones, 11 other subclasses of Flavonoids including, isoflavones, flavans, flavanones, flavanols, flavanolols, anthocyanidins, catechins (including proanthocyanidins), leukoanthocyanidins, dihydrochalcones, and aurones.
Cyanogenic glycoside
To get a better understanding of how secondary metabolites play a big role in plant defense mechanisms we can focus on the recognizable defense-related secondary metabolites, cyanogenic glycosides. The compounds of these secondary metabolites (As seen in Figure 1) are found in over 2000 plant species. Its structure allows the release of cyanide, a poison produced by certain bacteria, fungi, and algae that is found in numerous plants. Animals and humans possess the ability to detoxify cyanide from their systems naturally. Therefore, cyanogenic glycosides can be used for positive benefits in animal systems always. For example, the larvae of the southern armyworm consumes plants that contain this certain metabolite and have shown a better growth rate with this metabolite in their diet, as opposed to other secondary metabolite-containing plants. Although this example shows cyanogenic glycosides being beneficial to the larvae many still argue that this metabolite can do harm. To help in determining whether cyanogenic glycosides are harmful or helpful researchers look closer at its biosynthetic pathway (Figure 2). Past research suggests that cyanogenic glucosides stored in the seed of the plant are metabolized during germination to release nitrogen for seedling to grow. With this, it can be inferred that cyanogenic glycosides play various roles in plant metabolism. Though subject to change with future research, there is no evidence showing that cyanogenic glycosides are responsible for infections in plants.
Phytic acid
Phytic acid is the main method of phosphorus storage in plant seeds, but is not readily absorbed by many animals (only absorbed by ruminants). Not only is phytic acid a phosphorus storage unit, but it also is a source of energy and cations, a natural antioxidant for plants, and can be a source of myoinositol which is one of the preliminary pieces for cell walls.