Chemotaxonomy is a tool to assist in the accurate
positioning of a plant within a system; give illustrations
of how it may be useful in the quest for new medicines
or food.
Introduction
The science of taxonomy in plants is aimed at delivering a system for classifying all plant groups in a way which represents best the similarities and differences. The concept of classification of plant based on chemical character is not new as Greshoff in 1909 suggested that chemical
characters should be included in classification of taxa (Hegnauer, 1967). Chemotaxonomy was developed vastly through the 20th century although
it has advanced taxonomy through the study and identification of the similarities and differences of chemical substances in plants, which has thus enabled taxonomists to more accurately position plants. There are many different definitions for chemotaxonomy but an author (Hegnauer, 1967) described it in relation to plants as a ‘scientific investigation of the potentialities of chemical characters for the study of problems of plant taxonomy and plant phylogeny’. Chemotaxonomy not only deals with the investigation of plants metabolites and other products such as
carbohydrates and amino acids, it is also concerned with evolutionary change, chemical convergence and divergence in the plant. Biosynthetic pathways are also used to position plants. Analytical techniques such as chromatography and electrophoresis allow analysis of plants to be made quickly using minimal plant material and can detect small traces of chemicals. Further analysis can be carried out to give fingerprints and detailed compositions of samples using nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS) (Walton and Brown, 1999). Continued investigation has permitted the development of extensive data for classifying the position of plants, which can then be applied when looking for taxa with potential for medicinal compounds and foods.
Discussion
Classification of plants without the investigation of chemical constituents (chemotaxonomy) is assessed through examination of the morphology of the taxa that is under scrutiny. This would be inadequate in defining taxa accurately and therefore the study of chemotaxonomic classification has added considerable significance in positioning taxa within the appropriate
system (Bhargava, Patel and Desai, 2013). Plants of the same family commonly synthesise similar compounds as they often contain analogous enzymes, producing similar biosynthetic pathways. It is also possible for the same metabolites can be the product of two quite different pathways. Chemotaxonomic studies include the investigation of the pattern of
compound occurring in the plant and preferentially various explants such as the bark, wood, leaves, root, fruits and seeds. Holistic investigations are necessary in order to achieve substantial evidence for the taxa being studied, which should include testing on an acceptable number of
specimens of the plants in order to classify the plants as a whole.
Organic compounds within plants are recognised as primary metabolites, secondary metabolites and semantides. Primary metabolites such as acotinic acid and citric acid are present in the Krebs cycle and therefore may not be beneficial in systematically positioning a plant; however the varying quantities of these metabolites in different plants may be of use (Stace, 1989). Secondary metabolites have limited prevalence which makes them much more valuable in positioning plants. They have non-essential functions which are less fundamentally widespread in plants as with primary metabolites used for growth, reproduction and
photosynthesis (Stace, 1989). Secondary metabolite functions are, as well as regulating primary metabolic pathways, often evolved to aid plants to survive and thrive in their environment. Examples are pigments, tastes and scents which can attract pollinators or repel herbivores, insects and microorganisms, while others may act as poisons. These secondary metabolites as mentioned may be specific to a genus and therefore can act as reliable taxonomy markers. This can be of aid when screening taxa for the possibility of a particular secondary metabolite. The biosynthesis of secondary metabolites has a tendency to be in specialised cells and with not being used in metabolic functions are in smaller quantities to that of primary metabolites (Balandrin et al., 1985). Higher plants produce many organic compounds with the potential to be valuable medicinally and therefore commercially also, however it is still the case that many have still not been analysed for their chemically characterised (Balandrin et al., 1985). Chemotaxonomy is continuing to expand the portfolio of
biologically active compounds by positioning plants within appropriate systems.
The use of plants in pharmaceuticals is extensive with many medicinal products utilising chemicals naturally produced by plants or semi-synthetic modified plant chemicals. They may also be used in the development of insecticides or genetically modified foods that are less susceptible to being attacked by herbivores and insects. Some common types of secondary metabolites are alkaloids, flavonoids, terpenoids,
cyanogenic glycosides, glucosinolates and phenolic compounds (Stace, 1989). By exploiting chemotaxonomy, there is still scope for development of new medicines and food from plants.
Extensive use of chemotaxonomy requires a good understanding of the bio-chemistry and distribution of plants. Phytochemicals naturally
fluctuate in their distribution within a plant. The quantity and constitution of composites such as alkaloids, terpinoids and flavonoids and the many subclasses are controlled by the plants maturity, where the culture is taken from (e.g. leaves, roots, stem, and flower), the location it grows and its environment. Variation as a result of local ecology may be the result of the phenotypic plasticity as a result of being exposed to heterogeneity in the surrounding environment and understanding this can be vital for predicting changes in distribution, chemical composition, and crop yields as a result of climate change (Gratani, 2014), and therefore useful for future quests for new food and medicines from plants. Genetic variation in local populations is called polymorphism and is where variations lead to new races by migration and natural selection (Hegnauer, 1986). Plant species with extensive geographical range can have great variations in physiology, morphology and phenology, so will often produce closely related however different secondary metabolites. An example where this has proved useful is with taxol (paclitaxel) which is an effective anticancer drug. It is a taxane alkaloid (diterpinoid) extracted from the inner bark of the pacific yew tree, Taxus brevifolia, which are rare. A problem is that only low concentrations are stored in the bark and there are extremely high extraction costs. The extraction process is also destructive and leads to death of the yew trees, which with high demand means isolating it from this source is unsustainable and requires alternatives to obtain the drug especially for conservation of the species. One source where taxol can also be semi synthetically produced is by extracting 10-deacetylbaccatin (10-DAB) found in the needles of European yew trees, Taxus baccata. There are also cost extraction issues with this process and supplies can be affected by climatic changes, environmental damage and harvesting problems. New advancement in technologies means that there are new promising methods to sustainably make the precursor material such as plant cell-based fermentation at industrial levels (MacDonald, 2013). Without chemical taxonomic data carried out on the Taxus species it would not have been possible to sustain production of paclitaxel and this also led to the discovery of the a similar compound made from the Taxus baccata called docetaxel, which is also in clinical use as an effective anticancer drug.
Going back to polymorphism, an example of this is cyanogenesis that is the release of hydrogen cyanide (HCN) after cell damage. Usually
cyanogenesis depends upon the presence of cyanogenic glycosides and corresponding enzymatic hydrolysis in a given tissue. Plant families which contain cyanogenic glycosides are Fabaceae, Rosaceae, Linaceae,
Different L-amino acids like phenylalanine, tyrosine, valine, leucine, and isoleucine are precursors for the biosynthesis of cyanogenic glycosides, but they are restricted to particular family e.g. leucine to leguminosae and sapindaceae. The presence or lack or presence of cyanogenetic glycoside can therefore be used as a chemotaxonomic marker (Vetter, 2000).
Alkaloids are basic compounds, usually containing heterocyclic nitrogen with exceptions such as ephedrine and generally complex structures. They are naturally occurring so are produced from plants. There are various classifications of alkaloids; true alkaloids such as atropine have a nitrogen-containing heterocylic ring derived from amino acids; protoalkaloids – derived from amino acids but lacking nitrogen in a heterocyclic ring such as ephedrine; pseudo alkaloids not derived from amino acids. Most of them derived from- terpenes, sterols, nicotinic acid or purines. There are also false alkaloids which give a false positive test when subjected to alkaloid reagents Alkaloids come mostly from higher plants particularly in the dicotyledons, and are in abundance in the angiosperms. Alkaloids, due to their basic nature, generally exist as a salt of organic acid such as oxalic acid. Some alkaloids occur free in nature while a few occur as glycosides such as solanine in solanum (Hegnauer, 1963). It has been found that amino acids phenylalanine, ornithine, tryptophan, lysine, histidine, anthracitic acid are primary precursors of alkaloids in plants. Each of this amino acid can be regarded as starting point for synthesis of one or more types of alkaloids and therefore alkaloids can be placed into families corresponding to those six amino acids (Wink, 1999). Some
alkaloids only occur in certain plants e.g. Morphine from P. somniferum or strychnine is restricted to the strychnos class. So we can see there are instances and specifics which can be used in order to classify alkaloids using chemotaxonomy.
Hegnauer(1967) wrote of the interest in the steroidal alkaloids of Buxus sempcrvirens and how other species of Buxus and genera of the Buxaceae were studied for similar alkaloid substances. Similar compounds were detected in the genera Pachysandra and Sarcococca despite having differing morphology to the original plant where the first discovery was made.
Many drugs discovered many years ago still used today are from natural sources, such as quinine - Cinchona bark, morphine and codeine - latex of the Papaver somniferum, digoxin - Digitalis leaves. Newer drugs have also been discovered Anticancer drugs such as previously mentioned taxanes,
vinblastine and vincristine from Madagascar periwinkle - Catharanthus roseus and etoposide from American mayapple – Podophyllum peltatum. The resin podophyllin obtained from the root of the Podophyllum peltatum is toxic and is used clinically to remove warts. The major component of the resin is podophyllotoxin which inhibits cell division and while the resin is toxic, using the chemical characteristics and modifying them enabled the production of the semi-synthetic glucoside, etoposide (Phillipson, 2001).
Despite these innovations in phytochemistry many pharmaceutical companies have invested heavily in synthetic drugs more so than plant derived drugs recently (Walton and Brown, 1999). These produced many successful new medicines such as bisoprolol, ramipril, formeterol and lorazepam, all used for different treatments from hypertension to asthma. However often nature is able to produce unique chemical structures which may likely be undiscovered without chemotaxonomic research.
The secondary metabolite group terpinoids encompasses a vast range of terpenes and indicates that they are all based on a common chemical structure, isoprene, and have the formula C5H8.
The series includes essential oils such as the volatile substances
monoterpenes and sesquiterpenes, less volatile diterpenes, non-volatile triterpenoids and sterols and carotenoid pigments.
Artemisinin is a sesquiterpene endoperoxide that has been isolated from the Chinese antimalarial herb Artemisia annua. It has been shown to be an effective antimalarial against resistant strains of Plasmodium
falciparum the cause of human malignant cerebral malaria. Semi-synthetic derivatives including artemether have enhanced
pharmacokinetics and are also used clinically. Further studies of a range of plants used in traditional medicine for the treatment of malaria has led to the isolation of a series of other compounds with activity against P.
falciparum including isoquinoline and indole alkaloids, flavonoids, mono-, di- and sesquiterpenoids (Phillipson, 2001).
We can see discovery of active compounds from plants which are then characterised and classified may lead to new medicinal compounds or other useful products or they may just offer a starting compound which can be modified. Modifying existing structures can be less financially risky
for drug companies who may simply look to optimise the activity of a compound already known (Walton and Brown, 1999).
Conclusion
Plants are essentially chemical factories Walton and Brown wrote (1999) which naturally synthesise extremely diverse complex structures tailored for their needs. Chemotaxonomy has already proved to be of great
benefits in the discovery of drugs. Even when phytochemicals have been used for many years, with the new analysis methods at disposal we have been able to identify and isolate active chemicals that we want and refine them from unwanted chemicals which may be present. New complex secondary metabolites are being revealed and categorised all the time and so the understanding of biosynthesis and chemotaxonomy of plants is far from complete even for known plants. It is understood that when chemicals are detected within a sample that any not expected should be investigated further as even very small quantities may be the result of contamination or possibly indicative of the occurrence polymorphism due to ecological stresses from where it was sourced. With the changes in climate that are well documented and predicted to continue there may well be manifestations in plants through changes in biochemical as a result. An estimated only 20-30% of the world’s plants have undergone chemotaxonomy (Walton and Brown, 1999) and continued phytochemical and pharmacological studies will no doubt provide new sources for
medicines and foods. With the developments in technology it is growing more possible to use chemical data from plants and investigate if the phytochemicals of interest in them can be made in a more sustainable manner without the need to cultivate and harvest them from plant sources. The use of plant cell cultures is one aspect which is benefiting both chemical characterisation and industrial production. But with
certainty it can be said that any quests for new medicines or foods from plants will be more successful with a chemotaxonomy approach.
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