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Relevance of allelochemicals in disease control

The article covers use of natural products in disease and pathogen control, elaboration of the term of allelochemicals and the mode of action of the same and example of some botanical species which contain these allelochemicals

Biologically active natural products can be considered as bioregulators. Use of natural products in disease and pathogen control include the following:

* Allelochemicals :

These are novel compounds possessing strong toxic activity, often accompanying an antifeedant action. These chemicals are often isolated from plants of desert or tropical origin.

* Naturally occurring hormonal agents from insects and plants :

The juvenile, moulting, brain and diapause hormones, antihormones, such as the precocenes and the prostaglandin, are examples of these agents.

* Microbial and viral agents for control of pathogens and pests

* Natural products as inducers of resistance:

For example, the ''phytoalexins'' which are compounds in plants whose biosynthesis is elicited as a result of attack of pest.

* Induced autointoxicators:

The precursors are applied that develop activity only in the target species. Examples of this class are the furanocoumarins and acetylenes.

* Natural products that regulate plant growth:

This class is broadly divided into two groups.

The growth substances such as auxins, gibberllins, cytokinins, abscisic acid, ethylene and their analogues.

The secondary plant growth substances such as phenols, aliphatic and aromatic carboxylic acids and their derivatives, steroids, alkaloids, terpenoids, amino acids, and lipids.

* Disease control:

This category includes fungicides from tropical and subtropical plants. These fungicides are obtained naturally from plants in which they are produced as defence chemicals. They are being examined for their pharmaceutical and anticancer properties.

* Antibiotics:

These are produced by microbiological processes. Some antifungal antibiotics include cycloheximide, polyoxin, ezomycin, validamycin A, etc. Typical antibacterial antibiotics are streptomycin, tetracyclin, chloramphenicol, cellocidin and novobiocin.

* Elicitation of phytoalexins:

Phytoalexins, defined as low molecular weight products of plant biosynthesis that have antibiotic properties to one or several groups of micro-organisms, can be used to induce resistance in plants.

Thus natural products include thousands of alkaloids, terpenoids, phenolics and minor secondary chemicals. These substances have no known function in photosynthesis, growth or other basic aspect of plant physiology.

What are Allelochemicals?

With the expansion in the field of chemical ecology, the need for terms to describe chemical messenger was recognised The term "allelochemical" (alloioharmonen) is as ''chemicals significant to organisms of a species different from their source, for reasons other than food as such." In simpler words, Allelochemics means an interspecific chemical messenger.

The term is further subdivided into allomones and kairomones. ''Allomones'' (Greek-other) were defined as ''chemical substances produced or acquired by an organism, which on contacting an individual of another species in natural context, develop in the receiver, a behaviour or a physiological reaction adaptively favourable to the emitter.'' A ''Kairomone'' (Greek-opportunistic) is defined as ''A trans.-specific chemical messenger, the adaptive benefit of which faces on the recipient rather than the emitter.'' Allomone is an interspecific chemical messenger; adaptive advantage with emitter, while kairomone is an interspecific chemical messenger; adaptive advantage with receiver.

Production of Allelochemicals?

Allelochemicals are therefore biochemical agents which include representation from a broad spectrum of secondary plant products. While production of allelochemicals is a naturally occurring phenomenon, several factors in the environment alter amounts of such compounds produced by higher plants. These include ionising radiation, various mineral deficiencies, temperature stress, water stress and stress from allelochemicals and herbicides. Stress induced accumulation of allelochemicals may be extremely important in dictating the quantities of these agents in the environment.

Mode of communication of allelochemicals in the host plant to the invading pests

The main modes of perception of chemical messengers are by gustation and olfaction. The behaviour of the allelochemicals on pathogens or pests to protect the host plant may be categorised as:

* Food attractants and inhibitants/ deterrents:

Examples are L-glycosides as food attractants and glucosinolates in p-brassica Azadirachtin and Salannin in Neem which act as food deterrents.

* Oviposition attractants and deterrents:

Examples of this class are seen in pinene which is active in the stimulation of ovipositional behaviour in female species of Sprucebud worm, Choristeneura fumiferana. 1,8-cineole acts as oviposition deterrent against yellow fever mosquito, Aedes aegypti.

* Insect growth inhibitors:

Examples are seen in the linear coumarins extracted from plant extract of Thamnosma montans, which along with acting as insect growth inhibitors, exerts a number of other biological effects including dermal-photosensitization, antibacterial, antifungal, molluscicidal, allelopathic, insecticidal and insect antifeedant activities. Azadirachtin affects the juvenile hormone titres thus inhibiting the insect growth.

* Pheromones:

Examples are seen in pinene, the terpene of the host pine tree which along with the attractant pheromone ''Fontallin'' of the southern pine beetle (Dendroctonus frontallis Zimm.), released by the female, aggregates the sexual behaviour of the males on the tree.

* Defence chemicals:

Examples are terpenoid compounds, benzoquinones, steroids, aromatic compounds, aliphatic hydrocarbons, acids, esters, carbonyl compounds and various alkaloids.

Some botanical species possessing allelochemicals described below are from families Meliaceae, Annonaceae, Rutaceae, Asteraceae, Labiateae and Cannelaceae

Meliaceae

Neem tree-Azadirachta indica A. Juss. is a member of this family. Extracts from various parts of the tree have been shown to possess feeding deterrence, repellence, toxicity, growth inhibiting properties, etc. It is well known for its antipyretic, spermicidal and antimalarial properties in humans. Neem extracts are known to contain of mainly tetranortriterpenoids and the most active limonoid is Azadirachtin.

The Chinaberry tree ''Melia azadirach'' , another plant of this family, contains several limonoids common to Neem, except Azadirachtin. Main active ingredients of this are the meliacarpins. Due to extreme toxicity shown to warm blooded animals, these do not find use as pesticides.

Limonoids Hispidin A-C, isolated from Trichilia hispida Penning, were shown to have bioactivity.

From Carpa procera, Carapolides A-C were the main active ingredients, whereas from Carapa grandiflora Sprague, Carapolides D-F were the allelochemicals isolated.

Annonaceae

Numerous alkaloids are reported from several species of Annona. Liriodenine from Annona glabra seed bark is found to be an active tumour (KB) inhibitor, as are the other four alkaloids obtained from stems and leaves of Annona purpurea. Annonelliptine, an isoquinoline alkaloid isolated from Annona elliptica RE Fries, also possesses this activity. This family has also been found to have toxic effects on insects.

Rutaceae

Limonin, a limonoid isolated from several citrus species (specially orange and grape fruit), is the main active ingredient of this family. The compound is an effective antifeedant and toxicant though it''s activity is ten times lesser than that of Azadirachtin. Herculin, a pungent isobutylamide,(n-isobutyl-2,8-dodecadienamide), was found to be a toxicant and have rapid knockdown property.

Astreaceae

Germacranolides Schkuhrin-I and Schkuhrin-II isolated from the plant Schkuria pinnata (Lam) O.Kuntze, exhibited antifeedant activity, antibacterial activity against some gram-positive organisms and cytotoxicity (carcinostats).

The family mainly possesses Sesquiterpene lactones which are feeding deterrent for pests. Echinacein, the toxic component and echinolone, responsible for juvenilization, were obtained from Ochnopeltus fasciatus.

Malvaceae

Gossypol from Gossypium hirscitum L along with other related terpenoid, condensed tannins and certain monomer flavonoids were found to provide the necessary resistance against pests. Gossypol is presently under study in several parts of the world as a retractable antifertility agent for human males and is presently used in China for the same purpose.

Cannelaceae

The main active components are the sesquiterpenoids. Warbungal, a sesquiterpenoid dialdehyde, extracted from eastern African trees of the genera- Warbugia, has an antiyeast and antifungal activity in addition to being feeding deterrents.

Liberate

This family mainly consists of chlordane diterpenes and has juvenilizing effect on insects. Examples are isomeric ajugarins from Ajuga nipponnensis Makino and juvocimine-I and juvocimone-II from leaf oil of sweet Basil, Ocimum basilicum L.

Some Indian plants possessing allelochemicals

Some Indian plants which possess allelochemicals other than Neem are Pongamia glabra (Karanj), Schleichera trijuga (Kusum), Madhuca indica (Mowrah), Celastrus paniculatus (Malkanguni), Hydnocarpus wightiana (Chaulmoogra) and others.

Components of Malkanguni are responsible for the remedial properties of Rhuematism and Pellegra, as a curative for Beri-Beri, and is a brain tonic without any side effects. Chaulmoogra contains Hydnocarpic, Chaulmoogric and Gorlic acids as allelochemicals which are responsible for it''s therapeutic use in leprosy, as an antimicrobial agent and as a pheromone mimic. Extract from various parts of Karanj are used to treat auto immuno diseases, inflammatory diseases, and leprosy. It is used in agriculture as a nitrification inhibitor and as a pest regulator. Mahua cake is used as a wormicide. Saponins from Mahua are the main active ingredients(Fig.30). The cyanolipids in Kusum seed oil are responsible for the feeding and ovipositional stimulating properties .

The article is written with the intention to provide an insight into the vast area of allelochemicals and thus help to tap the rich potential nature has provided us with. This wealth of allelochemicals combats any situation in an eco-friendly way. These being natural products, degrade causing no harm to the end users. These materials don''t cause immunity. the wide spectrum of chemical structures present in these permit their use as target specific system that the world could make use of by its scientific approaches, to suit the appropriate needs and yet to remain safe.

The author is based in Mumbai and is a specialist in allelochemicals.

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