Allelopathy

History

The term of allelopathy was introduced for the first time by Molisch, scientific German, in 1937 to describe the harmful and beneficial biochemical interactions between all the types of plants including the micro-organisms. , in 1984, this definition in its monograph reinforces on the allelopathy (the first on this subject): “Any effect direct or indirect, positive or negative, of a plant (micro-organisms included) on another, by the means of biochemical compounds released in the Environment”.

These biochemical compounds are called compounds allelochimic. They can be classified mainly like secondary metabolites, which are generally regarded as being compounds not playing any part in the process of the Métabolisme essential with the survival of the plants.

One finds among these compounds of the phenolic acids, the flavonoïdes, the terpenoïdes, alkaloids, and the glucosinolates. These products allelochimic are present in practically all fabrics of the plant; Fruit S, Fleur S, Sheet S while passing by the Stem with the root S and rhizome S. Aussi on the level of pollen and seeds.

They are released from the plant to the environment by means of four ecological processes: volatilization, lixivation, exudat racinaire and decomposition of the residues of the plant.

The allelopathic interactions are often the result of action united several different compounds. The biological activities of the receiving plants are dependant on the concentration of the products allelochimic i.e. there is emission of an answer that when the concentration in products allelochimic reaches certain values threshold.

The interference which is established between close plants is allotted mainly for purposes of competition for the environmental resources: water, light and nutritive substances. Thus of many plant species Molécule S synthesize able to inhibit the Germination and the growth of the plants growing in their vicinity. Also for lack of mobility, the plants have to adapt to the predatory attacks of other organizations the such insects, the Champignon S and the Bactérie S; that by chemical mechanisms of defense being able to have several functions. They can be Insecticide S antimicrobic even for certain weedkillers. Currently, more than 30.000 secondary metabolites are known and this thanks to the phytochimic analysis of plants superiors.

Compounds allelopathic

One of the singularities of the plants is to form many compounds whose role, on the level of the cell, does not seem necessary while being able the being to the level of the whole plant. The fact that these compounds do not meet at all the species indicates that they do not enter the general metabolism and that they do not exert a direct function on the level of the fundamental activities of the vegetable organization: they are secondary metabolites.

Chemical nature

These compounds allelochimic are generally molecules of low molecular weight which can be absorbent or lipophilic. Among these compounds one finds acids phenolic, Quinone S and Terpène S. One can quote the Catéchine, the ellagic Acide, the Tellimagrandine, the Salicylic acid , the Ferulic acid among the Polyphénol S; the p-benzoquinone and the DMBQ among quinones; 1,8cinéole, 1,4cinéole, pinene among monoterpenes.

Mode of Action at the biochemical and cellular level

The compounds allelopathic behave like natural weedkillers; they often have several sites of action and the various effects on the target organizations. Certain allelochimic act by inhibiting the Photosynthèse what slows down the growth of the phototrophes. Thetrione (S) - hexahydroxyphenoyl-D-glucose (tellimagrandine II) inhibits the PSII by preventing the transfer of electrons between quinones Qa and Qb (Leu et al., 2002), just like the p-benzoquinone of the sorghum Sorghum bicolor. The cyanobactérine of Scytonema hofmannii inhibits the transport of electrons with-level of the site acceptor of the PSII. The fischerelline has of Fischerella sp stops the transport of electrons at four different places.

Phenols

The phenolic acids can disturb mineral absorption by the plant: the salicylic acid (o-hydroxybenzoate) and the ferulic acid (4-hydroxy-3-mthoxycinnamate) inhibit the absorption of K+ ions in the roots of Avena sativa. The degree of inhibition depends on the concentration of the phenolic acid and pH (the reduction in pH involves an increase in the absorption of the phenolic compounds and thus of inhibition). This disturbance is due to the fact that the phenolic acids depolarize the membrane potential of the cells racinaires what modifies the membrane permeability and thus the rate of emanation of ions, as well anions as cations. The extent of depolarization grows with the increase in the concentration in phenolic acids, especially with the salicylic acid.

Quinones

Quinones generate activated oxygens, responsible for their toxicity.

Certain substances act on the form of genes of the target organizations. The DMBQ (quinone) emitted by the hosts roots induces the development of plants parasite S by controlling the expression of certain genes, implied in the regulation of the cellular cycle, the synthesis of actine and tubuline, the extension of the vegetable walls and synthesis of GTP binding protein. The L-carvone of Mentha spicata, its derivatives (limonene, p-cymene and isoprene) and more generally the terpénoïdes with an unsaturated reason p-menthane induce the form of the genes bph bacteria of the kind Arthrobacter, persons in charge of the Catabolisme of PCB.Il exists probably a receiver with the structures isoprenes found in monoterpenes, person in charge of the regulation of the expression of these genes. But the various processes by which many plants select the bacterial catabolic genotypes in answer to the Pollution S are badly known.

Terpenes

Many classes of volatile monoterpenes inhibit the vegetable growth like the 1,8cineole, the 1,4cineole, the pulegone, alpha and beta pinene. The 1,4cineole inhibits the growth of the roots of certain grasses by inhibiting Asn-synthase with-level of the site of connection of the glutamine.

Metabolism

Expression according to fabrics and of the age

The inhibition of the vegetable growth is stronger during the first developmental stages of the transmitting plant. Young plants of cress ( Avena caudatus ) protect themselves while emitting by the roots of the lepidimoide and polysaccharides which affect the growth and the differentiation of the plants or the micro-organisms. The maximum degree of inhibition of the Sorgho is reached after 4 weeks of growth. The decomposition of the residues of the plant can is to inhibit or stimulate the growth of the close plants; the most severe inhibition appears at the stage earliest of the decomposition, then inhibition declines while stimulation emerges gradually.

Elicitation

The éliciteurs of the defense reactions have molecules able to induce at least one of the typical answers of defense, like the synthesis of phytoalexines, that in the absence of any infection. Two classes of éliciteurs were characterized: general éliciteurs such as those coming from disease-causing agents (exogenic) and those produced by the specific plants (endogenous) and éliciteurs. The elicitor S generals, of natures polysaccharidic, lipidic, or (glyco) proteinic do not reproduce the specificity of recognition gene for gene, contrary to the specific éliciteurs. Three major types éliciteurs of polysaccharidic nature were identified: β-1,3 and β-1,6 Glucane S and the Chitin coming from the fungic walls and the Oligogalacturonide S, residues of acids galacturonic in connection α-1,4 derived from the Pectin of the vegetable walls. The nature and the intensity of the answer of defense induced by these éliciteurs depend on their degree of Polymérisation and the plant. They probably intervene as signals of second generation in the cascade of reception-transductionparticipant to the expression of the answer of defense. Among the éliciteurs of lipidic nature, the arachidonic Acid and other unsaturated fatty-acids generate the Oxylipine S, effective in the activation of the synthesis of Phytoalexine S. The plants which influence the structure of the bacterial communities in answer to a pollution of the ground are those of which the roots are permeable with the pollutants.

Means of emission

They are released from the plant to the environment by means of four ecological processes: volatilization, exudat racinaire and decomposition of the residues of the plant. The maximum of effect occurs close to the roots.

Influence environmental factors on the action of the compounds

The degree of inhibition can depend on the pH of the medium which more or less facilitates the entry of allelochimic in the target cells. The poly acetylene and the Thiophène are more bioactifs after exposure to the GRAPE one. Their inhibiting effect is activated by the light. Certain substances have impact on the target organizations only when they are exposed to a constant contribution of made up coldly emitted. Effects of synergy between the various compounds present in the vegetable exudats can be observed. The negative effects on the target organizations, for example an inhibition, never reach 100% of effectiveness not to support the emergence of resistance.

Ecological interactions

The secondary metabolites are the principal tools of the coévolution alive plant-beings what gave place has a diversification as of these compounds. It is about a coévolution which applies to all the levels of organization of the alive one, of the Bacteria to Mushrooms, of the Insects to the Mammals which is expressed at all the stages of the development of the plant. Two axes of coévolution were privileged:

a first of opposition, which one can regard as a chemical war.

a second axis, of co-operation results in a partnership with the animals

Defense against the Pathogenic ones and the Predatory ones

Because of their immobility, the plants must use physical and chemical defenses of which toxic metabolites to survive the attacks of insects, bacteria and mushrooms and to take part in the competition for the light and the resources with the other plants. The compounds allelopathic of defense against the predatory ones can be insecticidal, of anti-fungic, anti-pathognènes (phytoalexines). There exist two types of defenses:

the direct defense which takes place when the compounds birds interact directly with the predator of the plant, ex: the acacia.

indirect defense, it does not have a direct influence on the herbivores but on their predatory enemies and the parasitoïdes. It is the case at the sheet of tobacco which after being infested by the caterpillar of Mandusca sexta will release from the volatile substances which attract the predatory ones of Manduca sexta.

The toxicity of a molecule is always relative and a molecule toxic or pushing back for certain species can be gravitational for others, which circumvented or diverted with their profit the ways of toxicity.

The synthesis and the effective use of chemical substances of defense are a permanent compromise between cost and benefit for the plant. These mechanisms are to be put in relation to the energy cost of the synthesis of the molecules of defense.

Competition

It is here about the strict version of the allelopathy: excretion or exudation by the plants of inhibiting substances which reduce or prevent the growth of other plants in the vicinity.

One regards the allelopathy as an active strategy of competition, because she exploits the capacity of the individuals to decrease the performances of other individuals. The function of relation of the plants rests on the extraordinary specificity of their secondary metabolites.

This is why perhaps specific inhibition and in certain cases, on the individuals of the same species more than the others.

Intraspecific competition

One can consider that it is not a question here really to enter in competition but to prevent an excessive growth in an hostile environment (deserted) under temporarily favorable conditions because the resources are function of the number of individuals of the same species present on the hostile territory. By limiting their growth these individuals can thus provide for their development while preserving their capacity with being competitive, to adapt to their medium. For example, if the shrubs of the desert answered immediately a strong rain by a rapid growth, they could exceed their capacity to survive one period of dryness, to which they are prepared by a weak development of their organization.

Interspecific competition in Terrestrial Environment

The competition between various species it is the interspecific competition. It is established for the appropriation of a resource present in quantity limited in the environment. It can be a question of a nutritive resource (light, water, rock salt), or of an appropriation of space. Because the plants being motionless require vast heat-transferring surfaces with the underground and air environment to arrive to their needs for fixed autotrophic organization. And more its heat-transferring surface is large plus the plant collects signals enabling him to modulate its development towards an effective exploitation of the resources of its medium.

The plant subjected to the competition protects and defends its heat-transferring surfaces thanks to secondary metabolites.

The majority of the individuals in competition are thus prone to an inhibition while the total production of biomass tends towards a maximum. One speaks about target plants which collect the toxic compounds.

The factors produced by the system racinaire play a big role here, with a weak contribution of the sheets.

The kinds Artemisia and Eucalyptus emit 1,8cineole, a powerful agent allelochimic which inhibits the growth of several grasses.

A plant can emit several different compounds allelopathic. A allelopathic substance is more or less specific with respect to the target organizations, it can act on several species, more or less distant phylogenetically.

The tree Ailanthus altissima emits ailanthone, inhibiter of growth of Brassica juncea, Eragrostif tef, Lemna minor and Lepidium satium. The extracts of this tree, which also contain like active compounds the quassinoïdes (degraded triterpenes), the amarolide, the acétyl amarolide and the 2-dihydroxyailanthone, have also an inhibiting effect on the growth of the insects Pieris sp, Platyedra sp and the plant louses.

The effects of these volatile substances released in the ground and the air are numerous: one can quote the inhibition of the mitosis on the level of the méristèmes racinaires, the reduction in the opening of the stomata, the inhibition of certain enzymes, the proteinic synthesis. The root S exudent a large variety of low-weight molecules molecular in the Rhizosphère. The rhizosphère is an important place of interaction between roots, Pathogène S, Microbe S beneficial and Invertébré S.

Interspecific competition in Aquatic environment

The allelopathy relates to also the watery world, at the angiospermes, the épiphytes and the phytoplankton.

The fresh water angiosperme Myriophyllum spicatum (Haloragaceae) emits polyphenols Algicide S and cyanobactéricides (ellagic Acide, Catéchine…), of which most active is the Tellimagrandine II, which inhibits the Photosynthèse Cyanobactérie S and others Phototrophe S and inactive the extracellular Enzyme S of these organizations by complexation.

The Cyanobactérie S Scytonema hofmannii (by the means of the Cyanobactérine) and Fischerella muscicola (via the Fischerelline has) act in the same way on photosynthesis.

The allelochimic interaction is also important for the competition in the Zooplancton, for example, the population of the Copépode Diaptomus tyrreli is reduced in the presence of substances emitted by the copépode Epischura nevadensis .

Case of Parasitism

The plants are Autotrophe S. Cependant some are unable only to be nourished and live in parasites. Parasitism is defined like a durable interspecific relation where one of the partners, the parasite, lives with depend on the second, the host who is injured by this association. The host thus represents the medium of life of the parasite. The installation of the parasitic relation constitutes a crucial stage in the life cycle of the parasite and depends on the meeting held on the two partners. The most known example is that of GUI but there are many parasites 3000 to 5000 species which are classified according to the term Hémiparasite or Holoparasite S.

The exit of the dormancy of the Seed S of the holoparasites of Striga asiatica, the development of haustorium, a pluricellular structure racinaire massive specialized which invades the hosts roots and is used as physiological conduit between the parasite and the host to steal the resources of the plant as well as the passage of the autotrophic life to the heterotrophic life by the development of the haustoria at the hémiparasites of the family of Scrophulariaceae is started by the application to the roots of the parasite of factors racinaires of the plant host. Several quinones and phenols cause this phenomenon while exploiting the osmotic potentials of the plant host. That will modify its structure and will give a signal to the parasite of launching the morphogenesis of haustorium.

The principal compound is the 2,6-dimethoxybenzoquinone (DMBQ). It is slackened in the Rhizosphère in the exudats racinaires or resulting from the oxidation of the phenolic acids, majority component of these exudats. Positive interaction, Co-operation

Answer to the contamination of the ground

The roots exert a selective influence on the bacterial communities which is partly specific plant.

The plants can increase the disappearance of the contaminants of the grounds by stimulating the microbial activity of degradation.

Many plants, in different environments and in answer to various pollutants, enrich the populations by endophytic bacteria and the mound racinaire in catabolic genotypes. Enrichment is depend on the nature and the quantity of contaminant but also on the species of plants. These bacteria protect probably the plants from the toxic effects of the pollutants.

The compounds allelopathic can thus play a part in the phytoremediation thanks to their important activity in the signals of information between the bacterium and the plant.

Symbiosis

The relations of the plants with the micro-organisms are not always conflict. Some are symbions quite as complex as the relations between disease-causing agents and the plant and with the consequences quite as important for agriculture.

There exist resemblances of structure and function between parasitism and symbiosis. Certain parasites can become symbions and conversely according to the environment, the physiological state of the plant and the genetic variability of the protagonists.

Among the majority of the plants which have symbiotic relations one distinguishes several symbions:

mycorhizes

Associations of a Mushroom in and of a root, the Mycorhize S are the most widespread symbiosis on ground. In addition to their role in the nutrition of the plant they contribute to protect the roots against an infection by pathogenic micro-organisms from the ground.

nodosities

Associations of a bacterium or a cyanobactery and generally of a root, they are more specific certain families of the plants.

The metabolites play a very important part in the process of recognition between symbions and hosts. Because this one implies a chemical dialog between the protagonists via molecular signals which are flavonoïdes.

The case of the Nodosité S is most known: the plant produces Flavonoïde S which attract the Bactérie S and stimulate their production of factors of coring. The plant perceives chemically these factors NOD by receivers and product in return more flanoïdes and initiates the nodisity.

For the mycorhizes the processes are the same ones except that the relation between mushroom and its host is not very specific. The way of indication of the endosymbioses is thus common practically common to the both mushroom and the bacterium.

Some compounds of the exudats racinaires can be used as natural substrate with the induction of bacterial genes of catabolism of the pollutants of the grounds. The L-carvone of Mentha spicata and others terpénoïdes is important inductors of the cometabolism of the PCB (biphénols polychlorés, pollutant) at Arthrobacter sp.

One observes an enrichment in phenotypes ntd Aa (2-nitrotoluene réductase) in the event of Pollution by thearomatic ones and an enrichment in phenotypes alk B (alkane monooxygénase) and ndo B (naphthalene dioxygénase) for a pollution with the Hydrocarbure S. Enrichment in Phénotype alk B occurs in the interior of the root (endophytic bacteria) while enrichment in ndo B occurs in the mound. Scirpus pungens exposed to oil enriches the ground in genotypes ndo B while the majority of the plants enrich it in alk B. the endophytic bacteria increase the capacity of the plants to resist pathogenic, herbivorous and other plants.

Ecology of the restoration

Certain associations plant-bacterium can increase the degradation of the pollutants and play a big role in the systems of phytoremediation

The identification of the substances which make it possible to stimulate the gene expression responsible for the biological breakdown of pollutants could allow the development of new approaches for the bioremediation of the contaminated grounds. The plants which secrete Monoterpène S could be used in situ for depollution by systems plants/bacterium of the grounds contaminated with the PCB.

Conclusion

For a durable agriculture and a reduction of the dependence to the synthetic chemicals, which cause a certain resistance, an increase in the cost and a contamination of the environment, the potential allelopathic can be used and this in several ways, for example in the use of compounds allelopathic like natural weedkillers or pesticides.

The management of bad grasses can be done by means of allelopathic plants used like vegetable cover, into under-semi or like cleaning intercalated culture. Indeed, the decomposition of the residues of the allelopathic plants can inhibit the germination and the growth of bad grasses while stimulating the growth of the crop plants. This decomposition can also be used as pesticides such as for example with the hairy bean decomposition (Mucura deeingiana) which reduces the development of nematodes phytopatogenes of roots of tomato of more than 50%.

The natural pesticides, or pesticides derived from natural products, contribute with the improvement of the production and the conservation of the environment while being the target of any organization, effective in the control of the harmful organizations, less toxic, and biodegradable at the same time. They can as be surer as the synthetic pesticides. Indeed, the repeated use of only one molecule being used as synthetic pesticide can lead to the development of resistance on behalf of the target populations, contrary to the natural pesticides which, in the mechanisms of defense of the plant, often are composed of a variety of toxins which thus allow a not very favourable adaptation of the organization targets.

The plants used as cover thus present a weak capacity of competition with respect to the cultures, while allowing a control of the adventitious flora.

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