The world of the viruses

What a virus?

The viruses are of all small particles of a hundred Nanomètre S length at least made up of a genetic information: a filament of Nucleic acid DNA or ARN. These particles are protected for the majority from a Capside (together of proteins), and sometimes from an additional envelope of lipidic nature . The viruses have the characteristic to penetrate the cell of their host in order to use the cytoplasmic contents of it and genetic to ensure to them multiplication. It is an essential condition so that they can develop, therefore one qualifies them “obligatory parasite S”

This viral multiplication will end up causing the metabolic modification or the destruction of the cell. The proliferation of the viruses inside vegetable fabrics can in certain cases not involve any visible Symptôme initially (phenomenon of masking) but very often the virus attacks appear by symptoms such as marblings or fasciations (Document)

living or not alive?

The origin of the viruses remains unknown and their position in the world living ready still with debate. Many authors agree to place the viruses apart from the alive world because they do not have the fundamental characteristic of all the living beings which is of being able to reproduce themselves, they are static and do not present any physiology of relation, nor of nutrition. On the other hand their genetic structure and their obligatory “parasitic” lifestyle cannot prevent us from finding a great affinity with the living organisms to them. The debate remains open.

The viruses have an evolutionary potential very important because their multiplication proceeds in the genetic intimacy of the host cell. Many rearrangements, changes, exchanges of Gène S are possible. Despite everything, the lines of virus evolved/moved independently from/to each other. Indeed like the majority of the Endoparasite S, the viruses multiply in isolation in their host S. the evolution in parallel of the viral strains and of the resistant hosts (Co-evolution) is at the origin of a great specialization of the viruses with respect to their host. One will find thus viruses able to tackle only one species or only one family of plants. The virus of the mosaic of the tobacco for example, is able to tackle the majority of the plants belonging only to the family of the Solanacée S (Tomate, Tabac, the Eggplant…)

How are the viruses classified?

The structure of the phytovirus is well defined. It is presented in the form of viral particles (or Virion S) which have isometric characteristics , rigid or filamentous sticks, as well as bacilliform forms . The organization of the phytovirus of categories, kind and family is primarily based on three criteria:

• the nature of the genetic material: DNA or ARN, mono or bicaténaire (a bit or two)

• the level of homology (resemblance resulting from a common ancestor) of genes. To be done, one sequence the genetic material of each virus. One compares the successions of the nitrogenized bases various viruses, one gathers in same a taxon the viruses to which the sequences are closest. According to the degree of homology one will define the kinds (for example Caulimovirus) then the families (for example Caulimoviridea).

However one always does not manage to gather all the kinds in families; there thus exist isolated kinds, like are Benyvirus or Pomovirus… the without-families to some extent.

• To a lesser extent one uses as criterion the shape of the viral particles. One uses this criterion for the group of the viruses with ARN monocaténaire messenger. This family is subdivided in isometric viruses, helicoid in stick or flexuous particles.

What a viroïde?

The viroïdes are distinguished from the viruses; they have a structure much simpler and the principle of their multiplication is not completely identical.

The viroïdes are made up exclusively of a circular ARN monocaténaire which has a very compact and rigid space structure. There is no Capside and even less envelope. The Viroïde S were classified in two families: the Pospoviroïdae and the Avsunviroïdae .

The ARN polymerase of the plant ensures the synthesis new viroïdes which accumulates then in the nucleole, the remainder of the core or the membrane of the Thylakoïde S of the chloroplasts. This accumulation of viroïdes inside the cell involves a metabolic dysfunction preventing for example the cellular multiplication. The multiplication of the viroïdes is supported by an increase in the duration of day and an increase in the temperature, for this reason they are implied in tropical diseases, Mediterranean or plants of ornament raised under greenhouse. One of these attacks causes “Cadang Cadang” which leads to a deterioration slow but lethal of the coconuts and which already decimated thousands of trees.

We have just presented the organization of the smallest agents to you known and perhaps most pathogenic. We will now discover how a viral particle can starting from one only individual, to produce different thousands of them by using the vegetable Cellule like source of matter and energy.

Viral multiplication

In this part we chose not to develop all the existing modes of multiplications. Only most significant and simplest were selected. There exists indeed of other modes of multiplication whose comprehension would require a more thorough knowledge in molecular biology.

How do the viruses with ARN of the messenger type multiply?

The introduction of a ARN monocaténaire (=simple bit) into the cell leads to the synthesis of many molecules of viral ARN and the synthesis of many proteins constitutive of the capsid. To be done, the virus uses the cellular machinery of its host: enzymes, reticulum, ribosomes of which itself is deprived. The viral ARN has a function messenger which leads to the formation of a foreign protein to the plant: an enzyme ARN viral Polymerase which thus will amplify creation again bit of viral ARN. The viruses with ARN, can have other pieces of ARN (satellite ARN). The satellite ARN depend on this ARN polymerase of the virus says “assisting virus”.

How do the viruses with DNA monocaténaire multiply?

Once transported in the core, the DNA viral is converted there into DNA bicaténaire. This one will be used as model for the replication and the transcription in ARN messenger. The viral proteins are synthesized in the cytoplasm starting from this ARN m, thus allowing the encapsidation of the DNA.

If the viruses are not put in the presence of cells to parasitize, they degenerate quickly. Once in contact with the cells of the plant host by the means of a mechanical agent (shears for example) or biological (a ravageur) they can be propagated in all the plant. This third stage will enable us to discover how that is possible.

Viral transmission

How the viruses move in the plant?

With short distance, from one cell to another, the viruses join proteins of movement with which they form a complex. This complex leads sometimes to the formation of tubules, ensures the passage of the viruses by the plasmodesmes With long distance, the virus uses the tubes of transport of the elaborate sap which are the cells of the phloem.

vertical transmission

How the viruses are transmitted of a generation of plant to the other? With some exceptions, the viruses are transmitted neither by the seeds nor by pollen. On the other hand the vegetative multiplication of the plants allows the infection of all the descent. The elimination of the fabrics virus diseases is done primarily by micro propagation by cutting and the culture of méristèmes

Horizontal transmission

How the viruses transmit plants to plants? The transmission of plants with plants can take place by mechanical inoculation at the time of a Clerc's Office or under the effect of the wind which makes rub two branches. Cultivation methods: cut, ploughings can also ensure the horizontal transmission. The mode of transmission most specific to the viruses remains the transmission by living organisms: Ravageurs, mushrooms or Cuscute (vegetable parasite). Among the ravageurs one will find all the ravageurs stitchers, nozzles: acarina, nematodes, and surtouts insects like the plant louses, cochineal, cicadelles (and aleurodes in the hot countries).

The viruses transmitted by the animals can be classified in two categories, the viruses not circulating and the circulating viruses.

Viruses not circulating

are carried by the oral parts of the animal one distinguishes among these types of germ:

• nonpersistent viruses: these viruses have one very short lifespan apart from the plant. The meal of acquisition is of very short duration (a few seconds) and must be inoculated very quickly to be able to be propagated.

• persistent semi viruses: these viruses have one lifespans longer apart from the plant (a few hours), moreover to be able to be propagated the meal of acquisition and the meal of inoculation must be longer (a few hours also)

Circulating viruses:

They circulate in the digestive system, the hemolymph then the salivary channel of the animal by which they are introduced into a new plant. These viruses are also qualified the persistent ones. The meal of acquisition and inoculation is several hours at several days separated by one long period from latency during which the insect cannot transmit the virus.

nematodes, vectors of wire father At a group of nematodes, Xiphinema, the transmission of a Népovirus is done according to the circulating mode but with a persistence which can go from a few weeks to a few months. If the nematode saw sufficiently a long time, it can ensure the transmission of the virus between two annual cultures.

The parameters which govern the expansion of a viral disease transmitted by vectors depends on the various factors relating to the biology of the vector itself. The knowledge of the parameters makes it possible to set up estimated systems making it possible to lead an effective fight: elimination of the sources of virus during inter culture or the interference with the behavior of the vectors.

How to fight against the phytovirus?

To date one does not know chemical substance able to ensure the curative fight, only the preventive fight perhaps considered.

Genetic selection:

The natural means of defense of the plants (protein synthesis of defenses, over-sensitiveness (see last sequence of the booklet) to fight against the viruses are of genetic nature. This definite aptitude concept of varietal resistance. The experiment proves that the more one plant is selected in an objective of output, and the more it is weakened. The geneticists were thus brought to create new varieties by introducing genes of resistance by the means of crossings and selection or by the means of technique of transgénèse. For example one crosses tomato seedlings Lycopersicon esculentum with wild species like Lycopersicon hirsutum or Lycopersicon peruvianum in order to improve resistance to the viruses of the Mosaïque of the tobacco, of the Marbrure of the tobacco and the Virus Y of the potato. In the same way crossing are carried out between the barley and the triticale in order to obtain varieties resistant to the virus of the yellowing mosaic.

Elimination of the sources of vectors:

For the nonpersistent viruses the propagation of the epidemic is made in a ray of a hundred put around the seedling virus disease.

• the fight will consist in eliminating the sources from virus like the plants perennial or bi-annual adventitious during interculture.
• One can in particular protect the culture by a hedge in the been windy areas. Indeed the wind is a factor of propagation of the insect in particular for the low market gardenings.
• One can also use an insecticide which intercepts the vector in vol.
• One can pulverize oil of synthesis on the seedlings in order to limit the fixing of virus to the oral apparatus of the insect.

For the viruses persisting the propagation of the viruses can be done on hundreds of km. The control of the population of phytophagous vector can sometimes be essential. In these cases there, the use of an insecticide on the foliage or in the ground can be recommended. This type of treatment intervenes in particular for the virus of the rolling up of potato or the virus of the Jaundice nanifiante of the Orge.

Cultures of méristèmes

The méristèmes are unscathed virus. On culture medium containing of the hormones and the inhibiters of replication, the culture of méristème allows the multiplication of healthy clones large scales.

Thermotherapy

Thermotherapy consists in exposing the plant to a strong temperature. It is able to destroy the viruses with the image of a fever at the human ones. For that one proceeds to hot baths or one places in an environment heated the plants “to treat”. This solution is considered only for small cultures.

strawberry plants with the heat! Seedlings of strawberry plants infected by the virus of the marbling can be cleansed by a stay from 3 to 4 weeks with 37/38 ° C. In general this technique of thermotherapy is reserved for the preventive fight to produce grafts or unscathed seedlings of virus.

The prémunition.

It consists in inoculating a viral strain producing of the symptoms attenuated in order to protect the plants against the later infection from severe stocks of the same virus or viroïde. However, it can exist of phenomenon of reversion which makes regress the stock securing in virulent stock. Moreover certain stocks securing can be virulent for other species. With dimensions random one of this technique appears as method transitory in waiting of other solutions cheaper and more practical to implement. At the present time the viruses being the subject of a prémunition on a commercial scale are:

• the Virus of the tristeza of the citrus fruits
• the Virus of the annular spot of the papaw
• the Virus of the yellow mosaic of the zucchini
• the Virus of the mosaic of the tobacco

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