Vegetable morphogenesis

The vegetable morphogenesis (of the Greek morphê/μορφη , form, and genesis/γένεσις , birth) constitutes the whole of the mechanisms which take part in the construction of a plant. It intervenes of the germination of seed until the death of the plant. The fundamental mechanisms of vegetable morphogenesis are common to all the plant species.

The construction of the plants

The morphological diversity of the plants

Different ports at of the same plants species

The port indicates the aspect, the general form of a Végétal. For example, a tree can be drawn up, in ball or flag. It results from the provision and on the development of the Organe S i.e. the trunk and the Branche S, the stem S and the Rameau X, Feuille S… the morphology depends on the genetic characteristics of the Espèce. There exist differences between the plants of the same species. All these differences are explained by interactions with the environment (shade of the trees, light, wind…).

Similar morphologies at plants of different species

Plants of different species can have a similar aspect if they are subjected to same a Environnement. That shows if one moves a Fleur mountain close to a flower of plain, this one modifies its aspect so much so that they resemble each other.

Genotype and phenotype

The morphology of the plants results from the expression of the Génotype of the species. This genetic information is carried by the Chromosome S in the core (let us recall that the vegetable cells are Eucaryote S). The realization of the genotype is modified by the factors of the environment.

Growth of a plant

The development of the network racinaire

The Embryon contained in the Graine comprises an outline of root. At the time of the Germination, this outline lengthens starting from its end and is inserted in the ground. It forms the principal root from which are formed the side ramifications: secondary roots.

The construction of the air parts

The Embryon carries small a Tige equipped with an outline of final bud. The stem develops starting from this apical bud . A stem is made of a succession of nodes and internodes. As for the sheets, they are inserted into the level of the nodes which also comprise a bud axillaire. It is from this one that develop the secondary branches. Under the moderated climate, the sheets generally have a limited lifespan. The buds are permanent structures but their operation is not continuous. They hatch spring ensuring the growth and the ramification of the stem. It is thus operation of the buds which the organization depends on the air parts.

Growth in thickness

One observes a growth in thickness only at the perennial plants (trees) and years by years, the Tige is transformed into trunk.

The operation of the zones of growth

Growth in length

The growth in length is localized with a few millimetres of the end of the plant. It associates two phenomena: the cellular division (Mitosis on the level of the Méristème) and elongation of the produced cells.

The méristème is a zone of division, located under a protective cap. The cell S there are low-size and are undifferentiated. They are thus likely to give any cellular type. It produces two types of cells: one which remains undifferentiated and which continues to divide and a cell which will cease dividing but which will take part in the structure of the root (which will undergo a lengthening in the zone of elongation). For the elongation: to see 3. More far from the end, the cells acquire specific characters in connection with their function: it is the zone of differentiation. For example of the cells take their function of transport of the Sève, hairs absorbing, conducting sap, storage of the Amidon…

Operation of the buds and cellular differentiation

The bud S are located at the end of the Tige S or the Rameau X (apical bud) and with the armpit of the sheets (bud axillaire). Each bud consists of a apical Méristème, of outline of overlapping sheets recovering this méristème apical and of meristematic cells located at the armpit of each outline of sheets.

The mitosis is a process common to the eucaryotes

A cell which divides by mitosis gives rise to two cells girls which have the same number of chromosomes as the cell mother. That supposes a copy of genetic information before the division of the two specimens during the mitosis. It is during the interphase that this duplication of chromosomes intervenes (and the replication of the DNA).

Course of the mitosis

Before the Mitosis, the Chromosome is duplicated, it is a double chromosome i.e. a chromosome with two Chromatide S thus two double helixes.

The Prophase

The chromosomes condense. They become visible in the cell. Between two poles of the cell, a Fuseau mitotic appears. The nuclear envelope disappears and the duplicated and very thick chromosomes are distributed randomly in the Cytoplasme.

The Metaphase

The chromosomes are aligned on the level of the equator of the spindle mitotic and form a figure called equatorial plate.

The Anaphase

For each duplicated chromosome, the two simple chromosomes thus with a Chromatide separate on the level from the Centromère. There are two batches identical of 2 chromosomes to two chromatides which migrate in directions opposed towards each cellular pole. As of the anaphase, the separation of the cytoplasm starts.

The Telophase

Each batch of simple chromosomes arrives at a pole of the cell and décondense. A nuclear envelope is formed around each batch and thus completes the formation of the two cores wire what marks the end of the mitosis. The spindle mitotic disappears and the continuous separation of the cytoplasm. Each phase is characterized by the state of the chromosomes and their localization in the cell.

The preparatory phase with the cellular division: the Interphase

The replication of the DNA is held at one time of the Interphase named Phase S. It follows a phase of growth of the G1 cell and precedes one second phase by growth called G2.

The replication, a conservative semi mechanism

The model of Watson and Crick: In 1953, they discover the architecture of DNA. Following that, they propose a model of replication, the conservative semi model. The DNA being a Double helix made up of two complementary bits which join according to the connections of Nucléotide S by complementarity of the nitrogenized bases (Adénine/Thymine, Guanine/Cytosine). Each of the two bits is used as matrix with the synthesis of a new bit. Each molecule of DNA thus contains an old bit and new.

Complex enzymatic which Catalysis reaction

The molecules of DNA in the course of replication can be observed with the Electron microscope. It is then possible to see zones called eye of Réplication where the molecule of DNA seems to be duplicated. An eye is formed by two forks of replication. The replication is carried out on the level of each fork and progresses on the two bits at the same time.

DNA polymerase is fixed on the molecule of DNA and opens it. The two bits separate. Free nucleotides complementary to each bit are placed opposite nucleotides of the two bits. The DNA polymerase binds free nucleotides by a Covalent bond: there is formation of two new bits. It moves along the DNA and at the end of the Phase S all the molecule of DNA is retorted except on the level of the Centromère which binds the two new molecules of DNA. The replication requires free nucleotides, DNA polymerase and energy. The DNA polymerase is a Enzyme which is endowed with a function of correction of error: if there exists an error, it will read again last nucleotide and will replace it.

Cellular modification at the time of the mitosis

The most important events and easiest to observe relate to the chromosomes but other cellular structures undergo important modifications.

Mitotic intervention of the Spindle

The chromosome S duplicated or simple move at the time of the Mitose thanks to the spindle mitotic which is a machine to move the chromosomes. It starts to be set up in Prophase and disappears in Télophase after having allowed the movement of the chromosomes during the Métaphase and the Anaphase.

The division of the cytoplasm: the cytodiérèse

At the plants, the Cytodiérèse is done by the appearance of a construction of a new wall at the equator of the cell.

Ches the animals, the Cytoplasme is divided into two by a simple throttling of the cytoplasm in the equatorial area of the spindle of division. The cells girls have all the Organite S necessary to their survival.

The systematic alternation of the replication of the DNA (in Interphase S) and of the division of two duplications (in Anaphase of Mitosis) ensure the conservation of genetic information during the successive cellular generations. Thus Interphase and mitosis constitutes the cellular cycle.

Growth and the control of morphogenesis

Cell multiplication

The vegetable cells have a extensible wall, tallies more or less rigid surrounding in a permanent way each cell. It is the existence of this wall which conditions the existence.

The primary wall is an extensible surface

It successively comprises several parts installation.

The average Lamelle is a primitive partition which is built and separated the two cells girls after the Mitose. On each side of the average plate, each cell girl in growth will work out her own wall.

The primary Paroi extensible has a thickness from 1 to 3 pm. It is made up of Polysaccharide S (sugar sequence): the Cellulose made of unit of glucose, the Hemicellulose, of made up the pectic and the Protéine S. the cellulose molecules form a network of microfibrils coated in a made paste mainly with hemicelluloses related to protein. This relatively soft paste makes the wall primary extensive.

the lengthening of the cells

The process of lengthening requires the intervention of two processes: a relaxation of the paste which links the Microfibrille S of Cellulose and the addition of new cellulose microfibrils (and others made up) to the primary wall. Thus the parietal Surface increases and the cell grows. The cell multiplication at the plants is not a simple swelling but a true growth.

In a cell which grows, of space new east creates. This space is mainly occupied by its vacuoles, cavities spared in the cytoplasm. The vacuoles contain a liquid rich in water in which many substances are dissolved (mineral ions: Potassium, Magnesium, amino-acid, Saccharose…). These dissolved substances create a water call of the outside of the cell towards the interior. It results a pressure from it: osmotic pressure of turgescence or which is exerted on all the surface of the wall via the Cytoplasme. This pressure of turgescence produces the force necessary to the swelling of the cell and is the engine of the cellular extension. The physiological state normal of a vegetable Cellule in growth or not is the turgescent state. When the cell loses water, the vacuole decreases by volume and the cell becomes plasmolysée. A Plasmolyze prolonged causes the death of the cell. In addition, the wall makes it possible the cell not to burst under the effect of this pressure. The proof is provided by a suspension of Protoplaste S. the protoplasts are private vegetable cells of their wall (attacks enzymatic). In a medium where water penetrates in the protoplasts, the latter burst.

The function of the skeletal Wall

Once the size of the acquired cell, the secondary Paroi is formed by successive deposits inside the primary wall. These deposits deprived of pectic compounds and proteins comprise extremely tight cellulose microfibrils the ones against the others. These deposits cannot increase any more.

Roles of the auxine in the cell multiplication

The auxine is a vegetable hormone

The Auxine is the first vegetable hormone discovered. Currently one calls hormone a substance which meets several conditions; to be manufactured by plant (thus not to be absorptive in the medium), to be active with very low dose (for the auxine 10-6g.mL-1), to convey information with cells sensitive to its action (target cells that it modifies) and to degrade themselves after having acted.

The auxine has several physiological effects

The manufacture of the auxine proceeds in the apexes of the stems starting from an Amino-acid , the Tryptophane. It migrates towards the target cells located at the base of the bodies, therefore the transport of the auxine is polarized. The auxine control the Cell multiplication by stimulating the increase in size due to the cellular elongation. In addition to this effect, the auxine acts on the cellular Différenciation by provocant the formation of root S side and conducting fabrics.

A double action on the cell multiplication

The auxine control growth by stimulating the cellular elongation. Its action is double: a short-term action on the plasticity of the wall. It lowers pH wall what causes its relaxation. An in the long run known action the expression of the Gene S coding the Protein S intervening in the cellular elongation. It stimulates the synthesis of specific ARN. The ARN are then translated into enzymatic proteins necessary to the manufacture of the Polysaccharide S (Cellulose, Hémicellulose…) wall.

Hormone and cellular development

The distribution of the auxine modifies the development

The Coléoptile of a Graminée is curved when it is enlightened laterally. This curve results only from one difference in growth between the enlightened face and the dark face by elongation of the cells. The long face lengthens of advantage owing to the fact that it contains much more auxine which stimulates the cellular elongation. The light causes a migration of the auxine towards the nonenlightened areas of the plant and thus allows a directed growth.

The final bud prevents the subjacent buds from developing. It is said that he exerts an apical predominance. She explains herself by the fact why the final bud manufactures auxine which directly inhibits the development of the other buds. The section of the final bud modifies the distribution of the auxine and raises this predominance.

The Organogenesis: a business of hormonal balances

The auxine is not the only vegetable hormone necessary for the development and the growth: the Cytokinine S (intervene in the cellular division), the Gibbérelline S, abcissic acid , the ethylene. The in vitro Culture of the vegetable cells illustrates well the combined intervention of these Hormone S. If one puts a fragment of plan with as much of auxine that from cytokinines, one will stimulate the cellular Division but there is no appearance of body; one obtains cal which are solid masses of undifferentiated cells. If one puts more auxine that of cytokinines, there is development of the root S. Contrary, one obtains buds.

The installation of the vegetable bodies is thus controlled by the proportions of the vegetable hormones auxine and cytokines primarily.

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