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There are five major classes of plant hormones. One of which is auxin. Generally theses hormones control development and plant growth, because they affect the elongation, division and differentiation of cells. Each of theses hormones has multiple effects. The effects depend on the stage the plant is at as well as the hormone concentration.
Auxin is any chemical substance that promotes elongation and coleoptiles. Also auxin stimulates differentiation and branching, the development of fruit, and the apical dominance. IAA or indoleacetic acid is a compound that is formed when the natural auxin is extracted from the plants. In addition to the natural auxin there are several other compounds. Most of which of these compounds are synthetic ones that have auxin activity that stimulate the elongation of cells in the young development of shoots.
The aprical meristem, in the shoot is the major site of the auxin synthesis. Hormones stimulate growth in the cells when the auxin from the shoot moves down to the region of cell elongation. The auxin can only have this effect over a certain concentration range. When the concentration is at a higher level the auxin may inhibit cell elongation. This can occur when a high level of auxin is induced during synthesis of another hormone, ethylene, which most commonly acts as an inhibitor of plant growth due to elongation.
Auxin transportation from the stem to the shoot apex moves at about 10mm per hour. This process is way too fast for diffusion. Yet this process is slower than translocation in phloem. From one cell to the next auxin seems to be transported directly through the parenchyma tissue. This moves from the shoot to the tip of the base. Polar transported is when unidirectional transport of auxin. The gravity has nothing to due with polar transport, but it is because auxin moves upwards in experiments where stem or coleoptiles segment is placed upside down. The polar transport requires auxin.
A key component in the growth response of the cells to auxin is the proton pumps located in the plasma membrane. The proton pumps are stimulated by the auxin, in the shoots region of elongation. This action lowers the pH in the wall. The fabric of the wall is loosened by the acidification of the wall. This causes cross links between cellulose micro fibrils to break. Once this has occurred the wall is more plastic like, which allows the cell to take up more water through elongation and osmosis. If some auxin is applied to young stems it will cause the wall to loosen and cell elongation within twenty minutes. To have sustained growth the cells must make more cytoplasm and wall material, also auxin stimulates long-term growth response.
Auxin also affects secondary growth by influencing the differentiation of secondary xylem: this is all in addition to stimulating cell elongation. Also auxin promotes the formation of the root at the cut of the base of the stem. When seeds develop they also synthesize auxin, which promotes the growth of fruit.
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