Auxins are plant hormones that accelerate, inhibit or modify growth. They are formed in the tips of a plant and transported down the stem to the roots. Auxin concentration is highest in the leaves and decreases towards the roots, influencing growth in different parts of the plant. Auxins cause positive phototropism in shoots and negative phototropism in roots, as well as affecting geotropism. They also inhibit side branch growth and stimulate adventitious roots and fruit formation.
Auxins are a type of hormone found in plants. Plant hormones are molecules produced by plants to accelerate, inhibit or modify its growth. In plants, growth occurs during cell division, cell elongation, and cell differentiation, and hormones can influence any or all of these different processes in plants.
Also called indoleacetic acid (IAA), this hormone has a very powerful effect on growth. It is one of the most common plant hormones and has been isolated from many different species. Studies on the response of plants to light, phototropism, are what led to their discovery and isolation. A tropism is the tendency of a plant to grow towards or away from something, and the two main stimuli that cause it in plants are light and gravity (geotropism).
These hormones are formed in the tips of a plant and then are transported down the stem to the roots. Consequently, the level of auxin concentration is highest in the leaves and, moving down the plant, decreases until it becomes lowest in the roots.
Growth in different parts of a plant is influenced by the concentration of auxin present. At high concentrations, the shoot of a plant is stimulated to grow, while the root is inhibited. When concentrations are low, the root is stimulated to grow, but the shoot is inhibited because the concentration level is too low. As such, growth is promoted in both parts of the plant by this hormone, but in different ways.
In plant shoots, the role of auxin is what causes positive phototropism, ie the growth of the plant towards the light. When light shines on a plant from one direction, it causes hormones to redistribute to the side in the shade. One function of the hormone is to cause cells to stretch. The redistribution causes the cells on the shaded side to stretch more than those on the side with the light shining on them, which causes the shoot to bend towards the light.
In roots, auxins cause negative phototropism, meaning they grow away from light. As in the shoot, the hormones redistribute themselves so that they are on the shaded side of the root. This causes a higher concentration of the hormone in these cells, which, as stated earlier, inhibits the growth of root cells. These cells will grow less than those on the side with light shining on them, which causes the root to bend away from the light.
Hormone redistribution may also explain the responses of shoots and roots to gravity. If a shoot or root is placed horizontally, it redistributes to the underside resulting in a higher concentration in those cells. In a shoot, the higher concentration causes increased cell growth, so the shoot bends up or against gravity, a negative geotropism. In roots, the higher concentration inhibits cells at the bottom, so cells at the top grow more, causing the root to bend down or with gravity, a positive geotropism.
Auxins have many functions beyond affecting root and shoot growth. They inhibit the growth of side branches from shoots, which is termed apical dominance, as the apex of the plant suppresses the lower parts of the plant. Apical dominance is the theory behind pruning because by removing the main stem, the source of auxin is removed, which encourages lateral shoots to grow lower.
These hormones stimulate the growth of adventitious roots, which are lateral roots that grow from the stem and are what allow cuttings to form their own root system after being removed from a plant. Auxins can also stimulate fruit formation without fertilization occurring. By treating unpollinated flowers with these hormones, they can be induced to form fruit.
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