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 affect 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 of 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 transported down the stem to the roots. As a result, the concentration level of auxin is the highest in the leaves and, moving down the plant, it decreases until it is the lowest in the roots.
Growth in different parts of a plant is affected based on the concentration of auxin present. At high concentration levels, 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 a different manner.
In plant shoots, the role of auxin is what causes a positive phototropism, or the plant to grow towards the light. When light is shone on a plant from one direction, it causes the hormones to redistribute to the shaded side. One function of the hormone is to cause cell elongation. The redistribution causes the cells on the shaded side to elongate 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 a negative phototropism, meaning that they grow away from the light. As in the shoot, the hormones redistribute so that they are found on the shaded side of the root. This causes a higher concentration of the hormone in these cells, which, as stated earlier, inhibits root cell growth. These cells will grow less than those on the side with the light shining on them, which causes the root to bend away from the light.
The redistribution of hormones can also explain the responses of shoots and roots to gravity. If a shoot or root is laid horizontally, they redistribute to the lower side resulting in a higher concentration in those cells. In a shoot, the higher concentration causes increased cell growth, so the shoot bends upwards or against gravity, a negative geotropism. In the roots, the higher concentration inhibits the cells on the underside so the cells on the top grow more, causing the root to bend downwards or with gravity, a positive geotropism.
Auxins have many functions besides affecting the growth of roots and shoots. They inhibit the growth of side branches from shoots, which is referred to as 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 side shoots to grow further down.
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.