Auxins are a type of hormone, or growth substance, 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. The hormones can affect any or all of these different processes in plants.

Auxin, or indoleacetic acid (IAA), has a very powerful effect on growth. It is one of the most common plant hormones and has been isolated from many different plants. Studies of the response of plants to light, phototropism, are what led to the discovery and isolation of auxins. A tropism is the tendency of a plant to grow towards or away from something. The two main stimuli that cause tropisms in plants are light and gravity (geotropism).

Auxins are formed in the tips of a plant and then transported down the stem to the roots. As a result, there is a concentration gradient of auxin within the plant from the tip down to the root. At the tip, the concentration level is the highest and as you move 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 auxin 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 auxin, 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 auxins to redistribute to the shaded side of the plant. One function of auxin 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. This causes the shoot to bend towards the light.

In roots, auxins cause a negative phototropism, meaning that the roots grow away from the light. As in the shoot, the auxins 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 auxins can also explain the responses of shoots and roots to gravity. If a shoot or root is laid horizontally, the auxins 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. This 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. By removing the main stem, the source of auxin is removed, which encourages side shoots to grow further down.

Auxins stimulate the growth of adventitious roots. Adventitious roots 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 auxin, they can be induced to form fruit.