Question

How does auxin promote phototropism?

Answer 1

The synthesis of plant hormones is mostly confined to specific plant tissues from where they are transported to the target tissues or the parts. The asymmetric distribution of these chemicals mediates the plant responses to stimuli.

Complete answer:
We will first learn the basics of tropic movements and then move to phototropism.
As well all know plants can not move from one place to another. However, plants exhibit growth movements.
When the growth movement of plants depends on the direction of the stimulus, it is called tropic movement. Here, the stimulus is a change in any external or internal conditions. For example, light, temperature, and gravity serve as stimuli.
As mentioned, tropic movements are observed in response to several stimuli. A tropic movement in response to light is called phototropism. However, a plant part may grow towards or away from the source of light. Some plant parts such as roots are underground. They grow under the soil and do not require light. Therefore, we can say that a plant part can grow away from or towards the light to exhibit negative or positive phototropism.
Now, let’s discuss auxins. Auxins are one of the plant hormones. As the term “hormone” suggests, auxins must be the chemical messengers that regulate several developmental aspects of the plants.
One of the effects of auxins on plants is cell elongation in stem and coleoptiles which means that auxins promote the cells of these plant parts to expand. Auxin's mediated cell elongation explains the basis of the phototropism. The asymmetric distribution of auxins between the shaded and light side of the plant part causes phototropism.
Let’s understand how auxins cause the positive phototropic movements of the stem.
When a plant stem is exposed to light from only one direction, auxins are transported laterally towards the shaded side of the stem. The buildup of auxins on the shaded side of the stem triggers the cell elongation. Since the cells in the shaded part of the stem elongate more than the cells on the light side, and the stem bends toward the light.

Note:
Auxins are mostly produced in the shoot apical meristem and are then transported in a unidirectional pattern. Therefore, the negative phototropism of roots is also mediated by auxins.