Question

How does auxin promote phototropism

Answer 1

Auxin is produced at the root of the plant, when the plant senses sunlight.
Due to production at roots, the roots bend towards the dark side and hence it causes the shoot to bend towards the light.
Therefore, promoting Phototropism.

Answer 2

When the tip of a tendril touches a support, then the auxins present in its tip move to that side of tip which is away from the support . Auxins promote growth . So due to more auxins in it , the side of tendril away from the support grows faster than the side which is in contact with the support and makes the tendril twirl around the support .