Final answer:
Auxin plays a crucial role in thigmotropism in pea tendrils by causing differential growth and coiling response when the tendril touches a support surface.
Step-by-step explanation:
In a pea tendril, auxin plays a crucial role in thigmotropism, which is the plant's response to touch or mechanical stimuli.
When the tendril touches a support surface, auxin is redistributed within the cells, causing differential growth and resulting in the tendril coiling around the support.
The cells on the side of the tendril in contact with the support surface contract, while cells on the opposite side expand, leading to the bending or coiling response.