Auxin is a plant hormone that causes the cells on the shady side of a plant shoot to elongate. The response enabled by auxin is known as -

phototropism

Explanation:

Auxin is a plant hormone that causes the cells on the shady side of a plant shoot to elongate. The response enabled by auxin is referred to as phototropism.

Phototropism simply refers to the growth of an organism towards light. It is usually common in plants, even though it can be seen in other organisms like fungi as well. The auxin in the plant cells that are farthest away from the light reacts when there's phototropism.

a symbiotic relationship with the mitochondria