Gravitropism: a response in which a plant grows towards (positive) or away (negative) from gravity. 
Auxins’ role in gravitropism
Tend to settle at the bottom end of the root.
However, this does not make the cells of the tip of the root grow longer.
Auxins prevent cells at bottom tip of root from growing, making cells at top of root grow faster.
When cells of top of the root grow faster, they push root deeper into soil and root gets longer.
The root grows in direction of the gravitational pull .
Auxins’ role in phototropism
If sun shines on right side of a plant’s shoot, auxins will accumulate on dark opposite left side.
Auxins accumulating makes cells on left side grow faster than cells on right side.
When left side of shoot starts growing faster than right side, therefore, ...
... shoot will start to bend to right side towards sunlight. 
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