Final answer:
AUX1/LAX are auxin influx carriers located on the plasma membrane of plant cells that play essential roles in phototropism and gravitropism by regulating the uptake of auxin into the cells, controlling the plant's directional growth and development.
Step-by-step explanation:
The auxin influx carrier, known as AUX1/LAX, is crucial in plant development processes such as phototropism and gravitropism. These carriers are located in the plasma membrane of plant cells, where they function to facilitate the uptake of the plant hormone auxin (indole-3-acetic acid or IAA) from the extracellular space into the cell. In phototropism, auxin is redistributed to the side of the plant that is away from light, leading to cell elongation and the plant bending towards the light. In gravitropism, auxin distribution within root cells causes differential growth, allowing roots to grow down into the soil and shoots to grow upwards. By regulating auxin levels, AUX1/LAX carriers play a major role in controlling the directional growth and various developmental stages of the plant.