How does auxin transport differ in roots and shoots?

Auxin transport in roots is primarily downwards (gravitropic), while in shoots it's upwards and towards light (phototropic).

Auxin, a plant hormone, plays a crucial role in the growth and development of plants. It is synthesised in the shoot apex and transported to other parts of the plant. However, the direction of this transport varies in roots and shoots due to their different growth responses.

In shoots, auxin transport is primarily upwards and towards the light, a phenomenon known as phototropism. This is because shoots grow towards the light to maximise photosynthesis. The auxin is transported from the shoot apex to the elongating regions, where it promotes cell elongation. This transport is facilitated by a group of proteins called PIN proteins, which are located on the plasma membrane of the plant cells. These proteins pump auxin out of the cells in a directional manner, allowing the hormone to move upwards in the shoot.

On the other hand, in roots, auxin transport is primarily downwards, a process known as gravitropism. Roots grow downwards into the soil to anchor the plant and absorb water and nutrients. The auxin is transported from the root tip upwards to the elongating regions, where it inhibits cell elongation. This is contrary to the effect of auxin in shoots. The downward transport of auxin in roots is also facilitated by PIN proteins, but their orientation in the cells is different from that in the shoot cells, resulting in the downward movement of the hormone.

In summary, the direction of auxin transport in plants is determined by the growth responses of the roots and shoots, and is facilitated by the orientation of PIN proteins in the plant cells. This differential transport of auxin in roots and shoots is crucial for the plant's adaptation to its environment and its overall growth and development.