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The key points covered of this video include:

1. Introduction to Auxins
2. The Actions of Auxins
3. Mechanisms Underlying Phototropism
4. Mechanisms Underlying Geotropism

Introduction to Auxins

We previously saw that directional plant responses are called tropisms and that these tropisms are controlled by plant growth factors. Indoleacetic acid (IAA) is an example of a plant growth factor which belongs to a group of substances called auxins. Auxins are responsible for controlling plant phototropic and geotropic responses by regulating cell elongation. In plants, IAA causes shoots to grow towards light whereas it causes the roots to grow in the opposite direction to light. IAA also causes the shoots to grow in the opposite direction to gravity whereas it causes the roots to grow in the direction of gravity. Therefore shoots are positively phototropic and negatively geotropic whereas roots are negatively phototropic and positively geotropic.

The Action of IAA


IAA has a number of different effects on cells which affect cell elongation. IAA causes an increase in the plasticity of plant cell walls so that they can stretch more easily when the cell elongates. The mechanism by which IAA is thought to cause increased plasticity is called the acid growth hypothesis. In the acid growth hypothesis, IAA causes protons to be actively transported into spaces in the cell wall. This causes proteins called expansins to be activated which loosens the cellulose in the cell wall and makes the cell wall more plastic.

Mechanisms Underlying Phototropism

If a light is shining on a plant from one side (unilateral light), the shoot will alter its direction of growth so that it grows towards the light. This response is due to the production of the auxin IAA in the tip of the shoot. Initially, IAA is transported from the tip of the shoot and distributed evenly throughout all regions down the shoot. However if light is shone on the shoot, it causes the IAA to move away from the illuminated side of the shoot and to the shaded side. This means there is a greater concentration of IAA on the shaded side of the shoot than the illuminated side of the shoot. In shoots, high concentrations of IAA cause cell elongation - therefore cell elongation occurs more quickly on the shaded side of the shoot. This causes the shoot to bend towards the light and this is a positive phototropism. In roots the opposite occurs - high concentrations of IAA inhibit cell elongation. When light is shone on the roots, it causes the IAA to move to the shaded side of the roots. This causes IAA to inhibit cell elongation on the shaded side - therefore cell elongation occurs more quickly on the illuminated side. This causes the roots to bend away from the light source and this is a negative phototropism.

Mechanisms Underlying Geotropism

IAA also causes roots to grow in the same direction as gravity which is called a positive geotropism. IAA is produced at the tip of the root and distributed evenly throughout the root. However gravity then causes the IAA to move from the upper side of the root to the lower side. This causes a high IAA concentration to build up in the lower side of the root so cell elongation is inhibited in the lower side. The cells on the upper side of the root therefore elongate faster than the cells on the lower side causing the root to grow down towards gravity. The opposite happens in shoots because IAA promotes cell elongation in shoots. This means in shoots gravity causes IAA to move to the lower side of the shoot. This causes cell elongation in the lower side of the shoot causing the shoot to grow upwards in the opposite direction to gravity.