1. Plant Responses
How plants move and communicate

2. Plant Hormones Plant hormones can be divided into two classes:
Growth promoters: Auxins, Gibberellins, Cytokinins
Growth inhibitors: Ethylene gas, Abscisic acid

3. Growth promoters Hormones can promote plant growth in two ways:
Stimulating cell division in meristems to produce new cells.
Stimulating elongation in cells.

4. Auxins stimulate genes in cells associated with plant growth.
Auxin activity
Auxins stimulate genes in cells associated with plant growth.

5. Auxin roles
Auxins carry out multiple roles having to do with plant growth including:
Tropisms
Apical dominance
Growth of adventitious roots
Fruit growth

6. Tropisms
Tropisms are the growth of a plant toward or away from a stimulus, including:
Phototropism: in response to light
Gravitropism: in response to gravity
Thigmotropism: in response to touch

7. Tropisms: cell elongation
In general, tropisms involve cell elongation or suppression of cell elongation on one side of a plant, causing the plant to grow in a particular direction.

8. Phototropism
Look at the sprouts in the bottom picture and the explanatory diagram at the top. Explain why the sprouts are all leaning in the same direction.

9. Gravitropism
In this Impatiens plant, shoots grow upwards and roots grow downwards in response to gravity.
On which side of the shoot and root do you think auxins are more concentrated?

10. Gravitropism in shoots
In shoots, auxins are more concentrated on the lower side of the stem, causing the cells there to elongate.
Why is this gravitropism and not phototropism?

11. Gravitropism in roots
In roots, however, auxin concentration on the lower side of the root suppresses cell elongation.
The upper side of the root continues to grow, causing the roots to bend downward.

12. Plastids and Gravitropism
How does a root “know” which way is down?
Plastids, particularly leucoplasts, in the root cap cell tend to settle on the bottom side of the cell. This stimulates the release of auxins.

13. Thigmotropism
In some plants, vining stems or tendrils will grow in response to touch.
Which side of the tendril is elongating? Where might the auxin be? (Remember, this is the shoot system.)