Plant Hormones and

Response to Stimuli

What are hormones and what do they do?

Regulate transcription by binding transcription factors to DNA: Initiated by hormones through second messengers.

Two second messenger pathways exist: One uses cyclic GMP or cGMP, the other uses Ca+Calmodulin to activate transcription. G-protein and protein kinases also involved.

The light sensitive receptor phytochrome may initiate the greening response.

Hormones are produced in one part of the body and have effects elsewhere in the body.

Very small amounts are required.

In plants, the response is often governed by the interaction of two or more hormones

Early Experiments of Phototropism

F. W. Went, 1926 Investigates PhototropismAgar block collected auxin, or Indole Acetic Acid (IAA)

In the dark, off-center block caused grass coleoptile to bend.

Proves chemical produced by the tip affects bending.

Acid Growth Hypothesis

Auxins (IAA)

Produced by embryo, meristem of apical buds, young leaves, tips of roots and shoots

Stimulates stem elongation in small concentrations.Works with gibberellin to “set” fruit.Most important application is spraying on Thompson seedless grapes to increase size.

Gibberellins(Gibberellic Acids)

Produced in roots and young leaves

Stimulate the growth of both leaves and stems but not roots.Enhance stem elongation.Promotes fruit development and seed germinationWorks with auxin

Effect of Gibberellin Treatment on Seedless Grapes

Ethylene Gas

Produces as stress response.Produced during fruit ripening.Produced due to auxin stimulation

Promotes fruit ripening.Effects similar to auxin.Stimulates production of flowers.Change in balance with auxin can cause leaf abscission (dropping of leaves).

Ethylene Induces the Triple Response of Germinating Seedlings to Circumvent Obstacles in Soil

CytokininsProduced in roots. Control cell division and

differentiation.Stimulates axillary bud growth.Reduces apical dominance.Antagonistic to auxin.

Abscisic Acid (ABA)

ABA is a growth inhibitor.

Maintains dormancy in seeds.

Works against many other growth hormones.

Enables plants to respond to drought.

Plant Responses to Stimuli• Phototropism – Response to light due to auxin: the higher auxin in the shady side of the stem, the more bending toward the light occurs.

•Gravitropism (geotropism) – Response to gravity: both auxin and gibberellin are involved.

•Thigmotropism – Response to touch in climbing vines.

Action Spectrum For Blue-Stimulated Phototropism

Photoperiodism

A flash of light during the night resets the biological clock.

Red light shortens the night length, and far-red restores the night length. The last flash determines the night length.