Plant Responses to Internal and External Signals

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Control of Apical Dominance

Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds

If the terminal bud is removed, plants become bushier.

Anti-Aging Effects

Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues.

Cytokinins

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Gibberellins

Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination.

Stem Elongation

Gibberellins stimulate growth of leaves and stems.

In stems, they stimulate cell elongation and cell division.

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Fruit Growth

In many plants, both auxin and gibberellins must be present for fruit to set.

Gibberellins are used in spraying of Thompson seedless grapes.

Germination

After water is imbibed, release of gibberellins from the embryo signals seeds to germinate.

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Effects of gibberellins on stem elongation and fruit growth

Gibberellin-induced stem

growth

(b) Gibberellin-induced fruit

growth

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Mobilization of nutrients by gibberellins during the germination of seeds such as barley

Gibberellins (GA)

send signal to

aleurone.

Aleurone secretes

-amylase and other enzymes.

Sugars and other

nutrients are consumed.

Aleurone

Endosperm

Water

Scutellum

(cotyledon)

Radicle

1

2

3

GA

GA

-amylase

Sugar

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Brassinosteroids

Brassinosteroids are chemically similar to the sex hormones of animals.

They induce cell elongation and division in stem segments.

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Abscisic Acid

Abscisic acid (ABA) slows growth.

Two of the many effects of ABA:

Seed dormancy

Drought tolerance.

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Seed Dormancy

Seed dormancy ensures that the seed will germinate only in optimal conditions.

In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold.

Drought Tolerance

ABA is the primary internal signal that enables plants to withstand drought.

Abscisic Acid = ABA

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Ethylene