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Agriculture research objective

More Yield
Quality of the produce
Less space and time more yield
To increase shelf life of the produce

Plant response

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Signal transduction pathways �link signal reception to response

Plants have cellular receptors that detect changes in their environment
For a stimulus to elicit a response, certain cells must have an appropriate receptor
Stimulation of the receptor initiates a specific signal transduction pathway
A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots

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These are morphological adaptations for growing in darkness, collectively called etiolation
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
A potato’s response to light is an example of cell-signal processing
The stages are reception, transduction, and response

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Fig. 39-2

(a) Before exposure to light

(b) After a week’s exposure to

natural daylight

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Fig. 39-3

CELL

WALL

CYTOPLASM

Reception

Transduction

Response

Relay proteins and

second messengers

Activation

of cellular

responses

Hormone or

environmental

stimulus

Receptor

Plasma membrane

1

2

3

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Reception and Transduction

Internal and external signals are detected by receptors, proteins that change in response to specific stimuli
Second messengers transfer and amplify signals from receptors to proteins that cause responses

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Fig. 39-4-3

CYTOPLASM

Reception

Plasma

membrane

Cell

wall

Phytochrome

activated

by light

Light

Transduction

Second messenger

produced

cGMP

Specific

protein

kinase 1

activated

NUCLEUS

1

2

Specific

protein

kinase 2

activated

Ca²⁺ channel

opened

Ca²⁺

Response

3

Transcription

factor 1

Transcription

factor 2

NUCLEUS

Transcription

Translation

De-etiolation

(greening)

response

proteins

P

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Response

A signal transduction pathway leads to regulation of one or more cellular activities
In most cases, these responses to stimulation involve increased activity of enzymes
This can occur by transcriptional regulation or post-translational modification

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Transcriptional Regulation

Specific transcription factors bind directly to specific regions of DNA and control transcription of genes
Positive transcription factors are proteins that increase the transcription of specific genes, while negative transcription factors are proteins that decrease the transcription of specific genes

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Post-Translational Modification of Proteins

Post-translational modification involves modification of existing proteins in the signal response
Modification often involves the phosphorylation of specific amino acids

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De-Etiolation (“Greening”) Proteins

Many enzymes that function in certain signal responses are directly involved in photosynthesis
Other enzymes are involved in supplying chemical precursors for chlorophyll production

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Plant Hormones

Hormones are chemical signals that coordinate different parts of an organism
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
Tropisms are often caused by hormones

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A Survey of Plant Hormones

In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

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Table 39-1

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Auxin

The term auxin refers to any chemical that promotes elongation of coleoptiles
Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA
Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell

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The Role of Auxin in Cell Elongation

According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane
The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric
With the cellulose loosened, the cell can elongate

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Fig. 39-8

Cross-linking

polysaccharides

Cellulose

microfibril

Cell wall

becomes

more acidic.

2

1

Auxin

increases

proton pump

activity.

Cell wall–loosening

enzymes

Expansin

Expansins separate

microfibrils from cross-

linking polysaccharides.

3

4

5

CELL WALL

Cleaving allows

microfibrils to slide.

CYTOPLASM

Plasma membrane

H₂O

Cell

wall

Plasma

membrane

Nucleus

Cytoplasm

Vacuole

Cell can elongate.

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Lateral and Adventitious Root Formation

Auxin is involved in root formation and branching

Auxins as Herbicides

An overdose of synthetic auxins can kill eudicots

Other Effects of Auxin

Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

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Cytokinins

Cytokinins are so named because they stimulate cytokinesis (cell division)

Control of Cell Division and Differentiation

Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Cytokinins work together with auxin to control cell division and differentiation

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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

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Gibberellins

Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of leaves and stems
In stems, they stimulate cell elongation and cell division
In many plants, both auxin and gibberellins must be present for fruit to set
Gibberellins are used in spraying of Thompson seedless grapes

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Fig. 39-10

Gibberellin-induced stem

growth

(b) Gibberellin-induced fruit

growth

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Fig. 39-11

Gibberellins (GA)

send signal to

aleurone.

Aleurone secretes

α-amylase and other enzymes.

Sugars and other

nutrients are consumed.

Aleurone

Endosperm

Water

Scutellum

(cotyledon)

Radicle

1

2

3

GA

α-amylase

Sugar