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Responses to LightPhotomorphogenesis refers to non-directional, light-triggered

development

Phototropisms are directional growth responses to light

Both compensate for plants’ inability to move

The phytochrome molecule exists in two interconvertible forms:

-Pr is the inactive form

-Absorbs red light at 660 nm

-Pfr is the active form

-Absorbs far-red light at 730 nm

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Responses to Light1. Seed germination

-Inhibited by far-red light and stimulated by red light in many plants

2. Shoot elongation

-Etiolation occurs when shoot internodes elongate because red light and active Pfr are not available

3. Detection of plant spacing

-Crowded plants receive far-red light bounced from neighboring plants

-This increases plant height in competition for sunlight

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Responses to LightPhytochromes are involved in many signaling

pathways that lead to gene expression

-Pr is found in the cytoplasm

-When it is converted to Pfr it enters the nucleus

-Pfr binds to transcription factors, leading to expression of light-regulated genes

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PhototropismsPhototropic responses including the bending of

growing stems to sources of light with blue wavelengths (460-nm range)

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Circadian Clocks

Circadian rhythms (“around the day”) are particularly common among eukaryotes

Have four characteristics:

1. Continue in absence of external inputs

2. Must be about 24 hours in duration

3. Cycle can be reset or entrained

4. Clock can compensate for differences in temperature

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Responses to GravityGravitropism is the response of a plant to the

gravitational field of the Earth

Four general steps of a gravitropic response:

1. Gravity is perceived by the cell

2. A mechanical signal is transduced into a gravity-perceiving physiological signal

3. Physiological signal is transduced to other cells

4. Differential cell elongation occurs in the “up” and “down” sides of root and shoot

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Responses to GravityShoots exhibit negative gravitotropismIn shoots, gravity is sensed in endodermal cells surrounding the vascular tissue

-Signaling is in the outer epidermal cells

Roots have a positive gravitropic response

In roots, the cap is the site of gravity perception

-Signaling triggers differential cell elongation and division in the elongation zone

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Response to GravityAuxin accumulates on lower

side of the stem

-Results in asymmetrical cell elongation and curvature of the stem upward

Lower cells in root cap are less elongated than those on upper side

-Upper side cells grow more rapidly causing the root to ultimately grow downward

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Responses to Mechanical StimuliThigmomorphogenesis permanent form

change in response to mechanical stresses

Thigmotropism directional growth of a plant in response to contact

Thigmonastic responses occur in same direction independent of the stimulus

Examples of touch responses:

-Snapping of Venus flytrap leaves curling of tendrils around objects

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Responses to Mechanical StimuliSome touch-induced plant movements involve

reversible changes in turgor pressure

-Touch triggers movement of ions to outer side of pulvini

-Water follows by osmosis

-Decreased interior turgor pressure causes the leaf to fold

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Responses to Mechanical StimuliSome turgor movements are triggered by light

-This movement maximizes photosynthesis

Bean leaves are horizontal during the day when their pulvini are rigid

-But become more or less vertical at night as the pulvini lose turgor

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Water and Temperature ResponsesAbscission involves changes at the petiole’s base

Protective layer = Consists of several layers of suberin-impregnated cells

Separation layer = Consists of 1-2 layers of swollen, gelatinous cells

ind and rainseparate the leaf from the stem

-Nutrient sinks are discarded, conserving resources

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Seed DormancyAccumulating food reserves, forming a protective seed

coat and dehydration

Offspring to wait until conditions for germination are optimal

-Legume seeds last decades or longer

-Seeds that are thousands of years old have been successfully germinated

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AuxinDiscovered in 1881 by Charles and Francis

Darwin

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Auxin In 1926, Frits Went performed an experiment that explained all of the previous results

-He named the chemical messenger auxin

-Cell elongation causes the plantto bend towards light

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AuxinWinslow Briggs later demonstrated that auxin molecules migrate away from the light into the shaded portion of the shoot

-Barriers in a shoot tip revealed equal amounts of auxin in both the light and dark sides

-However, different auxin concentrations produced different degrees of curvature

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How Auxin WorksIndoleacetic acid (IAA) is the most common natural

auxin

-Probably synthesized from tryptophan

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Synthetic Auxins

Naphthalene acetic acid (NAA) and indolebutyric acid (IBA) have many uses in agriculture and horticulture

-Prevent abscission in apples and berries

-Promote flowering & fruiting in pineapples

2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide commonly used to kill weeds

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Cytokinins Are purines that appear to be derivatives of adenine

Synthetic cytokinins

Cytokinins are produced in the root apical meristems and fruits

-Stimulate cell division and differentiation & promote the growth of lateral buds into branches

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GibberellinsNamed after the fungus Gibberella fujikuroi which

causes rice plants to grow very tall

Gibberellins belong to a large class of over 100 naturally occurring plant hormones

-All are acidic

-Have important effects on stem elongation

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BrassinosteroidsFirst discovered in the pollen of Brassica spp.

-Are structurally similar to steroid hormones-Elongation, cell division, stem bending, vascular tissue development, delayed senescence and reproductive development

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Oligosaccharins

Are complex plant cell wall carbohydrates that have a hormone-like function

-Can be released from the cell wall by enzymes secreted by pathogens

-Signal the hypersensitive response (HR)

In peas, oligosaccharins inhibit auxin-stimulated elongation of stems

-While in regenerated tobacco tissue, they inhibit roots and stimulate flowers

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EthyleneA gaseous hydrocarbon (H2C–CH2)

-thus retards their growth in lateral bud

-suppresses stem and root elongation

Ethylene controls leaf, flower and fruit abscission

It hastens fruit ripening

-Indeed, an antisense copy of the gene has been used to create transgenic tomato

-These stay fresh longer

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Abscisic AcidAbscisic acid is synthesized mainly in mature

green leaves, fruits and root caps

-induces formation of dormant winter buds

-necessary for dormancy in seeds

-important in the opening and closing of stomata

-Triggers movement of K+ out of guard cells