1 responses to light photomorphogenesis refers to non- directional, light-triggered development...
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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