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Sensory Systems in Plants
Chapter 41
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Responses to Light
Pigments other than those used in photosynthesis can detect light and mediate the plant’s response to it
Photomorphogenesis refers to non-directional, light-triggered development
Phototropisms are directional growth responses to light
Both compensate for plants’ inability to move
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Responses to Light
In Arabidopsis, five forms of phytochromes have been characterized: PHYA to PHYE -Involved in several plant growth responses
1. Seed germination -Inhibited by far-red light and stimulated
by red light in many plants
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Responses to Light
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 Light
Phytochromes 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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Phototropisms
Phototropic 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 Gravity
Gravitropism is the response of a plant to the gravitational field of the Earth -Shoots exhibit negative gravitotropism; roots have a positive gravitropic response
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Responses to Gravity
Four general steps lead to 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 Gravity
In shoots, gravity is sensed along the length of the stem in endodermal cells surrounding the vascular tissue -Signaling is in the outer epidermal cells
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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Stem Response to Gravity
Auxin accumulates on lower side of the stem -Results in asymmetrical cell elongation and curvature of the stem upward
Two Arabidopsis mutants, scarecrow (scr) and short root (shr) do not show a normal gravitropic response -Due to lack of a functional endodermis
and its gravity-sensing amyloplasts
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Root Response to Gravity
Lower cells in horizontally oriented root cap are less elongated than those on upper side -Upper side cells grow more rapidly causing the root to ultimately grow downward
Auxin may not be the long-distance signal
between the root cap and elongation zone -However, it has an essential role in root gravitotropism
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Responses to Mechanical Stimuli
Thigmomorphogenesis is a permanent form change in response to mechanical stresses
Thigmotropism is directional growth of a plant or plant part 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 Stimuli
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Responses to Mechanical Stimuli
Some touch-induced plant movements involve reversible changes in turgor pressure -If water leaves turgid cells, they may collapse, causing plant movements
-If water enters a limp cell, it becomes turgid and may also move
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Responses to Mechanical Stimuli
Some turgor movements are triggered by light -This movement maximizes photosynthesis
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Water and Temperature Responses
When water and temperature affect plants, responses can be short-term or long-term
Dormancy results in the cessation of growth during unfavorable conditions -Often begins with dropping of leaves
Abscission is the process by which leaves or petals are shed -One advantage is that nutrient sinks can be discarded, conserving resources
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Water and Temperature Responses
Abscission involves changes that occur in an abscission zone at the petiole’s base -Hormonal changes lead to differentiation of:
-Protective layer = Consists of several layers of suberin-impregnated cells
-Separation layer = Consists of 1-2 layers of swollen, gelatinous cells
-As pectins break down, wind and rain separate the leaf from the stem
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Seed Dormancy
Seeds allow plant offspring to wait until conditions for germination are optimal -Legume seeds often last decades and even longer without special care -Seeds that are thousands of years old have been successfully germinated
Essential steps leading to dormancy include: -Accumulating food reserves, forming a protective seed coat and dehydration
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Responses to Chilling
Plants respond to cold temperatures by: 1. Increasing number of unsaturated lipids in their plasma membranes 2. Limiting ice crystal formation to extracellular spaces 3. Producing antifreeze proteins
Some plants can undergo deep supercooling -Survive temperatures as low as –40OC
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Responses to High Temperatures
Plants produce heat shock proteins (HSPs) if exposed to rapid temperature increases -HSPs stabilize other proteins
Plants can survive otherwise lethal
temperatures if they are gradually exposed to increasing temperature -Acquired thermotolerance
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Hormones are chemicals produced in one part of an organism and transported to another part where they exert a response
In plants, hormones are not produced by specialized tissues -Seven major kinds of plant hormones -Auxin, cytokinins, gibberellins,
brassinosteroids, oligosaccharins, ethylene, and abscisic acid
Hormones and Sensory Systems
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Auxin
Discovered in 1881 by Charles and Francis Darwin -They reported experiments on the response of growing plants to light -Grass seedlings do not bend if the tip
is covered with a lightproof cap -They do bend when a collar is placed
below the tip
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Auxin
The Darwins hypothesized that shoots bend towards light in response to an “influence” transmitted downward from the tip
Thirty years later, Peter Boysen-Jensen and
Arpad Paal demonstrated that the “influence” was actually a chemical
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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 -It accumulated on the side of an oat
seedling away from light -Promoted these cells to grow faster
than those on the lighted side -Cell elongation causes the plant
to bend towards light
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Auxin
Winslow Briggs later demonstrated that auxin molecules migrate away from the light into the shaded portion of the shoot -Barriers inserted in a shoot tip revealed equal amounts of auxin in both the light and dark sides of the barrier -However, different auxin concentrations
produced different degrees of curvature
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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
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Cytokinins
Cytokinins are produced in the root apical meristems and developing fruits -Stimulate cell division and differentiation, in combination with auxin
Cytokinins promote the growth of lateral buds into branches -They inhibit the formation of lateral roots, while auxin promotes their formation
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Cytokinins
Cytokinins promote the synthesis or activation of cytokinesis proteins -They also function as anti-aging hormones
Plant tissue can form shoots, roots, or an
undifferentiated mass depending on the relative amounts of auxin and cytokinin
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Gibberellins
Named 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 and abbreviated GA -Have important effects on stem elongation
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Gibberellins
Adding gibberellins to certain dwarf mutants restores normal growth and development
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Gibberellins
GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes in the aleurone layer -When GA binds to its receptor, it frees GA-dependent transcription factors from a repressor -These transcription factors can now
directly affect gene expression
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Gibberellins
GAs hasten seed germination -They also function as pheromones in ferns
GAs are used commercially to extend internode length in grapes -The result is larger grapes
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Brassinosteroids
First discovered in the pollen of Brassica spp. -Are structurally similar to steroid hormones
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Brassinosteroids
Have a broad spectrum of physiological effects -Elongation, cell division, stem bending, vascular tissue development, delayed senescence and reproductive development
Additive effects with auxins and gibberellins
have been reported
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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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Ethylene
A gaseous hydrocarbon (H2C–CH2) Auxin stimulates ethylene production in the
tissues around the lateral bud and thus retards their growth
Ethylene also suppresses stem and root
elongation
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Ethylene
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 Acid
Abscisic acid is synthesized mainly in mature green leaves, fruits and root caps
There is little evidence that this hormone plays a role in abscission
Abscisic acid induces formation of dormant winter buds
It counteracts gibberellins, by suppressing bud growth and elongation, and auxin, by promoting senescence