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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

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Abscisic Acid

Abscisic acid is also necessary for dormancy in seeds -Prevents precocious germination called vivipary

Abscisic acid is important in the opening and

closing of stomata -Triggers movement of K+ out of guard cells