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

Photoperiodism response to changes in the length of day and night, it is nondirectional Phototropisms are directional growth responses to light

Both compensate for plants’ inability to move

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Responses to Light

Phytochrome (P) consists of two parts:

-Chromophore which is light-receptive

-Apoprotein which initiates a signal-transduction pathway

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Responses to Light

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

-Tagged by ubiquitin for degradation in the proteasome

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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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Responses to Light

Phytochrome also works through protein-kinase signaling pathways

-When Pr is converted to Pfr, its protein kinase domain causes autophosphorylation or phosphorylation of another protein

-This initiates a signaling cascade that activates 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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Phototropisms

A blue-light receptor phototropin 1 (PHOT1) has been characterized

-Has two regions

-Blue-light activates the light-sensing region of PHOT1

-Stimulates the kinase region of PHOT1 to autophosphorylate

-Triggers a signal transduction

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Phototropisms

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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 turgor movements are triggered by light

-This movement maximizes photosynthesis

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Responses to Mechanical Stimuli

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 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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How Auxin Works

Indoleacetic acid (IAA) is the most common natural auxin

-Probably synthesized from tryptophan

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How Auxin Works

The auxin receptor is the transport inhibitor response protein 1 (TIR1)

Two families of proteins mediate auxin-induced changes in gene expression

-Auxin responses factors (ARFs)

-Aux/IAA proteins

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How Auxin Works

1. Auxin binds TIR1 in the SCF complex if Aux/IAA is present

2. SCF complex tags Aux/IAA proteins with ubiquitin

3. These are degraded in the proteasome

4. Transcriptional activators of ARF genes are released from repression by Aux/IAA

5. Auxin-induced gene expression

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How Auxin Works

One of the downstream effects of auxin is an increase in plasticity of the plant cell wall

-The acid growth hypothesis provides a model linking auxin to cell wall expansion

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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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Cytokinins

The plant pathogen Agrobacterium introduces genes into the plant genome that increase the production of cytokinin and auxin

-Cause massive cell division and formation of a crown gall tumor

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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

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