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Chapter 28Lecture Outline

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When touched, the leaves of the sensitive plant (Mimosa pudica) quickly fold, and only slowly unfold

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Chapter 28

Flowering Plants: Behavior

Overview of Plant Behavioral Responses

Plant Hormones

Plant Responses to Light

Plant Responses to Gravity and Touch

Plant Responses to Attack

Chapter Outline:

Time-lapse photography shows that most plants are constantly in motion Bending, twisting, or rotating – known as nutation

Other examples of plant behavior: Shoots grow toward light and against gravity Roots grow toward water and toward gravity Seeds germinate when they detect light and moisture Flowers, fruit, and seeds grow only at the right season Plants respond to attack by microbes or animals

Overview of Plant Behavioral Responses

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Responses to internal and external stimuli

Internal stimuli Chemical signals – hormones, phytohormones or plant growth

substances Often interact with each other and external signals to

maintain homeostasis and progress through life stages

Environmental stimuli Light, atmospheric gases (CO2 and water vapor), temperature,

touch, wind, gravity, water, rocks, and soil stimuli Herbivores, pathogens, organic chemicals from neighboring

plants, and beneficial or harmful soil organisms Agricultural chemicals including hormones

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Biological stimuliPhysical stimuli

Environmental:

Light

Atmosphericgases includingCO2

Humidity

Temperature

Touch, wind

Gravity

Soil water

Rocks andOther barriers

Soilminerals

Internal:circadianrhythmshormones

Environmental:

Herbivores

Agriculturalhormoneapplications

Pathogens

Organicchemicalsemitted byother plants

Soilmicroorganisms

Plant responses

Receptor molecules located in plant cells sense various kinds of stimuli and lead to appropriate responses

ex: Phototropism – involves both a cellular perception of light and a growth response of stem tissue to an internal chemical signal (auxin)

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Plant signal transduction

Process in which a cell perceives a signal, switching on an intercellular pathway that leads to cellular response

Three stages1. Receptor activation

2. Transduction of the signal via second messengers

3. Cellular response via effector molecules

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Receptors or sensors Proteins that become activated when they receive a specific type

of signal

Messengers or second messengers Transmit messages from many types of activated receptors

Cyclic AMP, IP3, and calcium ions

Effectors Molecules that directly influence cellular responses Calcium-dependent protein kinases (CDPKs) are important Signal transduction ends when an effector causes a cellular

response

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BIOLOGY PRINCIPLE

Cells are the simplest units of life

Internal and environmental stimuli are received and elicit responses at the cellular level.

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Chemical signals transported within the plant body that bind to cellular receptors, thereby causing responses

About a dozen small molecules synthesized in metabolic pathways

Auxins, cytokinins, gibberellins, ethylene, abscisic acid and brassinosteroids

One hormone often has multiple effects

Different concentrations or combinations can produce distinct responses

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Plant Hormones

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Auxins

“Master” plant hormone

Indoleacetic acid (IAA) is one plant auxin

Promotes expression of diverse genes known as auxin-response genes Under low auxin conditions Aux/IAA repressors

prevent gene expression High enough auxin conditions cause breakdown of

repressors allowing gene expression

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Auxin transport

Produced in apical shoot tips and young leaves

Directionally transported

May enter cells by diffusion

AUX1 plasma membrane protein (auxin influx carrier) needed to transport IAA-

Apical end of cell

PIN proteins transport auxin out of cells (they are auxin efflux carriers) Basal end of cell

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Polar transport – auxin flows down in shoots into roots17

Auxin effects

Establishes the apical-basal polarity of seed embryos

Induces vascular tissue to differentiate

Mediates phototropism

Promotes formation of adventitious roots

Stimulates fruit development

Used for cloning in plant tissue culture18

Many effects of practical importance to humans Seedless fruit production

Stimulates flower ovaries to mature into fruits

Retards premature fruit drop

Root development on stem cuttings

Pinching topmost shoots produces bushy plants

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Cytokinins

Increase rate of cytokinesis or cell division

Root tips major production site

Also produced in shoots and seeds

At shoot and root tips, cytokinins influence meristem size, stem cell activity, and vascular tissue development

Also involved in root and shoot growth and branching, the production of flowers and seeds, and leaf senescence (aging)

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Gibberellins

Also known as gibberellic acids or GA

Produced in apical buds, roots, young leaves, and seed embryos

Foster seed germination

Enhance stem elongation and flowering

Retard leaf and fruit aging

Arise from stimulatory effects on cell division and elongation

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Plant gibberellin responses evolved in a step-wise manner

In flowering plants, gibberellin works by helping to liberate repressed transcription factors

In the absence of gibberellin, DELLA proteins prevent expression of gibberellin-responsive genes DELLAs function as brakes to restrain cell division and expansion

Gibberellin binds to GID1, leads to destruction of DELLAs

Compared flowering plant proteins to homologous proteins in bryophyte and lycophyte Necessary components (DELLAs and GID1 proteins) were

present earlier, but did not assemble into a growth regulation system until later in plant evolutionary history

EVOLUTIONARY CONNECTIONS

EVOLUTIONARY CONNECTIONS

Ethylene

Important in coordinating developmental and stress responses

Produced during seedling growth, flower development and fruit ripening

Important roles in leaf and petal aging and drop

Defense against osmotic stress and pathogen attack

Influences cell expansion, often with auxin

Cells tend to expand in all directions rather than elongating

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1. Ethylene prevents the seedling stem and root from elongating

2. Hormone induces the stem and root to swell radially, thereby increasing in thickness

3. Seedling stem bends so that a hook pushes up through the soil

Result from imbalanceof auxin

This response protects the delicate meristem from crusty soil

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Triple Reponse of seedlings

Stress hormones

Help plants respond to environmental stresses such as flooding, drought, high salinity, cold, heat, and attack by microorganisms and herbivores

Examples: Abscisic acid (ABA) Brassinosteroids

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Abscisic acid (ABA)

Slows or stops plant metabolism when growing conditions are poor

May induce bud and seed dormancy

Stimulates formation of protective scales around buds of perennial plants in preparation for winter

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Brassinosteroids

Found in seeds, fruits,shoots, leaves, and flowerbuds of all types of plants

Induce vacuole water intake and influence enzymes that alter cell-wall carbohydrates, thereby fostering cell expansion

Impede leaf drop

Stimulate xylem development

Can be applied to crops to help protect plants from heat, cold, high salinity, and herbicide injury 28

Based on the presence of light receptors within cells

Photoreceptors respond to light absorption by switching on signal transduction

Results in Sun tracking Phototropism Flowering Seed germination

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Plant Responses to Light

A plant “light-switch” Red- and far-red-light receptor Flips back and forth between 2 conformations

Pfr – conformation that only absorbs far-red light and activates cellular responses

When left in the dark, Pfr transforms to red light absorbing Pr

Pr can only absorb red light and cannot activate cellular responses

Plays a critical role in photoperiodism and plant responses to shading 30

Phytochrome

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DARKNESS

In darkness, seeds do notgerminate because phytochromeremains in the inactive Pr

conformation.

RedFarred

Red

Even a brief exposure to redlight generates the active Pfr

conformation of phytochrome,allowing seeds to germinate.

Exposure to far-red light afterred-light exposure converts activePfr to inactive Pr, so seeds do notgerminate.

RedFarred Red

RedFarred Red

Farred

The most recent light exposuredetermines whether phytochromeoccurs in the active Pfr or in theinactive Pr conformation. If in thelatter, most seeds do not germinate.

Exposure to red light after far-redlight switches phytochrome back tothe active Pfr conformation, soseeds germinate.

(1–5): © Prof. and Mrs. M. B.Wilkins/University of Glasgow

Photoperiodism

Phytochromes play a critical role

Influences the timing of dormancy and flowering

Flowering plants can be classified as long-day, short-day, or day-neutral according to the way their flowering responds to night length

Plants measure night length – not day length

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Long-day plants – flower in spring or early summer, when the night period is shorter (thus day length is longer) than a defined period

Short-day plants – flower only when the night length is longer than a defined period such as in late summer, fall, or winter, when days are short

Day-neutral plants – flower regardless of the night length, as long as day length meets the minimal requirements for plant growth

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Shading responses

Also mediated by phytochrome

Responses include Extension of leaves from shady portions of a dense

tree canopy into the light Growth that allows plants to avoid being shaded by

neighboring plants

Occur by the elongation of branch internodes

Leaves detect shade as an increased proportion of far-red light to red light

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Plant Responses to Gravity and Touch

Why do plant stems grow up and roots grow down?

Responses to gravity and touch

Gravitropism Growth in response to the force of gravity Shoots are said to be negatively gravitropic Most roots are positively gravitropic Statocytes contain starch-heavy plastids called

statoliths Heavy statoliths sink, causing changes in calcium ion

messengers, inducing lateral auxin transport Changes direction of root or shoot growth

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Roots encounter rocks as they grow down Touch response temporarily supersedes

the response to gravity Roots grow horizontally until they get around

the barrier, then downward growth in response to gravity resumes

More rapid responses, such as sensitive plant, based on changes in water content of cells

Cells in pulvinus become limp when touched Additional folding from electrical impulse

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Gravity and touch response are related

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Structural barriers (cuticles, epidermal trichomes, and outer bark) help to reduce infection and herbivore attack

Herbivore attack Wide variety of chemical defenses Make plants taste bad Some chemicals attract enemies of their attackers or

cause neighbor plants to produce defensive compounds

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Plant Responses to Attack

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Elicitors are molecules produced by bacterial and fungal pathogens that promote infection

Plants have several defense strategies Plasma membrane receptors that bind microbial

molecules, such as lipopolysaccharides or chitin Encoded by R genes (resistance genes)

MicroRNAs in the cytosol destroy the nucleic acids of invading viruses

Receptors in the cytosol recognize injected elictors, triggering the production of chemical defenses or programmed cell death

Pathogen attack

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Hypersensitive response (HR) Occurs when a plant recognizes a pathogen by

chemical means and responds in such a way that the disease symptoms are limited

Several components including increased production of hydrogen peroxide

Nitric oxide also produced

Synthesis of hydrolytic enzymes, defensive secondary metabolites, the hormone salicylic acid, and tough lignin in cell walls of nearby tissues

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Systemic acquired resistance (SAR) Localized hypersensitive response can result in the

production of alarm signals that travel to noninfected regions of a plant and induce widespread resistance to diverse pathogens

Jasmonic acid

May produce defensive enzymes or tannins (toxic to microorganisms)

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Methylsalicylate

Sites ofpathogenattack

Defensiveresponses

Defensiveresponses

Systemin

Jasmonicacid

Defensiveresponses

Systemin

Jasmonicacid

Defensiveresponses

SalicylicacidSalicylicacid