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Thought Question

Plants can’t fight or hide or run away, so how do they adapt to a changing environment?

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Lecture 7 Outline (Ch. 40, 41)I. Plant Defenses

A. Methods of Attack

B. Methods of Defense

II. Responses to Light

III. Circadian Rhythms

IV. Responses to Gravity

V. Responses to Touch

VI. Plant Hormones

A. Auxin

B. Gibberellins

C. Cytokinin

D. Ethylene

E. Abscisic Acid

• Plants are susceptible to physical stresses

Examples?• Other threats include: viruses, bacteria,

fungi, animals, and other plants– Take nutrient resources of plants or use

their cells– Some kill plant cells

immediately, leading to necrosis

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Plant Defenses

Alfalfa plant bug

• Why are nonnative invasive species especially problematic?

• Dermal tissue: 1st line of defense– secrete wax: protect from water loss and attack– Dermis covered with cutin or suberin – substances to

reinforce cell walls– Silica inclusions, trichomes, bark, and even thorns

can also offer protection

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Plant Defenses

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Plant Defenses

• Plant defenses aren’t always enough:– Mechanical wounds allow microbial entry– Parasitic worms can eat through plant cell walls

• Some form tumors on roots– In some cases simply having bacteria

on the leaf surface can increase damage

• Fungi can enter through stomata

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Plant Defenses

Phases of fungal invasion1. Windblown spore

lands on leaves2. Spore germinates

& forms adhesion 3. Hyphae grow

through cell walls and press against cell membrane

4. Hyphae differentiate

• Many plants produce toxins that kill herbivores, make them ill, or repel them with strong flavors or odors

• Some plants have antimicrobial peptides

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Toxin Defenses

• Secondary metabolites– Plants make defense

compounds via modified metabolism

– Alkaloids

[Wild tobacco has elevated nicotine levels lethal to tobacco hornworms]

– Tannins

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Toxin Defenses

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Toxin Defenses

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• Ricin: alkaloid produced by castor bean plant– 6X more lethal than cyanide– A single seed can kill a small child– Binds ribosomes - inhibits translation

Toxin Defenses

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Immediate Plant Responses

- Plants may produce protective compounds

- Plants may summon “bodyguards” when attacked

- Plants may warn other plants of attack

- Some plants move rapidly

• Some plants “recruit” animals in mutualism• Acacia trees and ants

– Small armies of ants protect Acacia trees from harmful herbivores

– Plant provides ants with food and shelter

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Animal “Body Guards”

Foolish katydid

– As caterpillar chews away, a wound response in the plant leads to release of a volatile compound

– Female parasitoid wasp is attracted– Lays fertilized eggs in caterpillar– Eggs hatch and larvae kill caterpillar

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Animal “Body Guards”

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Chemical Warnings

• Volatile chemicals released by plants boost defenses in neighbors

• Many virally-attacked plants produce salicylic acid– Activates an immune response

•• Attacked plant converts salicylic acid to methyl

salicylate (wintergreen) diffuses to air– Absorbed by neighboring healthy plants and

reconverted to salicylic acid (aspirin)

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• Tobacco plants produce salicylic acid to fight viral infections

Virus Infected Plant

Methyl salicylate

Salicylic acidproduction

Salicylic acidproduction

Salicylic acidproduction

Salicylic acidproduction

Chemical Warnings

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Touch Responses

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• Leaves have sensory “hairs” on inside– Fly triggers hairs - generates signal

• Cells in outer leaf epidermis pump H+ into cell walls

• Enzymes activated cells absorb water• Outer epidermal cells expand, close leaf

• Reopening leaves takes several hours

Venus fly trap

Touch Responses

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Self-Check

Defense Examples

Secondary metabolites

Recruited animals

Volatile chemicals

Movement

Sensory Systems in

Plants

Chapter 41

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Two major classes of light receptors:

Blue-light photoreceptors• stomatal movements• phototropism

Phytochromes – red/far-red receptor• shade avoidance response• photoperiodism

A phytochrome consists of two identical proteins joined

Photoreceptor activity.

Enzyme - kinase activity.

20Plant Timekeeping/Light Detection

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Plant Orientation

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Plant Responses to Light

• Blue light receptor: Directional growth responses• Connect environmental signal with cellular perception of the

signal, transduction into biochemical pathways, and ultimately an altered growth response

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Plant Responses to Light

• Blue light receptor: Embedded in cell membrane• When blue light detected, changes conformation,

signal transduction differential elongation

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Many legumes– Lower their leaves in the evening

and raise them in the morning

Noon Midnight

Circadian Rhythms• Cyclical responses to environmental stimuli

– approximately 24 hours long– entrained to external clues of the day/night cycle

• Phytochrome conversion marks sunrise and sunset– Providing the biological clock with environmental cues

Plant Timekeeping/Light Detection

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• Response to time of year (seasons)

• Photoperiod - relative lengths of night and day

• Triggers many developmental processes– Bud break

– Flowering

– Leaf drop in deciduous trees

• Are actually controlled by night length, not day length

• that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod

Photoperiodism

Plant Timekeeping/Light Detection

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• Leaves detect lengths of night/day– An internal biological clock– A light-detecting phytochrome

• Pigments found in leaves• Active/inactive depending

on light conditions

Still-unidentified chemical (florigens)

travel from leaf to bud to either trigger or inhibit flowering

Plant Timekeeping/Light Detection

• Response of a plant to the gravitational field of the Earth• Shoots exhibit negative gravitropism• Roots have a positive gravitropic response

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Response to Gravity

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Response to Gravity• Four general steps lead to a gravitropic response:

1. Gravity is perceived by the cell

2. Mechanical signal transduced into physiological signal

3. Physiological signal transduced inside cell & to other cells

4. Differential cell elongation occurs in the “up” and “down” sides of root and shoot

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Gravity Response

How Do Plants Detect Gravity?

• Starch-filled plastids– In specialized stem cells and root caps– Orient within cells toward gravity

• Changing plastid orientation triggers elongation

plastids

cell inroot cap

root

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Gravity Response

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Gravity Response

• 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:

Venus flytrap leaves

Tendrils around objects

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Thigmotropism

Often due to differential elongation or manipulated water/turgor pressure

Responses to Mechanical Stimuli• Mimosa leaves have

swollen structures called pulvini at base of leaflets

– Stimulation triggers electrical signal

– Triggers 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 Stimuli

• Bean leaves – Pulvini rigid during the day – Lose turgor at night

– Reduce transpiration during the night

– Maximize photosynthetic surface area during the day

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(Plant) Hormone: Chemicals made in one location and transported to other locations for action

Plant Hormones

Growth

Reproduction

Movement

Water balance

Dormancy

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Plant Hormone Overview

• Plants respond to stimuli and lead a stationary life

• Plants, being rooted to the ground– Must respond to whatever environmental change

comes their way

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Plant Hormones

Five major classes of plant hormones

• Hormone effects depend on– - target cell– - developmental stage of the plant– - amount of hormone– - presence of other hormones

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Plant Hormones

1. Auxins:

• Elongation of cells• Root elongation • stimulate (low

concentrations) inhibit (high concentrations)

• Vascular tissues and fruit development

• Responses to light (phototropism), gravity (gravitropism),

• and touch (thigmotropism)

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Expansin

CELL WALL

Cell wallenzymes

Cross-linkingcell wallpolysaccharides

Microfibril

H+ H+

H+

H+

H+

H+

H+

H+

H+

ATP Plasma membrane

Plasmamembrane

Cellwall

NucleusVacuole

Cytoplasm

H2O

Cytoplasm

Cell elongation in response to auxin

1 Auxinincreases the

activity ofproton pumps.

4 The enzymatic cleavingof the cross-linkingpolysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

2 The cell wallbecomes more

acidic.

5 With the cellulose loosened,the cell can elongate.

3 Wedge-shaped expansins, activatedby low pH, separate cellulose microfibrils fromcross-linking polysaccharides. The exposed cross-linkingpolysaccharides are now more accessible to cell wall enzymes.

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Other Auxin Stimulated Responses:

• Lateral / branching root formation• Promote fruit growth (tomato sprays)• As herbicide, overdose kills eudicots

Auxin is produced:

• At the shoot apex, seeds, other actively growing tissues.

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Plant Hormones

2. Gibberellins:

• Stem elongation, flowering, and fruit development

• Seed germination and bud sprouting

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• After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate

Gibberellins stimulate germination

Responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients inthe endosperm.

AleuroneEndosperm

Water

cotyledon

GAGA

amylase Sugar

embryo releases gibberellin as a signal

Nutrients absorbed from the endosperm by the cotyledon are consumed during growth of the embryo into a seedling.

Embryo

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Plant Hormones3. Cytokinins:

Anti- aging effects.

• Inhibit protein breakdown

• Stimulate RNA and protein synthesis

• Mobilize nutrients from surrounding tissues

58 day old cutting:Genetically engineered to express more cytokinin on right

(florist sprays)

• Stimulate cell division and differentiation

• Produced in actively growing tissues such as roots, embryos, and fruits

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Control of Apical Dominance

• Cytokinins and auxins interact in the control of apical dominance– The ability of a terminal bud to suppress development of

axillary buds• If the terminal bud is removed

– Plants become bushier

Axillary buds

“Stump” afterremoval ofapical bud

Lateral branches

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Plant Hormones4. Ethylene:

• Gas at room temperature• Promotes abscission (falling

off) of fruits, flowers, and leaves

• Required (with auxin) for fruit development

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Why will these ripe bananas help the green avocados ripen faster?

46Self-Check

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Plant Hormones

5. Abscisic Acid:

• Initiates closing stomata in water-stressed plants• Induces and maintains dormancy in buds and seeds

– (inhibits gibberellins)

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Two of the many effects of abscisic acid (ABA) are• Seed dormancy

– Ensures seeds germinate only when conditions are optimal• Drought tolerance

– Closes stomata, decreases shoot growthColeoptile

Abscisic Acid

Why is that one kernel (seed) germinating prematurely?

K+

K+

K+

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Self-Check

Hormone Name Functions

Auxin

Gibberellin

Cytokinin

Ethylene

Abscisic Acid

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Plant Orientation

Sprouts know where to go

• Auxin controls direction of sprouting seedling

• Distribution of auxin within shoot and root cells is influenced by gravity and light

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Opaque capover tip.

Plant Orientation

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Clear capover tip.

Opaque sleeveover bendingregion.

Plant Orientation

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Cellselongateslowly.

Cellselongaterapidly.

Plant Orientation

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Plant OrientationShoot Elongation

• In shoot, light and gravity cause auxin movement to the lower side

Auxin stimulates elongation of stem cells

Stem bends away from gravity & toward light

Due to gravity, auxin builds up on the lower side of the root

Auxin retards elongation of root cells, and the root bends toward gravity

Root Growth

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Senescence

• Process by which leaves, fruits, and flowers age rapidly

– Promoted by changes in hormone levels

• Cytokinin and auxin production decreases• Ethylene production increases

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• Proteins, starches, and chlorophyll broken down– Products stored in roots and

other permanent tissues

Senescence

Abscission

Ethylene stimulates production of enzyme that digests cell walls at base of petiole

Leaf falls when cells are sufficiently weakened

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leafpetiole

bud

abscission layer

Senescence

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• Period of reduced metabolic activity in which the plant does not grow and develop

Dormancy

Maintained by abscisic acid

Dormancy broken by: increased temperature, longer day length occur in the spring

Lecture 7 Summary1. Plant Physical & Biological Stresses (Ch. 40)

2. Methods of Defense (Ch. 40)- Toxins / volatiles- Animals- Movement

3. Responses to Light (Ch. 41)

- photoreceptors

- circadian rhythms

4. Responses to Gravity (Ch. 41)

- stems

- roots

5. Responses to Touch (Ch. 41)

6. Plant Hormones (Ch. 41)

- general functions

- role in cell elongation

- senescence

- dormancy