Plant Responses

AP Biology Chapter 39

Plants respond by signal transduction pathways just like we do!

• Plants have cellular receptors that detect changes in their environment

• For a stimulus to elicit a response, certain cells must have an appropriate receptor

• Stimulation of the receptor initiates a specific signal transduction pathway

Fig. 39-2

(a) Before exposure to light (b) After a week’s exposure to natural daylight

A potato’s response to light is an example of cell-signal processing

Fig. 39-4-3

CYTOPLASM

Reception

Plasmamembrane

Cellwall

Phytochromeactivated by light

Light

Transduction

Second messenger produced

cGMPSpecific protein

kinase 1 activated

NUCLEUS

1 2

Specific protein

kinase 2 activated

Ca2+ channel opened

Ca2+

Response3

Transcriptionfactor 1

Transcriptionfactor 2

NUCLEUS

Transcription

Translation

De-etiolation(greening)responseproteins

P

P

Signaling pathways due to Auxin

• A signal transduction pathway leads to regulation of one or more cellular activities

• In most cases, these responses to stimulation involve increased activity of enzymes involved in photosynthesis and chlorophyll production

• They may also lead to changes in gene expression.

The Discovery of Plant Hormones

• Any response resulting in curvature of organs toward or away from a stimulus is called a tropism

• Tropisms are often caused by hormones

• In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light

• They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present

• They postulated that a signal was transmitted from the tip to the elongating region

. F. Went concluded that the chemical was

auxin and that it migrated to the shady side of the plant and caused cell growth in that area.

Boysen-Jensen demonstrated that the substance was mobile and could move through a block of gelatin.

• But, maybe the light stimulates a GROWTH INHIBITOR on the lighted side!

Fig. 39-5a

RESULTS

Control

Light

Illuminatedside ofcoleoptile

Shadedside of coleoptile

A Survey of Plant Hormones

• In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells

• Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ

How does auxin work in stimulating cell elongation in phototropism?

AUXIN

• The term auxin refers to any chemical that promotes elongation of coleoptiles.

The Role of Auxin in Cell Elongation• According to the acid growth hypothesis, auxin

stimulates proton pumps in the plasma membrane

• The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric

• With the cellulose loosened, the cell can elongate

Fig. 39-8

Cross-linkingpolysaccharides

Cellulose microfibril

Cell wall becomes more acidic.

2

1 Auxin increases proton pump activity.

Cell wall–looseningenzymes

Expansin

Expansins separatemicrofibrils from cross-linking polysaccharides.

3

4

5

CELL WALL

Cleaving allowsmicrofibrils to slide.

CYTOPLASM

Plasma membrane

H2O

CellwallPlasma

membrane

Nucleus Cytoplasm

Vacuole

Cell can elongate.

Uses of auxin

• Cell elongation in phototropism and gravitropism

• root formation and branching• affects secondary growth by

stimulating cambium growth.• An overdose of synthetic auxins can

kill eudicots ! weedkillers

Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending

upon the tissue where it acts.

CYTOKININS

• Cytokinins are so named because they stimulate cytokinesis (cell division).

• Cytokinins retard the aging of some plant organs

Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds

• If the terminal bud is removed, plants become bushier

Fig. 39-9

(a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem

(b) Apical bud removed

Axillary buds

Lateral branches

“Stump” afterremoval ofapical bud

Gibberellins

Gibberellins or gibberellic acid (GA) have a variety of effects, such as stem elongation, fruit growth, and seed germination

Fig. 39-11

Gibberellins (GA)send signal toaleurone.

Aleurone secretes -amylase and other enzymes.

Sugars and other nutrients are consumed.

AleuroneEndosperm

Water

Scutellum (cotyledon)

Radicle

12 3

GA

GA

-amylaseSugar

Seed Germination

Abscisic Acid

• Abscisic acid (ABA) slows growth• Two of the many effects of ABA:

– Seed dormancy• In some seeds, dormancy is broken

when ABA is removed by heavy rain, light, or prolonged cold– Drought tolerance

• ABA is the primary internal signal that enables plants to withstand drought

Ethylene

• Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection

• Also induces leaf fall (abscision) and fruit ripening.

The dosage effect of ethylene on impatiens.

Plants not exposed to ethylene (A). Plants exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D).

Initially only open flowers abscised, then buds began to abscise. After three days of exposure, all flowers and buds had been shed

Light Cues in Plants

• Effects of light on plant morphology are called photomorphogenesis

Fig. 39-16b

Light

Time = 0 min

(b) Coleoptile response to light colors

Time = 90 min

Effects of light on plant morphology are called photomorphogenesis

Phytochromes as Photoreceptors

• Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life

• These responses include seed germination and shade avoidance• Phytochromes exist in two photoreversible states, with

conversion of Pr to Pfr triggering many developmental responses

Fig. 39-19

Synthesis

Pr

Far-redlight

Slow conversionin darkness(some plants)

Enzymaticdestruction

Responses:seed germination,control offlowering, etc.

Pfr

Red light

Absorption of red light causes the Pr to change to Pfr. Far-red light reverses the conversion. Mostly, it is the Pfr that switches on physiological and developmental responses.

Fig. 39-17

Dark (control)

RESULTS

DarkRed

Red Far-red Red Dark Red Far-red Red Far-red

Red Far-red Dark

How does the order of red and far-red illumination affect seed germination?

a) red-light ?b) Far-red ?c) Determing factor?d) Are the effects

reversible?

a. Simulatesb. Inhibitsc. Final-light

exposured. yes

Biological Clocks and Circadian Rhythms

• Many plant processes oscillate during the day• Many legumes lower their leaves in the

evening and raise them in the morning, even when kept under constant light or dark conditions

Fig. 39-20

Noon Midnight

Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year

• Photoperiodism is a physiological response to photoperiod

• Some processes, including flowering in many species, require a certain photoperiod

Critical Night Length• In the 1940s, researchers discovered

that flowering and other responses to photoperiod are actually controlled by night length, not day length

Fig. 39-2124 hours

Light

Criticaldark period

Flashof light

Darkness

(a) Short-day (long-night) plant

Flashof light

(b) Long-day (short-night) plant

What does this experiment indicate?

Red light (received by phytochromes) can interrupt the nighttime portion of the photoperiod

Fig. 39-22

24 hours

R

RFR

RFRR

RFRRFR

Critical dark period

Short-day(long-night)

plant

Long-day(short-night)

plant

A flash of far-red can reverse the effect though.

Other Responses:Gravity

• Response to gravity is known as gravitropism• Roots show positive gravitropism; shoots

show negative gravitropism• Plants may detect gravity by the settling of

statoliths, specialized plastids containing dense starch grains

Fig. 39-24

Statoliths20 µm

(b) Statoliths settling(a) Root gravitropic bending

Mechanical Stimuli

• The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance

• Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

Fig. 39-25

• Thigmotropism is growth in response to touch

• It occurs in vines and other climbing plants• Rapid leaf movements in response to

mechanical stimulation are examples of transmission of electrical impulses called action potentials

Fig. 39-26

(a) Unstimulated state

Leafletsafter stimulation

Pulvinus(motororgan)

(c) Cross section of a leaflet pair in the stimulated state (LM)

(b) Stimulated state

Side of pulvinus withflaccid cells

Side of pulvinus withturgid cells

Vein

0.5

µm

How plants react to environmental stresses

• Drought: close stomata, slow leaf growth, reduce exposed surface, deep roots

• Heat stress – heat shock proteins protect them

• Cold – alter lipids in cell membrane• Salt – increased solute conc in cells• Flooding – make air spaces in root

cortex

How plants resist herbivores and pathogens

• Physical and chemical defenses

• Recruit predatory animals

• Immune system – gene for gene recognition, hypersensitive response, system acquired response, salicylic acid*

*In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments.

Beware!Chemical Defenses

• Physical Defenses

Recruiting predatory animals

• Ants and acacia tree

Fig. 39-28

Recruitment of parasitoid wasps that lay their eggs within caterpillars

Synthesis and release of volatile attractants

Chemical in saliva

Wounding

Signal transduction pathway

1 1

2

3

4

Fig. 39-29

Signal

Hypersensitiveresponse

Signal transduction pathway

Avirulent pathogen

Signal transduction

pathway

Acquired resistance

R-Avr recognition andhypersensitive response

Systemic acquiredresistance

Recognizing plant pathogens