Review of photosynthesis and plant adaptations to dry hot climates.

Review of photosynthesis and plant adaptations to dry hot climates

Review: Photosynthesis uses light energy to make food molecules

A summary of the chemical processes of photo-synthesis

Figure 7.11

Light

Chloroplast

Photosystem IIElectron transport

chains Photosystem I

CALVIN CYCLE Stroma

Electrons

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration

Cellulose

Starch

Other organic compounds

Many plants make more sugar than they need

– The excess is stored in roots, tuber, and fruits

– These are a major source of food for animals

C4 and CAM plants have special adaptations that save water

Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle

In these types of plants, stomata on the leaf surface close when the weather is hot

This causes a drop in CO2 and an increase in O2 in the leaf

Photorespiration may then occur

Photorespiration in a C3 plant

CALVIN CYCLE

2-C compound

Figure 7.12A

EXAMPLES: wheat, barley, potatoes and sugar beet.

Some plants have special adaptations that enable them to save water

CALVIN CYCLE

4-C compound

Figure 7.12B

– Special cells in C4 plants—corn, crabgrass and sugarcane—incorporate CO2 into a four-carbon molecule

– This molecule can then donate CO2 to the Calvin cycle 3-C sugar

In C4 plants, the bundle sheath cells contain chloroplasts; carbon is fixed in mesophyll cells, then transported to bundle sheath cells where Calvin Cycle reactions occur in the absence of oxygen.

The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism

CALVIN CYCLE

4-C compound

Figure 7.12C

– They open their stomata at night and make a four-carbon compound

– It is used as a CO2 source by the same cell during the day

3-C sugar

Night

Day

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 39Chapter 39

Plant Responses to Internal and External Signals


• Overview: Stimuli and a Stationary Life

• Plants, being rooted to the ground

– Must respond to whatever environmental change comes their way


• For example, the bending of a grass seedling toward light

– Begins with the plant sensing the direction, quantity, and color of the light

Figure 39.1


• Concept 39.1: Signal transduction pathways link signal reception to response

• Plants have cellular receptors

– That they use to detect important changes in their environment

• For a stimulus to elicit a response

– Certain cells must have an appropriate receptor


• A potato left growing in darkness

– Will produce shoots that do not appear healthy, and will lack elongated roots

• These are morphological adaptations for growing in darkness

– Collectively referred to as etiolation

Figure 39.2a

(a) Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.


• After the potato is exposed to light

– The plant undergoes profound changes called de-etiolation, in which shoots and roots grow normally

Figure 39.2b

(b) After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.


• The potato’s response to light

– Is an example of cell-signal processing

Figure 39.3

CELLWALL

CYTOPLASM

1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane


Reception

• Internal and external signals are detected by receptors

– Proteins that change in response to specific stimuli


Transduction

• Second messengers

– Transfer and amplify signals from receptors to proteins that cause specific responses


Figure 39.4

1 Reception 2 Transduction 3 Response

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

cGMP

Second messengerproduced

Specificproteinkinase 1activated

Transcriptionfactor 1 NUCLEUS

P

P

Transcription

Translation

De-etiolation(greening)responseproteins

Ca2+

Ca2+ channelopened

Specificproteinkinase 2activated

Transcriptionfactor 2

• An example of signal transduction in plants

1 The light signal isdetected by thephytochrome receptor,which then activatesat least two signaltransduction pathways.

2 One pathway uses cGMP as asecond messenger that activatesa specific protein kinase.The otherpathway involves an increase incytoplasmic Ca2+ that activatesanother specific protein kinase.

3 Both pathwayslead to expressionof genes for proteinsthat function in thede-etiolation(greening) response.


Response

• Ultimately, a signal transduction pathway

– Leads to a regulation of one or more cellular activities

• In most cases

– These responses to stimulation involve the increased activity of certain enzymes


Transcriptional Regulation

• Transcription factors bind directly to specific regions of DNA

– And control the transcription of specific genes


Post-Translational Modification of Proteins

• Post-translational modification

– Involves the activation of existing proteins involved in the signal response


De-Etioloation (“Greening”) Proteins

• Many enzymes that function in certain signal responses are involved in photosynthesis directly

– While others are involved in supplying the chemical precursors necessary for chlorophyll production


• Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli

• Hormones

– Are chemical signals that coordinate the different parts of an organism


The Discovery of Plant Hormones

• Any growth response

– That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism

– Is often caused by hormones


• Charles Darwin and his son Francis

– Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century


Figure 39.5

In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.

EXPERIMENT

In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.

CONCLUSION

RESULTS

Control Darwin and Darwin (1880) Boysen-Jensen (1913)

Light

Shadedside ofcoleoptile

Illuminatedside ofcoleoptile

Light

Tipremoved

Tip coveredby opaquecap

Tipcoveredby trans-parentcap

Base coveredby opaqueshield

Light

Tip separatedby gelatinblock

Tip separatedby mica


• In 1926, Frits Went

– Extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin.

The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.

Excised tip placedon agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Control(agar blocklackingchemical)has noeffectControl

Offset blockscause curvature

RESULTS

CONCLUSION

In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.

EXPERIMENT

Figure 39.6


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 concentrations

– But a minute amount can have a profound effect on the growth and development of a plant organ


Auxin

• The term auxin

– Is used for any chemical substance that promotes cell elongation in different target tissues


• Auxin transporters move auxin from shoot to base

– Move the hormone out of the basal end of one cell, and into the apical end of neighboring cells

Figure 39.7

EXPERIMENT

Cell 1

Cell 2100 m

Epidermis

Cortex

Phloem

Xylem

PithBasal end

of cell

25 m

To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They useda greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems.

RESULTS The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells.

CONCLUSION The results support the hypothesis that concentration of the auxintransport protein at the basal ends of cells is responsible for polar transport of auxin.


The Role of Auxin in Cell Elongation

• According to a model called the acid growth hypothesis

– Proton pumps play a major role in the growth response of cells to auxin


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

Figure 39.8

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.


Lateral and Adventitious Root Formation

• Auxin

– Is involved in the formation and branching of roots

• An overdose of auxins

– Can kill eudicots

• Auxin affects secondary growth

– By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem


Cytokinins

• Cytokinins

– Stimulate cell division

– Are produced in actively growing tissues such as roots, embryos, and fruits

– Work together with auxin


Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in the control of apical dominance

– The ability of a terminal bud to suppress development of axillary buds

Figure 39.9a

Axillary buds


• If the terminal bud is removed

– Plants become bushier

Figure 39.9b

“Stump” afterremoval ofapical bud

Lateral branches


Anti-Aging Effects

• Cytokinins retard the aging of some plant organs

– By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues


Gibberellins

• Gibberellins have a variety of effects

– Such as stem elongation, fruit growth, and seed germination


Stem Elongation

• Gibberellins stimulate growth of both leaves and stems

• In stems

– Gibberellins stimulate cell elongation and cell division


Fruit Growth

• In many plants

– Both auxin and gibberellins must be present for fruit to set


• Gibberellins are used commercially

– In the spraying of Thompson seedless grapes

Figure 39.10


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

Germination

Figure 39.11

2 2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water

Scutellum(cotyledon)

GA

GA

-amylase

Radicle

Sugar

1 After a seedimbibes water, theembryo releasesgibberellin (GA)as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.


2 The aleurone responds by synthesizing and secreting digestive enzymes thathydrolyze stored nutrients inthe endosperm. One exampleis -amylase, which hydrolyzesstarch. (A similar enzyme inour saliva helps in digestingbread and other starchy foods.)

Aleurone

Endosperm

Water

Scutellum(cotyledon)

GA

GA

-amylase

Radicle

Sugar

2 1 After a seedimbibes water, theembryo releasesgibberellin (GA)as a signal to thealeurone, the thinouter layer of theendosperm.

3 Sugars and other nutrients absorbedfrom the endospermby the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.


Brassinosteroids

• Brassinosteroids

– Are similar to the sex hormones of animals

– Induce cell elongation and division


Abscisic Acid

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

– Seed dormancy

– Drought tolerance


Seed Dormancy

• Seed dormancy has great survival value

– Because it ensures that the seed will germinate only when there are optimal conditions


• Precocious germination is observed in maize mutants

– That lack a functional transcription factor required for ABA to induce expression of certain genes

Figure 39.12

Coleoptile


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


The Triple Response to Mechanical Stress

• Ethylene induces the triple response

– Which allows a growing shoot to avoid obstacles

Figure 39.13 Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.CONCLUSION

Germinating pea seedlings were placed in thedark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.

EXPERIMENT

All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.RESULTS

0.00 0.10 0.20 0.40 0.80

Ethylene concentration (parts per million)


• Ethylene-insensitive mutants

– Fail to undergo the triple response after exposure to ethylene

Figure 39.14a

ein mutant


• Other types of mutants

– Undergo the triple response in air but do not respond to inhibitors of ethylene synthesis

Figure 39.14b

ctr mutant


• A summary of ethylene signal transduction mutants

Figure 39.15

ControlEthylene

added

Ethylenesynthesisinhibitor

Wild-type

Ethylene insensitive(ein)

Ethyleneoverproducing (eto)

Constitutive tripleresponse (ctr)


Apoptosis: Programmed Cell Death

• A burst of ethylene

– Is associated with the programmed destruction of cells, organs, or whole plants


Leaf Abscission

• A change in the balance of auxin and ethylene controls leaf abscission

– The process that occurs in autumn when a leaf falls

Figure 39.16

0.5 mm

Protective layer Abscission layer

Stem Petiole


Fruit Ripening

• A burst of ethylene production in the fruit

– Triggers the ripening process


Systems Biology and Hormone Interactions

• Interactions between hormones and their signal transduction pathways

– Make it difficult to predict what effect a genetic manipulation will have on a plant

• Systems biology seeks a comprehensive understanding of plants

– That will permit successful modeling of plant functions


• Concept 39.3: Responses to light are critical for plant success

• Light cues many key events in plant growth and development

• Effects of light on plant morphology

– Are what plant biologists call photomorphogenesis


• Plants not only detect the presence of light

– But also its direction, intensity, and wavelength (color)

• A graph called an action spectrum

– Depicts the relative response of a process to different wavelengths of light


• Action spectra

– Are useful in the study of any process that depends on light

Figure 39.17

Wavelength (nm)

1.0

0.8

0.6

0.2

0450 500 550 600 650 700

Light

Time = 0 min.

Time = 90 min.

0.4

400Pho

totr

opic

eff

ectiv

enes

s re

lativ

e to

436

nm

Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.

EXPERIMENT

The graph below shows phototropic effectiveness (curvature per photon) relativeto effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.

RESULTS

CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.


• Research on action spectra and absorption spectra of pigments

– Led to the identification of two major classes of light receptors: blue-light photoreceptors and phytochromes


Blue-Light Photoreceptors

• Various blue-light photoreceptors

– Control hypocotyl elongation, stomatal opening, and phototropism


Phytochromes as Photoreceptors

• Phytochromes

– Regulate many of a plant’s responses to light throughout its life


Phytochromes and Seed Germination

• Studies of seed germination

– Led to the discovery of phytochromes


• In the 1930s, scientists at the U.S. Department of Agriculture

– Determined the action spectrum for light-induced germination of lettuce seeds

Dark (control)

Dark

Dark


Figure 39.18

Dark (control)

Dark Dark Red Far-redRed

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

CONCLUSION

EXPERIMENT

RESULTS

During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light.

The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposedto red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).

Red light stimulated germination, and far-red light inhibited germination.The final exposure was the determining factor. The effects of red and far-red light were reversible.


• A phytochrome

– Is the photoreceptor responsible for the opposing effects of red and far-red light

A phytochrome consists of two identical proteins joined to formone functional molecule. Each of these proteins has two domains.

Chromophore

Photoreceptor activity. One domain,which functions as the photoreceptor,is covalently bonded to a nonproteinpigment, or chromophore.

Kinase activity. The other domainhas protein kinase activity. Thephotoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.

Figure 39.19


• Phytochromes exist in two photoreversible states

– With conversion of Pr to Pfr triggering many developmental responses

Figure 39.20

Synthesis

Far-redlight

Red light

Slow conversionin darkness(some plants)

Responses:seed germination,control offlowering, etc.

Enzymaticdestruction

PfrPr


Phytochromes and Shade Avoidance

• The phytochrome system

– Also provides the plant with information about the quality of light

• In the “shade avoidance” response of a tree

– The phytochrome ratio shifts in favor of Pr when a tree is shaded


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

Figure 39.21

Noon Midnight


• Cyclical responses to environmental stimuli are called circadian rhythms

– And are approximately 24 hours long

– Can be entrained to exactly 24 hours by the day/night cycle


The Effect of Light on the Biological Clock

• Phytochrome conversion marks sunrise and sunset

– Providing the biological clock with environmental cues


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


Photoperiodism and Control of Flowering

• Some developmental processes, including flowering in many species

– Requires 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

Figure 39.22

During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.

24 h

ours

Darkness

Flash oflight

Criticaldarkperiod

Light

(a) “Short-day” plantsflowered only if a period ofcontinuous darkness waslonger than a critical darkperiod for that particularspecies (13 hours in thisexample). A period ofdarkness can be ended by abrief exposure to light.

(b) “Long-day” plantsflowered only if aperiod of continuousdarkness was shorterthan a critical darkperiod for thatparticular species (13hours in this example).


• Action spectra and photoreversibility experiments– Show that phytochrome is the pigment that receives

red light, which can interrupt the nighttime portion of the photoperiod

Figure 39.23

A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods,researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-redlight canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruptionof dark periods.

24

20

16

12

8

4

0

Hou

rs

Short-day (long-night) plant

Long-day (short-night) plant

R RFR FR

R

R RFRRFR

Crit

ical

dar

k pe

riod


A Flowering Hormone?

• The flowering signal, not yet chemically identified

– Is called florigen, and it may be a hormone or a change in relative concentrations of multiple hormones


Figure 39.24

To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted toa plant that had not been induced.

EXPERIMENT

RESULTS

CONCLUSION Both plants flowered, indicating the transmission of a flower-inducingsubstance. In some cases, the transmission worked even if one was a short-day plantand the other was a long-day plant.

Plant subjected to photoperiodthat induces flowering

Plant subjected to photoperiodthat does not induce flowering

Graft

Time(severalweeks)


Meristem Transition and Flowering

• Whatever combination of environmental cues and internal signals is necessary for flowering to occur

– The outcome is the transition of a bud’s meristem from a vegetative to a flowering state


• Concept 39.4: Plants respond to a wide variety of stimuli other than light

• Because of their immobility

– Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms


Gravity

• Response to gravity

– Is known as gravitropism

• Roots show positive gravitropism

• Stems show negative gravitropism


• Plants may detect gravity by the settling of statoliths

– Specialized plastids containing dense starch grains

Figure 39.25a, b

Statoliths20 m

(a) (b)


Mechanical Stimuli

• The term thigmomorphogenesis

– Refers to the changes in form that result from mechanical perturbation


• Rubbing the stems of young plants a couple of times daily

– Results in plants that are shorter than controls

Figure 39.26


• Growth in response to touch

– Is called thigmotropism

– 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

Figure 39.27a–c

(a) Unstimulated (b) Stimulated

Side of pulvinus withflaccid cells

Side of pulvinus withturgid cells

Vein

0.5 m(c) Motor organs

Leafletsafterstimulation

Pulvinus(motororgan)


Environmental Stresses

• Environmental stresses

– Have a potentially adverse effect on a plant’s survival, growth, and reproduction

– Can have a devastating impact on crop yields in agriculture


Drought

• During drought

– Plants respond to water deficit by reducing transpiration

– Deeper roots continue to grow


Flooding

• Enzymatic destruction of cells

– Creates air tubes that help plants survive oxygen deprivation during flooding

Figure 39.28a, b

Vascularcylinder

Air tubes

Epidermis

100 m 100 m(a) Control root (aerated) (b) Experimental root (nonaerated)


Salt Stress

• Plants respond to salt stress by producing solutes tolerated at high concentrations

– Keeping the water potential of cells more negative than that of the soil solution


Heat Stress

• Heat-shock proteins

– Help plants survive heat stress


Cold Stress

• Altering lipid composition of membranes

– Is a response to cold stress


• Concept 39.5: Plants defend themselves against herbivores and pathogens

• Plants counter external threats

– With defense systems that deter herbivory and prevent infection or combat pathogens


Defenses Against Herbivores

• Herbivory, animals eating plants

– Is a stress that plants face in any ecosystem

• Plants counter excessive herbivory

– With physical defenses such as thorns

– With chemical defenses such as distasteful or toxic compounds


Recruitment ofparasitoid waspsthat lay their eggswithin caterpillars

4

3 Synthesis andrelease ofvolatile attractants

1 Chemicalin saliva

1 Wounding

2 Signal transductionpathway

• Some plants even “recruit” predatory animals

– That help defend the plant against specific herbivores

Figure 39.29


Defenses Against Pathogens

• A plant’s first line of defense against infection

– Is the physical barrier of the plant’s “skin,” the epidermis and the periderm

• Once a pathogen invades a plant

– The plant mounts a chemical attack as a second line of defense that kills the pathogen and prevents its spread


• The second defense system

– Is enhanced by the plant’s inherited ability to recognize certain pathogens


Gene-for-Gene Recognition

• A virulent pathogen

– Is one that a plant has little specific defense against

• An avirulent pathogen

– Is one that may harm but not kill the host plant


• Gene-for-gene recognition is a widespread form of plant disease resistance

– That involves recognition of pathogen-derived molecules by the protein products of specific plant disease resistance (R) genes


Figure 39.30a

Receptor coded by R allele

(a) If an Avr allele in the pathogen corresponds to an R allelein the host plant, the host plant will have resistance,making the pathogen avirulent. R alleles probably code forreceptors in the plasma membranes of host plant cells. Avr allelesproduce compounds that can act as ligands, binding to receptorsin host plant cells.

• A pathogen is avirulent

– If it has a specific Avr gene corresponding to a particular R allele in the host plant

Signal molecule (ligand)from Avr gene product

Avr allele

Plant cell is resistantAvirulent pathogen

R


• If the plant host lacks the R gene that counteracts the pathogen’s Avr gene

– Then the pathogen can invade and kill the plant

Figure 39.30b

No Avr allele;virulent pathogen

Plant cell becomes diseased

Avr allele

No R allele;plant cell becomes diseasedVirulent pathogen

Virulent pathogen

No R allele;plant cell becomes diseased

(b) If there is no gene-for-gene recognition because of one ofthe above three conditions, the pathogen will be virulent,causing disease to develop.

R


3 In a hypersensitiveresponse (HR), plantcells produce anti-microbial molecules,seal off infectedareas by modifyingtheir walls, andthen destroythemselves. Thislocalized responseproduces lesionsand protects otherparts of an infectedleaf.

4 Before they die,infected cellsrelease a chemicalsignal, probablysalicylic acid.

6 In cells remote fromthe infection site,the chemicalinitiates a signaltransductionpathway.

5 The signal is distributed to the rest of the plant.

2 This identification step triggers a signal transduction pathway.

1 Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells.

7 Systemic acquiredresistance isactivated: theproduction ofmolecules that helpprotect the cellagainst a diversityof pathogens forseveral days.

Signal

7

6

54

3

2

1

Avirulentpathogen

Signal transductionpathway

Hypersensitiveresponse

Signaltransduction

pathway

Acquiredresistance

R-Avr recognition andhypersensitive response

Systemic acquiredresistanceFigure 39.31

Plant Responses to Pathogen Invasions

• A hypersensitive response against an avirulent pathogen

– Seals off the infection and kills both pathogen and host cells in the region of the infection


Systemic Acquired Resistance

• Systemic acquired resistance (SAR)

– Is a set of generalized defense responses in organs distant from the original site of infection

– Is triggered by the signal molecule salicylic acid

Review of photosynthesis and plant adaptations to dry hot climates.

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