Plant Response to Internal and External Signals

Chapter 39

Figure 39.3

CYTOPLASMCELLWALL

Plasma membrane

second messengers

Relay proteins andReceptor

Hormone orenvironmentalstimulus

Activationof cellularresponses

Transduction ResponseReception1 2 3

An Example:• A potato left growing in darkness produces shoots that

look unhealthy, and it lacks elongated roots• These are morphological adaptations for growing in

darkness, collectively called etiolation• After exposure to light, a potato undergoes changes

called de-etiolation, in which shoots and roots grow normally

Reception

• Internal and external signals are detected by receptors, proteins that change in response to specific stimuli

• In de-etiolation, the receptor is a phytochrome capable of detecting light

Figure 39.4-1

Reception

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

1

Transduction

• Second messengers transfer and amplify signals from receptors to proteins that cause responses

• Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP)

• The phytochrome receptor responds to light by– Opening Ca2+ channels, which increases Ca2+ levels

in the cytosol– Activating an enzyme that produces cGMP

Figure 39.4-2

Reception

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

1 Transduction

cGMP

Secondmessengerproduced

Proteinkinase 1

Proteinkinase 2

2

Ca2+ channelopened

Ca2+

Response• 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• This can occur by

– 1. transcriptional regulation • specific transcription factors bind directly to specific regions of DNA and control transcription of

genes• some transcription factors are activators that increase the transcription of specific genes• other transcription factors are repressors that decrease the transcription of specific genes

– 2. post-translational modification• involves modification of existing proteins in the signal response• modification often involves the phosphorylation of specific amino acids• the second messengers cGMP and Ca2+ activate protein kinases directly• protein phosphatases “switch off” the signal transduction pathways by dephosphorylating

proteins

Figure 39.4-3

Reception

CYTOPLASM

Plasmamembrane

Phytochromeactivatedby light

Cellwall

Light

1 Transduction

cGMP

Secondmessengerproduced

Proteinkinase 1

2

Ca2+ channelopened

Ca2+

De-etiolation(greening)responseproteins

Translation

Transcription

Transcriptionfactor 2

NUCLEUSTranscriptionfactor 1

Response3

P

P

Proteinkinase 2

De-Etiolation (“Greening”) Proteins

• De-etiolation activates enzymes that– Function in photosynthesis directly– Supply the chemical precursors for chlorophyll

production– Affect the levels of plant hormones that regulate

growth

Hormones help coordinate growth, development, & responses to stimuli

• Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant

• Produced in very low concentrations, but can have profound effects on growth and development– Each hormone has multiple effects, but multiple

hormones can influence a single process• Plant responses depend on amount and

concentration of specific hormones and often on the combination of hormones present

A Survey of Plant Hormones

• The major plant hormones include– Auxin*– Cytokinins*– Gibberellins*– Abscisic acid*– Ethylene*– Brassinosteroids– Jasmonates – Strigolactones

Table 39.1

Auxins: any chemical that promotes elongation of coleoptiles

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

• Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light (late 1800s)– 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• In 1913, Peter Boysen-Jensen demonstrated that the

signal was a mobile chemical substance

Figure 39.5

Control

Light

Shadedside

Illuminatedside

Boysen-Jensen

LightLight

Darwin and Darwin

Results

Tipremoved

Opaquecap

Trans-parentcap

Opaqueshieldovercurvature

Gelatin(permeable)

Mica(impermeable)

Auxin II

• Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA

• Auxin is produced in shoot tips and is transported down the stem

• Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell

• 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

Figure 39.7

CELL WALL

CYTOPLASM

Plasma membrane

H+

H+

H+

H+

H+H+

H+

H+

H+

ATP

Low pHactivatesexpansins.

Acidityincreases.

Protonpumpactivityincreases.

Cell wall-looseningenzymes cleavecross-linkingpolysaccharides.

Plasmamembrane

Cellwall

H2O

Nucleus Cytoplasm

Vacuole

Sliding cellulosemicrofibrils allowcell to elongate.

1

2

3

4

5

Cytokinins

• Stimulate cytokinesis • Cytokinins are produced in actively growing tissues such as

roots, embryos, and fruits• Cytokinins work together with auxin to control cell division and

differentiation• Cytokinins, auxin, and strigolactone 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

• Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

Figure 39.8

(a) Apical bud intact (not shown in photo)

(b) Apical bud removed

(c) Auxin added to decapitated stem

Lateral branches

“Stump” afterremoval ofapical bud

Axillary buds

Gibberellins: Stem Elongation

• Gibberellins have a variety of effects

• Stem elongation– Produced in young roots

and leaves– Stimulate growth of

leaves and stems• In stems, they stimulate

cell elongation and cell division

Gibberellins: Fruit Growth

• Fruit Growth– In many plants, both

auxin and gibberellins must be present for fruit to develop

– Used in spraying of Thompson seedless grapes

Gibberellins: Seed Germination

• Seed Germination– After water is imbibed, release of gibberellins from the

embryo signals seeds to germinate

Abscisic Acid• Abscisic acid (ABA) slows growth• Seed dormancy

– Ensures that the seed will germinate only in optimal conditions

– Dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold

– Precocious (early) germination can be caused by inactive or low levels of ABA

• Drought tolerance– Primary internal signal that enables

plants to withstand drought– Accumulation causes stomata to

close rapidly

Ethylene

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

• The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening

Ethylene Response to Mechanical Stress

• Ethylene induces the triple response, which allows a growing shoot to avoid obstacles– The triple response

consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth

Ethylene: Senescence & Leaf Abscission

• Senescence is the programmed death of cells or organs

• A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

• A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

Ethylene: Fruit Ripening

• A burst of ethylene production in a fruit triggers the ripening process

• Ethylene triggers ripening, and ripening triggers release of more ethylene

• Fruit producers can control ripening by picking green fruit and controlling ethylene levels

Response to Light• Light cues many key events in plant growth and development• Effects of light on plant morphology are called photomorphogenesis• Plants detect the presence of light, its direction, intensity, and

wavelength (color)– An action spectrum depicts relative response of a process to different

wavelengths– Action spectra are useful in studying any process that depends on light

• Different plant responses can be mediated by the same or different photoreceptors– Blue-light photoreceptors

• control hypocotyl elongation, stomatal opening, and phototropism

– Phytochromes• Pigments that regulate many of a plant’s responses to light throughout its life• These responses include seed germination and shade avoidance

Phytochromes and Seed Germination

• Many seeds remain dormant until light and other conditions are near optimal

• Red light increased germination, while far-red light inhibited germination

• The effects of red and far-red light are reversible; the final light exposure determines the response

• The photoreceptors responsible for the opposing effects of red and far-red light are phytochromes

Figure 39.16

Red Dark Red Far-red Dark

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

Results

Dark (control)

How Does It Work?• Phytochromes exist in two photoreversible

states, with conversion of Pr to Pfr triggering many developmental responses

• Red light triggers the conversion of Pr to Pfr • Far-red light triggers the conversion of Pfr to Pr • The conversion of Pr to Pfr is faster than the

reverse process• Sunlight, containing both red and far-red light,

increases the ratio of Pfr to Pr and triggers germination

Figure 39.17

Synthesis

Red light

Pr Pfr

Slow conversionin darkness(some species)

Enzymaticdestruction

Responses to Pfr:• Seed germination• Inhibition of verticalgrowth and stimu-lation of branching

• Setting internal clocks• Control of flowering

Far-red light

Phytochromes and Shade Avoidance

• The phytochrome system also provides the plant with information about the quality of light

• Leaves in the canopy absorb red light• Shaded plants receive more far-red than red light• In the “shade avoidance” response, the

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

• This shift induces vertical growth

Biological Clocks and Circadian Rythms

• Many plant processes oscillate during the day in response to light and temperature changes

• Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”– Can be set to exactly 24 hours by the day/night cycle– May depend on synthesis of a protein regulated through

feedback control and may be common to all eukaryotes• Phytochrome conversion marks sunrise and sunset,

providing the biological clock with environmental cues

Photoperiodism and Flowering

• 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– Plants that flower when a light period is shorter than a critical

length are called short-day plants– Plants that flower when a light period is longer than a certain

number of hours are called long-day plants– Flowering in day-neutral plants is controlled by plant maturity,

not 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

• Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness

• Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness

Effects of Red Light• Red light can interrupt the

nighttime portion of the photoperiod– A flash of red light followed

by a flash of far-red light does not disrupt night length

• Action spectra and photoreversibility experiments show that phytochrome is the pigment that detects the red light– Some plants flower after

only a single exposure to the required photoperiod

– Other plants need several successive days of the required photoperiod

Responses to Stimuli Other than Light

• Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms

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

gravitropism– Plants may detect gravity by the settling of statoliths,

dense cytoplasmic components• Thigmotropism is growth in response to touch

– It occurs in vines and other climbing plants

Response to Abiotic Stresses• During drought, plants reduce transpiration by closing stomata, reducing

exposed surface area, and in some species, shedding leaves• Enzymatic destruction of root cortex cells creates air tubes that help plants

survive oxygen deprivation during flooding• Salt can lower the water potential of the soil solution and reduce water uptake

– Produce solutes tolerated at high concentrations– Keeps the water potential of cells more negative than that of the soil solution

• Cold temperatures decrease membrane fluidity– Altering lipid composition of membranes is a response to cold stress– Freezing causes ice to form in a plant’s cell walls and intercellular spaces

• Many plants have antifreeze proteins that prevent ice crystals from growing and damaging cells

• Excessive heat can denature a plant’s enzymes– Transpiration helps cool leaves by evaporative cooling– Heat-shock proteins help protect other proteins from heat stress

Figure 39.UN05

Environmental Stress Major Response

Drought

Flooding

Salt

Heat

Cold Adjusting membrane fluidity; avoidingosmotic water loss; producing antifreezeproteins

Synthesis of heat-shock proteins, which reduceprotein denaturation at high temperatures

Avoiding osmotic water loss by producingsolutes tolerated at high concentrations

Formation of air tubes that help roots surviveoxygen deprivation

ABA production, reducing water loss byclosing stomata

Figure 39.25

Vascularcylinder

Air tubes

Epidermis

(a) Control root(aerated)

(b) Experimental root(nonaerated)

100 µm100 µm

Defense Against Pathogens• A plant’s first line of defense against infection is the barrier presented by

the epidermis and periderm• Pathogens can enter through wounds or natural openings, such as

stomata• The first line of immune defense depends on the plant’s ability to

recognize pathogen-associated molecular patterns (PAMPs)– These molecular sequences are specific to certain pathogens

• PAMP recognition starts a chain of signaling events leading to the production of antimicrobial chemicals and toughening of the cell wall– Pathogens that have evolved the ability to deliver effectors into plant cells &

can suppress PAMP-triggered plant immunity• Effectors are pathogen-encoded proteins that cripple the host’s innate immune system

• A second level of plant immune defense evolved in response to these pathogens

The Hypersensitivity Response

• The hypersensitive response– Causes cell and tissue death near the infection site– Induces production of enzymes that attack the

pathogen– Stimulates changes in the cell wall that confine the

pathogen

Systemic Acquired Resistance

• Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response

• Methylsalicylic acid is synthesized around the infection site and carried in the phloem to other remote sites where it is converted to salicylic acid

• Salicylic acid triggers the defense system to respond rapidly to another infection

Figure 39.26a

Signal transduction pathway

Hypersensitiveresponse

Signal

Pathogen

R protein

Effector protein

Hypersensitiveresponse

Signal transduction

pathway

Acquiredresistance

Systemic acquiredresistance

5

63

4

2

1

7