Chapter 39 Campbell

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

• 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 __________ to elicit a __________

– 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

• After the potato is exposed to light

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

• The potato’s response to light

– Is an example of cell-signal processing

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 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (layer 1)

Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (layer 2)

Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (layer 3)

• An example of signal transduction in plants

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 (“__________”) 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 __________ 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 What part of a __________ senses light, and how is the signal transmitted?

39.05 Phototropism

• In 1926, Frits Went

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

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

• Cell elongation in response to auxin

Lateral and Adventitious Root Formation

• Auxin

– Is involved in the formation and branching of roots

Auxins as Herbicides

• An overdose of auxins

– Can kill eudicots

Other Effects of Auxin

• Auxin affects secondary growth

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

Cytokinins

• Cytokinins

– Stimulate cell division

Control of Cell Division and Differentiation

• Cytokinins

– 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

• If the terminal __________ is removed

– Plants become bushier

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

Germination

• After water is imbibed, the release of gibberellins from the embryo

– Signals the seeds to break dormancy and germinate

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 __________ 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

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

• Ethylene-insensitive mutants

– Fail to undergo the triple response after exposure to ethylene

• Other types of mutants

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

Figure 39.15 Ethylene signal transduction mutants can be distinguished by their different responses to experimental treatments

• A summary of ethylene signal transduction mutants

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

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 __________ 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

Figure 39.17 What wavelengths stimulate phototropic bending toward light?

• Action spectra

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

Figure 39.18 How does the order of red and far-red illumination affect seed germination?

• A phytochrome

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

Unnumbered figure page 804

• Phytochromes exist in two photoreversible states

– With conversion of Pr to Pfr triggering many developmental responses

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

– __________ during the day

• Many legumes

– Lower their leaves in the evening and raise them in the morning

• 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

• Action spectra and photoreversibility experiments

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

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

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 __________ by the settling of statoliths

– Specialized plastids containing dense starch grains

39.25 Gravitropism

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

• Growth in response to touch

– Is called __________

– 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

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

39.27 Mimosa Leaf

Flooding

• Enzymatic destruction of cells

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

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 __________

– With physical defenses such as thorns

– With chemical __________ such as __________ or toxic compounds

• Some plants even “recruit” predatory animals

– That help defend the plant against specific herbivores

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

• A pathogen is avirulent

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

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

– Then the pathogen can invade and kill the plant

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 __________ acid

Animation 38.1(a) Tropisms

Animation 38.1(b) Phototropism Experiments

Animation 38.2 Went’s Experiment

Animation 38.3 Auxin Affects Cell Walls

Video 38.1 Germination of soybean plants

Video 38.2 Time-lapse of acorn germination and growth

Video 38.3 Time-lapse of flower and fruit formation

Video 38.4 Time-lapse of bud burst in plants

Video 38.5 Gravitropism in a root

Video 38.6 Vine thigmotropism

Video 38.7 Thigmotropism of Mimosa