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CHAPTER 39

PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS

Fig. 39-1

Concept 39.1: Signal transduction pathways

• 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

• 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

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

• The stages are reception, transduction, and response

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General Model for Signal Transduction Pathways

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 responses

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Reception-Transduction

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Second Messengers

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Reception-Transduction-Response

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3. Response:• Cellular response is primarily accomplished by two

mechanisms: (1) increasing or decreasing mRNA production, or (2) activating existing enzyme molecules

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II. Concept 39.2: Plant Hormones

A. Hormones are defined as chemical messengers that coordinate different parts of a multicellular organism•They are produced by one part of the body and transported to

anotherB. A trophism is a plant growth response from hormones that

results in the plant growing either toward (positive) or away (negative) from a stimulus•Phototrophism is the growth of a shoot in a certain direction in

response to light. •Positive phototropism is the growth of a plant toward light•Negative phototropism is the growth of a plant away from light

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Phototropism in Grass Seedlings

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C. Discovery of Auxin1. Charles and Francis Darwin (1881)

•Concluded that coleoptile tips were responsible for sensing light and producing a substance that was transported to elongating region

2. Peter Boysen-Jensen•Demonstrated that the substance for elongation was mobile

3. F. W. Went (1926)•Named substance for elongation—auxin

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F. W. Went’s Experiment

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

Table 39-1

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D. Actions of Plant Hormones

1. Six Classes of Plant Hormones:a. Auxin (natural auxin—IAA–indoleacetic acid)

• Stimulates cell elongation• Promotes root formation• Regulates fruit development• Enhances apical dominance

b. Cytokinins• regulate cell division• Anti-aging effects (keeps cut flowers fresh)• Slow apoptosis

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Apical Dominance—Auxin/Cytokinin Control

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c. Gibberellins• Promote stem elongation• Promote seed germination• Contributes to fruit growth

d. Brassinosteroids•Promote cell elongation and division•Promotes xylem differentiation•Slow leaf abscission

e. Abscisic acid•Promotes seed dormancy until optimum conditions•Drought tolerance (closes stomata during water stress)

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Effect of Gibberellin (on right)

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f. Ethylene (gas)• Promotes fruit ripening• Prepare for leaf abscission• Initiates triple response (growth maneuver so a

shoot can avoid an obstacle)

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III. Concept 39.3: Plant Responses to Light

A. Photomorphogenesis is the term used to describe the effects of light on plant morphology

B. There are two major classes of light receptors:1. Blue-light photoreceptors initiate a number of plant responses to light including phototropisms and the light-induced opening of the stomata2. Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life

•Responses include seed germination and shade avoidance

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•Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses•Phytochromes absorb mostly red light

Fig. 39-UN3

Photoreversible states of phytochrome

PrPfr

Red light

Far-red light

Responses

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

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C. Circadian rhythms are physiological cycles that have a frequency of about 24 hours and that are not paced by a known environmental clock.• In plants, the surge of Pfr at dawn resets the biological

clock.• The combination of a phytochrome system and a

biological clock allow the plant to accurately assess the amount of daylight or darkness and hence the time of the year

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D. Photoperiodism is defined as a physiological response to a photoperiod (the relative lengths of night and day). • Important in plant life cycles such as flowering• It is night length—not day length—that controls flowering

and certain other response to photoperiod.•Short-day plants require a period of continuous darkness

longer than a critical period (length of day) in order to flower. These plants usually flower in late summer, fall, and winter

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•Long-day plants flower only if a period of continuous darkness was shorter than a critical period. They often flower in late spring or early summer. They are actually short-night plants.

•Day-neutral plants can flower in days of any length.

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E. Responses to Other Environmental Stimuli 1. Gravitropism is a plant’s response to gravity

•Roots show positive gravitropism•Shoots show negative gravitropism•Auxins play a key role in gravitropism

2. Thigmotropism is directional growth as a response to touch•Ex: tendrils

Fig. 39-26ab

(a) Unstimulated state (b) Stimulated state

Environmental Stresses

• Environmental stresses have a potentially adverse effect on survival, growth, and reproduction

• Stresses can be abiotic (nonliving) or biotic (living)• Abiotic stresses include drought, flooding, salt stress,

heat stress, and cold stress

Concept 39.5: Plants respond to attacks by herbivores and pathogens

• Plants use defense systems to deter herbivory, prevent infection, and combat pathogens

Defenses Against Herbivores

• defenses such as thorns and chemical defenses such as distasteful or toxic compounds

• Some plants even “recruit” predatory animals that help defend against specific herbivores

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

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Defenses Against Pathogens

• A plant’s first line of defense against infection is the epidermis and periderm

• If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread

• A virulent pathogen is one that a plant has little specific defense against

• An avirulent pathogen is one that may harm but does not kill the host plant

The Hypersensitive Response

• The hypersensitive response– Causes cell and tissue death near the infection site– Induces production proteins, which attack the pathogen– Stimulates changes in the cell wall that confine the

pathogen