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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

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chapter 39



concept 39 1 signal transduction pathways
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

(b) After a week’s exposure to

natural daylight

(a) Before exposure to light

A potato’s response to light is an example of cell-signal processing
  • The stages are reception, transduction, and response
  • Internal and external signals are detected by receptors, proteins that change in response to specific stimuli
  • Second messengers transfer and amplify signals from receptors to proteins that cause responses

3. Response:

  • Cellular response is primarily accomplished by two mechanisms: (1) increasing or decreasing mRNA production, or (2) activating existing enzyme molecules
ii concept 39 2 plant hormones
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 another

B. Atrophismis 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

C. Discovery of Auxin

1. 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
a survey of plant hormones
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
d actions of plant hormones
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

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. Abscisicacid

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

f. Ethylene (gas)

  • Promotes fruit ripening
  • Prepare for leaf abscission
  • Initiates triple response (growth maneuver so a shoot can avoid an obstacle)
iii concept 39 3 plant responses to light
III. Concept 39.3: Plant Responses to Light

A. Photomorphogenesisis 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 stomata

2. Phytochromesare pigments that regulate many of a plant’s responses to light throughout its life

  • Responses include seed germination and shade avoidance

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



Red light




biological clocks and circadian rhythms
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




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

D. Photoperiodismis 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

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 plantscan flower in days of any length.

E. Responses to Other Environmental Stimuli

1. Gravitropismis a plant’s response to gravity

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

2. Thigmotropismis directional growth as a response to touch

  • Ex: tendrils

Fig. 39-26ab

(b) Stimulated state

(a) Unstimulated state

environmental stresses
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
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 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


in saliva




Signal transduction



defenses against pathogens
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 avirulentpathogen is one that may harm but does not kill the host plant
the hypersensitive response
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