What are the responses of plants to environmental cues and how do hormones mediate them

1
What are the responses of plants to environmental
cues and how do hormones mediate them?

• Chapter 39
• 6th Edition Campbell

2
I. Signal Transduction in Plants
Pale, unexpanded leaves adaptations for growing
underground
Green leaves, sturdy stems long roots from a
week of daylight
3
Why the change in the potatoes?

• The transformation began with the reception of
  light by specific pigment
• These changes in morphology and biochemistry are
  called greening

4
Cell Signal Processing 3 stages
5
Step 1 - Reception

• Signals (internal/external) are detected by
  receptors
• Receptor for greening is phytochrome
• Most receptors are in the cell membrane
• Phytochrome is in the cytoplasm
• Phytochrome functions in light detection in the
  greening process

6
Step 2 Signal Transduction

• Involves second messengers (are chemicals
  produced in the cytoplasm)
• They amplify the signal from the receptor to
  proteins that will cause the response
• Transduction helps to regulate cellular activity

7
Step 3 - Response

• Leads to an increased activity of enzymes by
  stimulating the transcription of mRNA for the
  enzymes
• OR
• Activates existing enzymes by adding a phosphate
  group (phosphorylation)
• Figure 39.3

8
II. Plant Responses to Hormones

• Tropism growth toward or away from a stimulus
• Phototropism growth of shoot toward light
• The curving of a plant results from differential
  growth of cells
• Dark side elongates faster

9
Phototropism proven through experiments

• Charles Darwin and son, 1880
• Used grass seedlings
• Each plant has a sheath covering called
  coleoptile
• Concluded that the shoot will bend toward light
  only if the tip were present or uncovered

10
(No Transcript)
11

• Peter Boysen-Jensen
• Added to Darwins experiment in 1913
• Concluded that the signal allowing the shoot to
  grow toward the light was a mobile chemical
  substance

12

• F.W. Went, modified Boysen-Jensens experiment in
  1926
• Removed the tip and put in on agar
• Chemical in the tip diffused into agar and
  therefore should be able to substitute tip for
  agar

13

• Concluded that agar DID contain the chemical from
  the tip
• Concluded that the chemical stimulated growth
• Called the chemical (hormone) Auxin (in Greek
  means to increase)

14
(No Transcript)
15
Plant Hormones

• Using pages 808 817, determine the location of
  the six plant hormones discussed in the book,
  their major functions, and gather one other
  important fact about them

16
Responses to Light

• Light is required for photosynthesis
• Light cues key events in growth and development
• Photomorphogenesis effect of light on plant
  morphology
• Light reception allows plants to measure days and
  seasons

17
Blue Light is Most Effective in

• Phototropism
• Pigment Phototropin
• Inhibiting hypocotyl elongation
• Pigment Cryptochrome
• Opening stomates
• Pigment Zeaxanthin

18
Circadian Rhythms

• Many physiological processes in plants continue
  to occur in growth chambers where external
  factors are controlled
• The rhythm is not paced by a known environmental
  variable
• Bean Plants (Figure 39.2)

19
(No Transcript)
20
Desynchronized with the Natural Environment

• If left in darkness, the bean plant will still
  rise and lowers its leaves, but not at the exact
  time as under natural conditions
• Similar to jetlag

21
Physiological Response to the length of light
Photoperiodism

• Short Day Plant
• Require few hours of light in order to flower
• Poinsettias (late summer, fall, winter)
• Long Day Plant
• Longer than a certain of light hours
• Spinach, iris (spring, early summer)
• Day Neutral Plant
• Flowers when mature, not dependent on light
• Tomatoes, rice, dandelions

22
Wait, Its not Day LengthIts Night Length!

• Short Day Plant is actually a Long Night Plant
  and vice versa
• Plants require a certain of continuous hours in
  darkness in order to produce flowers

23
Chrysanthemum (Short Day)

• Usually flower in the fall
• The floriculture industry has prolonged this
  blooming by interrupting each night with a flash
  of light
• Therefore, one long night equals two short nights
• Flowering is held off until Mothers Day in May

24
Other Environmental Stimuli

• Gravitropism response to gravity
• Roots have positive response
• Shoots have negative response
• Auxins play a role in this tropism

25

• Thigmomorphogenesis
• Changes in form of plant as a result of
  mechanical stress
• Mechanical stimulation activates signal
  transduction, increases calcium, activates genes,
  which affects the cell wall properties
• Strong wind stockier trunk

26

• Thigmotropism
• Growth in response to touch
• Vines use other structures to their advantage in
  an effort to reach the top of the canopy

27
How do plants respond to abiotic stresses?

• Pages 825 827
• Drought, flooding, salt, heat, cold

28
How do plants respond to biotic stresses?

• Page 827 829
• Herbivores and pathogens (bacteria, viruses,
  fungus)