1. © 2017 Pearson Education, Inc.

2. Stimuli and a Stationary Life
Plants receive signals from the environment and respond by altering growth and development
For example, the bending of a seedling toward a host plant occurs in response to chemicals released by the host
© 2017 Pearson Education, Inc.

3. Concept 39.1: Signal transduction pathways link signal reception to response
A potato left growing in darkness produces pale stems, unexpanded leaves, and short 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
A potato’s response to light is an example of cell signal processing
The stages are reception, transduction, and response
© 2017 Pearson Education, Inc.

4. Before exposure to light After a week’s exposure to natural daylight
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes
Before exposure to light
After a week’s exposure to natural daylight
© 2017 Pearson Education, Inc.

5. 1 2 3 Activation of cellular responses
WALL
CYTOPLASM 1
Reception 2
Transduction 3
Response
Receptor
Relay proteins and
Activation
of cellular
responses
second messengers
Figure 39.3 Review of a general model for signal transduction pathways
Hormone or
environmental
stimulus
Plasma membrane
© 2017 Pearson Education, Inc.

6. Reception
Signals are detected by receptors, proteins that change in shape in response to specific stimuli
In de-etiolation, the receptor is a phytochrome capable of detecting light
© 2017 Pearson Education, Inc.

7. 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:
Calcium ions (Ca2+)
Cyclic GMP (cGMP)
© 2017 Pearson Education, Inc.

8. 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
© 2017 Pearson Education, Inc.

9. 1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM
NUCLEUS
Plasma
membrane
cGMP
Protein
kinase 1 P
Second
messenger
produced
Transcription
factor 2
Phytochrome
activated
by light P
Cell
wall
Protein
kinase 2
Transcription
Light
Translation
Figure 39.4_3 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response (step 3)
De-etiolation
(greening)
response
proteins
Ca2+ channel
opened
Ca2+
© 2017 Pearson Education, Inc.

10. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant
Plant hormones are also called plant growth regulators
© 2017 Pearson Education, Inc.

11. Plant hormones help coordinate growth, development, and responses to stimuli
Plant hormones are 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
© 2017 Pearson Education, Inc.

12. A Survey of Plant Hormones
The major plant hormones include
Auxin
Cytokinins
Gibberellins
Abscisic acid
Ethylene
Brassinosteroids
Jasmonates
Strigolactones
© 2017 Pearson Education, Inc.

13. 1. Auxin
Any response resulting in curvature of organs toward or away from a stimulus is called a tropism
The term auxin refers to any chemical that promotes elongation of coleoptiles (growing tips)
Auxin also alters gene expression and stimulates a sustained growth response
© 2017 Pearson Education, Inc.

14. The Role of Auxin in Cell Elongation
According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane
Proton pumps move H+ into the cell, lowering the pH
Reduced pH activates expansins, enzymes that loosen the fabric of the cell wall
Osmotic uptake of water into the cell increases turgor pressure
Increased cell wall plasticity combined with increased turgor pressure enable the cell to elongate
© 2017 Pearson Education, Inc.

15. Cell wall before auxin activates proton pumps
Loosening of cell wall, enabling elongation
Cellulose
microfibrils 1
Auxin increases
activity of proton
pumps. 2
Low pH activates
expansins (red).
Elongation
PLANT CELL
WALL H+ H+ H+ H+
Nucleus
Vacuoles H+ H+ H+ H+ H+ H+ H+
Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis H+ H+
Cellulose microfibril
Cross-linking
polysaccharide 3
Polysaccharides are cleaved by cell wall-loosening
enzymes (purple), loosening the microfibrils (brown).
© 2017 Pearson Education, Inc.

16. Practical Uses for Auxins
The auxin stimulates adventitious roots and is used in vegetative propagation of plants by cuttings
Synthetic auxins used in herbicides such as 2,4-D kill eudicots by causing a hormonal overdose;
The plants (weeds) “grow” themselves to death
Monocots (grass) are able to inactivate these hormones
© 2017 Pearson Education, Inc.

17. 2. Cytokinins
So named because they stimulate cytokinesis (cell division)
Produced in actively growing tissues such as roots, embryos, and fruits
Work together with auxin to control cell division and differentiation
Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
© 2017 Pearson Education, Inc.

18. 2. Cytokinins Control of Apical Dominance
Apical dominance is a terminal bud’s ability to suppress development of axillary buds
It is under the control of sugar, cytokinins, auxin, and strigolactone
Removal of the apical bud increases sugar availability and decreases auxin and strigolactone levels, initiating axillary bud growth
© 2017 Pearson Education, Inc.

19. Plant with apical bud intact Plant with apical bud removed
Topmost axillary buds grow
and take over as the new
apical bud.
Apical bud
Figure 39.8 Effects on apical dominance of removing the apical bud
Limited growth
of axillary buds
Plant with apical
bud intact
Plant with apical
bud removed
© 2017 Pearson Education, Inc.

20. 3. Gibberellins
Variety of effects, such as stem elongation, fruit growth, and seed germination
Produced in young roots and leaves
Stimulate growth of leaves and stems by enhancing cell elongation and cell division
In many plants, both auxin and gibberellins must be present for fruit to develop
Gibberellins are used in spraying of Thompson seedless grapes
© 2017 Pearson Education, Inc.

21. Rosette form (left) and gibberellin-induced bolting (right)
Grapes from control vine
(left) and gibberellin-treated
vine (right)
Figure 39.9a Effects of gibberellins on stem elongation and fruit growth (part 1: bolting)
Rosette form (left) and gibberellin-induced bolting (right)
© 2017 Pearson Education, Inc.

22. Germination
After water is imbibed, release of gibberellins from the embryo signals seeds to germinate
Aleurone
Endosperm
α-amylase
Sugar GA
Water
Scutellum
(cotyledon)
Radicle 3 2 1
Figure Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
© 2017 Pearson Education, Inc.

23. 4. Abscisic Acid
(ABA) slows growth, often by antagonizing the actions of growth hormones and effects seed dormancy and drought tolerance
Seed dormancy increases the likelihood that the seed will germinate only in optimal conditions
Many dormant seeds germinate when ABA is removed or inactivated
The ratio of ABA to gibberellins often affects whether seeds will break dormancy
© 2017 Pearson Education, Inc.

24. 4. Abscisic Acid Drought Tolerance
ABA is the primary internal signal that enables plants to withstand drought
ABA accumulation in wilting leaves causes stomata to close rapidly
Transport of ABA from water-stressed root systems to leaves can act as an “early warning system”
© 2017 Pearson Education, Inc.

25. 5. 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 (cells dying)
leaf abscission (leaves falling)
fruit ripening
© 2017 Pearson Education, Inc.

26. 5. Ethylene
Ethylene is produced when a seedling tip pushes against an obstacle
The production of ethylene induces a triple response:
stem elongation is slowed
stem thickens
stem begins to grow horizontally
Vertical growth resumes when the effects of the ethylene wear off
© 2017 Pearson Education, Inc.

27. Ethylene concentration (parts per million)
Figure The ethylene-induced triple response
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
© 2017 Pearson Education, Inc.

28. Senescence and Leaf Abscission
5. Ethylene
Senescence and Leaf Abscission
Senescence: the programmed death of certain cells or organs or entire plants
A burst of ethylene is associated with the onset of apoptosis, programmed cell death
Leaf Abscission: A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls
© 2017 Pearson Education, Inc.

29. 5. Ethylene Fruit Ripening
In many cases, 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
© 2017 Pearson Education, Inc.

30. 6. Brassinosteroids
Chemically similar to cholesterol and the sex hormones of animals
They induce cell elongation and division in stem segments and seedlings at low concentration
They slow leaf abscission and promote xylem differentiation
© 2017 Pearson Education, Inc.

31. 7. Jasmonates
They are produced in response to wounding and are involved in controlling plant defenses
Jasmonates also regulate many other physiological processes, including:
Nectar secretion Fruit ripening
Pollen production Flowering time
Seed germination Root growth
Tuber formation Tendril coiling
© 2017 Pearson Education, Inc.

32. 8. Strigolactones Strigolactones are xylem-mobile chemicals that
Stimulate seed germination
Suppress adventitious root formation
Help control apical dominance
© 2017 Pearson Education, Inc.

33. Circadian rhythms
Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”
These cycles can be free-running, varying from 21 to 27 hours, when organisms are kept in a constant environment
The 24-hour period arises from the transcription of “clock genes” regulated through negative-feedback loops
© 2017 Pearson Education, Inc.

34. A Flowering Hormone?
Photoperiod is detected by leaves, which cue buds to develop as flowers
The flowering signal molecule is called florigen
© 2017 Pearson Education, Inc.

35. Gravity 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
© 2017 Pearson Education, Inc.

36. Mechanical Stimuli
The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance
For example, rubbing stems of young plants a couple of times daily results in plants that are shorter than controls
© 2017 Pearson Education, Inc.

37. Mechanical Stimuli Thigmotropism is growth in response to touch
It occurs in vines and other climbing plants
Some plants undergo rapid leaf movements in response to mechanical stimulation
For example, Mimosa pudica folds its leaflets and collapses in response to touch
The touch response results from the transmission of electrical impulses called action potentials
© 2017 Pearson Education, Inc.

38. Mechanical Stimuli (a) Unstimulated state (leaflets spread apart)
Figure Rapid turgor movements by the sensitive plant (Mimosa pudica)
(a) Unstimulated state (leaflets spread apart)
(b) Stimulated state (leaflets folded)
© 2017 Pearson Education, Inc.

39. Environmental Stresses
Environmental stresses have a potentially adverse effect on survival, growth, and reproduction
Stresses can be biotic (living) or abiotic (nonliving)
Biotic stresses include herbivores and pathogens
Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress
© 2017 Pearson Education, Inc.

40. Drought
Plants may wilt or die when water loss by transpiration exceeds water absorption
During drought, plants reduce transpiration by closing stomata, reducing exposed surface area, and in some species, shedding leaves
© 2017 Pearson Education, Inc.

41. Flooding
Waterlogged soils lack the air spaces needed to provide oxygen for cellular respiration in roots
Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding
Some plants, such as mangroves, also produce aerial roots
© 2017 Pearson Education, Inc.

42. Salt Stress
Sodium and some other ions are toxic to plants in high concentrations
Salt can also lower the water potential of the soil solution and reduce water uptake
Plants respond to salt stress by producing solutes tolerated at high concentrations
This process keeps the water potential of cells more negative than that of the soil solution
© 2017 Pearson Education, Inc.

43. Heat Stress Excessive heat can denature a plant’s enzymes
Transpiration helps cool leaves by evaporative cooling
Heat-shock proteins are produced at temperatures above 40ºC to help protect other proteins from heat stress
© 2017 Pearson Education, Inc.

44. Cold Stress
Cold temperatures decrease membrane fluidity altering lipid composition of
Ice formation during freezing reduces water potential outside the cell
Plants, and many other organisms, have antifreeze proteins that hinder the formation of ice crystals
Frost tolerance can be increased in some crop plants by engineering antifreeze genes into their genomes
© 2017 Pearson Education, Inc.

45. Concept 39.5: Plants respond to attacks by pathogens and herbivores
1. Cellular-Level Defenses
Cells may be specialized to store chemical deterrents, or produce irritants
2. Organ-Level Defenses
Leaves can be modified into spines and bristles
Some species have leaves that appear partially eaten; others have structures that mimic insect eggs
3. Community-Level Defenses
Some plants “recruit” predatory animals that help defend against specific herbivores
© 2017 Pearson Education, Inc.