1. Plant Growth and Development
Chapter 31

2. Overview: The Race to Live
Young seedlings must outcompete their neighbors in the race for resources in order to survive
Unlike animals, which respond through movement, plants must respond to environmental challenges by altering their growth and development 2

3. Hormones and Development
Chemical signals that modify or control one or more specific physiological processes within a plant
Help to coordinate growth, development and responses to environmental stimuli
Produced in very low concentrations
a minute amount can greatly affect growth and development of a plant organ
Synthesized or stored in one part of the plant
Travel within phloem, xylem or cell to cell

4. Plant Hormones Auxins Cytokinins Gibberellins Brassinosteroids
Abscisic Acid
Ethylene

5. Auxins
Auxin’s effects

6. Auxins Promote stem lengthening (elongation)
Play a role in responses to gravity and light
Indoleacetic acid (IAA) is the most common auxin in nature
Produced in shoot tips and is transported down the stem
Certain synthetic auxins are used as herbicides

7. Auxins The role of auxin in cell elongation
Polar transport of auxin stimulates proton pumps in the plasma membrane
Proton pumps lower the pH in the cell wall, activating expansins
enzymes that loosen the wall’s fabric
with the cellulose loosened, the cell can elongate 7

8. 4 3 H 2 H H H H H H H 1 ATP H 5
CELL WALL 4
Cell wall-loosening
enzymes cleave
cross-linking
polysaccharides. 3
Low pH
activates
expansins.
H2O
Cell
wall H
Plasma
membrane 2
Acidity
increases. H H H H H H H
Figure 31.5 Cell elongation in response to auxin: the acid growth hypothesis 1
Proton
pump
activity
increases.
Nucleus
Cytoplasm
Plasma membrane
Vacuole
ATP H 5
Sliding cellulose microfibrils
allow cell to elongate.
CYTOPLASM 8

9. Plant Tropisms
Adjustment of plant growth toward or away from an environmental stimulus
Gravitropism
Phototropism
Thigmotropism

10. Gravity and statolith distribution
Gravitropism
Gravity and statolith distribution

11. Gravitropism
Roots tend to grow toward pull of gravity; shoots grow against it
Gravitational field is sensed via position of statoliths (a type of amyloplast)
Auxin is involved in response; causes asymmetric cell elongation

12. Gravitropism

13. Gravitropism

14. Phototropism
Phototropism

15. Phototropism Change in growth in response to light
Controlled by the flow of auxin produced in the plant tip

16. Phototropism
The coleoptile bends after the auxin diffuses from tip to cells on shaded side
Rays of sunlight strike one side of a coleoptile

18. Thigmotropism Growth in response to contact with a solid object
Allows vines and tendrils to wrap around supports
Cells not on the contact side elongate, causing stem to curl
Auxin and ethylene may be involved

19. Thigmotropism

20. Cytokinins Promote cell division
Work together with auxin to control cell division and differentiation
Most abundant in
roots
shoot meristems
maturing fruits
In mature plants, produced in roots and transported to shoots

21. Cytokinins Anti-aging effects inhibits protein breakdown
stimulates RNA and protein synthesis
Used to artificially extend the shelf life of cut flowers; delays leaf death

22. Gibberellins In nature, gibberellin:
helps seeds and buds break dormancy
makes stems lengthen
Enhances cell elongation and cell division
influences flowering
fruit formation
Works with auxin
Applied by growers to enhance stem length, control ripening

23. Seed Germination
Process by which the plant embryo resumes growth after seed dispersal
Depends upon environmental factors
temperature
soil moisture
oxygen levels

24. Germination
Gibberellin acts in the endosperm’s aleurone, a protein-storing layer
Transcription of the gene for amylase
Sugar monomers released are transported to fuel plant growth

25. Splitting the Seed Coat
Imbibition
water molecules move into a seed
As water moves in, the seed swells and the coat ruptures

26. Aleurone Endosperm Water Radicle Scutellum (cotyledon) GA GA -amylase
Sugar GA GA
Water
Figure 31.7 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
Radicle
Scutellum
(cotyledon) 26

27. Plant Development
Plant development

28. Brassinosteroids
Chemically similar to cholesterol and sex hormones of animals
Induce cell elongation division in stem segments and seedlings
They slow leave abscission and promote xylem differentiation

29. Abscisic Acid (ABA) Causes the suspension of growth
Two of the many effects of ABA:
Plays a role in drought response
ABA accumulation causes stomata to close
Promotes dormancy of buds and seeds
a predictable period of metabolic inactivity
short days; long, cool nights trigger dormancy
used to induce dormancy in plants to be shipped

30. Daylength and dormancy

31. Breaking Dormancy
Seeds and buds respond to environmental cues by resuming growth
ABA is removed
heavy rains, light, prolong cold

32. Ethylene Produced in response to stress drought flooding
mechanical pressure
injury
infection
The effects of ethylene include response to:
mechanical stress
senescence
leaf abscission
fruit ripening

33. Mechanical Stress
Stem elongation and plant growth patterns can be affected by winds or other mechanical stress
trees in windswept areas tend to be stubbier
shaking inhibited growth of tomato plants
The triple response to mechanical stress
slowing of stem elongation
a thickening of the stem
horizontal growth

34. Senescence Senescence
Sum total of processes that lead to death of a plant or some of its parts
Factors that influence senescence:
decrease in daylight is recurring factor
wounds, drought, or nutritional deficiencies can also bring it about

35. control (pods
not removed)
experimental plant
(pods removed)

36. Control of Abscission Abscission
dropping of flowers, fruits, or leaves
What brings it about?
auxin production declines
cells in abscission zone produce ethylene
enzymes digest cell walls that attach leaf or fruit to plant

37. Abscission

38. Table 31.1 Overview of plant hormones38

39. Signal Transduction Pathway of cell communication in plants 3 steps
Reception
Transduction
Response

40. cytoplasm of target cell
Signal Transduction
signal
reception
signal
transduction
cellular response
Activated receptor activates enzymes and other molecules, which enter reactions that induce or inhibit some cellular activity
Hormone binds to and activates cell receptor
cell wall
membrane receptor
cytoplasm of target cell

41. Biological Clocks Internal timing mechanisms
trigger shifts in daily activity
help induce seasonal adjustments
Phytochrome is part of the switching mechanism
pigments that regulate many of a plant’s responses to light throughout its life
responses include seed germination and shade avoidance
blue-green plant pigment

42. Phytochrome conversions

43. Phytochrome
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses
Red light triggers the conversion of Pr to Pfr
Far-red light triggers the conversion of Pfr to Pr
The conversion to Pfr is faster than the conversion to Pr
Sunlight increases the ratio of Pfr to Pr and triggers germination 43

44. Interconversion of Phytochrome
Flowering response experiments

45. Vernalization Low temperature stimulation of flowering
Seasonal responses