1. Lecture 10 Outline (Ch. 39, 36) I. Plant Hormones
II. Plant Orientation/Shape
III. Plant Timekeeping (?!)
Senescence & Dormancy
“Fast” Responses
Plant transport
A. Water pressure
B. Xylem
C. Phloem
Lecture Concepts

2. Plant Hormones (Plant) Hormone: Chemicals made in one location and
transported to other locations for action
Growth
Reproduction
Movement
Water balance
Dormancy

3. Plant Hormone Overview
Plants respond to stimuli and lead a stationary life
Plants, being rooted to the ground
Must respond to whatever environmental change comes their way
Figure 38.9a Two common types of seed germination

4. Plant Hormones
Five major classes of plant hormones (table 44-1 summary)
Hormone effects depend on
- target cell
- developmental stage of the plant
- amount of hormone
- presence of other hormones

5. Plant Hormones 1. Auxins: Elongation of cells Root elongation
stimulate (low concentrations) inhibit (high concentrations)
Vascular tissues and fruit development
Responses to light (phototropism), gravity (gravitropism),
and touch (thigmotropism)

6. Cell elongation in response to auxin
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Expansin
CELL WALL
Cell wall enzymes
Cross-linking cell wall polysaccharides
Microfibril H+
ATP
Plasma membrane
Plasma
membrane
Cell
wall
Nucleus
Vacuole
Cytoplasm
H2O
4 The enzymatic cleaving of the cross-linking
polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand.
2 The cell wall becomes more acidic.
1 Auxin increases the activity of proton pumps.
5 With the cellulose loosened,
the cell can elongate.
Figure 39.8

7. Other Auxin Stimulated Responses:
Lateral / branching root formation
Promote fruit growth (tomato sprays)
As herbicide, overdose kills eudicots
Auxin is produced:
At the shoot apex, seeds, other actively growing tissues.
In a variety of molecular structures.

8. Plant Hormones 2. Gibberellins:
Stem elongation, flowering, and fruit development
Seed germination and bud sprouting

9. Gibberellins stimulate germination
After water is imbibed, the release of gibberellins from the embryo
Signals the seeds to break dormancy and germinate
Responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm.
Aleurone
Endosperm
Water
cotyledon GA
amylase
Sugar
embryo releases gibberellin as a signal
Nutrients absorbed from the endosperm by the cotyledon are consumed during growth of the embryo into a seedling.
Figure 39.11
Embryo

10. Plant Hormones 3. Cytokinins:
Stimulate cell division and differentiation
Produced in actively growing tissues such as roots, embryos, and fruits
Anti- aging effects.
Inhibit protein breakdown
Stimulate RNA and protein synthesis
Mobilize nutrients from surrounding tissues
(florist sprays)
58 day old cutting:
Genetically engineered to express more cytokinin on right

11. Control of Apical Dominance11
Control of Apical Dominance
Cytokinins and auxins interact in the control of apical dominance
The ability of a terminal bud to suppress development of axillary buds
If the terminal bud is removed
Plants become bushier
“Stump” after removal of apical bud
Lateral branches
Axillary buds
Figure 39.9

12. Plant Hormones 4. Ethylene: Gas at room temperature
Promotes abscission (falling off) of fruits, flowers, and leaves
Required (with auxin) for fruit development

13. 13
Self-Check
Why will these ripe bananas help the green avocados ripen faster?

14. Plant Hormones 5. Abscisic Acid:
Initiates closing stomata in water-stressed plants
Induces and maintains dormancy in buds and seeds
(inhibits gibberellins)

15. Abscisic Acid 15 Two of the many effects of abscisic acid (ABA) are
Seed dormancy
Ensures seeds germinate only when conditions are optimal
Drought tolerance
Closes stomata, decreases shoot growth K+
Mutation that prevents the synthesis of ABA
Why is that one kernel (seed) germinating prematurely? 15

16. Sprouts know where to go
Plant Orientation
Sprouts know where to go
Auxin controls direction of sprouting seedling
Distribution of auxin within shoot and root cells is influenced by gravity and light

17. Plant Orientation Opaque cap over tip.
FIGURE E44-1 Tip doesn't bend in dark

18. Plant Orientation Clear cap over tip. Opaque sleeve over bending
region.
FIGURE E44-2 Tip perceives direction of light

19. Plant Orientation Cells elongate rapidly. Cells elongate slowly.
FIGURE E44-3 Cells elongate on shaded side

20. Plant Orientation Shoot Elongation Root Growth
In shoot, light and gravity cause auxin movement to the lower side
Auxin stimulates elongation of stem cells
Stem bends away from gravity & toward light
Root Growth
Due to gravity, auxin builds up on the lower side of the root
Auxin retards elongation of root cells, and the root bends toward gravity

21. How Do Plants Detect Gravity?
Plant Orientation
How Do Plants Detect Gravity?
Starch-filled plastids
In specialized stem cells and root caps
Orient within cells toward gravity
Changing plastid orientation may trigger high levels of auxin
plastids
cell in
root cap
root 21

22. Plant Timekeeping/Light Detection22

23. Plant Timekeeping/Light Detection23
Plant Timekeeping/Light Detection
Two major classes of light receptors:
Blue-light photoreceptors
stomatal movements
phototropism
Phytochromes – red/far-red receptor
shade avoidance response
photoperiodism
A phytochrome consists of two identical proteins joined
Photoreceptor activity.
Enzyme - kinase activity.
Figure 39.18

24. Plant Timekeeping/Light Detection
Circadian Rhythms
Cyclical responses to environmental stimuli
approximately 24 hours long
entrained to external clues of the day/night cycle
Phytochrome conversion marks sunrise and sunset
Providing the biological clock with environmental cues
Many legumes
Lower their leaves in the evening and raise them in the morning
Pulvinus: tip of petiole/base of blade
Noon
Midnight
Figure 39.20 24

25. Plant Timekeeping/Light Detection
Photoperiodism
Response to time of year (seasons)
Photoperiod - relative lengths of night and day
Triggers many developmental processes
Bud break
Flowering
Leaf drop in deciduous trees
Are actually controlled by night length, not day length
that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod »

26. Plant Timekeeping/Light Detection
Leaves detect lengths of night/day
An internal biological clock
A light-detecting phytochrome
Pigments found in leaves
Active/inactive depending on light conditions
Still-unidentified chemical (florigens)
travel from leaf to bud to either trigger or inhibit flowering 26

27. Cytokinin and auxin production decreases Ethylene production increases
Senescence
Process by which leaves, fruits, and flowers age rapidly
Promoted by changes in hormone levels
Cytokinin and auxin production decreases
Ethylene production increases

28. Senescence Lack of nutrients Lack of sugars
Cut flowers undergo senescence due to:
Reduced water uptake
Lack of nutrients
Lack of sugars

29. Senescence Proteins, starches, and chlorophyll broken down
Products stored in roots and other permanent tissues
Abscission
Ethylene stimulates production of enzyme that digests cell walls at base of petiole
Leaf falls when cells are sufficiently weakened

30. Dormancy
Period of reduced metabolic activity in which the plant does not grow and develop
Maintained by abscisic acid
Dormancy broken by: increased temperature, longer day length occur in the spring

31. Immediate Plant Responses
Plants may produce protective compounds
Plants may summon “bodyguards” when attacked
Plants may warn other plants of attack
Some plants move rapidly

32. Immediate Plant Responses
Chemical Warnings
Volatile chemicals released by plants boost defenses in neighbors
Many virally-attacked plants produce salicylic acid
Activates an immune response
Attacked plant converts salicylic acid to methyl salicylate (wintergreen)  diffuses to air
Absorbed by neighboring healthy plants and reconverted to salicylic acid (aspirin)
Barley and willow, alder, and birch trees warn members of their own species
Sagebrush and lima bean plants warn members of differing species (wild tobacco and cucumber plants, respectively) 32

33. Immediate Plant Responses
Some plants respond to attack by releasing volatile chemicals
Chemicals attract parasitic wasps and predaceous mites that feed on plant predators

34. Immediate Plant Responses
Touch generates an electrical signal
Increases permeability to ions (K+) of “motor cells” at bases of leaflets and petiole
K+ flow out of motor cells; water follows
Motor cells shrink  leaflets and petiole droop
Sensitive plant (mimosa)

35. Immediate Plant Responses
Leaves have sensory “hairs” on inside
Fly triggers hairs - generates signal
Cells in outer leaf epidermis pump H into cell walls
Enzymes activated cells absorb water
Outer epidermal cells expand, close leaf
Reopening leaves takes several hours
Venus fly trap

36. Self-Check Hormone Name Functions Auxin Gibberellin Cytokinin Ethylene
Abscisic Acid

37. Transport in vascular plants occurs on three scales
Transport in Plants
Physical forces drive the transport of materials in plants over a range of distances
Transport in vascular plants occurs on three scales
Transport of water and solutes by individual cells, such as root hairs
Short-distance transport of substances from cell to cell at the levels of tissues and organs
Long-distance transport within xylem and phloem at the level of the whole plant 37

38. Osmosis : movement of water
Transport in Plants
To survive
Plants must balance water uptake and loss
Osmosis : movement of water
Water potential : measure water movement due to solute concentration & pressure
designated as psi (ψ)
Water flows from regions of high water potential to regions of low water potential

39. Transport in Plants The solute potential (ψs) of a solution
Pressure potential (ψp)
Is the physical pressure on a solution
Therefore, the water potential equation is:
Ψ = ψs + ψp
psi is measured in megapascals (MPa)
0 MPa = the water potential of pure water in a container open to the atmosphere »

40. The addition of solutes
Transport in Plants
The addition of solutes
Reduces water potential (Figure 36.8)»
0.1 M
solution
H2O
Pure
water
P = 0
S = 0.23
 = 0.23 MPa
 = 0 MPa
(a)
Figure 36.8a

41. Application of physical pressure
Transport in Plants
Application of physical pressure
Increases water potential »
H2O
P =
S = 0.23
 = 0 MPa
 = 0 MPa
(b)
H2O
P =
S = 0.23
 = MPa
 = 0 MPa
(c)
Figure 36.8b, c

42. Transport in Plants Water potential
Affects uptake and loss of water by plant cells
If a flaccid cell is placed in an environment with a higher solute concentration
The cell will lose water and become plasmolyzed (Figure 36.9)»
Figure 36.9a
0.4 M sucrose solution:
Initial flaccid cell:
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
P = 0
S = 0.7
S = 0.9
 = 0.9 MPa
 = 0.7 MPa

43. Transport in Plants
If the same flaccid cell is placed in a solution with a lower solute concentration
The cell will gain water and become turgid »
Distilled water:
Initial flaccid cell:
Turgid cell
at osmotic equilibrium
with its surroundings
P = 0
S = 0.7
S = 0
P = 0.7
Figure 36.9b
 = 0.7 MPa
 = 0 MPa
 = 0 MPa
These process occur in cells that can affect tissues, which can in turn affect the entire plant (e.g., wilting)

44. 44 Uses of turgor pressure: Inexpensive cell growth
Hydrostatic skeleton
Phloem transport

45. Transport in Plants 45 Water molecules are attracted to:
Each other (cohesion)
Solid surfaces (adhesion)

46. cytoplasmic continuum apoplast
Transport in Plants
Most plant tissues
cell walls and cytosol are continuous from cell to cell
cytoplasmic continuum
called the symplast
apoplast
continuum of cell walls
Plus extracellular spaces »

47. 47
Transport in Plants
How do water and minerals get from the soil to the vascular tissue?
Apoplastic
Symplastic
transmembrane

48. Transport in Plants 48 Symplast Apoplast Endodermis  Xylem
What happens to psi between soil and endodermis?
Where is osmosis occurring?

49. 49
Transport in Plants
Transpiration = loss of water from the shoot system to the surrounding environment.
What drives water loss?

50. 50
Transport in Plants
Tension in water in mesophyll cell walls

51. 51 Bulk Flow = movement of fluid due to pressure gradient
Transpiration drive bulk flow of xylem sap.
Water is PULLED up a plant.
Ring/spiral wall thickening protects against vessel collapse

52. Xylem Sap Ascent by Bulk Flow: A Review
The movement of xylem sap against gravity
Is maintained by the transpiration-cohesion-tension mechanism
Stomata help regulate the rate of transpiration
Leaves generally have broad surface areas
And high surface-to-volume ratios
Both of these characteristics
Increase photosynthesis
Increase water loss through stomata

53. 53
H+ pumped out
K+ flow in
H2O flow in
stomata open
Stomata Control
Why?
Why?
K-channels, aquaporins and radially oriented cellulose fibers play important roles.

54. 54
Transport in Plants
What happens if rate of transpiration nears zero?
Guttation

55. Transport in Plants 55 Direction is source to sink
Near source to near sink
Adjacent sieve tubes can flow in different directions.
Are tubers and bulbs sources or sinks?
Phloem tissue

56. Transport in Plants 56 Phloem sap composition: Sugar (mainly sucrose)
amino acids
hormones
minerals
enzymes
Phloem sap under positive pressure

57. In studying angiosperms
Pressure Flow
Vessel
(xylem)
H2O
Sieve tube
(phloem)
Source cell
(leaf)
Sucrose
Sink cell
(storage
root) 1
Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the
sieve-tube members. This causes the tube to take up water by osmosis. 2 4 3
This uptake of water generates a positive pressure that forces the sap to flow along
the tube.
The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink.
In the case of leaf-to-root
translocation, xylem recycles water from sink
to source.
Transpiration stream
Pressure flow
Figure 36.20
In studying angiosperms
Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure

58. Lecture 10 concepts
Name the five major plant hormones & list two roles for each one.
Explain how plants get their roots to grow down and their shoots to grow up.
Define: thigmotropism, phototropism, gravitropism
What happens (hormonally) if you cut the growing top off of a plant? What shape does the plant take?
How does day length relate to flowering?
Define senescence. What happens (hormones) to cause leaf senescence?
Give two examples (be specific) of how plants can “quickly” respond to their environment.
Describe fluid movement in plants: local, bulk transport.
How is water balance regulated in plants?
How is water transported? From where to where?
How are sugars transported? From where to where?