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Lecture 10 Outline (Ch. 39, 36)

I. Plant Hormones

II. Plant Orientation/Shape

III. Plant Timekeeping (?!)

IV. Senescence & Dormancy

V. “Fast” Responses

VI. Plant transport

A. Water pressure

B. Xylem

C. Phloem

IV. Lecture Concepts

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(Plant) Hormone: Chemicals made in one location and transported to other locations for action

Plant Hormones

Growth

Reproduction

Movement

Water balance

Dormancy

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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

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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

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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)

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Expansin

CELL WALL

Cell wallenzymes

Cross-linkingcell wallpolysaccharides

Microfibril

H+ H+

H+

H+

H+

H+

H+

H+

H+

ATP Plasma membrane

Plasmamembrane

Cellwall

NucleusVacuole

Cytoplasm

H2O

Cytoplasm

Cell elongation in response to auxin

Figure 39.8

1 Auxinincreases the

activity ofproton pumps.

4 The enzymatic cleavingof the cross-linkingpolysaccharides allowsthe microfibrils to slide.The extensibility of thecell wall is increased. Turgorcauses the cell to expand.

2 The cell wallbecomes more

acidic.

5 With the cellulose loosened,the cell can elongate.

3 Wedge-shaped expansins, activatedby low pH, separate cellulose microfibrils fromcross-linking polysaccharides. The exposed cross-linkingpolysaccharides are now more accessible to cell wall enzymes.

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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.

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Plant Hormones

2. Gibberellins:

• Stem elongation, flowering, and fruit development

• Seed germination and bud sprouting

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• After water is imbibed, the release of gibberellins from the embryo– Signals the seeds to break dormancy and germinate

Gibberellins stimulate germination

Responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients inthe endosperm.

AleuroneEndosperm

Water

cotyledon

GAGA

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

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Plant Hormones3. Cytokinins:

Anti- aging effects.

• Inhibit protein breakdown

• Stimulate RNA and protein synthesis

• Mobilize nutrients from surrounding tissues

58 day old cutting:Genetically engineered to express more cytokinin on right

(florist sprays)

• Stimulate cell division and differentiation

• Produced in actively growing tissues such as roots, embryos, and fruits

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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

Figure 39.9

Axillary buds

“Stump” afterremoval ofapical bud

Lateral branches

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Plant Hormones4. Ethylene:

• Gas at room temperature• Promotes abscission (falling

off) of fruits, flowers, and leaves

• Required (with auxin) for fruit development

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Why will these ripe bananas help the green avocados ripen faster?

13Self-Check

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Plant Hormones

5. Abscisic Acid:

• Initiates closing stomata in water-stressed plants• Induces and maintains dormancy in buds and seeds

– (inhibits gibberellins)

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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

Abscisic Acid

Why is that one kernel (seed) germinating prematurely?

K+

K+

K+

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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

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Opaque capover tip.

Plant Orientation

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Clear capover tip.

Opaque sleeveover bendingregion.

Plant Orientation

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Cellselongateslowly.

Cellselongaterapidly.

Plant Orientation

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Plant OrientationShoot Elongation

• 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

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

Root Growth

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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 inroot cap

root

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Plant Timekeeping/Light Detection

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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

23Plant Timekeeping/Light Detection

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Many legumes– Lower their leaves in the evening

and raise them in the morning

Noon Midnight

Circadian Rhythms

Figure 39.20

• 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

Plant Timekeeping/Light Detection

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• 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 »

Photoperiodism

Plant Timekeeping/Light Detection

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• 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

Plant Timekeeping/Light Detection

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Senescence

• Process by which leaves, fruits, and flowers age rapidly

– Promoted by changes in hormone levels

• Cytokinin and auxin production decreases• Ethylene production increases

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Senescence

• Cut flowers undergo senescence due to:

–Reduced water uptake

•Lack of nutrients

•Lack of sugars

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• Proteins, starches, and chlorophyll broken down– Products stored in roots and

other permanent tissues

Senescence

Abscission

Ethylene stimulates production of enzyme that digests cell walls at base of petiole

Leaf falls when cells are sufficiently weakened

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• Period of reduced metabolic activity in which the plant does not grow and develop

Dormancy

Maintained by abscisic acid

Dormancy broken by: increased temperature, longer day length occur in the spring

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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

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Chemical Warnings

• Volatile chemicals released by plants boost defenses in neighbors

• Many virally-attacked plants produce salicylic acid– Activates an immune response

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• Attacked plant converts salicylic acid to methyl salicylate (wintergreen) diffuses to air– Absorbed by neighboring healthy

plants and reconverted to salicylic acid (aspirin)

Immediate Plant Responses

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• Some plants respond to attack by releasing volatile chemicals

Immediate Plant Responses

• Chemicals attract parasitic wasps and predaceous mites that feed on plant predators

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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)

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• 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

Immediate Plant Responses

Venus fly trap

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Self-Check

Hormone Name Functions

Auxin

Gibberellin

Cytokinin

Ethylene

Abscisic Acid

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Physical forces drive the transport of materials in plants over a range of distances

Transport in vascular plants occurs on three scales

1. Transport of water and solutes by individual cells, such as root hairs

2. Short-distance transport of substances from cell to cell at the levels of tissues and organs

3. Long-distance transport within xylem and phloem at the level of the whole plant

Transport in Plants

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• 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

Transport in Plants

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• 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 »

Transport in Plants

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• The addition of solutes– Reduces water potential (Figure 36.8)»

Figure 36.8a

0.1 Msolution

H2O

Purewater

P = 0

S = 0.23

= 0.23 MPa = 0 MPa

(a)

Transport in Plants

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• Application of physical pressure– Increases water potential »

H2O

P = 0.23

S = 0.23

= 0 MPa = 0 MPa

(b)

H2O

P = 0.30

S = 0.23

= 0.07 MPa = 0 MPa

(c)

Figure 36.8b, c

Transport in Plants

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• 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 cellat osmotic equilibriumwith its surroundings

P = 0

S = 0.7

P = 0

S = 0.9

P = 0

S = 0.9

= 0.9 MPa

= 0.7 MPa

= 0.9 MPa

Transport in Plants

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• 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 cellat osmotic equilibriumwith its surroundings

P = 0

S = 0.7

P = 0

S = 0

P = 0.7

S = 0.7

Figure 36.9b

= 0.7 MPa

= 0 MPa

= 0 MPa

Transport in Plants

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44Uses of turgor

pressure:

• Inexpensive cell growth

• Hydrostatic skeleton

• Phloem transport

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Water molecules are attracted to:• Each other (cohesion)• Solid surfaces (adhesion)

Transport in Plants

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Most plant tissues–cell walls and cytosol are continuous

from cell to cellcytoplasmic continuum

– called the symplastapoplast

– continuum of cell walls– Plus extracellular spaces »

Transport in Plants

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How do water and minerals get from the soil to the vascular tissue?

• Apoplastic• Symplastic• transmembrane

Transport in Plants

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Symplast Apoplast Endodermis Xylem

What happens to psi between soil and endodermis?

Where is osmosis occurring?

Transport in Plants

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Transpiration = loss of water from the shoot system to the surrounding environment.

What drives water loss?

Transport in Plants

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Tension in water in mesophyll cell walls

Transport in Plants

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51Bulk 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

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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

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53Stomata ControlH+ pumped out

K+ flow in

H2O flow in

stomata open

Why?

Why?

K-channels, aquaporins and radially oriented cellulose fibers play important roles.

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What happens if rate of transpiration nears zero?

• Guttation

Transport in Plants

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Phloem tissue

• 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?

Transport in Plants

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Phloem sap composition:

• Sugar (mainly sucrose)• amino acids• hormones• minerals• enzymes

Phloem sap under positive pressure

Transport in Plants

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Vessel(xylem)

H2O

H2O

Sieve tube(phloem)

Source cell(leaf)

Sucrose

H2O

Sink cell(storageroot)

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Sucrose

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

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1

2

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 tubeat the sink.

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In the case of leaf-to-roottranslocation, xylem recycles water from sinkto source.

Tra

nsp

irat

ion

str

eam

Pre

ssu

re f

low

Figure 36.20

Pressure Flow

• In studying angiosperms

– Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure

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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?