Lecture Notes: Plant Reproduction, Bio 102

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Bio 102 Syllabus

Outline: Control Systems in Plants, Chapter 39

I. Reminder: Seed Germination:

Why does stem grow up and roots grow down?

II. Phototropism: Discovery of Plant Hormones

Hormone:

1. A hormone is a chemical secreted by cells in one part of

the body that is transported ... to other parts of the body,
where it affects particular target cells.

IV-A. Plant Hormones: Auxin, Cytokinin, Gibberellin, Abscisic Acid, Ethylene, Brassinolide, Florigens.

IV-B. Mechanism of Action: Signal Transduction

IV-C. Some Specific Examples:

Lecture: Control Systems In Plants; Hormones Chpt. 39

I. Reminder: Seed Germination

Coleoptile

3. QUESTION: why does stem grow up and roots grow down?

II. Phototropism: Discovery of Plant Hormones

A. Phototropism:

1. tropism:

2. Phototropism:

3. Phototropism in Plants:

B. Darwin and Son (Charles and Francis), (England, 1880s):

1. Observation: illuminate grass coleoptiles

2. Experiments: Shine light...

	EXPERIMENT	OBSERVATION
a.	cut tip:	no bend
b.	cover tip opaque cap:	no bend
c.	cover tip clear cap:	bend
d.	cover base, opaque shield:	bend

3. HYPOTHESIS: TIP FACTOR

light sensitive region is in tip, signal is transmitted down

C. Peter Boysen-Jensen (Denmark, 1913) (35.2, p.752)

1. Test Darwins' hypothesis

	EXPERIMENT	OBSERVATION
a.	cut tip:	NO elongation NO bend
b.	replace tip:	elongation in dark bending in light
c.	separate tip and stump with pourous gelatin	elongation and bending
d.	separate tip and stump with non-pourous mica	NO elongation NO bending

2. CONCLUSION: TIP FACTOR

D. Frits Went - (Netherlands, 1920s)

1. Isolation of factor:

2. Named Factor: "auxin" (Greek - "to increase")

E. Kenneth Thimann - Cal Tech - purified - molecular structure.
F. Hormone:

1. A hormone is "a chemical secreted by cells in one part of

the body that is transported ... to other parts of the body,
where it affects particular target cells."

2. act over distance (in animals, neurotransmitters vs. hormones)
3. release site >>>>> target cells

all/many cells in organism exposed, only specific cells sensitive

IV-A. Plant Hormones

BIG 5 (35.1p.753)	ACTION	ACTION
1. Auxin (shoot tip)	cell elongation phototropism/gravitropism inhibit lateral bud sprouting (apical dominance)	vascular tissue, fruit development Retard senescence Stimulate ethylene prod.
2. Cytokinin (roots)	stimulate lateral bud sprouting prevent senescence	promote cell division Stimulate fruit, endosperm &embryo development
3. Gibberellin (meristem, young leaves)	seed & bud germination stem elongation	stimulate flowering fruit development.
4. Abscisic Acid (leaves, stems, green fruit)	close stomata water stress	promotes dormancy in seeds and buds
5. Ethylene (ripe fruit, old leaves)	promotes fruit ripening leaf abscission	inhibits stem elongation

Others... ACTION ACTION

1. Brassinolide
(steroid) apical dominance
growth & fertility light regulated gene expression

2. Florigens
uncharacterized regulate flowering

(regulated by daylength)

IV-B. Mechanism of Action: Signal Transduction

A. "Signal Transduction"

1. Conversion of signal of one form to signal of another form.
2. Example: Radio Speaker: How is Sound Made?

3. Hormones Act on Target Cells (35.19, p. 772)

a. Hormones are External Signals

b. Target Cells Express Specific Receptors for hormone

ONLY CELLS EXPRESSING RECEPTOR SEE HORMONE

- most hormones can not cross cell membrane

act as EXTERNAL SIGNALS

c. Signal Transduction...

Extracellular		Membrane		Intracellular
Hormone (Ligand)	>	Receptor	>	Cytoplasmic Response
"First" Messenger	>	Receptor	>	"Second Messenger"

d. Some possible Cellular Responses: activation of...

⁺

e. Membrane soluble hormones: steroids

f. in general, the cellular mechanisms of most plant hormones
are poorly understood.

g. Auxin: Mode of Action in Cell Elongation (35.4, p. 754; 35.5, p.755)

- synthesized in apical meristem
- unidirectional movement down-stem through parenchyma cells

-- chemiosmosis (35.4, p. 754)

ATP dependent proton pumps, pump out H⁺

⁺

- auxin stimulates (activates) proton pumps (35.5, p. 755)

^-8

^-3

₂

IV-C. Some Specific Examples:

Each hormone has multiple actions
Many hormones act together to effect specific responses.
One hormone may regulate relase of another, and vice versa.
A. AUXIN

1. Synthesized in growing shoot tips - migrates down

2. some actions :

b. Opposite effects in shoot and root

(Apical Dominance)

B. CYTOKININ

1. released from roots (tips?), migrates upwards through xylem
2. some actions:

C. Auxin + Cytokinin: opposing gradients - hormonal interactions

- auxin down from tip

- cytokinin up from root

- where cytokinin exceeds auxin: branching

- parenchyma in culture (see book for more info.)

V. PLANT MOVEMENT

Tropisms

Mechanism of Response = Differential Rate of Cell Elongation along the opposite side of "growth".

Mechanisms of Signal Recognition and Signal Transduction differ for each tropism and differ among plant species.

Phototropism

grass - asymmetrical distribution of auxin
other plants - not auxin, but asymetrical distribution of certain growth inhibitors

Gravitotropism

statoliths, specialized plastids containing dense starch grains - root cap.
Hypothesis: aggrigation of statoliths triggers redistribution of Ca++ stimulating lateral transport of auxin. Accumulation of Ca++ and auxin stimulates elongation.
Experiments on mutant plants (Arabidposis & tobacco) lacking statoliths - plants still show partial gravitotropism - hypothesis is in process of revision (p. 762-763).

Thigmotropism

vines attach tendrils to verticle surfaces - mechanical stimulation
also - wind stimulated for some plants
thigmomorphogenesis - increased production of ethylene in response to chronic mechanical stimulation

Rapid Leaf Movements - Electrophysiology of Plants

Venus Fly Traps

Sensitive plant

VI. PHOTOPERIODISM

"Biological Clocks" common feature of single and multicellular organisms - mechanisms remain poorly understood. Drosophila (insect) and Neurospora (bread mold), mechanism involves transcription factors regulating other transcription factors. Clock in plants not known.

Plants determine season to flower based on Night Length

1920s - mutant tobacco plant would only flower if day length appropriately short (WW Garner & HA Allard)
1940s - Experiments identified plants sensitive to NIGHT LENGTH (not day length) short dark pulse interrupting light period - no effect short light pulse interupting dark period - reveral of effect (NIGHT LENGTH SHORTENED)

MECHANISM OF PHOTOPERIODISM
Phytochromes: the light receptor protein

Homodimers, each with two activities: photodetection activity (chromophore) and kinase activity
Two states: wavelength dependent: Red (660nm) vs. FarRed (730nm)
P_R <--> P_FR
Synthesized as P_R
DAY: light converts some to P_FR
Dynamic equilibrium established between P_R and P_FR based on ration of R vs. FR in environment (e.g. FR>R under forest canopy because of filtering by leaves)
NIGHT: P_FR shifts back to P_R turnover and new synthesis of P_R biochemical conversion of P_R to P_FR
Leaves: the photodetector organ.
Hypothetical pathway controlling Flowering...
1. Light signal "read" by leaf
2. Phytochrome activates kinase signal cascade
3. Day length determined by Phytochrome compared with biological clock
4. Leaf releases signal (factor - hormone(s)?)
5. Factor travels from leaf and activates flowering bud
6. Conversion of vegitative tissue to meristem: expression of Meristem Identification Genes (p. 406-410)
7. Activate Flowering Bud: expression of Organ Identificaiton Genes (p. 406-410)
Phytochromes are not the Biological Clock. However, Phytochromes may entrain the Biological Clock, providing critical "real" information regarding when Dark Period starts and ends.

VI. DEFENSE

ENVIRONMENTAL STRESS
Water Deficit
- low water, reduce transpiration
  turgor drops, stomata guard cells close
  increase synthesis/release of abscisic acid from mesophyll cells >> reinforce stomata closure
- low water, reduce growth (due to turger drop - reduced cell elongation)
  reduce surface area
- wilting often done with shape changes that reduce surface area (turgor)
Oxygen Deprivation:
- Excess soil water >> no needed O₂ in soil.
- Mangroves - modified roots grow upwards to "tap" air.
- Other plants (corn) may modifiy roots forming air tubes.
Salt Stress
- Problems: osmotic water loss, compromise mineral exchange
- Solutions: not many
  - Modest salinity - formation of precipitable solutes
  - Special Halophytes (e.g. Salicornia or Pickleweed): salt glands pump salts out through leaves
Heat Stress
- Problem: inactivation, denaturation of proteins; shift in kinetics of complex metebolic pathways
- Solutions:
  - increase transpiration to increase evaporative cooling
  - Produce Heat Shock Proteins (>40^oC): "Chaparone Proteins", which complex with other proteins stabilizing folding, maintaining activity.
Cold Stress
- Problem: Membrane fluidity decreases. Freezing leads to ice crystal formation which distrupts molecular structures.
- Solution:
  - alter lipid composition (if temperature change is gradual)
  - Freezing adaptations: increase solute in cytosol, maintain freezing only in extracellular spaces. Heat of freezing can act to indirectly keep freezing from spreading.
Herbivory
- Problems: many animals, but especially insects.
- Solutions:
  - Physical "armor" - thorns, etc.
  - Taste deterents, repellents, toxins - inducable
  - Parasite Recruitment
PATHOGENS: Virsues, Bacteria, Fungi
First Line of Defense: Surface
- Easy to pass via Stomata, other breaks
Second Lind of Defense: poorly understood mechanistically - very descriptive
- Genetic / Biochemical Competition between host and pathogen
  - Example: Dr. Vance's Research - Viruses attack tobacco plant. Plant gene produces protein which supresses the expression of foreign proteins; Virus (tobacco mosic virus) has countered by evolving a gene that supresses the plant supressor. And so it goes...
- Antimicrobials: Phytoalexins, PR proteins (pathogenesis related). Some directly attack bacteria; others recruit neighboring cells into response.
  - R-Avr: Host finds a defense (resistance gene-R), pathogen counter attacks (antiresistance gene-Avr)
  - Hypersensitive Response: R may be a membrane receptor and Avr a pathogenic ligand (attached to cell surface). R-activation may induce Hypersensitive Response (HR) - inhanced productions of phytoalexins, PR proteins, etc.
  - local, specific, physically contained
  - Systemic acquired resistance (SAR): local site of infection recruits neighboring tissue.
    - Chemical Signals - alarm hormones released from site of infection and stimulating the production of phytoalexins and PR proteins.
    - Salicylic Acid (a.k.a. aspirin) candidate alarm hormone.

Vocabulary

Dormancy
germination
Coleoptile
phototropism
gravitropism
hormone
auxin
gibberellin
cytokinin
abscisic acid
ethylene
florigen
Brassinolide
apical dominance
biological clock
phytochrome
senescence
statolith
abscission layer
freeze tolerant
draught tolerant
transduction
hormone - ligand
receptor
membrane impermeable
membrane receptor
second messenger
receptor
actions :
membrane ion channels
metabolic processes in cell
gene expression
membrane permeable - cytosolic
pH
negative log [H⁺]
acid / base
proton pump
protonation/deprotonation Tropisms
Phototropism
Gravitotropism
statoliths
Thigmotropism
thigmomorphogenesis
action potential
Photoperiod
Biological Clocks
transcription factor
Phytochromes
P_R
P_FR
Homodimer
chromophore
kinase activity
wavelength
Meristem Identification Genes
Organ Identificaiton Genes
Environmental Stress
turgor
water stress
Oxygen Deprivation
Salt Stress
salt glands
Heat Shock Proteins
Cold Stress
Herbivory
Pathogens
Viruses
Antimicrobials
Phytoalexins
PR proteins
R-Avr
Hypersensitive Response
Systemic Acquired Resistance
Alarm Hormones
Salicylic Acid

Others...	ACTION	ACTION
1. Brassinolide (steroid)	apical dominance growth & fertility	light regulated gene expression
2. Florigens uncharacterized	regulate flowering	(regulated by daylength)