Outline: Control Systems in Plants, Chapter 35

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.

V. Mechanism of Action: Signal Transduction

VI. Some Specific Examples:

A. AUXIN
B. CYTOKININ
C. Auxin + Cytokinin: opposing gradients - hormonal interactions

Lecture: Control Systems In Plants; Hormones Chpt. 35

I. Reminder: Seed Germination

1. Embryo absorbs water, swells, bursts open seed coat.
2. Shoot tip pushes up through soil -

- Monotcots (e.g. grass): Coleoptile : cap protecting apical meristem
- Dicots: embryonic stem bends and the bend leads, protecting tip

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

II. Phototropism: Discovery of Plant Hormones

A. Phototropism:

1. tropism: Directional Growth (gravi-, thigmo-, photo-)

(growth can be by cell division or by change in cell shape)

2. Phototropism: Growth toward or away from light

(positive or negative phototropism).

3. Phototropism in Plants: (35.1 p. 751)

Stem bends toward light: elongation of cells away from light
Assymetrical, physiological response.

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

1. Observation: illuminate grass coleoptiles (35.1, p.751)

a. bend towards light
b. bend is few mm. down from tip.

2. Experiments: Shine light... (35.2, p.752)


	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

- in dark, chemical diffuses down
- in light, some asymmetry somehow induced

D. Frits Went - (Netherlands, 1920s)

1. Isolation of factor: (35.3, p. 752)

a. Place Cut Tips on agar for few hours
b. Chemical transferred to agar - agar induces stump response

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

4. all multicellular organisms
5. chemical signals that coordinate the parts of the organism

IV. Plant Hormones

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

V. 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?

a. Electrical Signal In
b. Sound Signal out
c. Speaker is a transducer: converts electrical signal into sound signal

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

a. Hormones are External Signals

- hormones are released from specific locations
- circulate around organism

b. Target Cells Express Specific Receptors for hormone

- all cells are exposed to hormone
- if a cell has a receptor for hormone, cell can "see" 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...

i. membrane bound ion channel (fast, short lasting) (e.g. H⁺ pumps)
ii. specific cytoplasmic metabolic pathways
iii. genes - regulation of gene expression (slow, long lasting)

e. Membrane soluble hormones: steroids

- bind to intracellular receptor protein
- Typically activate/regulate gene expression

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⁺


cell outside more acidic (low pH) than inside
cell outside + relative to inside (-)

external auxin, neutral charge at low external pH,

auxin easily diffuses across membrane into cell

internal auxin, negative charge at high internal pH (loses H⁺)

charged auxin CAN NOT diffuse across membrane
BUT charge drives auxin out of cell via transport proteins
transport proteins ONLY located down-stem end of cell

thus, unidirectional "polar transport" of auxin down-stem

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

-- concentration dependent (10^-8 - 10^-3 M)
-- proton pumps distributed lateral cell membrane
-- H+ pumped out lowers pH, activates cellulase enzyme
-- cellulase locally degrades cellulose
-- ungirdled cell swells (uptake of H₂O by osmosis)

and elongates along long axis.

VI. 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

Produced in response to gravity and/or light

2. some actions :

a. Cell elongation (growing tip, root)
b. Opposite effects in shoot and root

Inhibits axillary / lateral buds in shoots (Apical Dominance)
Stimulates growth in roots.

B. CYTOKININ

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

a. stimulates sprouting of axial (lateral) buds (competing with auxin)
b. promotes cell division

C. Auxin + Cytokinin: opposing gradients - hormonal interactions

- auxin down from tip

--suppresses axillary / lateral buds from forming branches

apical dominance

-- auxin influence falls with distance (concentration dependent)

as shoot elongates, auxin inhibition of lateral buds decreases

-- different cellular responses in root cells than in shoot cells

- cytokinin up from root

-- stimulates bud sprouting in shoots

- where cytokinin exceeds auxin: branching

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


no hormone: no growth
auxin alone: no growth
[cytokinin] + [auxin]: cells divide, form mass

[cytokinin] > [auxin]: shoot buds develop from mass
[auxin] > [cytokinin]: roots develop from mass

Vocabulary