Signaling

1
Signaling
2
Signaling (hormones, light, etc)
Reception
Transduction
Response
Signal
No response
Receptor
Relay proteins
Signal
Receptor
Differential gene expression
3
Major signals that control plant growth and
development

• Environmental signals
• - Light
• - Gravity
• - Temperature
• - Humidity
• - etc

4
Major signals that control plant growth and
development

• Internal signals Plant Hormones
• - AUXIN
• - CYTOKININ
• - ETHYLENE
• - ABSCISIC ACID
• - GIBBERELLIC ACID

5
Auxin
Note several different auxins are known to date
(natural as well as synthetic). IAA is the most
common natural auxin found in plants.
6
Auxin effects

• - promotes cell elongation
• - inhibits lateral meristem activity
• - promotes root formation

7
Auxin and differential growthGravitropic growth
responses of Arabidopsis seedlings
Cotyledons (embryonic leaves)
Turn seedling 90o
Hypocotyl (embryonic stem)
Root
Hypocotyl shows a negative gravitropic response
Root shows a positive gravitropic response
Areas of differential growth (one side grows
faster than the other)
8
Differential growth
a
Rate of cell elongation is higher on the a-side
of the coleoptile compared to the b-side. This
leads to differential growth increased growth
rate on one side of plant organ, results in
curvature of the organ.
b
9
Auxin and shoot apical dominance

• Decapitation of the apical bud releases the
  lateral buds. In the absence of auxin coming from
  the shoot apex, lateral buds become active
  leading to branching (and a more bushy shoot
  development)

10
Example Auxin and lateral root formation in
Arabidopsis
The synthetic auxin 2,4-D promotes lateral root
formation in Arabidopsis
Note 2,4-D is also used as a herbicide because
it completely inhibits growth at higher
concentrations.
11
Example Auxin promotes adventitious root
formation from Ilex opaca (Holly) shoots.
Shoots form roots at their bases faster when the
bases are treated with auxin. The ends of these
shoots were dipped for 5 seconds in solutions
containing (from left to right) 0, 0.1 and 0.5
auxin. They were then rooted in moist vermiculite
for 2 weeks.
12
Cytokinin
Zeatin
Zeatin is one of many natural cytokinins found in
plants
13
Cytokinin effects

• - promotes cell division/shoot formation
• - promotes lateral meristem activity
• - controls sink/source identity of plant organs
• - delays senescence

14
auxin
cytokinin
15
Cytokinin and shoot apical dominance

• By increasing the cytokinin concentration in the
  shoot, lateral buds become active resulting in
  increased branching (and a more bushy shoot
  development)

Cytokinin
16
The effect of cytokinin on senescence. Cytokinin
applied to the right-hand primary leaf of this
bean seedling inhibited its senescence. The
left-hand did not get cytokinin.
Fig. 15-13, p. 246
17
Gibberellin
Gibberellic acid 3
Note several different gibberellins are known to
date (natural as well as synthetic). GA3 is the
most common natural gibberellin found in plants.
18
Gibberellin effects

• - promotes stem elongation growth
• - promotes seed germination

19
Gibberellins promote stem elongation in many
plant species
Pea seedlings
Pea seedlings treated with GA3
20
Gibberellins and world food production

• Norman Borlaug
• Nobel Peace Prize 1970
• Developed high-yielding wheat strains
• Disadvantages
• Strains require high levels of fertilizer
  (containing N, see lecture on absorption and
  transport of minerals)
• Expensive (requires fossil fuels)
• Create pollution

21
Coordination of Development via Hormone action

• The major plant hormones
• - Auxins
• - Cytokinins
• - Gibberellins
• - Abscisic acid
• - Ethylene

Hormones that promote/control growth (direction)
Survival hormones (tend to inhibit growth)
22
Ethylene
23
Ethylene effects

• - inhibits cell expansion
• - accelerates senescence
• - accelerates fruit ripening

24
Ethylene effects on etiolated seedlings
Arabidopsis seedlings grown in the dark display
an etiolated growth pattern 1) unexpanded
cotyledons 2) Apical hook 3) long thin
hypocotyl
Exposure to ethylene during growth in the dark
results in 1) Exagerated apical
hook curvature 2) Much shorter and
thicker hypocotyl
25
Ethylene and senescence
Solution that contains STS, an inhibitor of
ethylene action. STS delays floral senescence.
26
Ethylene and fruit ripening

• Ripening of fruit stimulated by ethylene
• Ethylene is THE most damaging hormone in
  agriculture (accelerates ripening and
  consequently rotting of fruits)
• Involves
• Conversion of starch or organic acids to sugars
• Softening of cell walls to form a fleshy fruit
• Rupturing of cell membrane with resulting loss of
  cell fluid to form dry fruit
• Overripe fruit is potent source of ethylene
• Promotes ripening of adjacent fruits

27
Abscisic acid
28
Abscisic acid effects

• - promotes stomatal closure
• - inhibits seed germination

29
Abscisic Acid and drought stress
Abscisic acid is a signal of this emergency
situation. Under drought conditions, wilted
mesophyll cells of a leaf rapidly synthesize and
excrete abscisic acid (ABA). This ABA diffuses
to the guard cells, where an ABA receptor
recognizes the presence of the hormone and acts
to release K, Cl-, and as a result H2O, thus
rapidly reducing turgor pressure and closing the
stomata
30
Abscisic Acid and germination
Wild type (normal) Corn seeds attached . Majority
of seeds are dormant they contain ABA that
prevents germination.
ABA insensitive corn. Majority of seeds are
already germinating while still attached to the
parent plant because of a defect in ABA
sensitivity.