Chapter 39: Plant responses to
External and Internal Signals
Summary of key points
•Plant responses; tropism and phototropism:
•Experiments in phototropism and The Went experiment.
•Role of auxins in cell elongation in young developing shoots
•Role of cytokinins in stimulating cell division and differentiation
•Role of gibberellins in cell elongation and cell division in stems and
leaves, role in fruit growth and seed germination.
•Role of Abscisic acid seed dormancy and drought stress
•Role of ethelyne gas stimulates triple response to mechanical damage, and
stimulates apoptosis (programmed cell death), leaf abscission and fruit
•Response to gravity and Gravitropism
•Response to mechanical stimuli and Thigmotropism
•Response to stress such as drought, flooding, salt and heat.
Why do plants bend towards the
• Study done in grasses.
• Grasses grown in the dark or uniform light the
coleoptile (outer sheath) grown straight.
• When exposed to light from one side, the
coleoptile grows towards it…
• Cells on the dark side elongate faster than the cells
on the illuminated side.
• If the tip of the coleoptile is removed or covered
you do not get bending.
Boysen-Jensen demonstrated that a chemical
made in the tip caused the bending effect
The hormone is
which promotes cell
growth towards the light.
Just like the
need to know
• All control plant growth and development
by altering cell division, elongation and
• All have multiple effects depending upon
site of action, concentration and
developmental stage of plant.
Auxin = Indoleacetic Acid (IAA)
• Major function: Affects cell elongation in
• Transported by a polar mechanism.
Auxin and Cell Elongation
• Works at concentrations of 10-8 to 10-4 M.
• Anything higher, ethylene gas produced
which inhibits cell elongation.
• Acid growth hypothesis.
Acid Growth Hypothesis
• Stimulates cytokinesis and cell division.
• Produced in actively growing regions
(roots, shoots and embryos)
• Cell culture in vitro:
+cytokinins, -auxin cells grow large, but no division
+ cytokinins,+ auxin cell division
Ratio of cytokinin and auxin causes plant
cells to differentiate.
Terminal Bud Terminal Bud
• Terminal bud intact
inhibits auxiliary buds
because auxin transport • Terminal bud removed, you
down. remove repression of auxiliary
• Favors shoot to lengthen. bud growth by auxin.
Cytokinins moving upward
from roots counteract and • Cytokinins still produced which
cause auxiliary bud causes auxiliary buds to develop.
• If balanced, shoot growth
upward at the expense of
lateral growth of auxiliary
• Explains why auxiliary buds
closer to the root are more
likely to grow.
Gibberellins: Effect growth in stem
and leaves but no effect in roots
• Gibberillins stimulate Stem Elongation and
• Fruit growth
Abscisic Acid (ABA)
• Generally slows down plant growth
• Antagonist of growth hormones (Auxin, cytokines
• Seed dormancy: ABA levels are high seed is
dormant and the seed matures. Remember that
Gibberillins induce germination. So its really the
ratio of ABA:Gibberillins that controls when seeds
• Also, controls Drought Stress. Causes the stomata
to close rapidly. Do you remember how? (hint:
Ethylene Gas has 4 major effects on
• Triple Response to mechanical stress.
• Apoptosis: programmed cell death
• Leaf Abscission (falling off)
• Fruit Ripening
Triple response to Mechanical Stress
• Consider a pea plant pushing up from the
soil then encountering a rock. What
happens? Production of ethylene gas.
• 1.) Slows Stem elongation 2.) Thicken
stem to make stronger 3.) Curves stem to
start growing horizontally.
elongation Thicken stem
Ethylene Gas controls apoptosis
• What are some reasons why a plant would
want to intentionally destroy cells?
• Leaf in the Fall
• Annual flowering plant
• Xylem vessel elements when its living
contents are destroyed.
• Essential nutrients are salvaged before
• Make the fruit sweet to be attractive to
• Coordinated with seed maturation
• Ethylene gas triggers starch and acids to
become sugars, making the fruit attractive.
• Positive feedback in plants. Even spreads
from fruit to fruit.
• Similar to cholesterol
• Induce cell elongation
• Retards leaf abscission and promotes xylem
Plant Responses to Light
• Light has many roles other than photosynthesis.
• Light has effects upon growth and development
• Light allows for a plant to “tell time” seasonally
• Two types of photoreceptors; Blue –light receptors
and phytochromes (red light receptors)
Action Spectrum in the blue range
induced phototropism Receptor =
Time 90 min.
The Phytochrome Switch and Seed
• Red light (660nm) promotes germination; Far Red
(730 nm) inhibits germination; It turns out that the
last flash is the one that counts.
How does this happen?
What is a tree is growing under the
canopy of others (in the shade)?
• Less red light is getting to the plant
(chlorophyll of the canopy is absorbing it),
more far red is getting through.
• Thus, there will be more of the
phytochrome Pr form than Pfr. This
stimulates the tree to grow taller.
• In the sun, it is the reverse; more Pfr from
and this stimulates branching.
The equilibrium between Pr Pfr allows the plants to
have a circadian rhythm.
• Phytochrome is synthesized in its
Pr form. Any Pfr will be converted
back to Pfr for at night.
• The sun rises and there is a rapid
conversion to the Pfr form.
• In this sense plants can tell how
long the day is and how long the
night is. Photoperiod
• Through this they can track the
• How do plants know when to start making
leaves in the spring/ How do they know
when to flower when insects will be
• Answer: They can tell by sensing the
amount of light in a given day.
Control of Flowering
• Short day plants Need daylight to be shorter
than a certain amount of light (generally 14
hours). Really “long night” plants. Need
continuous darkness for a set period.
• Long day plants Need daylight to be longer than
a certain amount of light. “short night” plants
• Day neutral plants Flower independent of
daylength. When they reach maturity, they flower.
• In reality, its night length that controls flowering.
Reversible effects of red and far red
light on photoperiodic response. The
far red flash cancels the red flash.
Other responses to stimuli
• Gravitopism: Response to gravity, causes a
root from a new germinating seed to grow
down and the shoot to grow up, regardless
of initial position in the soil.
• Thigmotroptism: Response to touch or