Transcript Plant Response to Signals

Plant
Response to
Signals
Ch 39
Plant Response
 Stimuli

& a Stationary Life
animals respond to stimuli by changing
behavior
 move
toward positive stimuli
 move away from negative stimuli

plants respond to stimuli by adjusting
growth & development
Signal Transduction Pathway model
 signal
triggers receptor
 receptor triggers internal cellular messengers &
then cellular response
 receptor
 signal
pathway
(2° messengers)
 response
What kinds of
molecules are
the receptors?
Signal Transduction Pathway example
1. Light signal is detected by the
phytochrome receptor, which
then activates at least 2 signal
2. One pathway uses cGMP as a 2nd
transduction pathways
messenger
to activate
3. Both pathways
leada protein
kinase.
to expression of genes
The
pathway
forother
proteins
that involves
increases
Ca2+ that
function in
in cytoplasmic
greening
activates
different
responseaof
plant. protein kinase.
Signal Transduction Pathway example
1. Light signal is detected by the
phytochrome receptor, which
then activates at least 2 signal
2. One pathway uses cGMP as a 2nd
transduction pathways
messenger
to activate
3. Both pathways
leada protein
kinase.
to expression of genes
The
pathway
forother
proteins
that involves
increases
Ca2+ that
function in
in cytoplasmic
greening
activates
different
responseaof
plant. protein kinase.
Plants do not have brains
 Or

nervous systems for that matter,
So how do they communicate with itself
and coordinate beneficial responses?
Plant hormones
 Chemical
signals that coordinate different
parts of an organism





only tiny amounts are required
produced by 1 part of body
transported to another part
binds to specific receptor
triggers response in target cells & tissues
2005-2006
Plant hormones
 auxins
 cytokinins
 gibberellins
 abscisic
acid
 ethylene
Hormones in review:





AUXINS __ Promote cellular elongation __ by softening of
cell walls __ Involved in phototropism __ Involved in
geotropism __ Involved in apical dominance
CYTOKININS __ Promotes lateral growth_ growth in size of
leaf cells __ Stimulate cell division (hence name) __ Release
buds from apical dominance
GIBERELLINS __ seasonal growth___ Stimulate cell
elongation __ Produce bolting in biennials __ Stimulate
production of starch digestion enzymes in some seeds
ABSCISIC ACID __ opposite of giberellins__causes slow
down or “cut off” of growth__Promotes stomatal closure __
__ Promotes seed and bud dormancy _
ETHYLENE __ Promotes ripening of fruit
Response to light:
Phototropism
 Growth
towards light
 Hormone: Auxin


asymmetrical distribution of auxin, moves away
from sunny side of stem (-ve phototropism, -ve
gravitropism)
cells on darker side elongate faster
than cells on brighter side
2005-2006
Apical dominance
 Controls
cell division &
differentiation
 axillary
buds do no grow while
apical bud exerts control
shoot
root
Figure 39.8
Cell elongation in response to auxin: the acid growth hypothesis
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
Expansin
CELL WALL
Cellulose
microfibril
H2O
H
Plasma
membrane
H
H
H
ATP
H
H
H
Cell wall
H
H
Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole
Gibberellins
 Family

of hormones
over 100 different gibberellins identified
 Effects


fruit growth
seed germination
plump grapes in grocery
stores have been treated
with gibberellin hormones
while on the vine
Abscisic acid (ABA)
 Effects


slows growth
seed dormancy
 high

concentrations of Abscisic acid
germination only after ABA is inactivated down or
leeched out
 survival
value:
seed will germinate only
under optimal conditions


light, temperature, moisture
drought tolerance
 rapid
stomate closing
Ethylene
 Ethylene
is a hormone gas released by plant cells
 Multiple effects

response to mechanical stress
 triple




response
slow stem elongation
thickening of stem
curvature to stem growth
leaf drop (like in Fall)
 apoptosis

fruit ripening
Apoptosis & Leaf drop:
combination of hormones
 Ethylene
& auxin
 many
events in plants
involve apoptosis
(pre-programmed cell
death)
 death
of annual plant after
flowering
 differentiation of xylem
vessels

loss of cytosol
 shedding
of autumn leaves
What is the
evolutionary
advantage of
loss of leaves
in autumn?
Fruit ripening
 Adaptation
 hard,
tart fruit protects
developing seed from herbivores
 ripe, sweet, soft fruit attracts
animals to disperse seed
 Ethylene
 triggers
ripening process
 breakdown

softening
 conversion

of cell wall
of starch to sugar
sweetening
 positive
feedback system
 ethylene
triggers ripening
 ripening stimulates more ethylene production
2005-2006
Applications
 Truth

in folk wisdom…..
one bad apple spoils the whole bunch
 ripening
apple releases ethylene to speed ripening of
fruit nearby
 Ripen
green bananas by bagging them with an
apple
 Climate control storage of apples

high CO2 storage = reduces ethylene production
Plant stimuli
Flowering Response

Triggered by photoperiod


relative lengths of day & night
night length—“critical period”— is trigger
Plant is
sensitive to
red light
exposure
What is the
evolutionary
advantage of
photoperiodism?
Short-day plants
Long-day plants
Synchronizes
plant responses
to season
Circadian rhythms
 Internal
(endogenous) 24-hour cycles
4 O’clock
Noon
Morning glory
Midnight
2005-2006
Response to gravity

How does a sprouting shoot “know” to grow towards
the surface from underground?

environmental
cues?



roots = positive
gravitropism
shoots = negative
gravitropism
settling of statoliths
(dense starch
grains) may
detect gravity
2005-2006
Response to touch
 Thigmotropism
Mimosa (Sensitive plant)
closes leaves in response to
touch
Caused by changes in
osmotic pressure =
rapid loss of K+ =
rapid loss of H2O =
loss of turgor in cells
2005-2006
Plant defenses
 Defenses
against herbivores
Plant defenses
 Defenses
against herbivores
Parasitoid wasp larvae
emerging from a
caterpillar
coevolution
Any Questions??