Plant Responses to Internal and
External Signals
Control systems…
• Keep track of
the time of day
• Notice seasons
• Sense gravity
• Sense direction
of sunlight
Signal Transduction Pathways Link
Signal Reception to Response
Reception
• Cell signals are detected by
receptors that undergo changes in
shape as a response to a specific
stimulus
Transduction
• Multistep pathway that amplifies
the response using relay proteins
and second messengers
Response
• Activation of cellular responses
– Activating existing enzyme molecules
– Increasing or decreasing mRNA
production
Signal Transduction Pathways Link
Signal Reception to Response
Etiolation
• Morphological
adaptations for
growing in darkness
De-Etiolation
• Greening
• Profound changes
that occur due to
exposure to light
Plant Hormones Help Coordinate
Growth, Development, and
Responses to Stimuli
Hormone
• compound produced by one part of an
organism that is transported and triggers a
response in another part
• Animals transport in circulatory system
• Plants are different b/c…
– No circulatory system
– Only act locally
– Present in high concentrations
– Referred to as plant growth regulators
Discovery of Plant Hormones
Tropism
• any growth response
that results in a
plant’s organs curving
toward or away from
a stimulus
• Phototropism
What part of a grass
coleoptile senses
light, and how is the
signal transmitted?
• Only the tip senses
light
Discovery of
Plant Hormones
Does asymmetrical
distribution of a
growth promoting
chemical cause a
coleoptile to grown
toward the light?
• A coleoptile grows
toward light because
its dark side has a
higher concentration
of auxin (growth
promoting hormone)
Survey of Plant Hormones
Survey of Plant Hormones
Apical dominance: with apical bud (left), apical bud removed (right)
Controlled by cytokinins, auxin, and strigolactones
Effects of Gibberellins
Effects of Abscisic Acid
Effects of Brassinosteroids
[Photo: The level of
brassinosteroids regulates
both the size and senescence
of tobacco. With low levels,
tobacco is dwarfed (some as
small as 10 inches tall; see
plant in front) and the leaves
do not senesce, while at
normal levels of
brassinosteroids, tobacco
stands almost 6 feet tall and
the leaves turn yellow as
they age (plant in back).
Effects of Strigolactones
Figure 1: Rice plants showing
the impact of strigolactone on
branching—normal plant (left);
mutant plant which produces
low levels of strigolactone
(center); mutant plant treated
with strigolactone (right).
Ethylene Induced Triple Response
Abscission caused by a change in the ratio of ethylene to auxin
Responses to Light are
Critical for Plant Success
Photomorphogenesis
• Effects of light on plant morphology
Plants detect…
• Presence of light
• Direction of light
• Intensity of light
• Wavelength of light
Two major classes of light receptors
• Blue light photoreceptors
• phytochromes
Blue-Light Photoreceptors
Blue light triggers…
• Phototropism
• Opening of stomata
• Slowing of hypocotyl elongation
Phytochromes
Protein containing a chromophore responsible
for a plant’s response to the photoperiod
• Alternate between 2 forms (Pr and Pfr)
• Pr only converts to Pfr in presence of light
• Pfr triggers many plant responses
• Pfr degrades back to Pr at night
Biological Clocks & Circadian Rhythms
Circadian rhythms
• Cycle of about 24 hours
• Does not require environmental cues
• Internally set
• Daily signals from the environment can set
the circadian clock to about 24 hrs.
Photoperiodism and Response to the
Seasons
Physiological response to the photoperiod
Critical night length
• day length does not trigger flowering
• If daytime is interrupted, no effect on flowering
• If night period is interrupted by a short period
of light, plants do not flower
– Could be effected by a single exposure or may
require several exposures
Vernalization
• Use of pretreatment with cold to induce
flowering
Photoperiodic control of flowering
Reversible effects of red and far-red light on photoperiodic response
Photoperiodism
Short day plants
• night is longer than a critical length
• Flower in late summer, fall, winter
• Mums, poinsettias, soybeans
Long day plants
• night is shorter than a critical length
• Flower in late spring, early summer
• Radishes, lettuce, irises
Day Neutral plants
• unaffected by photoperiod
• triggered by maturity
• Tomatoes, rice, dandelions
Evidence for a flowering hormone(s)
Flowering locus T (FT) gene\
•activated in leaf cells and the
FT protein travels through the
symplastic route to the apical
meristems and induces
flowering
Tropisms
• Growth responses that result in
curvatures of the whole plant
toward/away from a stimulus
Phototropism
• stimulus is light
Tropisms
Gravitropism
•stimulus is gravity detected by statholiths
•Dense cytoplasmic components that settle at
the bottom of the cell due to gravity
•Positive – downward (roots)
•Negative – upward (shoots)
Tropisms
Thigmomorphogenesis
• Refers to changes in form that result from
mechanical perturbation
Thigmotropism
• stimulus is touch
Turgor Movements
Turgor Movements
• Reversible movements
caused by changes in
turgor pressure
Rapid Leaf movements
• reduce water loss or
protect from herbivores
• touch causes leaf to
collapse (causes a rapid
loss of turgor pressure by
cells causing them to
become flaccid)
• motor cells lose K+
• 10 minute restoration
Mimosa clip
Turgor Movements
Sleep movements
• lowering of leaves to
vertical position in
evening
• raising leaves to a
horizontal position in
the morning
• one side of plant is
turgid while the other
is flaccid
– Daily changes in
turgor pressure
Environmental Stress
• An environmental condition that can have an
adverse effect on a plant’s growth,
reproduction, & survival
Abiotic
• Water deficit, flooding, salt stress, heat,
cold
Biotic
• Herbivores, pathogens
Drought
• Control systems in both leaves and roots
Leaves/Shoots
• Guard cells lose turgor and close
• Mesophyll releases abscisic acid
• Young leaf growth is inhibited
• Wilting reduces surface area
Roots
• Shallow root growth inhibited; deeper
roots continue to grow
Flooding
(Oxygen Deprivation)
• Waterlogged soil
lacks air spaces to
hold oxygen
• May form air tubes
from roots to the
surface
• Submerged roots
may be continuous
with aerial roots
Salt Stress
• Lowers water potential of soil causing a water
deficit even if enough water is present
• Produce compatible solutes in response to
moderately saline soils
– Keeps water potential of cells more negative
than the soil solution w/o admitting toxic
quantities of salt
Heat Stress
• Transpiration reduces temperature and
keeps enzymes from denaturing
• Produce shock proteins
– a back up plan to transpiration
Cold Stress
• Subfreezing temps cause ice crystals to
form in protoplast
– death
• Lipids become locked and causes a loss
of fluidity in membranes
• Alter lipid composition by increasing
saturated fatty acids
Herbivores
Chemical
• Distasteful
• Toxic
Structural
• Thorns, etc.
Recruit predatory animals
• Plants attract wasps that
lay eggs in caterpillars
Canavanine
• Replaces arginine
• Proteins cannot be
made…insect dies
Defense Against Pathogens
Virulent pathogens
• Plants have virtually no defense
Avirulent pathogens
• Mildly harm but do not kill
Gene-for-gene recognition
• Resistance to a disease depends on a
precise match up between an allele in
a plant and an allele in the pathogen
Defense Against Pathogens
Hypersensitive response
• chemical signaling system to resist
infection
• Phytoalexins
– Compounds with fungicidal and bactericidal
properties
Systemic acquired resistance
• protects unaffected tissues from a
pathogen spreading
Defense responses against an avirulent pathogen