Stephanie Bobbitt
Chapter 39) Plant Responses to Internal and External Signals p. 817-827
- Plant Responses to Light
o photomorphogenesis: effects of light on plant morphology
o action spectrum: graph that relates physiological response to wavelengths of light; used to determine
which responses are mediated by the same photoreceptor (pigment)
o Blue-light photoreceptors are a heterogeneous group of pigments
 blue light initiates phototropism, slowing of hypocotyl elongation in seedling, opening of stomata
 cryptochromes: pigment for the inhibition of hypocotyl elongation; phototropin: pigment for
phototropism; zeaxanthin: pigment for stomatal opening
o Phytochromes function as photoreceptors in many plant responses to light
 phytochromes: pigments that regulate a plant’s response to light
 The Phytochrome Switch and Seed Germination
 effects of red and far-red light in a plant are reversible
 photoreceptor for red and far-red light is a phytochrome
 chromophore of phytochrome goes back and forth between two isomeric forms – one absorbs
red light, and the other absorbs far-red light (called photoreversible)
 plants make
phytochrome as
red phytochrome
(Pr), but when
phytochrome is
exposed to red
light, the red
phytochrome is
converted into farred phytochrome
(Pfr); this is what
can be used to
trigger
germination
 The Phytochrome Switch and Shade Avoidance
 during the day, Pr and Pfr reach an equilibrium; if it is shady in a forest, Pr is favored because the
forest screens out more red light than far-red light  change induces the tree to grow vertically;
opposite case (direct sunlight / Pfr is favored), branching growth is stimulated
o Biological clocks control circadian rhythms in plants and other eukaryotes
 physiological processes in plants still occur under artificially constant conditions (no difference
between day and night)
 circadian rhythms: physiological cycles with a 24 hour frequency that are not directed by an
experimental variable
 biological clocks are internal and have 21-27 hour periods
 sweeping leaves of sleep movements are “hands” of the biological clock – if a leaf is forced to be still,
when released, move to the appropriate position of the day
 hypothesis: biological timekeeping is dependent on synthesis of a protein that regulates its own
production through feedback control – protein is a transcription factor that inhibits transcription of
the gene that encodes for the transcription factor
o Light entrains the biological clock
 when a plant away from environmental cues, the plant desynchronizes with its natural environment
 phytochrome and blue-light photoreceptors entrain circadian rhythms
 Pr is favored in dark, degradative enzymes destroy more Pfr than Pr; Pfr converts into Pr at sundown
in some plant species; when sun rises, there is a sudden increase in Pfr that resets the clock
o Photoperiodism synchronizes many plant responses to changes of season
 photoperiod: environmental stimulus plants use to detect the time of year
 photoperiodism: physiological response to a photoperiod (ex: flowering)
 Photoperiodism and the Control of Flowering
 short-day plant: plants that require a light period shorter than a critical length to flower
 long-day plant: plants that flower when the light period is longer than a certain number of hours
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

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day-neutral plant: flower when they reach a certain stage of maturity
short-day, long-night plants flower when night exceeds a critical dark period; a flash of light
interrupting the dark period stops flowering
long-day, short-night plants flower only if the night is shorter than a critical dark period; night
can be shortened with a flask of light
Critical Night Length
o night length controls flowering and photoperiod responses
o Cocklebur plant needs 8 hours of continuous darkness in order to flower
red light is the most effective color in interrupting the night-time part of photoperiod
a flash of red
shortens the
dark period,
and a
subsequent
flash of farred light
cancels the
effect of the
red flash
o winter wheat
example:
does not
flower unless
it is exposed
to several
weeks of
temperature
below 10°C,
called
vernalization
Is There a Flowering
Hormone?
 leaves detect
photoperiods and
send signal to
buds to cue them to developing into flowers
o
o


-
-
researchers believe the flowering signal is a hormone or change in the concentrations of two or
more hormones
 Meristem Transition from Vegetative Growth to Flowering
 environmental cues and internal signals trigger flowering, which is a result of a bud’s meristem
going from a vegetative state to a flowering state
 vegetative to flowering requires genes to be switched on
Plant Responses to Environmental Stimuli Other Than Light
o Plants respond to environmental stimuli through a combination of developmental and physiological
mechanisms
 Responses to Gravity
 gravitropism: a response in relation to gravity
 roots display positive gravitropism; shoots display negative gravitropism
 auxin is important to gravitropism
 statoliths: specialized plastids with dense starch grains; plants use these to tell up from down
 Responses to Mechanical Stimuli
 thigmomorphogenesis: changes in form that result from mechanical disruption
 mechanical stimulation activates a signal-transduction pathway that increases the cytoplasmic
calcium that mediates the activation of genes
 thigmotropism: directional growth of a plant in relation to touch
 stimulus transmission is very rapid – when one leaf is touched, the adjacent leaves respond
 action potentials: rapid change in membrane potential of an excitable cell caused by stimulus
 Responses to Stress
 stress ex: flooding, drought, extreme temperatures
 environmental stresses contribute to determining geographic ranges of plants
 Drought
o plant is stressed by water deficiency because it loses too much water by transpiration
o water deficit causes guard cells to close stomata and stimulates increased synthesis and
release of abscisic acid to keep stomata closed
o leaves wilt into a shape that reduces transpiration
 Flooding
o plant can suffocate because soil lacks air spaces needed for oxygen for cellular respiration
o oxygen deprivation stimulates the production of ethylene, which causes some cells to
undergo apoptosis, creating air tubes
 Salt Stress
o salt lowers water potential and sodium can be toxic when the concentration is too high
o plants produce solutes for the salt
 Heat Stress
o heat can denature enzymes and damage metabolism
o transpiration functions as evaporative cooling
o heat-shock proteins: special proteins that plant cells synthesize a lot of when the weather is
too warm; may help prevent denaturing
 Cold Stress
o when too cold, membranes are not as fluid, altering solute transport
o plant response is to alter the lipid composition of membranes
o freezing is an extreme case of cold stress; certain plants have developed special adaptations
to survive through freezing
Plant Defense: Responses to Herbivores and Pathogens
o plant is at the bottom of the food chain, plants can be infected by viruses, etc.
o Plants deter herbivores with both physical and chemical defenses
 physical defense ex: thorns; chemical defense ex: production of bad-tasting or toxic compounds
 canavanine: molecule that takes the place of arginine, which kills the organism that ate the
canavanine-containing plant
 some plants have predatory animals to help defend the plant
 jasmonic acid: important molecule of plant defense
o Plants use multiple lines of defense against pathogens
 plant’s first defense = epidermis of primary plant body and periderm of secondary plant body
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Gene-for-Gene Recognition
 avirulent: pathogens that gain access to host to perpetuate itself without severely
damaging/killing the plant host
 gene-for-gene recognition: depends on match-up between genetic allele in the plant and
pathogen; specific resistance to a plant disease is based on this
Hypersensitive Response
 elicitors: usually cellulose fragments (oligosaccharins) induce production of phytoalexins
(antimicrobial compounds)
 PR proteins: pathogenesis related proteins; can be made by genes activated by infection
 hypersensitive response (HR): enhanced production of phytoalexins and PR proteins and the
response with the infection is more effective
System Acquired Resistance
 hypersensitive response includes a production of chemical signals that “sound the alarm”
 systemic acquired resistance (SAR): response providing protection against different pathogens
 salicylic acid: one hormone responsible for activating SAR