Plant Responses to Internal

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Plant Responses to Internal External Signals

• Chapter 39

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I. Signal Transduction and Plant Responses

• Plants receive signals and respond to them
• Receptors detect a change in the environment
• Cells respond to this change and eventually a
  response occurs.

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• A potato left growing in darkness
• Will produce shoots that do not appear healthy,
  and will lack elongated roots
• These are morphological adaptations for growing
  in darkness
• Collectively referred to as etiolation

(a) Before exposure to light. Adark-grown potato
has tall,spindly stems and nonexpandedleavesmor
phologicaladaptations that enable theshoots to
penetrate the soil. Theroots are short, but
there is littleneed for water absorptionbecause
little water is lost by theshoots.
Figure 39.2a
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• After the potato is exposed to light
• The plant undergoes profound changes called
  de-etiolation, in which shoots and roots grow
  normally

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• The potatos response to light
• Is an example of cell-signal processing

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Reception

• Signals are detected by receptors
• Receptors - proteins that change shape in
  response to a specific stimulus
• Receptor for greening in plants - phytochrome
  (contains a light-absorbing pigment attached to a
  specific protein)

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Transduction

• Secondary messengers - small, internally produced
  chemicals that transfer and amplify the signal
  from the receptor to proteins that cause the
  specific response.

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• An example of signal transduction in plants

Figure 39.4
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• Phytochrome - interacts with guanine-binding
  proteins (G-proteins).
• In the greening response, a light-activated
  phytochrome interacts with an inactive G-protein,
  leading to the replacement of guanine diphosphate
  by guanine triphosphate on the G-protein.
• This activates the G-protein, which activates
  guanyl cyclase, the enzyme that produces cyclic
  GMP, a second messenger.
• Second messengers include two types of cyclic
  nucleotides, cyclic adenosine monophosphate
  (cyclic AMP) and cyclic guanosine monophosphate
  (cyclic GMP).

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• Phytochrome activation also induces changes in
  cytosolic Ca2.
• Ca2 binds directly to small proteins called
  calmodulins which bind to and activate several
  enzymes, including several types of protein
  kinases.
• Activity of kinases, through both the cyclic GMP
  and Ca2-calmodulin second messenger systems
  leads to the expression of genes for proteins
  that function in the greening response.

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Response

• Have regulation of 1 or more activities
• Usually - increased activity of certain enzymes.
• This occurs through two mechanisms stimulating
  transcription of mRNA for the enzyme or by
  activating existing enzyme molecules
  (post-translational modification).

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• Transcriptional Regulation
• Transcription factors bind directly to specific
  regions of DNA and control the transcription of
  specific genes
• Some of the activated transcription factors
  increase transcription of specific genes, others
  deactivate negative transcription factors which
  decrease transcription

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• Post transcriptional modification of proteins
• Involves the activation of existing proteins
  involved in the signal response
• Modify mostly with phosphorylation

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• De-Etiolation Greeningproteins
• Include enzymes that function in photosynthesis
  directly or that supply the chemical precursors
  for chlorophyll production.
• Others affect the levels of plant hormones that
  regulate growth.

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II. Plant Responses to Hormones

• Hormones chemical signals that coordinate parts
  of an organism made at one part of the body and
  transported to another where it binds and causes
  a response
• Only need a minute amount
• Can change due to environment

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Discovery of Plant Hormones

• Discovered the use of hormones through many
  experiments
• Plants always grow towards light
• Phototropism - growth of a shoot toward light
• Found that the coleoptile while growing will
  curve towards the light.

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• Darwin and his son did experiments
• Observed that a grass seedling bent toward light
  only if the tip of the coleoptile was present.
• This response stopped if the tip were removed or
  covered with an opaque cap (but not a transparent
  cap).
• While the tip was responsible for sensing light,
  the actual growth response occurred some distance
  below the tip.
• They postulated that some signal was transmitted
  from the tip downward.

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Figure 39.5
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• Peter Boysen-Jensen
• Demonstrated that the signal was a mobile
  chemical substance.
• Separated the tip from the remainder of the
  coleoptile by a block of gelatin, preventing
  cellular contact, but allowing chemicals to pass.
• These seedlings were phototropic.
• However, if the tip were segregated from the
  lower coleoptile by an impermeable barrier, no
  phototropic response occurred.

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• F.W. Went
• Extracted the chemical messenger for phototropism
• He named it auxin.
• Modifying the Boysen-Jensen experiment, he placed
  excised tips on agar blocks, collecting the
  hormone.
• If an agar block with this substance were
  centered on a coleoptile without a tip, the plant
  grew straight upward.
• If the block were placed on one side, the plant
  began to bend away from the agar block

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Figure 39.6
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Plant Hormones

• Help coordinate growth and development by
  affecting division, elongation and
  differentiation
• Help with responses to environmental stimuli
• Produced in small amounts
• All are small and can move through the cell walls
• Each hormone has multiple effects, depending on
  its site of action, its concentration, and the
  developmental stage of the plant.

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Plant Hormones(Table 39.1 page 808)

• Auxin
• Cytokinins
• Gibbererellins

• Abscisic Acid
• Ethylene
• Brassionosteroids

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A Survey of Plant Hormones
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Auxin

• First plant hormone discovered
• Promotes the elongation of coleoptiles in
  developing shoots
• Transported directly through the parencyma tissue
  to other cells

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• Auxin transporters
• Move the hormone out of the basal end of one
  cell, and into the apical end of neighboring cells

EXPERIMENT
Figure 39.7
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Role of Auxin in Cell Elongation

• Apical meristem major site of auxin synthesis
• As auxin moves from the apex down to the region
  of cell elongation, the hormone stimulates cell
  growth.
• Auxin stimulates cell growth only over a certain
  concentration range, from about 10-8 to 10-4 M.
• At higher concentrations, auxins may inhibit cell
  elongation, probably by inducing production of
  ethylene, a hormone that generally acts as an
  inhibitor of elongation.

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• Acid growth hypothesis
• In a shoots region of elongation, auxin
  stimulates plasma membrane proton pumps,
  increasing the voltage across the membrane and
  lowering the pH in the cell wall.
• Lowering the pH activates expansin enzymes that
  break the cross-links between cellulose
  microfibrils.
• Increasing the voltage enhances ion uptake into
  the cell, which causes the osmotic uptake of
  water
• Uptake of water with looser walls elongates the
  cell

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• Also alters gene expression rapidly, causing
  cells in the region of elongation to produce new
  proteins within minutes

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• Cell elongation in response to auxin

Figure 39.8
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Lateral Adventitious Root Formation

• Auxin helps with vegetative propagation
• Used from plants after cutting
• Using auxin causes roots to form near the cut
  surface

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Auxins as Herbicides

• 2,4-dinitrophenol (2,4-D) synthetic auxin
• Monocots (maize or turfgrass) can rapidly
  inactivate these synthetic auxins.
• Eudicots cannot and die from a hormonal overdose.

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Other Effects of Auxins

• Affects secondary growth by inducing cell
  division in cambium and by inducing
  differentiation of secondary xylem development
• Developing seeds synthesize auxin, which promotes
  the growth of fruit
• Ex if sprayed on tomato plants induces fruit
  development without the step of fertilization.
  Can grow seedless tomatoes.

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Cytokinins

• Growth regulators
• Stimulate cytokinesis or cell division
• Most common zeatin

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Control of Cell Division and Differentiation

• Are produced in actively growing tissues such as
  roots, embryos, and fruits
• Work together with auxin
• When used without auxin, they have no effect
• Need to be in a specific ratio to work

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Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in
  the control of apical dominance
• The ability of a terminal bud to suppress
  development of axillary buds
• Direct Inhibition Hypothesis - proposed that
  auxin and cytokinin act antagonistically in
  regulating axillary bud growth.
• Auxin levels would inhibit axillary bud growth,
  while cytokinins would stimulate growth.

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• If the terminal bud is removed (auxin is removed)
• Plants become bushier

Figure 39.9b
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• Hypothesis is not necessarily true
• Recent studies have shown auxin levels increase
  in the axillary buds of decapitated plants

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Anti-Aging Effects

• Cytokinis retard the aging of some plant organs
  by inhibiting protein breakdown
• Leaves removed from a plant and dipped in a
  cytokinin solution stay green much longer.
• Slows deterioration of leaves on intact plants.
• Florists use cytokinin sprays to keep cut flowers
  fresh.

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Gibberellins

• More than 100 different types
• Chemical secreted that produces hyperelongation
  of stems
• Such as stem elongation, fruit growth, and seed
  germination

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Stem Elongation

• Roots young leaves site of production
• Gibberellins stimulate growth in both leaves and
  stems but have little effect on root growth.

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• In stems, gibberellins stimulate cell elongation
  and cell division.
• One hypothesis proposes that gibberellins
  stimulate cell wall loosening enzymes that
  facilitate the penetration of expansin proteins
  into the cell well.
• So auxin, by acidifying the cell wall and
  activating expansins, and gibberellins, by
  facilitating the penetration of expansins, act
  together to promote elongation.

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• Ex of gibberellins at work
• After treatment with gibberellins, dwarf pea
  plant grow to normal height.
• However, if applied to normal plants, there is
  often no response

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Fruit Growth

• Both auxin and gibberellins must be present for
  fruit to set.
• Spray gibberellins to seedless grapes causing
  them to grow much larger

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Germination

• Embryo of seeds has a large supply of
  gibberellins
• After hydration of the seed, the release of
  gibberellins from the embryo signals the seed to
  break dormancy and germinate
• Support the growth of cereal seedlings by
  stimulating the synthesis of digestive enzymes
  that mobilize stored nutrients

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Brassinosteroids

• Are similar to the sex hormones of animals
• Induce cell elongation and division
• Also stop leaf abscission and promote xylem
  differentiation

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Abscisic Acid (ABA)

• Slows down growth
• Seed dormancy
• Drought tolerance
• Antagonizes the actions of the growth hormones
• Ratio of abscisic acid to growth hormones that
  determines the growth

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Seed Dormancy

• Dormancy is important
• Germinate only when conditions are right
• ABA allows for dormancy by inhibiting germination
  and by producing enzymes to help control
  dehydrations
• Many types of dormant seeds will germinate when
  ABA is removed or inactivated

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• Precocious germination is observed in maize
  mutants
• That lack a functional transcription factor
  required for ABA to induce expression of certain
  genes

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Drought Stem

• ABA - primary internal signal that enables plants
  to withstand drought.
• When a plant begins to wilt, ABA accumulates in
  leaves and causes stomata to close rapidly,
  reducing transpiration and preventing further
  water loss

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Ethylene

• Produced in response
• Stress (drought)
• Fruit ripening
• Programmed cell death
• To high concentration of externally applied auxin

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Triple Response to Mechanical Stress

• Allows a growing shoot to avoid obstacles
• 3 parts to the response - stem elongation slows,
  the stem thickens, and curvature causes the stem
  to start growing horizontally

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• Steps -
• As the stem continues to grow horizontally, its
  tip touches upward intermittently.
• If the probes continue to detect a solid object
  above, then another pulse of ethylene is
  generated and the stem continues its horizontal
  progress.
• If upward probes detect no solid object, then
  ethylene production decreases, and the stem
  resumes its normal upward growth.

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• Ethylene-insensitive mutants
• Fail to undergo the triple response after
  exposure to ethylene
• Lack a receptor

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• Other types of mutants
• Undergo the triple response in air but do not
  respond to inhibitors of ethylene synthesis

Figure 39.14b
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• Other mutants undergo the triple response in the
  absence of physical obstacles.
• Some mutants (eto) produce ethylene at 20 times
  the normal rate.
• Other mutants (constitutive triple-response (ctr)
  mutants) undergo the triple response in air but
  do not respond to inhibitors of ethylene
  synthesis.
• Ethylene signal transduction is permanently
  turned on even though there is no ethylene
  present

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• A summary of ethylene signal transduction mutants

Figure 39.15
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Apoptosis - Programmed Cell Death

• Burst of ethylene - the programmed destruction of
  cells, organs, or whole plants
• New enzymes are made to help break down many
  chemical components, including chlorophyll, DNA,
  RNA, proteins, and membrane lipids

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Loss of Leaves (abscission)

• A change in the balance of ethylene and auxin
  controls abscission.
• An aged leaf produces less and less auxin and
  this makes the cells of the abscission layer more
  sensitive to ethylene.

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• When an autumn leaf falls, the breaking point is
  an abscission layer near the base of the petiole.
• The parenchyma cells here have very thin walls,
  and there are no fiber cells around the vascular
  tissue.
• The abscission layer is further weakened when
  enzymes hydrolyze polysaccharides in the cell
  walls.
• The weight of the leaf, with the help of the
  wind, causes a separation within the abscission
  layer.

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Fruit Ripening

• Immature fruits - tart, hard, and green but
  become edible at the time of seed maturation,
  triggered by a burst of ethylene production.
• Enzymatic breakdown of cell wall components
  softens the fruit, and conversion of starches and
  acids to sugars makes the fruit sweet.

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• A chain reaction occurs during ripening ethylene
  triggers ripening and ripening, in turn, triggers
  even more ethylene production
• Because ethylene is a gas, the signal to ripen
  even spreads from fruit to fruit.
• One bad fruit can spoil others
• By keeping ethylene from fruit, ripening is
  prevented till needed

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Systems Biology and Hormone Interactions

• Interactions between hormones and their signal
  transduction pathways
• Make it difficult to predict what effect a
  genetic manipulation will have on a plant
• Systems biology seeks a comprehensive
  understanding of plants
• That will permit successful modeling of plant
  functions

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III. Plant Responses to Light

• Photomorphogenesis effects of light on plant
  morphology
• Plants detect the light its direction,
  intensity wavelength
• Action spectrum - the measure of physiological
  response to light wavelength
• Photosynthesis has 2 peaks red and blue

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• Observations led researchers to two major classes
  of light receptors
• Blue-light
• Phytochromes - absorb mostly red light

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Blue Light Photoreceptors

• Control hypocotyl elongation, stomatal opening,
  and phototropism
• 3 types of pigment that determine blue light
• Cryptochromes (for the inhibition of hypocotyl
  elongation)
• Phototropin (for phototropism),
• Zeaxanthin (carotenoid-based photoreceptor for
  stomatal opening).

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Phytochromes

• Regulate many of a plants responses to light
  throughout its life

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Switch and Seed Germination

• Experiment - exposed seeds to a few minutes of
  monochromatic light of various wavelengths and
  stored them in the dark for two days and recorded
  the number of seeds that had germinated under
  each light regimen.
• While red light increased germination, far red
  light inhibited it and the response depended on
  the last flash

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During the 1930s, USDA scientists briefly
exposed batches of lettuce seeds to red light or
far-red light to test the effects on germination.
After the light exposure, the seeds were placed
in the dark, and the results were compared with
control seeds that were not exposed to light.
EXPERIMENT
The bar below each photo indicates the sequence
of red-light exposure, far-red light exposure,
and darkness. The germination rate increased
greatly in groups of seeds that were last
exposedto red light (left). Germination was
inhibited in groups of seeds that were last
exposed to far-red light (right).
RESULTS
Red light stimulated germination, and far-red
light inhibited germination.The final exposure
was the determining factor. The effects of red
and far-red light were reversible.
CONCLUSION
Figure 39.18
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• Photoreceptor responsible for these opposing
  effects of red and far-red light is a -
  phytochrome.
• It consists of a protein (a kinase) covalently
  bonded to a nonprotein part that functions as a
  chromophore, the light absorbing part of the
  molecule.
• The chromophore reverts back and forth between
  two isomeric forms with one (Pr) absorbing red
  light and becoming (Pfr), and the other (Pfr)
  absorbing far-red light and becoming (Pr).

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• Interconversion between isomers acts as a
  switching mechanism that controls various
  light-induced events in the life of the plant.
• The Pfr form triggers many of the plants
  developmental responses to light.
• Exposure to far-red light inhibits the
  germination response.

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• Plants synthesize phytochrome as Pr and if seeds
  are kept in the dark the pigment remains almost
  entirely in the Pr form.
• If the seeds are illuminated with sunlight, the
  phytochrome is exposed to red light (along with
  other wavelengths) and much of the Pr is
  converted to (Pfr), triggering germination

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Switch and Shade Avoidance

• Provides plants with information about the
  quality of light.
• During the day, with the mix of both red and
  far-red radiation, the Pr ltgtPfr photoreversion
  reaches a dynamic equilibrium.
• Plants can use the ratio of these two forms to
  monitor and adapt to changes in light conditions

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Biological Clocks

• Many plant processes, such as transpiration and
  synthesis of certain enzymes, oscillate during
  the day.
• Due to changes in light levels, temperature, and
  relative humidity that accompany the 24-hour
  cycle of day and night.
• Even under constant conditions in a growth
  chamber, many physiological processes in plants
  continue to oscillate with a frequency of about
  24 hours.

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• Circadian rhythms - physiological cycles with a
  frequency of about 24 hours and that are not
  directly paced by any known environmental
  variable
• All research thus far indicates that the
  oscillator for circadian rhythms is endogenous
  (internal).

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• If an organism is kept in a constant environment,
  its circadian rhythms deviate from a 24-hour
  period, with free-running periods ranging from 21
  to 27 hours
• Not synchronized with the outside world
• Can interrupt a biological rhythm but like
  clockwork it goes right back
• May be due to a transcription factor by a
  positive feedback

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• Many legumes
• Lower their leaves in the evening and raise them
  in the morning

Figure 39.21
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Light Entrains the Biological Clock

• Light controls the clock
• Both phytochrome and blue-light photoreceptors
  can entrain circadian rhythms of plants
• Involves turning cellular responses off and on by
  means of the Pr ltgt Pfr switch.

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• In darkness, the phytochrome ratio shifts
  gradually in favor of the Pr form
• When the sun rises, the Pfr level suddenly
  increases This sudden increase in Pfr each day at
  dawn resets the biological clock.
• Plants use the length of days to mark seasons

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Photoperiodism

• Physiological response to photoperiod (relative
  lengths of night and day)
• Ex - flowering

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Photoperiodism Control of Flowering

• Flowering - requires a certain photoperiod
• Garner Allard
• Looked at a mutant variety of tobacco (Maryland
  Mammoth)
• Seasoned in the winter instead of the summer
• In light-regulated chambers, they discovered that
  this variety would only flower if the day length
  was 14 hours or shorter, which explained why it
  would not flower during the longer days of the
  summer

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• Short day plant - it required a light period
  shorter than a critical length to flower
• Long day plant - will only flower when the light
  period is longer than a critical number of hours
• Day-neutral plants - flower when they reach a
  certain stage of maturity, regardless of day
  length

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Critical Night Length

• 1940s - researchers discovered that it is
  actually night length, not day length, that
  controls flowering and other responses to
  photoperiod
• Short-day plants are actually long-night plants,
  requiring a minimum length of uninterrupted
  darkness
• Long-day plants are actually short-night plants.
• A long-day plant grown on photoperiods of long
  nights that would not normally induce flowering
  will flower if the period of continuous darkness
  are interrupted by a few minutes of light

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• Long-day and short-day plants are distinguished
  not by an absolute night length but by whether
  the critical night lengths sets a maximum
  (long-day plants) or minimum (short-day plants)
  number of hours of darkness required for
  flowering
• Red light - most effective color in interrupting
  the nighttime portion of the photoperiod
• Plants measure night length very accurately
• Humans can exploit the photoperiodic control of
  flowering to produce flowers out of season.

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Is There a Flowering Hormone?

• The flowering signal, not yet chemically
  identified
• Is called florigen, and it may be a hormone or a
  change in relative concentrations of multiple
  hormones

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Meristem Transition

• Outcome of flowering is the transition of a buds
  meristem from a vegetative state to a flowering
  state.
• Requires that meristem-identity genes that
  specify that the bud will form a flower must be
  switched on.
• Organ-identity genes are activated in the
  appropriate regions of the meristem.

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Plant Responses to Gravity

• Gravitropism responses to gravity
• Roots demonstrate positive gravitropism
• Shoots exhibit negative gravitropism.
• Gravitropism ensures that the root grows in the
  soil and that the shoot reaches sunlight
  regardless of how a seed happens to be oriented
  when it lands
• Auxin helps with this

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• Plants may detect gravity by the settling of
  statoliths
• Specialized plastids containing dense starch
  grains

Figure 39.25a, b
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Plant Responses to Mechanical Stimuli

• Thigmomorphogenesis changes in form that result
  from mechanical perturbation
• Plants are very sensitive to mechanical stress
• Rubbing the stems of young plants a few times
  results in plants that are shorter than controls.

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• Mechanical stimulation activates a
  signal-transduction pathway that increase
  cytoplasmic calcium, which mediates the activity
  of specific genes, including some which encode
  for proteins that affect cell wall properties.

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• Thigmotropism directional growth in response to
  touch (vines)
• Some plants undergo rapid leaf movement
• When touched, collapses due to turgor pressure.

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Environmental Stresses

• Drought
• Plants respond to water deficit by reducing
  transpiration
• Guard cell lose turgor and the stomata close.
• Stimulates increased synthesis and release of
  abscisic acid in a leaf, which also signals guard
  cells to close stomata.
• Deeper roots continue to grow in search of water

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• Flooding
• Can suffocate because the soil lacks the air
  spaces that provide oxygen for cellular
  respiration in the roots.
• Some plants are adapted to very wet habitats
  some plants may produce ethylene in the roots
  causing some cortical cells to undergo apoptosis,
  which creates air tubes that function as
  snorkels.

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• Salt Stresses
• Excess of sodium chloride threatens plants
  because
• They lower the water potential of the soil
  causing plants to lose water to the environment
  rather than absorb it.
• Sodium and certain other ions are toxic to plants
  when their concentrations are relatively high.

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• Heat stresses
• Heat harms and kills because denatures enzymes
  and damages the metabolism
• Some plants have heat shock proteins plant
  cells begin to make these at about 40ºC help to
  prevent denaturing

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• Cold Stress
• Causes a change in fluidity of the membranes
• When the temperature becomes too cool, lipids are
  locked into crystalline structures and membranes
  lose their fluidity, solute transport and the
  functions of other membrane proteins are
  adversely affected.
• One solution is to alter lipid composition in the
  membranes, increasing the proportion of
  unsaturated fatty acids, which have shapes that
  keep membranes fluid at lower temperatures.
• Takes a couple of days for this

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• At subfreezing temperatures ice forms in the
  cells walls and intercellular spaces of most
  plants.
• Solutes in the cytosol depress its freezing
  point.
• This lowers the extracellular water potential,
  causing water to leave the cytoplasm and,
  therefore, dehydration.

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V. Plant Defense

• Need ways to protect themselves from pathogens

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Plants Deter Herbivores

• Do so both physically (thorns) and chemically
  (toxins)
• Canavanine an amino acid
• Some plants produce this
• If an insect eats a plant containing canavanine,
  canavanine is incorporated into the insects
  proteins in place of arginine insect dies

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• Some plants even recruit predatory animals that
  help defend the plant against specific
  herbivores.
• Some plants make a volatile molecules which are
  warning signs for nearby plants of the same
  species

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Plants Use Multiple Lines of Defense

• First line of defense epidermis periderm
• Stuff can enter by injuries or openings
• Second line of defense a chemical attack that
  occurs once the substance enters

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Gene-for-Gene Recognition

• Plants generally resistant to pathogens because
  can recognize pathogens
• Virulent pathogen plant has little defense
  against
• Avirulent pathogens - gain enough access to its
  host to perpetuate itself without severely
  damaging or killing the plant

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• Gene-for-gene recognition is a widespread form of
  plant disease resistance
• That involves recognition of pathogen-derived
  molecules by the protein products of specific
  plant disease resistance (R) genes

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Hypersensitive Response

• Plants can initiate a chemical attack in response
  to molecular signals released from cells damaged
  by infection.
• Molecules (elicitors - cellulose fragments called
  oligosaccharins released by cell-wall damage)
  induce the production of antimicrobial compounds
  (phytoalexins)

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• A pathogen is avirulent
• If it has a specific Avr gene corresponding to a
  particular R allele in the host plant

Signal molecule (ligand) from Avr gene product
R
Avr allele
Avirulent pathogen
Plant cell is resistant
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• If the plant host lacks the R gene that
  counteracts the pathogens Avr gene
• Then the pathogen can invade and kill the plant

R
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Plant Response

• Localized and specific
• A hypersensitive response against an avirulent
  pathogen
• Seals off the infection and kills both pathogen
  and host cells in the region of the infection
• Elicitors cause a broad type of host defense
  response
• Stimulate the production of phytoalexins
• Also PR proteins are made by activated genes
  can attack molecules in the cell wall or spread
  the news

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4 Before they die,infected cellsrelease a
chemicalsignal, probablysalicylic acid.
3 In a hypersensitiveresponse (HR), plantcells
produce anti-microbial molecules,seal off
infectedareas by modifyingtheir walls, andthen
destroythemselves. Thislocalized
responseproduces lesionsand protects
otherparts of an infectedleaf.
5 The signal is distributed to the
rest of the plant.
Signal
5
4
Signaltransductionpathway
6
Hypersensitiveresponse
3
6 In cells remote fromthe infection site,the
chemicalinitiates a signaltransductionpathway.
Signal transductionpathway
Acquiredresistance
2
7
2 This identification step triggers a
signal transduction pathway.
7 Systemic acquired resistance isactivated
theproduction ofmolecules that helpprotect the
cellagainst a diversityof pathogens forseveral
days.
1
Avirulentpathogen
1 Specific resistance is based on the
binding of ligands from the pathogen to
receptors in plant cells.
R-Avr recognition and hypersensitive response
Systemic acquired resistance
Figure 39.31
120

• Infection also stimulates cross-linking of
  molecules in the cell wall and deposition of
  lignins.
• This sets up a local barricade that slows spread
  of the pathogen to other parts of the plant.

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• If the pathogen is avirulent based on an R-Avr
  match, the localized defense response is more
  vigorous - hypersensitive response (HR).
• Enhanced production of phytoalexins and PR
  proteins, and the sealing response that
  contains the infection is more effective.
• After cells at the site of infection mount their
  chemical defense and seal off the area, they
  destroy themselves.

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Systemic Acquired Resistance (SAR)

• Nonspecific, providing protection against a
  diversity of pathogens for days
• Production of chemical signals that spread
  throughout the plant, stimulating production of
  phytoalexins and PR proteins
• Hypersensitive response, triggered by R-Avr
  recognition, results in localized production of
  antimicrobial molecules, sealing off the infected
  areas, and cell apoptosis

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• Hormone for activating SAR - salicylic acid
• Similar to aspirin

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