Sensory Systems in Plants

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Sensory Systems in Plants

• Chapter 41

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Responses to Light

• Pigments other than those used in photosynthesis
  can detect light and mediate the plants response
  to it
• Photoperiodism response to changes in the length
  of day and night, it is nondirectional
  Phototropisms are directional growth responses to
  light
• Both compensate for plants inability to move

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Responses to Light

• Phytochrome (P) consists of two parts
• -Chromophore which is light-receptive
• -Apoprotein which initiates a signal-transduction
  pathway

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Responses to Light

• The phytochrome molecule exists in two
  interconvertible forms
• -Pr is the inactive form
• -Absorbs red light at 660 nm
• -Pfr is the active form
• -Absorbs far-red light at 730 nm
• -Tagged by ubiquitin for degradation in the
  proteasome

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Responses to Light

• In Arabidopsis, five forms of phytochromes have
  been characterized PHYA to PHYE
• -Involved in several plant growth responses
• 1. Seed germination
• -Inhibited by far-red light and stimulated by
  red light in many plants

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Responses to Light

• 2. Shoot elongation
• -Etiolation occurs when shoot internodes
  elongate because red light and active Pfr are
  not available
• 3. Detection of plant spacing
• -Crowded plants receive far-red light bounced
  from neighboring plants
• -This increases plant height in
  competition for sunlight

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Responses to Light

• Phytochromes are involved in many signaling
  pathways that lead to gene expression
• -Pr is found in the cytoplasm
• -When it is converted to Pfr it enters the
  nucleus
• -Pfr binds to transcription factors, leading
  to expression of light-regulated genes

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Responses to Light

• Phytochrome also works through protein-kinase
  signaling pathways
• -When Pr is converted to Pfr, its protein kinase
  domain causes autophosphorylation or
  phosphorylation of another protein
• -This initiates a signaling cascade that
  activates transcription factors leading to
  expression of light-regulated genes

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Phototropisms

• Phototropic responses including the bending of
  growing stems to sources of light with blue
  wavelengths (460-nm range)

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Phototropisms

• A blue-light receptor phototropin 1 (PHOT1) has
  been characterized
• -Has two regions
• -Blue-light activates the light-sensing region
  of PHOT1
• -Stimulates the kinase region of
  PHOT1 to autophosphorylate
• -Triggers a signal transduction

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Phototropisms
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Circadian Clocks

• Circadian rhythms (around the day) are
  particularly common among eukaryotes
• Have four characteristics
• 1. Continue in absence of external inputs
• 2. Must be about 24 hours in duration
• 3. Cycle can be reset or entrained
• 4. Clock can compensate for differences in
  temperature

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Responses to Gravity

• Gravitropism is the response of a plant to the
  gravitational field of the Earth
• -Shoots exhibit negative gravitotropism roots
  have a positive gravitropic response

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Responses to Gravity

• Four general steps lead to a gravitropic
  response
• 1. Gravity is perceived by the cell
• 2. A mechanical signal is transduced into a
  gravity-perceiving physiological signal
• 3. Physiological signal is transduced to other
  cells
• 4. Differential cell elongation occurs in the
  up and down sides of root and shoot

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Responses to Gravity

• In shoots, gravity is sensed along the length of
  the stem in endodermal cells surrounding the
  vascular tissue
• -Signaling is in the outer epidermal cells
• In roots, the cap is the site of gravity
  perception
• -Signaling triggers differential cell elongation
  and division in the elongation zone

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Stem Response to Gravity

• Auxin accumulates on lower side of the stem
• -Results in asymmetrical cell elongation and
  curvature of the stem upward
• Two Arabidopsis mutants, scarecrow (scr) and
  short root (shr) do not show a normal gravitropic
  response
• -Due to lack of a functional endodermis and
  its gravity-sensing amyloplasts

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Root Response to Gravity

• Lower cells in horizontally oriented root cap are
  less elongated than those on upper side
• -Upper side cells grow more rapidly causing the
  root to ultimately grow downward
• Auxin may not be the long-distance signal between
  the root cap and elongation zone
• -However, it has an essential role in root
  gravitotropism

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Responses to Mechanical Stimuli

• Thigmomorphogenesis is a permanent form change in
  response to mechanical stresses
• Thigmotropism is directional growth of a plant or
  plant part in response to contact
• -Thigmonastic responses occur in same direction
  independent of the stimulus
• Examples of touch responses
• -Snapping of Venus flytrap leaves
• -Curling of tendrils around objects

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Responses to Mechanical Stimuli
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Responses to Mechanical Stimuli

• Some turgor movements are triggered by light
• -This movement maximizes photosynthesis

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Responses to Mechanical Stimuli

• Bean leaves are horizontal during the day when
  their pulvini are rigid

-But become more or less vertical at night as
the pulvini lose turgor
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Water and Temperature Responses

• When water and temperature affect plants,
  responses can be short-term or long-term
• Dormancy results in the cessation of growth
  during unfavorable conditions
• -Often begins with dropping of leaves
• Abscission is the process by which leaves or
  petals are shed
• -One advantage is that nutrient sinks can be
  discarded, conserving resources

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Water and Temperature Responses

• Abscission involves changes that occur in an
  abscission zone at the petioles base
• -Hormonal changes lead to differentiation of
  -Protective layer Consists of several layers
  of suberin-impregnated cells
• -Separation layer Consists of 1-2 layers of
  swollen, gelatinous cells
• -As pectins break down, wind and rain
  separate the leaf from the stem

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Seed Dormancy

• Seeds allow plant offspring to wait until
  conditions for germination are optimal
• -Legume seeds often last decades and even longer
  without special care
• -Seeds that are thousands of years old have been
  successfully germinated
• Essential steps leading to dormancy include
• -Accumulating food reserves, forming a
  protective seed coat and dehydration

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Responses to Chilling

• Plants respond to cold temperatures by
• 1. Increasing number of unsaturated lipids in
  their plasma membranes
• 2. Limiting ice crystal formation to
  extracellular spaces
• 3. Producing antifreeze proteins
• Some plants can undergo deep supercooling
• -Survive temperatures as low as 40OC

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Responses to High Temperatures

• Plants produce heat shock proteins (HSPs) if
  exposed to rapid temperature increases
• -HSPs stabilize other proteins
• Plants can survive otherwise lethal temperatures
  if they are gradually exposed to increasing
  temperature
• -Acquired thermotolerance

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Hormones and Sensory Systems

• Hormones are chemicals produced in one part of an
  organism and transported to another part where
  they exert a response
• In plants, hormones are not produced by
  specialized tissues
• -Seven major kinds of plant hormones
• -Auxin, cytokinins, gibberellins,
  brassinosteroids, oligosaccharins, ethylene,
  and abscisic acid

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Auxin

• Discovered in 1881 by Charles and Francis Darwin
• -They reported experiments on the response of
  growing plants to light
• -Grass seedlings do not bend if the tip is
  covered with a lightproof cap
• -They do bend when a collar is placed below
  the tip

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Auxin

• The Darwins hypothesized that shoots bend towards
  light in response to an influence transmitted
  downward from the tip
• Thirty years later, Peter Boysen-Jensen and Arpad
  Paal demonstrated that the influence was
  actually a chemical

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Auxin

• In 1926, Frits Went performed an experiment that
  explained all of the previous results
• -He named the chemical messenger auxin
• -It accumulated on the side of an oat seedling
  away from light
• -Promoted these cells to grow faster
  than those on the lighted side
• -Cell elongation causes the plant to
  bend towards light

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Auxin

• Winslow Briggs later demonstrated that auxin
  molecules migrate away from the light into the
  shaded portion of the shoot
• -Barriers inserted in a shoot tip revealed equal
  amounts of auxin in both the light and dark sides
  of the barrier
• -However, different auxin concentrations
  produced different degrees of curvature

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How Auxin Works

• Indoleacetic acid (IAA) is the most common
  natural auxin
• -Probably synthesized from tryptophan

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How Auxin Works

• The auxin receptor is the transport inhibitor
  response protein 1 (TIR1)
• Two families of proteins mediate auxin-induced
  changes in gene expression
• -Auxin responses factors (ARFs)
• -Aux/IAA proteins

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How Auxin Works

• 1. Auxin binds TIR1 in the SCF complex if
  Aux/IAA is present
• 2. SCF complex tags Aux/IAA proteins with
  ubiquitin
• 3. These are degraded in the proteasome
• 4. Transcriptional activators of ARF genes are
  released from repression by Aux/IAA
• 5. Auxin-induced gene expression

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How Auxin Works

• One of the downstream effects of auxin is an
  increase in plasticity of the plant cell wall
• -The acid growth hypothesis provides a model
  linking auxin to cell wall expansion

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Synthetic Auxins

• Naphthalene acetic acid (NAA) and indolebutyric
  acid (IBA) have many uses in agriculture and
  horticulture
• -Prevent abscission in apples and berries
• -Promote flowering fruiting in pineapples
• 2,4-dichlorophenoxyacetic acid (2,4-D) is a
  herbicide commonly used to kill weeds

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Cytokinins

• Are purines that appear to be derivatives of
  adenine

Synthetic cytokinins
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Cytokinins

• Cytokinins are produced in the root apical
  meristems and developing fruits
• -Stimulate cell division and differentiation, in
  combination with auxin
• Cytokinins promote the growth of lateral buds
  into branches
• -They inhibit the formation of lateral roots,
  while auxin promotes their formation

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Cytokinins

• Cytokinins promote the synthesis or activation of
  cytokinesis proteins
• -They also function as anti-aging hormones
• Plant tissue can form shoots, roots, or an
  undifferentiated mass depending on the relative
  amounts of auxin and cytokinin

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Cytokinins

• The plant pathogen Agrobacterium introduces genes
  into the plant genome that increase the
  production of cytokinin and auxin

-Cause massive cell division and formation of a
crown gall tumor
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Gibberellins

• Named after the fungus Gibberella fujikuroi which
  causes rice plants to grow very tall
• Gibberellins belong to a large class of over 100
  naturally occurring plant hormones
• -All are acidic and abbreviated GA
• -Have important effects on stem elongation

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Gibberellins

• Adding gibberellins to certain dwarf mutants
  restores normal growth and development

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Gibberellins

• GA is used as a signal from the embryo that turns
  on transcription of genes encoding hydrolytic
  enzymes in the aleurone layer
• -When GA binds to its receptor, it frees
  GA-dependent transcription factors from a
  repressor
• -These transcription factors can now directly
  affect gene expression

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Gibberellins

• GAs hasten seed germination
• -They also function as pheromones in ferns

GAs are used commercially to extend internode
length in grapes -The result is larger grapes
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Brassinosteroids

• First discovered in the pollen of Brassica spp.
• -Are structurally similar to steroid hormones

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Brassinosteroids

• Have a broad spectrum of physiological effects
• -Elongation, cell division, stem bending,
  vascular tissue development, delayed senescence
  and reproductive development
• Additive effects with auxins and gibberellins
  have been reported

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Oligosaccharins

• Are complex plant cell wall carbohydrates that
  have a hormone-like function
• -Can be released from the cell wall by enzymes
  secreted by pathogens
• -Signal the hypersensitive response (HR)
• In peas, oligosaccharins inhibit auxin-stimulated
  elongation of stems
• -While in regenerated tobacco tissue, they
  inhibit roots and stimulate flowers

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Ethylene

• A gaseous hydrocarbon (H2CCH2)
• Auxin stimulates ethylene production in the
  tissues around the lateral bud and thus retards
  their growth
• Ethylene also suppresses stem and root elongation

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Ethylene

• Ethylene controls leaf, flower and fruit
  abscission
• It hastens fruit ripening
• -Indeed, an antisense copy of the gene has been
  used to create transgenic tomato
• -These stay fresh longer

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Abscisic Acid

• Abscisic acid is synthesized mainly in mature
  green leaves, fruits and root caps
• There is little evidence that this hormone plays
  a role in abscission
• Abscisic acid induces formation of dormant winter
  buds
• It counteracts gibberellins, by suppressing bud
  growth and elongation, and auxin, by promoting
  senescence

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Abscisic Acid

• Abscisic acid is also necessary for dormancy in
  seeds
• -Prevents precocious germination called vivipary
• Abscisic acid is important in the opening and
  closing of stomata
• -Triggers movement of K out of guard cells

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