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Plant Responses to Internal


Title: Control Systems in Plants Author: Authorized User Last modified by: CCSD Created Date: 3/24/2007 12:07:52 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Plant Responses to Internal

Plant Responses to Internal External Signals
  • For the readings, pay special attention to the

Review of Signal Transduction Pathways
  • Reception signal is detected by some sort of
  • Transduction signal is carried from the
    receptor to the nucleus
  • This is where you have secondary messengers
    transfer the signal
  • Protein kinase, or
  • The cGMP pathway (like the cAMP pathway)
  • Response activates different transcriptional
    factors or enzymes

Example Potatoes
  • Potatoes grow beneath the soil in a dark
    environment (they produce many short stems that
    lack leaves in the hope that the stem will break
    through the soil surface)
  • The adaptations to grow in the dark is known as
  • When a stem breaks through the surface, leaves
    expand, roots elongate, and the plant produces
  • This process is called de-etiolation

  • A photoreceptor, phytochrome, detects that light
    after the shoot breaks through the soil
  • Phytochromes are not located on the cell surface,
    but are instead located in the cytoplasm
  • When light strikes the phytochrome, it causes a
    conformational change that causes a transduction
    pathway to begin

  • Reception can be from a VERY weak light source,
    therefore you need to amplify the signal
    through transduction
  • The phytochrome, when activated, causes an
    increase in the concentration of cGMP (like cAMP)
    and calcium ion.
  • cGMP activates protein kinases (through
  • The increased calcium concentration with the
    activation of kinases leads to a response

  • Many transcriptional factors are activated in
  • Some are activated by phosphorylation
  • Some are activated by cGMP
  • Some are activated by calcium
  • In addition to transcriptional factors,
    post-translational enzymes are also activated to
    modify the created proteins
  • Most of the proteins created are associated with
    photosynthesis and chlorophyll production

  • How hormones coordinate growth, development and
    response to environment

Plant Hormones
  • Hormones are chemical signals that coordinate the
    various parts of an organism
  • A hormone is a compound produced in one part of
    the organism which is then transported to other
    parts of the organism, where it triggers
    responses in target cells and tissues
  • Many hormones are effective in VERY small
  • Many times, hormone concentrations are dependent
    on environmental stimuli

Example with Light
  • Example of the action of hormones
  • Auxin is a hormone that induces a plant to move
    towards or away from a stimuli, tropism
  • Stimulus Light
  • Response the plants growth pattern will cause
    the growth shoot to move towards or away from the
  • Phototropism growth towards the light
  • Negative phototropism growth away from the light

Plant Hormones
  • There are 5 major classes of plant hormones, each
    with specific functions
  • Auxin
  • Cytokinins
  • Gibberellins
  • Ethylene
  • Abscisic acid
  • NOTE Many hormones interact with each other to
    enhance or inhibit their activities

  • Found
  • In the embryo of seeds, meristems of apical buds
    and young leaves
  • Function
  • Stimulates stem elongation and root growth
    (causes the root cells to elongate)
  • Stimulates development of fruit
  • Involved in phototropism and gravitropism,
    response of a plant to the effects of gravity

  • Found
  • Made in the roots and transported to other organs
    of the plant
  • Function
  • Affect the growth and differentiation of roots
  • Stimulates cell division and growth (in
    conjunction with auxins)
  • Stimulates germination, growth from a seed
  • Delay senescence, or the aging of the plant

  • Found in meristems of apical buds and roots,
    young leaves and embryos
  • Function
  • Promote seed and bud germination, stem elongation
    and leaf growth
  • Stimulate flowering and fruit development
  • Affect root growth and differentiation

  • Found in tissues of ripe fruit, nodes of stems,
    and aging leaves and flowers
  • Function
  • Opposes some of the effects of auxin (feedback)
  • Promotes fruit ripening
  • Senescence (aging) is at least party caused by
  • One bad apple spoils the whole bunch

Abscisic Acid
  • Found in leaves, stems, roots, and green fruit
  • Function
  • Induces seed dormancy
  • Anti-gibberellin
  • Inhibits cell growth
  • Anti-cytokinin
  • Inhibits fruit ripening
  • Anti-ethylene
  • Closes stomata during water stress, allowing many
    plants to survive droughts

  • How plants respond to various factors

  • Tropisms are growth responses that result in
    curvatures of whole plant organs toward or away
    from a stimuli
  • There are three major stimuli that induce
  • Light (Phototropism)
  • Gravity (Gravitropism)
  • Touch (Thigmotropism)

  • Phototropism is the growth of a shoot towards
  • This is primarily due to the action of auxin
  • Auxin elongates the cells on the non-light side

Light Receptors There are 2 main types of
  • Blue-light photoreceptor These receptors absorb
    mostly blue light
  • May be responsible for opening stomata, and
    inhibit hypocotyl elongation in seedlings
    breaking ground
  • Phytochromes These receptors absorb mostly red
  • Responsible for de-etiolation, seed germination,
    and avoid shade

Circadian Rhythms
  • The production of enzymes, hormones and other
    processes oscillate during the day
  • This is due to many environmental factors
  • Light levels, temperature, humidity
  • There are other processes that occur with a
    frequency of every 24 hours that are not
    dependent on environment circadian rhythms

Biological Clocks/Circadian Rhythms
  • A physiological cycle with a frequency of about
    24 hours is called a circadian rhythm
  • Even without external, environmental cues,
    circadian rhythms persist in humans and in all
  • Example jet lag in humans, leaf position in bean
  • It is believed these are due to some internal
    biological clock that regulate these processes
    (these work independent of the day/night cycle)

  • A physiological response to day length (differs
    in winter, summer, spring, and fall) is known as
  • Short-day plants
  • Require a shorter light period
  • Flower in later summer/fall/winter
  • Example poinsettias
  • Long-day plants
  • Require a longer light period
  • Flower in late spring/early summer
  • Example spinach
  • Day-neutral plants
  • Are unaffected by photoperiod
  • Example tomatoes
  • But its actually the night that matters!! (if
    there is even a little sunlight during the
    night the flowers will not bloom)

Other Factors that Affect Flowering
  • In addition to photoperiod, some plants need
    additional environmental cues to induce flowering
  • Example Some plants need to be exposed to
    critical temperature ranges
  • Vernalization the need to be exposed to long
    periods of cold temperatures to induce
    flowering (this occurs in winter wheat)

Missing Flower Hormone
  • It is believed that the photoperiod is detected
    by some chemical signal located in the leaves,
    florigen (not yet found)
  • If all of the leaves are removed from the plant,
    it is no longer affected by photoperiod

Plant Defenses
  • Plants defend themselves against herbivores in
    several ways
  • Physical defenses, such as thorns
  • Chemical defenses, such as
  • producing distasteful/toxic
  • compounds
  • Can use chemicals to attract
  • insects to help defend the plant
  • Wasps

Plant Defenses
  • Chemical warning systems
  • When there is an infestation by insects, plants
    can release a chemical signal that causes other
    plants to activate defense genes to counteract
    the infesting organisms

Plant Defenses
  • Defense against pathogens is also important for
    plant survival
  • First line of defense is the plants skin
  • Plant dermis, cuticle, bark
  • If a plant becomes infected, they release a
    series of chemicals that destroy the pathogen
    (much like our immune system)
  • If a pathogen is able to avoid or suppress a
    plants defenses, the pathogen is said to be
  • Many times the pathogen weakens, but does not
    kill, the plant so that the pathogen may survive
  • This condition is called avirulent

Pathogen Detection
  • At the genetic level, plant disease resistance
    can begin with gene-for-gene recognition
  • The plant is able to recognize the protein
    products of the pathogen and able to mount a
    defense against the disease
  • Another detection method are molecules called
  • Example oligosaccharins are molecules that
    derive from damage cell walls
  • They can also mount a defense against disease

Plant Response to Pathogen
  • Once warned, plants can release chemicals that
    can fight an invader
  • Phytoalexins are a group of compounds that are
    antimicrobial (the equivalent of our B and
  • There are also a general group of proteins (PR,
    or pathogenesis-related, proteins) that are
    antimicrobial or act as messengers to activate
    further defense
  • If the pathogen is avirulent, then there may be a
    more aggressive, localized, response called
    hypersensitive response

General Defense
  • A hypersensitive response may produce a chemical
    signal that alerts the rest of the plant
  • As a result, more phytoalexins and PR proteins
    can be released to produce a nonspecific defense,
    system acquired resistance (SAR)
  • A hormone that is thought to produce this
    resistance is salicylic acid