Plant Hormones

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Plant Hormones
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Plant tropisms

• Growth in a particular direction in response to
  an external stimulus

Response to gravity is called gravitropism To
light is phototropism To touch is
thigmotropism Responses may be positive or
negative
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Early experiments

• Canary grass coleoptiles

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Classes Hormones

• Five classes are identified
• Auxins
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic Acid

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Auxin

• Primary form is Indole acetic acid (IAA)
• Photo-, Gravi-, and Thigmotropisms come about in
  large part due to auxin effects
• Regulate growth primarily by promoting cell
  elongation with some differentiation.

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Auxin production and transport

• Produced in shoot apical tips, leaves, seeds
• Moves from tip to base
• Moves primarily through parenchyma cells
  surrounding vascular tissue

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Fritz Went Experiments
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Phototropism mechanism
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Auxin Mechanism

• IAA stimulates H pumps in the cell membrane.
• H pumps secrete H into the cell wall,
  decreasing its pH.
• This acidifies the cell wall which activates
  pH-dependent enzymes and breaks bonds between
  cellulose microfibrils.
• The wall "loosens" because of the broken bonds
  and the turgor pressure expands the cell.

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Gravitropism

• Root shoot differential growth in response to
  gravity
• Auxin in higher amounts on lower side of organ
• Roots negative response
• Root more sensitive to auxin - inhibits elongation

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Organ Response to Hormone
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Gravity sensing mechanism

• Root cap cells contain amyloplasts (statoliths)
  containing starch grains
• Density causes movement through cytoplasm to
  lower part of cell

Statoliths at bottom of cells of pea root
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Apical dominance
Control

• Auxin production transport from tip inhibits
  lateral bud growth
• Pinching the tip releases buds for growth
• The actual mechanism is not simplistic IAA may
  induce ethylene production which inhibits lateral
  bud growth. Cytokinins which move apically may
  actually be of greater importance.

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Leaf senescence
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Senescence

• Shorter days of fall, drought, or the lack of
  nutrients cause lower auxin production
• A "senescence factor" stimulates cells to form
  ethylene which produces cellulase (an enzyme
  that breaks down cellulose) and pectinase.
• Middle lamella is digested causing cells to
  separate causing abscission.
• Ratio of auxin to cytokinin may play a role.

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Other effects of auxin

• Stimulates development of fruit
• Can stimulate lateral root formation
• May stimulate adventitious root formation in stems

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Gibberellins

• Translocated in xylem pholem
• Formed in young leaves, apical tips, embryo
• Effects
• Bolting
• Can overcome dwarfing in some plants
• Stimulates flowering in some plants
• Affects fruit development
• Stimulates germination of seeds

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Difference Auxin Gibberellin

• Gibberellin controls elongation in the mature
  regions of trees and shrubs
• auxin regulates elongation in grass seedlings and
  herbs.
• Gibberellin stimulates cell division and
  elongation
• auxin stimulates only cellular elongation.
• Plants can tolerate high levels of gibberellin
  but not of auxin.
• Gibberellin has little effect on roots
• auxin has more of an effect on roots.

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Gibberellin example

• Effect on cabbage
• Treated once/week for 2 months
• Evidence that cabbage comes from a tall, spindly
  ancestor

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Dwarf Pea

• Control Gibberillin added

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Cytokinins

• Formed in roots
• Translocated upward in xylem
• Effects
• Stimulates cell division
• Shoot root differentiation
• Stimulates growth of lateral buds leaf
  expansion
• Chloroplast development
• Delays leaf senescence
• Often the auxin/cytokinin ratio is important

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Abscisic acid

• Seed maturation stomatal function
• May aid onset of seed dormancy
• Transported from leaves in phloem
• Stress hormone - effects help protect plant
  from unfavorable conditions
• Levels increase in response to cold, drought, and
  high salt levels

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Ethylene

• Gas - diffuses through tissues
• Stimulates abscission and fruit ripening
• Used in commercial ripening for bananas green
  picked fruit
• Involved in leaf abscission flower senescence
• Primarily synthesized in response to stress

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Photoperiodism classes

• Response of plants to length of day is called
  photoperiodism
• Flowering is a photoperiodism response

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Florigen - hypothetical hormone

• Proposed to regulate the initiation of flowering

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Critical Periods

• Long-day plants flowers when night length is
  less than critical period
• Flower in spring early summer when days grow
  long
• Short-day plants flowers when uninterrupted
  darkness is longer than critical period
• Chrysanthemum more than 10-12 hrs. light keeps
  them from flowering

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Long day plants
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Short day plants
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Potato
Radish
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Other day classes

• Day-neutral plants
• tend to flower independent of day length
• Long-short or short-long
• a proper sequence is needed for flowering to
  begin
• Intermediate-day plants
• two critical periods - day length must be between
  them

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Timing Mechanism?
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Phytochrome
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Dark etiolation
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Florigen - the trigger?

• Florigen has been proposed as the hormone that
  triggers flowering
• No hormone has ever been isolated
• Critical period inductions given to one part of
  the plant can trigger flowering in another part.
• illumination of a leaf on a plant for the proper
  photoperiod can start flowering

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Grafting experiments

• One plant given proper period (short day)
• Flowering begins
• Later flowering starts in other plant
• See Fig. 35.14

Graft union
Light tight barrier
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Phytochrome system

• Plants can sense light both in quantity and
  quality
• Plants respond to changes in light quality with
  different types of growth
• Much of the sensing seems to come in the red part
  of the spectrum
• Red light (660 nm) and far red light (gt700 nm)
  can be differentiated
• The pigment system responsible is called the
  phytochrome system

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Summary

• Critical lengths can vary from species to species