Plant Hormones

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Plant Hormones
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Plant Hormones

• There are five major types of plant hormones
• Gibberelins
• Cytokinins
• Ethylene
• Abcisic Acid
• Auxins
• The structure and function of each type of
  hormone will be described

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Gibberellins
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Overview

• Gibberellins (GAs) regulate and influence
• cell elongation
• seed germination
• dormancy
• flowering
• sex expression
• enzyme induction
• leaf and fruit senescence.

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Germination

• Signal starch hydrolysis through inducing the
  synthesis of the enzyme a-amylase in the aleurone
  cells
• Gibberellins produced in the scutellum diffuse to
  the aleurone cells where they stimulate the
  secretion a-amylase
• a-Amylase then hydrolyses starch into glucose
• Gibberellins cause higher levels of transcription
  of the gene coding for the a-amylase enzyme

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Gibberellins Chemical Structure

• Gibberelins have complex ring structures
• Typically contain carboxylic acid groups
• Many specific gibberelins exist
• Numeric naming system (i.e. GA)
• May be classified into two structural types
• C-19 Gibberelins (19 carbon)
• C-20 Gibberelins (20 carbon)

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Gibberellins Chemical Structure

• Type 1 19 Carbon Gibberelins

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Gibberellins Chemical Structure

• Type 2 20 Carbon Gibberelins

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Cytokinins
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Cytokinins

• Found in a variety of plants and have many
  functions
• Synthesized in meristematic tissues in roots and
  transported to aboveground organs
• Regulate growth and development of tissue
  primarily by promoting cell division
• Involved in germination, shoot differentiation,
  leaf senescence
• Interacts with other plant hormones for some
  functions

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Cytokinins Function

• Regulates apical dominance and lateral root
  initiation
• Slows down senescence (plant aging) and
  chlorophyll degradation in aging leaves
• Regulates growth of dicot seedlings in the dark
  (in combination with ethylene)
• Involved in development of sex organs and male
  sterility
• Synthesized in meristematic tissues in roots and
  transported to aboveground organs

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Cytokinins

• Cytokinins contain adenine
• Two structure types
• Isoprenoid
• Isoprene structural units
• Aromatic
• Contain aromatic groups

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Cytokinins Isoprenoid
Isoprene units
adenine
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Cytokinins Aromatic
adenine
Aromatic group
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Cytokinins Aromatic
Aromatic group
adenine
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Ethylene
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Ethylene

• Universally produced by all plants
• Angiosperms, Gymnosperms, Ferns, Mosses,
  Liverworts
• Also found in some fungi, yeast and bacteria
• Important roles in
• Abscission
• Germination
• Senescence
• Stress
• response to pathogens

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Ethylene and Fruit Ripening

• Helps fruits go through color change, softening
  of walls, conversion of starch to sugar
• Ethylene is produced in low amounts throughout
  plant life
• some climacteric plants have sudden peaks in
  ethylene synthesis which signals ripening changes
• Ethylene gas is sprayed on fruit crops to ripen
  at same time

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Ethylene and Stress

• Some stress situations trigger ethylene
  production
• exposure to heat/cold
• physical damage
• attack by fungal or bacterial pathogens
• flooding that limits oxygen
• Similar to Abscisic acids stress response

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Growth and Messaging

• Ethylene and growth
• Promotes root growth and root hair growth
• Can cause asymmetric growth in stems and leaves
• Ethylene regulates seedlings horizontal growth
  apical hook formation Triple response of
  seedlings grown in dark
• Can act as second messenger
• Auxin, cytokinin can cause ethylene production in
  seedlings

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Ethylenes triple response
Apical hook formation
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Ethylene Chemical Structure

• Ethylene is a very small, simple molecule
  compared to other plant hormones
• Two carbons sharing a double bond
• Ethylene is a gas at room temperature

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Abscisic Acid
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Abscisic Acid (ABA)

• Found universally in plants and algae
• Many functions! Important roles in
• plant development
• bud seed dormancy
• Germination
• cell division
• leaf senescence
• Abscission
• cellular response to stress

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Abscisic Acid

• Acts as a general inhibitor of growth and
  metabolism
• Inhibits growth in hypocotyls, epicotyls, leaves,
  coleoptiles
• Seed dormancy
• ABA promotes seed dormancy so plant seeds can
  withstand desiccation

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ABA as a Stress Hormone

• ABA increases with various environmental or
  biological plant stresses
• Excess heat, pests, excess salt and/or
  dehydration
• Wilted plants have high levels of ABA
• In a drought, ABA increases in some plants,
  causing the stomata to close, preventing water
  loss
• ABA can also produces osmolytes that protect cell
  membranes from dehydration

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Abscisic Acid Chemical Structure

• Abscisic acid is a carboxylic acid

Carboxylic acid
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Auxins
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Its All in the Name

• Auxins from the Greek word a??a?? "I grow or
  increase".
• They were the first of the major plant hormones
  to be discovered.

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Overview

• essential for cell growth
• affects both cell division and cellular
  expansion.
• may promote axial elongation (as in shoots),
  lateral expansion (as in root swelling), or
  isodiametric expansion (as in fruit growth)
• auxin-promoted cellular expansion occurs in the
  absence of cell division.
• auxin-promoted cell division and cell expansion
  may be closely sequenced within the same tissue
  (root initiation, fruit growth)

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Important Functions

• coordination of many growth and behavioral
  processes in the plant life cycle
• stimulate or inhibit the expression of specific
  genes.
• coordinate development at all levels in plants,
  from the cellular level through organs and
  ultimately the whole plant.

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Master Hormone

• indole-3-acetic acid (IAA).
• the most important member of the auxin family
• the most potent native auxin
• generates the majority of auxin effects in intact
  plants

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Working Together

• patterns of active transport are complex
• typically act in concert with, or in opposition
  to other plant hormones
• auxins and other plant hormones nearly always
  interact to determine patterns of plant
  development.

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Auxin Shared Functions

• stimulates cell elongation by stimulating wall
  loosening factors, such as elastins, to loosen
  cell walls (with gibberellins)
• stimulates cell division (with cytokinins)
• applied to callus, rooting can be generated (with
  cytokinin)
• xylem tissues can be generated (with cytokinins)

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More Auxin Shared Functions

• promotes femaleness in dioecious flowers (with
  ethylene)
• inhibits or promotes leaf and fruit abscission
  (with ethylene)
• stimulate cell division in the cambium andin
  tissue culture (with cytokinins)

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Auxin Functions

• Stimulate cell elongation
• stimulate differentiation of phloem and xylem
• Stimulate root initiation on stem cuttings and
  lateral root development in tissue culture
• mediate the tropistic response of bending in
  response to gravity and light
• suppresses growth of lateral buds
• delay leaf senescence

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More Auxin Functions

• can induce fruit setting and growth in some
  plants
• involved in assimilate movement toward auxin,
  possibly by an effect on phloem transport
• delay fruit ripening
• promote flowering in Bromeliads
• stimulate growth of flower parts
• stimulate the production of ethylene at high
  concentrations
• inhibit growth by closing the stoma during water
  stress.

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Auxins Chemical Structure

• Many naturally occurring auxins exist, along with
  many synthetic auxins used in agriculture
• Most naturally occurring auxins contain an indole
  ring group or a phenyl group
• Auxins (natural and synthetic) are carboxylic
  acids
• Halides are also seen in both natural and
  synthetic auxins

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Naturally Occurring Auxins
Carboxylic acid
IAA, the most important member of the auxin
family
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Naturally Occurring Auxins
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Synthetic Auxins
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Synthetic Auxins
Ether linkage
halogens
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Sources

• Wikipedia, Auxin, 2010, http//en.wikipedia.org/wi
  ki/Auxin
• Campbell, Neil A., and Jane B. Reece. Biology.
  6th ed. Boston Benjamin-Cummings Company, 2001.
• Delker, C., Raschke, A. and Quint, M., 2008,
  Auxin dynamics the dazzling complexity of a
  small molecules message, Planta, vol 227,
  929-941.
• Gibberellins A Short History, from
  http//www.plant-hormones.info, the home since
  2003 of a website developed by the now-closed
  Long Ashton Research Station
• Wikipedia, Gibberellin, 2010, http//en.wikipedia.
  org/wiki/Gibberellin
• Koning, Ross E. 1994. Auxins. Plant Physiology
  Information Website. http//plantphys.info/plant_p
  hysiology/auxin.shtml. (4-7-2010).
• Litwak, G. 2005. Plant hormones. Elsevier
  Academic Press San Diego, CA.
• Raghavan, V. 1997. Molecular embryology of
  flowering plants. Cambridge University Press. New
  York, NY.
• Srivastava, LM. 2002. Plant growth and
  development hormones and environment. Elsevier
  Science San Diego, CA.
• http//www.plant-hormones.info/auxins.htm the
  home since 2003 of a website developed by the
  now-closed Long Ashton Research Station

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• Photo credits
• http//humankinetics.files.wordpress.com/2009/07/f
  resh-fruit.jpg
• http//www.nature.com/emboj/journal/v22/n6/thumbs/
  7595043f4.jpg
• http//plantphys.info/plant_physiology/images/trip
  leresponse.gif
• http//farm4.static.flickr.com/3657/3513022448_e7b
  b1c305e_m.jpg
• http//www.hiltonpond.org/images/FreezeHackberry01
  .jpg