Plant Hormones 101

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Plant Hormones 101

• MUPGRET Workshop

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What are hormones?

• a group of naturally occurring organic compounds
  that influence physiological processes at low
  concentrations.
• a substance that is transferred from one part of
  an organism to another.

From Plant Hormones Biosynthesis, Signal
Transduction, Action! 2005. Peter Davies, ed.
From Went and Thimann. 1937. Phytohormones.
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Hormones

• Can be synthesized locally or transported to
  their site of action.
• They differ from mammalian hormones in this
  respect.

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The first hormone

• Auxin
• Fits the original definition of a hormone.
• Transported from site of synthesis to site of
  action.

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Sachs

• Hypothesized root forming and flowering forming
  compounds move through the plants and cause
  morphological changes in 1880.

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Darwin

• First to observe phototropism, the bending of a
  plant to light.
• Also in 1880.
• Hypothesized that redistribution of a chemical
  within the young seedling caused it to bend
  toward the light.

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Auxin

• Darwins compound that caused phototrophism.
• Isolated several years later by extracting the
  chemicals that diffused from cut coleoptiles into
  blocks of agar.
• More about this compound tomorrow when Dr. Hagen
  talks.

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Classical Plant Hormones

• Auxin
• Gibberellins
• Cytokinins
• Abscisic Acid
• Ethylene

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New generation plant hormones

• Brassinosteriods
• Salicylic acid
• Peptides
• Polyamines

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• http//www.plant-hormones.info/Index.htm

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Auxin

• Indole-3-acetic acid (IAA) and its conjugates.
• Synthesized from tryptophan or indole in leaf
  primodia, young leaves and developing seed.

IAA Structure
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Auxin Transport

• In vascular cambium and procambial strands.
• Possibly between epidermal cells.
• Transport to root may occur through phloem.

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Auxin Stimulates

• Cell enlargement
• Stem growth
• Cell division in cambium
• Phloem and xylem differentiation

• Root initiation
• Root branching
• Phototropism
• Floral organ growth

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Auxin functions to

• Delay leaf senescence
• Promote apical dominance
• Delay fruit ripening

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Auxin mutant
Brachytic 2 (br2) mutant in maize.
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Gibberellin

• Family of 125 compounds.
• Gibberellin acid (GA3)is the most common.
• Synthesized in young shoots and developing seed.

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Gibberellins

• Chloroplast is the initial site of synthesis.
• Transported in phloem and xylem.

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Gibberellin promotes

• Stem elongation
• Cell division and elongation in stems
• Germination if cold or light treatment is
  required.
• Enzyme production, eg. a-amylase.
• Fruit set

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Gibberellin mutant

• Dwarf 8 (D8) mutant in maize.
• The same gene in wheat is responsible for the
  Green Revolution.
• Introduction of this gene into cultivated wheat
  earned Norman Borlaug the Nobel Prize.

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Cytokinins

• Adenine derivatives.
• Zeatin is the most common.
• Synthesized in root tips and developing seed.
• Transported in xylem.

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Cytokinin promotes

• Cell division if auxin is present.
• Photomorphogenesis, eg. Crown gall formation
• Lateral bud growth
• Leaf expansion by cell enlargement
• Stomatal opening
• Chloroplast development

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Cytokinin delays

• Leaf senescence

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Ethylene

• H2CCH2

• Gas
• Synthesized from methionine
• Most tissues can synthesize ethylene in response
  to stress.

Chemical Structure
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Ethylene

• Transport occurs by diffusion.
• Not absolutely required for growth.
• Mutants with non-functional ethylene gene develop
  normally.

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Ethylene causes

• The triple response in dark
• Reduced stem elongation
• Stem thickening
• Lateral growth

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Ethylene stimulates

• Defense response to wounding or disease
• Release from dormancy
• Shoot growth and differentiation
• Root growth and differentiation
• Adventitious root formation
• Leaf and fruit abscission

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Ethylene stimulates

• Flower opening
• Fruit ripening

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Abscisic Acid

• Synthesized from glyceraldehyde-3-phosphate
  through carotenoid pathway.
• Synthesized in roots, mature leaves, and seeds.

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Abscisic Acid

• Synthesis increases in response to drought.
• Transported from roots in xylem.
• Transported from shoots in phloem.

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ABA stimulates

• Stomatal closure
• Root growth under water stress.
• Storage protein synthesis in seeds.
• Breaking dormancy
• Defense response

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ABA inhibits

• Shoot growth under water stress.

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Abscisic Acid Mutant
viviparous 5 (vp5) mutant in maize.
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Polyamines
H N

• Aliphatic amines.
• Putrescine, spermidine and spermine are most
  common.
• Effective at low concentrations.
• Mutants have abberant development.

NH2
H2N
Spermidine structure
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Brassinosteroids

• Sixty steroidal compounds.
• Effective at low concentrations.

Brassinolide structure
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Brassinosteriods stimulate

• Cell division
• Cell wall loosening
• Vascular differentiation
• Ethylene biosynthesis

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Brassinosteroids

• Required for fertility
• Inhibit root development and growth

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Jasmonates

• Jasmonic acid is most common
• Methyl esters
• Scented

Methyl jasmonate
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Jasmonate function

• Induce tuberization
• Important in plant defense
• Inhibit growth
• Inhibit germination
• Promote senescence
• Promote pigmentation

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Salicylic Acid

• Synthesized from phenylalanine.
• Promotes production of pathogenesis related
  proteins.
• Can reverse effects of ABA in some cases.

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Signal Peptides

• Small molecules that can be transported
  throughout the plant and effect development.
• Involved in defense response
• Help determine cell fate
• Involved in self-incompatibility
• Involved in nodule formation in legumes