Regulating Growth and Development: The Plant Hormones

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Regulating Growth and Development The Plant
Hormones

• Chapter 27

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Hormones

• Hormone- gr. to excite. Organic substances
  produced in small amounts that regulate and
  coordinate metabolism, growth, and morphogenesis.

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Five Classes of Plant Hormones

• Auxins
• Leaf primordia, young leaves, developing seeds.
• Polarly (unidirectionally) and nonpolarly.
• Cytokinins
• Root tips.
• From roots to shoots via xylem.
• Ethylene
• Most tissues in response to stress.
• Diffusion from site of synthesis.
• Abscisic acid
• Mature leaves and roots, seeds.
• From leaves in phloem and from the roots in the
  xylem.
• Gibberellins
• Young tissues of the shoot and developing seeds.
• Xylem and phloem.

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• Auxin is the only plant hormone known to be
  transported polarly.
• Polar transport is basipetal (toward the base).

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Auxin Provides Chemical Signals That Communicate
Information Over Long Distances

• Apical Dominance- inhibitory influence of the
  apical bud upon lateral buds.

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Auxin Plays a Role in the Differentiation and
Regeneration of Vascular Tissue
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Auxin Promotes Fruit Development
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Auxin Promotes the Formation of Lateral and
Adventitious Roots
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Auxin Prevents Abscission
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Synthetic Auxins Kill Weeds- Herbicides
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Ethylene Plays a Role in Fruit Ripening
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Ethylene May Inhibit or Promote Cell Expansion
inhibit
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Cytokinins Promote Cell Division
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Cytokinins Delay Leaf Senescence
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Cytokinins Promote the Growth of Lateral
Buds
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Abscisic Acid (ABA) Prevents Seed Germination
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Abscisic Acid Plays a Role as a Root-to-Shoot
Signal
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Abscisic Acid is Responsible for Stomatal
Closure
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• Gibberellin (GA) causes dwarf mutants to grow
  tall.
• Gibberellin causes hyperelongation of shoots by
  stimulating cell division and elongation.

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• Gibberellin Plays Multiple Roles in
• 1) Breaking Seed Dormancy
• 2) In Germination

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• Gibberellin Can Cause Bolting

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Gibberellin Affects Fruit Development
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Additional Chemical Signals

• Brassinosteroids- required for normal growth of
  most plant tissue.
• Salicylic acid- signal in defense responses to
  plant pathogens.
• Jasmonates- plant growth regulation and defense.
• Polyamines- growth and development mitosis and
  meiosis.
• Systemin- long-distance signal that activates
  chemical defenses against herbivores.
• Nitric oxide- signal in hormonal and defense
  responses.