Regulating Growth and Development:
The Plant Hormones
Chapter 27
Hormones
• Hormone- gr. to excite.
Organic substances
produced in small
amounts that regulate
and coordinate
metabolism, growth,
and morphogenesis.
Six Major 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.
• Brassinosteroids
– Young tissues and throughout the plant.
– They act locally.
• Auxin is the only plant
hormone known to be
transported polarly.
• Polar transport is
basipetal (toward the
base).
Auxin Provides Chemical Signals
That Communicate Information
Over Long Distances
• Apical Dominance- inhibitory influence of
the apical bud upon lateral buds.
Auxin Plays a Role in the Differentiation
and Regeneration of Vascular Tissue
Auxin Promotes Fruit Development
Auxin Promotes the Formation of
Lateral and Adventitious Roots
Auxin Prevents Abscission
Synthetic Auxins Kill Weeds- Herbicides
Ethylene Plays a Role in
Fruit Ripening
Ethylene May Inhibit or Promote
Cell Expansion
= inhibit
Cytokinins Promote Cell Division
Cytokinins Delay Leaf Senescence
Cytokinins Promote the
Growth of Lateral Buds
Abscisic Acid (ABA) Prevents
Seed Germination
Abscisic Acid Plays a Role as a
Root-to-Shoot Signal
Abscisic Acid is Responsible
for Stomatal Closure
• Gibberellin (GA) causes
dwarf mutants to grow tall.
• Gibberellin causes
hyperelongation of shoots
by stimulating cell division
and elongation.
Gibberellin Plays Multiple Roles in
1) Breaking seed dormancy
2) In germination
Gibberellin Can Cause Bolting
Gibberellin Affects Fruit Development
Brassinosteroids
Additional Chemical Signals
• 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.
External Factors and Plant
Growth- Chapter 28
• Tropism- directional growth in response
to an external stimulus.
Frits Went
• Phototropism- the curving of a growing shoot
toward light.
– Auxin?
Phytochrome and Photoperiodism
• Gravitropism- the
response of shoot
or root to gravity.
– 1st Cytokinin
– 2nd Auxin
• Thigmotropismthe response to
contact with a
solid object.
• Heliotropism- the orientation of leaves
and flowers to the sun.
– Turgor pressure changes of the pulvinus.
• Pulvinusjointlike
thickening at
the base of
the petiole or
petiolule.
Growth response unrelated to the
direction of the external stimulus
• Nastic Movement- gr. nastos= closedpressed.
– Nyctinasty
– Thigmonasty
• Thigmomorphogenesis
• Nyctinasty- the up and down movement of
leaves in response to daily rhythms (night and
day).
• Thigmonasty- nastic movements resulting
from touch.
• Thigmomorphogenesisthe alteration of plant
growth patterns in
response to mechanical
stimuli.