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.

• Phototropism- the curving of a growing shoot toward light.– Auxin?

Frits Went

Phytochrome and Photoperiodism

• Gravitropism- the response of shoot or root to gravity.– 1st Cytokinin– 2nd Auxin

• Thigmotropism- the response to contact with a solid object.

• Heliotropism- the orientation of leaves and flowers to the sun.– Turgor pressure changes of the pulvinus.

• Pulvinus- jointlike thickening at the base of the petiole or petiolule.

Growth response unrelated to the direction of the external stimulus

• Nastic Movement- gr. nastos= closed-pressed.– 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.

• Thigmomorphogenesis- the alteration of plant growth patterns in response to mechanical stimuli.