20: Plant Hormones

Why do leaves fall off a tree?

How are fruit production companies able to grow fruit in the tropics and ship them to United States without spoiling? Why are fruits almost always at the correct ripeness when you purchase them?

How do you grow plants from cuttings?

What does all this have to do with biotechnology?

A. Plant Hormones: What Are They?

  • Plant hormones are organic compounds produced in small quantities that have variable impacts on growth, development, and life cycle events (like flowers)

  • There are several categories of them

A substance called a hormone is produced endogenously. A plant growth regulator (PGR) is any substance, natural, or synthetic, that is applied to plants to modulate their growth and development

  • e.g. “rooting hormones”

B. Discovery of Plant Hormones

The observation of how plants move and grow in response to light direction, called phototropism (photo-: light, -tropism: growth response to a directional stimulus), initiated a series of experiments over nearly 50 years that led to the discovery of auxin.

  • Charles Darwin and his son, Francis Darwin

    • Established that the shoot apex of seedlings is where light is perceived and has an effect in other points of the shoot

  • Peter Boysen-Jensen

    • Established that a water-soluble chemical is responsible for moving the stem to induce bending effect

  • Frits Went

    • Isolated chemical and found that growth effects were induceable without light

C. Auxin

AUXIN: “to increase”

Indole-3-acetic acid (IAA) is the most common form of auxin. There are several others.

Produced in: seed embryo, buds, young flowers, shoot apical meristem

Some Major Activities/Effects/Functions:

  1. Stimulate cell expansion all over plant

  2. Induces root growth and branching from pericycle → application can induce roots to form on cutting

  3. Promote differentiation/specialization of vascular tissues

  4. Prevents axillary bud and branching near stem tips → apical dominance

Synthetic auxins:

  • 2, 4-D → stimulate growth in low concentrations but herbicidal in high concentrations

D. Gibberellins (Gibberellic Acids or GA)

Discovery: “Foolish Seedling Disease” of Rice caused by Gibberella fujikuroi (a fungus)

About 110 known natural forms: GA, GA_2, GA_3, …

Produced in: seed embryos during germination, young shoots, flowers

Some Major Activities/Effects/Functions:

  1. Promoting cell division and elongation in shoots → most effective with auxins, especially at internodes

  2. Stimulates flower and cone development, stimulates seed germination

  3. Application during flowering and young fruit development → larger fruits spread further apart→ grapes

E. Cytokinins

Produced in: roots and germinating seeds → transported to shoots via xylem

Some Major Activities/Effects/Functions:

  1. Promote cell division all over the plant

  2. Delay aging and death (senescence) of vegetative organs and flowers)

  3. Stimulate branching of shoot growth → regulated by auxin throughout apical dominance

F. Abscisic Acid

Produced or found in: mature leaves, roots, fruits, seeds before germination

  • 1 molecule, “the conservation hormone”

Some Major Activities/Effects/Functions:

  1. Induce and maintain seed dormancy → blocks GA activity

  2. Induce guard cells to close during periods of drought/heat

  3. Influences the rate or amount of H2O or sugar transport to fruits and seeds

G. Ethylene

  • C_2H_4 → single molecule → a gas

Where did the expression, “one bad apple soils the whole barrel” come from?

  • stress from damage/ripening → ethylene released → absorbed by nearby fruits → induces surrounding fruits to produce ethylene which has a ripening effect

Produced in: potentially in all living cells

Some Major Activities/Effects/Functions:

  1. Can be applied to induce ripening in some fruits

  2. Injury/disturbance/stress results in production

  3. Stimulates essence → promote leaf and fruit drop

  4. Accumulates when plants encounter a physical barrier → stimulate pathways that lead to stronger cell walls