Plant Hormones - Auxin + Cytokinin

A hormone is a signaling molecule that regulates gene expression and can target specific areas of the plant.

5 Main Classes of Plant Hormones

Auxins, Cytokinin’s, Ethelene, Abscisic acid, Gibberellins

Auxins - “to increase” in Greek

Discovered by Peter Went (1826)

  • Produced from the tip of leaves and moves from the tip → base of the plant.

Examples:

  • Indoleacetic Acid (IAA) - naturally occurring plant hormone that plays a crucial role in regulating plant growth and development.

  • Synthetic Auxins - Synthesized from tryptophan (an amino acid)

    • 2,4-D (used in herbicides)

    • 2,4,5-T (banned, part of Agent Orange)

Phototropism: The tendency of a plant to bend towards light.

  • Studied by Darwin & Francis, experimented using oat stems.

    • Bending occurs due to auxin distribution from the light to the shaded side of the plant.

  • The coleoptile tip detects light.

    • Auxin moves away from light to the shaded side → uneven cell elongation → bending toward light.

Auxin Polar Transport

  • Moves only in one direction (tip → base): basipetal transport.

  • Uses carrier proteins:

    • AUX1 = influx (brings auxin into cell)

    • PIN proteins = efflux (exports auxin from cell base)

  • Together it creates polar movement.

The Functions of Auxin

  • Phototropism

  • Apical dominance: apical bud suppresses lateral buds

    • Remove apical bud → lateral buds grow → bushier plants

  • Wound Healing: Regeneration of Xylem

  • Abscission (Fruit/Leaf dropping)

    • Auxin delays leaf drop (opposes ethylene)

    • Too much auxin → triggers ethylene → promotes drop

  • Parthenocarpy: makes seedless fruit

  • Root Formation

    • IBA (indole butyric acid) used in horticulture as “rooting hormone” to grow roots from cuttings.

Cytokinin - connected to cell division

Discovery:

  • Found in Coconut Milk and Studied by Overbeek & Skoog

    • Stimulated cell division in tissue culture

  • Skoog: “Resembled Adenine” (He never found it)

    • Miller then synthesized Kinetin: 1st artificial Cytokinin.

    • A Natural Cytokinin Found: Zeatin found in Zea Mays “corn“

Function - Promotes cell division + growth

  • Works w/ Auxin to control development

    • If BOTH hormones are LOW = No Growth

    • If BOTH hormones are HIGH = Undifferentiated Mass (callus)

    • Auxin HIGH = Root Formation

    • Cytokinin HIGH = Shoot/Bud Formation (overrides apical dominance by promoting lateral bud growth)

Experiments

  • Overexpress cytokinin oxidase → breaks down cytokinins → stunted plants (no division/growth)

  • Normal cytokinin → healthy plant.

  • Reduced cytokinin → small, undeveloped plants.

  • Regulate root growth and shoot development.

  • Can delay senescence (aging).