Plant Hormones: Auxin, Cytokinins, Gibberellins, ABA

Key Plant Hormones and Their Roles:

• Auxin:

  • Produced in shoot and root tips.
  • Influences cell division, cell elongation, root initiation, apical dominance, and responses to light and gravity.
  • Stimulates gibberellin biosynthesis
  • Suppresses ethylene and abscisic acid (ABA) production.

• Cytokinins:

  • Found in meristematic regions, roots, young leaves, and developing fruits.
  • Regulates cell division, organ differentiation, stomatal movement and delays senescence.
  • Mediates auxin transport.
  • Can induce resistance to pathogens.

• Gibberellins (GA):

  • Promotes stem elongation.
  • Reverses dwarfism.
  • Induces bolting (sudden elongation of internodes before flowering).
  • Delays senescence.
  • Induces parthenocarpy.
  • Breaks seed dormancy.
  • Stimulates the production of hydrolytic enzymes.

• Abscisic Acid (ABA):

  • Involved in seed dormancy and responses to water stress.
  • Mediates stomatal closure under water-deficient conditions.
  • Affects seed coat growth characteristics.
  • ABA controls embryo dormancy.
  • Gibberellins control embryo germination.

• Ethylene:

  • Controls fruit ripening.
  • Promotes the abscission of leaves, flower parts, and fruit along with cytokinins.

Hormone Interactions and Transport:

• Hormones are transported within the plant through cytoplasmic streaming, diffusion, and vascular tissues (xylem and phloem).
• Plant cells respond to hormones at specific stages of their growth cycle, with the greatest effects occurring during specific periods.
• Plants regulate hormone levels through biosynthesis, storage, inactivation, catabolism, and conjugation with carbohydrates, amino acids, or peptides.
• Plant hormones frequently regulate the concentrations of other plant hormones.
  • Cytokinin is only capable of promoting cell division when it is also in the presence of auxin.