Lecture 6: Plant Hormones and Development

Lecture 6: Regulation of Developmental and Differentiation Hormones

• The lecture covers plant growth regulators (phytohormones)
• Students will learn to list these regulators and explain their functions
• Students will learn how hormones affect plant development
• Plant hormones are chemicals that alter growth and maintenance within a plant
• These substances are produced in low concentrations
• Hormones are mobile, and act over short and long distances
• Plant hormones are regulated in various tissues undergoing different development stages
• Two main groups of plant hormones: endogenous and exogenous
• Endogenous hormones are produced by the plant, and affected by its environment.
• Exogenous hormones are applied to a plant, and affect it in the same way as endogenous hormones.

Plant Hormones

• Plant hormones (or phytohormones) have multiple effects
• Send chemical signals to cells to regulate activity
• Activate or deactivate genes that encode specific enzymes
• Regulate cell division, elongation, and differentiation
• Respond to signals from the plant and its environment

Auxin

• Chemically, auxin is indoleacetic acid (IAA) derived from indole or tryptophan
• Synthetic auxins also exist, such as NAA and 2,4-D
• Auxins primarily move from the apical (top) part of a plant, downwards
• Influences plant growth and development
• Functions:
  • Stimulate cell elongation, cell division in cambium, phloem differentiation, and xylem development.
  • Roots can be stimulated to develop from stem cuttings in tissue culture
  • Delays leaf senescence and suppression of lateral bud development

Callus Formation

• Plant growth regulators in tissue culture medium can induce callus formation (+ 2,4-D)
• Stages in callus formation include cell separation, cell suspension, cell clumping, cell divisions, heart stage embryo, torpedo shape embryo, mature embryo, and germination.
• 2,4-D can be substituted to induce embryos from callus.

Auxin (Further functions)

• Suppresses lateral bud growth: apical dominance
• Inhibits or promotes fruit and leaf abscission
• Promotes flowering in Bromeliads
• Influences phototropism (plant bending toward light)

Cytokinin

• Derivatives of purine adenine
• Found in actively dividing tissues of seeds, fruits, leaves, and root tips, wound tissue sites
• Promotes cell division and callus formation
• Roots supply cytokinins upwards to the shoots
• Cytokinins migrate through xylem tissue
• Promotes cell division by speeding up progression from G2 phase to mitosis phase
• Acts with auxin to promote elongation and expansion
• Effects:
  • Promotes axillary (lateral) bud growth; over-rides auxin's inhibiting effect
  • Delays (retards) leaf senescence
  • Promotes germination of some seeds in the absence of light

Gibberellin

• Named after fungus Gibberella fujikuroi
• Family of compounds
• Many are inactive precursors to active forms
• Biosynthesis occurs in proplastids, chloroplasts, and plastids
• Accelerates metabolic pathways (respiration, protein synthesis)
• Functions:
  • Signals germination, breaks seed dormancy
  • Promotes mobilization of food materials (e.g. starch in cereal grains) in the embryo
  • Plays a role in metabolizing lipids to glucose and then to sucrose

Gibberellin (Further functions)

• Works with auxins to promote rapid elongation and stem tissue division.
• Can overcome genetic dwarfism
• Can stimulate parthenocarpic fruit development in some species
• Induces flowering in some plants (especially biennial plants and winter crops).
• Vernalization in plants is the cold period required for flowering

Abscisic Acid (ABA)

• Natural plant growth retardant
• Often acts as an antagonist to gibberellins and auxin
• Biosynthesis occurs primarily in leaves
• Transported through xylem and phloem
• Functions:
  • Stimulates stomata closure (water stress response)
  • Enhances drought resistance
  • Promotes stomata closure during water deficit by activating potassium ion transport
• Has important role in seed dormancy
  • Maintaining high ABA levels in mature seeds and inhibiting germination
  • Degraded by environmental factors such as light and enzymes triggering germination

Abscisic Acid (ABA) (Further functions)

• Inhibits shoot growth, but may promote growth of roots
• Induces seeds to synthesize storage proteins, and inhibits the effects of gibberellins on stimulating a-amylase
• Has a role in seed dormancy
• Low levels of ABA in maturing seeds can promote premature germination.
• Induces gene transcription, especially for proteinase inhibitors in response to plant wounding and pathogens, which explains a defense role.

Ethylene

• Sole plant growth regulator that is a gas (at room temperature)
• Produced in many plant organs, including wounded and bruised tissues, and commonly studied in fruits
• Synthesized from the amino acid methionine
• High auxin concentrations promote the conversion of intermediate substances into ethylene
• Toxic substances like air pollutants can trigger the conversion to ethylene
• Functions:
  • Stimulation of fruit ripening (bananas, honeydew, melon, tomatoes)
  • Causes softening of fruit through degradation of cell walls
  • Causes leaf and fruit abscission
• Affects sex expression, promotes female flower production in some species, while high gibberellins promote male flower formation
• Ethylene gas breaks down chlorophyll, causing the degreening of fruits like citrus