Chap. 12 : Plant Hormones

• Plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells

  
• Plant hormones are produced in very low concentrations but a minute amount can have a profound effect on the growth and development of a plant organ
• Auxin


  Any chemical substance that promotes cell elongation in different target tissues

  
• Cell elongation in response to auxin

  1. Auxin increases the activity of proton pumps

  2. The cell wall becomes more acidic

  3. Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides

  4. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes

  5. With the cellulose loosened, the cell can elongate

  
• Role of auxin


  • Involved in the formation and branching of roots
  • An overdose of auxins can kill eudicots
  • Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

  
• Cytokinins


  Stimulate cell division<|>Are produced in actively growing tissues such as roots, embryos, and fruits<|>Work together with auxin<|>Control apical dominance - the ability of a terminal bud to suppress development of axillary buds<|>Retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

  
• Gibberellins


  • Stimulate growth of both leaves and stems by stimulating cell elongation and cell division
  • In many plants, both auxin and gibberellins must be present for fruit to set
  • After water is imbibed, there is release of gibberellins from the embryo which gives signals the seeds to break dormancy and germinate
• Abscisic acid (ABA)


  Induces seed dormancy to ensure the seed will germinate only when there are optimal conditions<|>The primary internal signal that enables plants to withstand drought by causing stomata to close rapidly, reducing transpiration and preventing further water loss
• Ethylene


  Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection<|>Induces the triple response in growing shoots to allow them to avoid obstacles - stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally<|>Associated with the programmed destruction (apoptosis) of cells, organs, and whole plants<|>A burst of ethylene production in fruit triggers the ripening process, softening the fruit and converting starches and acids to sugar
• All the major plant hormones - auxin, cytokinins, gibberellins, abscisic acid, and ethylene - have a variety of effects on plant growth and development