Plant hormones

1
Plant Hormones

• Plant hormones

• Organic compounds produced in low concentrations

• Produced in one part of plant (i.e. source)

• Transported to another part of plant (i.e. target)

• Cause physiological or developmental responses
  (stimulatory or inhibitory)

• Also called plant growth regulators

2
Plant Hormones

• Major types of plant hormones

• Auxins

• Cytokinins

• Gibberellins

• Ethylene

• Abscisic acid

3
Plant Hormones

• Auxins

• Types of auxins used in horticulture

• Natural auxin is indole-3-acetic acid (IAA)

• IAA is broken down in sunlight

• Synthetic auxins used in horticulture

• Indole-3-butyric acid (IBA)

• Naphthaleneacetic acid (NAA)

• 2, 4-dichlorophenoxyacetic acid (2, 4-D)

• Site of auxin production in plants

• Shoot and root tips (apical meristems)

• Young, expanding leaves

• Young seeds

4
Plant Hormones

• Auxins

• Action of auxins in plants

• Stimulate cell elongation

• Bend toward light because more auxin remains in
  cells on opposite side of light source

• Etiolated plants have long internodes because
  auxin not quickly broken down

• Promotes apical dominance

• Pinching bud removes source of auxin and releases
  axillary buds from apical dominance

• Promotes growth of adventitious roots

• We used both IBA and NAA with geranium cuttings

5
Plant Hormones

• Auxins

• Actions of auxins in plants

• Promotes or inhibits abscission of leaves,
  flowers and fruit

• NAA applied shortly after bloom is used to thin
  apples

• NAA applied during fruit development inhibits
  abscission of apples

• Auxin concentration determines if action is
  stimulatory or inhibitory

• Low concentration promotes adventitious root
  growth, but high concentration inhibits root
  growth of cuttings

• 2, 4-D is used as herbicide by applying at high
  concentrations

6
Plant Hormones

• Gibberellins

• Types of gibberellins used in horticulture

• Several different gibberellins (GA) produced by
  plants

• Large, complicated molecules not synthesized

• Commercial gibberellins produced by fungus

• Site of gibberellin production in plants

• Shoot and root tips (apical meristems)

• Young, expanding leaves

• Embryos

• Fruits

• Tubers

7
Plant Hormones

• Gibberellins

• Action of gibberellins in plants

• Stimulate cell elongation

• Dwarf plants treated with gibberellins produce
  normal growth

• Applied to grapes to elongate the peduncle (stem
  of flower cluster) and pedicels (stem of single
  flower), making looser cluster

• Promotes cell division in vascular cambium

• Promotes seed germination

• causes production of enzymes that break down
  starch into energy needed for growth

• Used by beer brewers to stimulate sugar
  production in barley malt (sugar is converted
  into alcohol during fermentation)

• Influences flower and fruit development

8
Plant Hormones

• Cytokinins

• Types of cytokinins used in horticulture

• Several natural forms

• Zeatin first cytokinin isolated

• Synthetic cytokinins used in horticulture

• Benzyladenine (BA)

• Site of cytokinin production in plants

• Embryos

• Young leaves and fruit

• Apical meristems of roots

9
Plant Hormones

• Cytokinins

• Actions of cytokinins in plants

• Promotes cell division (cytokinesis)

• Contributes to cell enlargement (in leaves)

• Stimulates differentiation of cells (with auxins)

• High cytokinin and low auxin promotes shoot
  initiation in tissue culture, whereas reverse
  combination promotes root formation

• Moderate levels of both hormones promotes callus
  growth

• Delays senescence in leaves (maintains and
  promotes synthesis of chlorophyll)

10
Plant Hormones

• Ethylene

• Gas at physiological temperatures

• Liquid form used in horticulture (ethephon)

• Site of ethylene production in plants

• Throughout plants

• Actions of ethylene in plants

• Inhibits root and shoot elongation by blocking
  transport of auxins from apical meristems

• Stimulates leaf curling (epinasty)

11
Plant Hormones

• Ethylene

• Actions of ethylene in plants

• Induces adventitious root formation by blocking
  (and accumulating) auxin at tip of stem cutting

• Enhances flow of latex in rubber trees

• Enhances flowering in pineapples

• Stimulates abscission of leaves and fruit

• Used as a harvest aid for cherries

• Promotes fruit ripening (apple, tomato, citrus,
  coffee)

• Promotes senescence of flowers

12
Plant Hormones

• Abscisic Acid (ABA)

• Similar structure as gibberellins

• Site of abscisic acid production in plants

• All organs (e.g. roots, leaves, stems, fruits)

• Actions of abscisic acid in plants

• Counteracts effects of auxins and gibberellins

• Maintains dormancy in seeds and buds

• Stimulates guard cells to close stomates (to
  conserve water)