growth regulators in plants

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. Botany 130, Lecture 16
Plant hormones
Coleoptiles
as a model
system

7. Botany 130, Lecture 16
Plant hormones
Phototropism experiments with
coleoptiles

8. Botany 130, Lecture 16
Plant hormones
Frits Went’s
experiments

9. Botany 130, Lecture 16
Plant hormones
Frits Went’s
experiments

10. Botany 130, Lecture 16
Plant hormones
Frits Went’s
experiments

11. Botany 130, Lecture 16
Plant hormones
Polar Auxin Transport
Fig. 28-3

12. Botany 130, Lecture 16
Plant hormones
Auxin promotes rooting
Fig. 28-6

13. Botany 130, Lecture 16
Plant hormones
Fig. 28-5

14. 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

15. 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

16. Gibberellins

17. Botany 130, Lecture 16
Plant hormones

18. 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

19. 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)

20. Botany 130, Lecture 16
Plant hormones
Auxin
Cytokinin
Fig. 28-9

21. 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)

22. 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
• Stimulates abscission of leaves and fruit
– Used as a harvest aid for cherries
• Promotes fruit ripening (apple, tomato, citrus, coffee)
• Promotes senescence of flowers
• Enhances flowering in pineapples

23. 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)