1. Internal Control of Development: The Plant Growth Regulators
Angiosperms VII
Internal Control of Development:
The Plant Growth Regulators

2. The Growth Regulators Often referred to as “hormones”
adapted from animal physiology
animal hormones are produced in one place (gland) and exert an effect some other location
plant “hormones” don’t always work like that
“growth regulator” better descriptive term

3. Five Basic Groups Auxins Cytokinins Ethylene Abscisic Acid
Gibberellins

4. Early Work on Phototropism
Charles Darwin and his son Francis published early work on the problem:
“Power of Movement in Plants” (1880)
Worked with oat (Avena) Coleoptiles
Demonstrated importance of the “tip” in the plant response

5. Early Darwin Experiments

6. Discovery of Auxin Fritz Went (1928)
Found that he could collect the substance responsible for the bending response
Developed the Avena Coleoptile Curvature Test (first bioassay)
Went called the unknown substance AUXIN

7. Coleoptile Curvature Test

8. What is Auxin?
German workers in the 1930’s identified Went’s auxin as indole-acetic acid (IAA)
Lucky discovery based on study of urine (so the story goes) in pregnant women

9. Formula of IAA

10. Effects of IAA In Phototropism
Causes local elongation of cells on the shaded side
This unequal elongation causes the bending of the stem (coleoptile) toward the light
Mechanism of action is called the acid growth hypothesis

11. Acid Growth Hypothesis
IAA triggers H+ ion transport which lowers the pH
This drop in pH loosens cell wall structure probably via proteins (expansins) which “break” bonds holding the cellulose microfibrils
This creates a “loosening” of the cell wall structure so turgor pressure can “expand” the cells
The effect is quite rapid

12. Cell Wall Expansion

13. Natural vs. Synthetic Auxins
IAA (indole-acetic acid) is the naturally occurring growth regulator
Synthetic auxins
substances which will cause bending the Avena Coleoptile Curvature Test
examples include: indole-butyric acid (IBA), naphthalene acid acid (NAA), 2,4-D and 2,4,5-T

14. AUXIN RESPONSES Apical Dominance
IAA produced in the shoot apex inhibits the development of lateral (axillary) buds
However, it may be that high IAA concentrations stimulate ethylene production which actually inhibits the bud development
Concept used frequently in horticulture (creation of “bushy” shrubs)

15. Other Auxin Responses (cont.)
Abscission
Actively growing leaves and fruits produce large amounts of auxin (IAA) which is transported to the stem
This inhibits abscission of leaves and fruits
Environmental or age changes stimulate production of ethylene which stimulates production of abscission zone forming enzymes

16. Other Auxin Responses (cont.)
Differentiation of Vascular Tissue
Auxin + gibberellins and/or various concentrations of sucrose can stimulate development of xylem/phloem (either or both)

17. Other Auxin Responses (cont.)
Fruit Development
Seeds (result of fertilization) are a source of auxin, which in turn stimulates the formation (not ripening) of the fruit
May form parthenocarpic fruits (tomato and cucumber)

18. Other Auxin Responses (cont.)
Adventitious Root Formation
Several synthetic auxins (especially IBA) are used commercially to stimulate root development in “cuttings”
Some plants produce enough IAA in the shoot or leaves to stimulate the root formation in a cutting without additional hormone

19. Other Auxin Responses (cont.)
Weed Killers and Defoliants
2,4-D and 2,4,5-T effective against dicots
manufacture as part of “Agent Orange” produced toxic trace molecule dioxin (a carcinogen)

20. Cytokinins Discovered in the 1940’s in attempting plant tissue culture
Found that coconut milk stimulated cell division
Trail led to “old herring sperm DNA”
Isolated “kinetin” and dubbed the group of growth regulators cytokinins (after cytokinesis in cell division)

21. Cytokinins (cont.)
Zeatin the first naturally-occurring cytokinin to be isolated
Most cytokinins are produced in roots, but also in seeds, fruits and young leaves
Effects include:
stimulation of cell division
retard senescence in leaf tissues (once used as a bioassay)

22. Cytokinins (cont.)
with IAA, stimulate formation of either roots or shoot
HIGH IAA, low cytokinin = ROOTS
HIGH CYTOKININ, low IAA = SHOOTS

23. Cytokinins (cont.) generally a “juvenile” hormone = keeps things young
used commercially to keep cut foliage “green and fresh”, but NOT for human consumption (a suspected carcinogen because of its nucleotide structure)

24. Strange Observations?
Burning of “illuminating gas” in Europe in 1800s caused trees near street lights to become defoliated on one side
Oranges can cause rapid ripening of bananas (don’t store them together)
The ancient Chinese burned incense in special huts to ripen fruit
“One bad apple can spoil a whole bushel”

25. Ethylene A gas (H2C=CH2), unusual for a growth regulator
Produced in most CLIMATERIC fruits like apples, oranges, tomatoes, bananas
Used widely in the commercial fruit industry (here in Omaha)
The apples you buy in March were probably picked in September

26. Other Ethylene Responses
Promotes flowering in some plants like mangos and pineapples
Some growers may actually set fires near crops
May induce senescence in some flowers (orchids)
Generally promotes leaf and fruit abscission
Also involved in in monocot sex expression flowers, stem elongation (shaking response inhibits normal elongation), waterlogging effects (epinasty)

27. Abscisic Acid (ABA)
First extracted from dormant buds and called dormin
Later, found to be chemically identical with another compound called abscisic acid (unfortunate choice since it is not involved in abscission)
Involved in closure of stomata (guard cells) by stimulating loss of K+ ions (followed by water loss and closure)

28. Gibberellins
Discovered by E. Kurosawa studying “foolish seedling” disease of rice
Fungus, Gibberella fujikuroi, found to be disease agent
Could induce symptoms (stem elongation) from fungus extract
Later, found same substances in plants themselves

29. Effects of Gibberellins
Growth of Intact Plants
elongation AND cell division throughout the plant (unlike auxin)
overcomes genetic dwarfing

30. Gibberellins and Mendel
One of the 7 pairs of traits that Mendel studied in peas as he worked out the basic rules of inheritance was dwarf-tall.
The recessive gene - today called le - turns out to encode an enzyme that is defective in enabling the plant to synthesize GA.
The dominant gene, Le, encodes a functioning enzyme permitting normal GA synthesis and making the "tall" phenotype.

31. Effects of Gibberellins (cont.)
Seed Germination (grasses)
produced by the embryo and stimulates the aleurone layer to synthesize amylases

32. Starch Digestion in Seeds
Treated with 1 ppm GA
Treated with 1 ppb
Treated with water

33. Effects of Gibberellins (cont.)
Used in production of sugarcane (increases biomass)
Mechanism of action not involved in cell wall acidification
May overcome light or cold requirements for seed germination
used in brewing industry to help germinate barley and produce the “malt”

34. Effects of Gibberellins (cont.)
Flowering/Bolting of Biennials
can substitute for “winter” cold period for bolting (flowering) in rosette biennials

35. Effects of Gibberellins (cont.)
Fruit Formation
used to produce larger fruits in open clusters in Thompson seedless grapes