1. Plant Hormones

2. Plant Hormones There are five major types of plant hormones:
Gibberelins
Cytokinins
Ethylene
Abcisic Acid
Auxins
The structure and function of each type of hormone will be described

3. Gibberellins

4. Overview Gibberellins (GAs) regulate and influence: cell elongation
seed germination
dormancy
flowering
sex expression
enzyme induction
leaf and fruit senescence.
Gibberellins regulate growth and influence various developmental processes including cell elongation, seed germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence. 4

5. Germination
Signal starch hydrolysis through inducing the synthesis of the enzyme α-amylase in the aleurone cells
Gibberellins produced in the scutellum diffuse to the aleurone cells where they stimulate the secretion α-amylase
α-Amylase then hydrolyses starch into glucose
Gibberellins cause higher levels of transcription of the gene coding for the α-amylase enzyme
Gibberellins are involved in the natural process of breaking dormancy and various other aspects of germination. Before the photosynthetic apparatus develops sufficiently in the early stages of germination, the stored energy reserves of starch nourish the seedling. Usually in germination, the breakdown of starch to glucose in the endosperm begins shortly after the seed is exposed to water. It is believed that gibberellins in the seed embryo signal starch hydrolysis through inducing the synthesis of the enzyme α-amylase in the aleurone cells. In the model for gibberellin-induced production of α-amylase, it is demonstrated that gibberellins (denoted by GA) produced in the scutellum diffuse to the aleurone cells where they stimulate the secretion α-amylase. α-Amylase then hydrolyses starch, which is abundant in many seeds, into glucose that can be utilized in cellular respiration to produce energy for the seed embryo. Studies of this process have indicated that gibberellins cause higher levels of transcription of the gene coding for the α-amylase enzyme, in order to stimulate the synthesis of α-amylase. 5

6. Gibberellins: Chemical Structure
Gibberelins have complex ring structures
Typically contain carboxylic acid groups
Many specific gibberelins exist
Numeric naming system (i.e. GA#)
May be classified into two structural types:
C-19 Gibberelins (19 carbon)
C-20 Gibberelins (20 carbon)

7. Gibberellins: Chemical Structure
Type 1: 19 Carbon Gibberelins

8. Gibberellins: Chemical Structure
Type 2: 20 Carbon Gibberelins

9. Cytokinins

10. Cytokinins Found in a variety of plants and have many functions
Synthesized in meristematic tissues in roots and transported to aboveground organs
Regulate growth and development of tissue primarily by promoting cell division
Involved in germination, shoot differentiation, leaf senescence
Interacts with other plant hormones for some functions

11. Cytokinins Function
Regulates apical dominance and lateral root initiation
Slows down senescence (plant aging) and chlorophyll degradation in aging leaves
Regulates growth of dicot seedlings in the dark (in combination with ethylene)
Involved in development of sex organs and male sterility
Synthesized in meristematic tissues in roots and transported to aboveground organs

12. Cytokinins Cytokinins contain adenine: Two structure types: Isoprenoid
Isoprene structural units:
Aromatic
Contain aromatic groups

13. Cytokinins: Isoprenoid
Isoprene units
adenine

14. Cytokinins: Aromatic
adenine
Aromatic group

15. Cytokinins: Aromatic
Aromatic group
adenine

16. Ethylene

17. Ethylene Universally produced by all plants
Angiosperms, Gymnosperms, Ferns, Mosses, Liverworts
Also found in some fungi, yeast and bacteria
Important roles in:
Abscission
Germination
Senescence
Stress
response to pathogens

18. Ethylene and Fruit Ripening
Helps fruits go through color change, softening of walls, conversion of starch to sugar
Ethylene is produced in low amounts throughout plant life
some “climacteric” plants have sudden peaks in ethylene synthesis which signals ripening changes
Ethylene gas is sprayed on fruit crops to ripen at same time

19. Ethylene and Stress Some stress situations trigger ethylene production
exposure to heat/cold
physical damage
attack by fungal or bacterial pathogens
flooding that limits oxygen
Similar to Abscisic acid’s stress response

20. Growth and Messaging Ethylene and growth Can act as second messenger
Promotes root growth and root hair growth
Can cause asymmetric growth in stems and leaves
Ethylene regulates seedlings’ horizontal growth & apical hook formation… = “Triple response” of seedlings grown in dark
Can act as second messenger
Auxin, cytokinin can cause ethylene production in seedlings

21. Ethylene’s “triple response”
Apical hook formation

22. Ethylene: Chemical Structure
Ethylene is a very small, simple molecule compared to other plant hormones
Two carbons sharing a double bond
Ethylene is a gas at room temperature

23. Abscisic Acid

24. Abscisic Acid (ABA) Found universally in plants and algae
Many functions! Important roles in:
plant development
bud & seed dormancy
Germination
cell division
leaf senescence
Abscission
cellular response to stress

25. Abscisic Acid Acts as a general inhibitor of growth and metabolism
Inhibits growth in hypocotyls, epicotyls, leaves, coleoptiles
Seed dormancy
ABA promotes seed dormancy so plant seeds can withstand desiccation

26. ABA as a Stress Hormone
ABA increases with various environmental or biological plant stresses
Excess heat, pests, excess salt and/or dehydration
Wilted plants have high levels of ABA
In a drought, ABA increases in some plants, causing the stomata to close, preventing water loss
ABA can also produces osmolytes that protect cell membranes from dehydration

27. Abscisic Acid Chemical Structure
Abscisic acid is a carboxylic acid
Carboxylic acid

28. Auxins

29. It’s All in the Name
“Auxins” from the Greek word αυξανω = "I grow or increase".
They were the first of the major plant hormones to be discovered.
Auxins derive their name from the Greek word αυξανω ("auxano" -- "I grow/increase"). They were the first of the major plant hormones to be discovered. 29

30. Overview essential for cell growth
affects both cell division and cellular expansion.
may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth)
auxin-promoted cellular expansion occurs in the absence of cell division.
auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth)
On the cellular level, auxin is essential for cell growth, affecting both cell division and cellular expansion. Depending on the specific tissue, auxin may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth). In some cases (coleoptile growth) auxin-promoted cellular expansion occurs in the absence of cell division. In other cases, auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth). 30

31. Important Functions
coordination of many growth and behavioral processes in the plant life cycle
stimulate or inhibit the expression of specific genes.
coordinate development at all levels in plants, from the cellular level through organs and ultimately the whole plant.
Auxins have an essential role in coordination of many growth and behavioral processes in the plant life cycle. Auxins directly stimulate or inhibit the expression of specific genes. Auxins coordinate development at all levels in plants, from the cellular level through organs and ultimately the whole plant. 31

32. Master Hormone indole-3-acetic acid (IAA).
the most important member of the auxin family
the most potent native auxin
generates the majority of auxin effects in intact plants
The most important member of the auxin family is indole-3-acetic acid (IAA). It generates the majority of auxin effects in intact plants, and is the most potent native auxin. 32

33. Working Together patterns of active transport are complex
typically act in concert with, or in opposition to other plant hormones
auxins and other plant hormones nearly always interact to determine patterns of plant development.
Auxin’s patterns of active transport through the plant are complex. They typically act in concert with, or in opposition to other plant hormones. In a living plant, it appears that auxins and other plant hormones nearly always interact to determine patterns of plant development. 33

34. Auxin Shared Functions
stimulates cell elongation by stimulating wall loosening factors, such as elastins, to loosen cell walls (with gibberellins)
stimulates cell division (with cytokinins)
applied to callus, rooting can be generated (with cytokinin)
xylem tissues can be generated (with cytokinins)
Auxin stimulates cell elongation by stimulating wall loosening factors, such as elastins, to loosen cell walls. The effect is stronger if gibberellins are also present. Auxin also stimulates cell division if cytokinins are present.
When auxin and cytokinin are applied to callus, rooting can be generated if the auxin concentration is higher than cytokinin concentration. Xylem tissues can be generated when the auxin concentration is equal to the cytokinins. 34

35. More Auxin Shared Functions
promotes femaleness in dioecious flowers (with ethylene)
inhibits or promotes leaf and fruit abscission (with ethylene)
stimulate cell division in the cambium andin tissue culture (with cytokinins)
Auxin promotes (via ethylene production) femaleness in dioecious flowers. They can inhibit or promote (via ethylene stimulation) leaf and fruit abscission. They stimulate cell division in the cambium and, in combination with cytokinins, in tissue culture. 35

36. Auxin Functions Stimulate cell elongation
stimulate differentiation of phloem and xylem
Stimulate root initiation on stem cuttings and lateral root development in tissue culture
mediate the tropistic response of bending in response to gravity and light
suppresses growth of lateral buds
delay leaf senescence
Auxins contribute to a myriad of plant functions. They stimulate cell elongation , differentiation of phloem and xylem and root initiation on stem cuttings and lateral root development in tissue culture. They play a major role in plant growth by mediating the tropistic response of bending in response to gravity and light. The auxin supply from the apical bud suppresses growth of lateral buds. Auxins also delay leaf senescence. 36

37. More Auxin Functions
can induce fruit setting and growth in some plants
involved in assimilate movement toward auxin, possibly by an effect on phloem transport
delay fruit ripening
promote flowering in Bromeliads
stimulate growth of flower parts
stimulate the production of ethylene at high concentrations
inhibit growth by closing the stoma during water stress.
Auxins can induce fruit setting and growth in some plants. They are involved in assimilate movement toward themselves, possibly by an effect on phloem transport. They delay fruit ripening. They promote flowering in Bromeliads and stimulate growth of flower parts. Auxins stimulate the production of ethylene at high concentrations. And they inhibit growth by closing the stoma during water stress. 37

38. Auxins: Chemical Structure
Many naturally occurring auxins exist, along with many synthetic auxins used in agriculture
Most naturally occurring auxins contain an indole ring group or a phenyl group
Auxins (natural and synthetic) are carboxylic acids
Halides are also seen in both natural and synthetic auxins

39. Naturally Occurring Auxins
Carboxylic acid
Note carboxylic acids, and both of these contain indole rings
=IAA, the most important member of the auxin family

40. Naturally Occurring Auxins
A natural auxin with a phenyl ring

41. Synthetic Auxins

42. Synthetic Auxins Ether linkage halogens
Note the ester linkage seen in this synthetic auxin (not in natural)

43. Sources Wikipedia, Auxin, 2010, http://en.wikipedia.org/wiki/Auxin
Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2001.
Delker, C., Raschke, A. and Quint, M., 2008, Auxin dynamics: the dazzling complexity of a small molecule’s message, Planta, vol 227,
Gibberellins: A Short History, from the home since 2003 of a website developed by the now-closed Long Ashton Research Station
Wikipedia, Gibberellin, 2010,
Koning, Ross E Auxins. Plant Physiology Information Website. ( ).
Litwak, G Plant hormones. Elsevier Academic Press: San Diego, CA.
Raghavan, V Molecular embryology of flowering plants. Cambridge University Press. New York, NY.
Srivastava, LM Plant growth and development: hormones and environment. Elsevier Science: San Diego, CA.
the home since 2003 of a website developed by the now-closed Long Ashton Research Station

44. Photo credits: