1. University of Arkansas
Plant Hormones
Anjali More
University of Arkansas

2. Why do plants need hormones?
Hormones enable plants to:
Respond to environmental factors and changes
Direct developmental processes

3. Why do plants need hormones?
Light
Parasites
Temperature
Pathogens
Humidity
Insects
Stress
Toxins
Oxygen

4. What are hormones? Harman - “to set in motion”
A chemical messenger from one cell (or group of cells) to another
Signal molecules produced at specific locations
Found in low concentrations
Cause altered processes in target cells at other locations
Found in multicellular organisms

5. What are plant hormones?
Occur in small amounts
Organic compounds
Synthesized by plants
Active at low concentrations
Promote or inhibit growth and developmental responses
Often show a separation from the site of production and the site of action
Plant hormones do not always have all these characteristics
Plant growth regulators or plant growth substances

6. Plant hormones are chemical messengers
Physiological
response
Hormone synthesis
Message

7. General Plant Hormones
Auxins
Cytokinins
Abscisic acid
Ethylene
Gibberellins
Jasmonic acid
Salicylic acid
Classical phytohormones

8. Auxins Auxein - to grow First plant hormone discovered
Indole-3-acetic acid (IAA)
Auxein - to grow
First plant hormone discovered
Occurs in very low concentrations
Confers apical dominance
Regulates developmental processes, e.g. cell division, cell elongation etc
Auxin – important for root development

9. Cytokinins Stimulates cell division Lateral bud development
Delays senescence and promotes nutrient uptake
Effect of cytokinin application on leaf senescence
Rost et al., 1998

10. Abscisic acid ‘Abscisin II’- role in abscission
Released during desiccation (of vegetative tissue)
Produced in response to stress
Synthesized in green fruits and seeds
General growth inhibitor – inhibits fruit ripening
ABA – The stress hormone
jan01/acid0101.htm?pf=1

11. Ethylene Fruit ripening Opening of flowers Induces seed germination
Initiation of stem elongation and bud development
Tomato
Ethylene treated
Banana

12. Gibberellins
Ubiquitous in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns
Many forms exist, named GA…GAn in the order of discovery

13. Gibberellins uninfected infected
Discovered in association with foolish seedling disease of rice (“Bakanae”) caused by the fungus, Gibberella fujikuroi. The fungus produces GA.
uninfected
infected
Yabuta and Sumiki, 1938

14. Gibberellins Synthesized in the apical portions of both stem and roots
Important effect on stem elongation in plants
Application of gibberellins promotes internode elongation
Involved in many other aspects of plant growth
science/ html

15. Functions of gibberellins
- GA
+ GA
Fewer flowers and larger fruits
Delayed harvesting
Increased fruit size
GAs are commercially used for increased fruit size in table grapes and to regulate citrus flowering and rind maturation
Cell elongation
Seed germination
Flower induction
Breaking dormancy
Fruit growth – seedless grapes

16. Gibberellin mutants
Elongated mutants – constitutive GA response (e.g. spy in arabidopsis) or enhanced GA response (e.g. lv in pea)
Dwarf mutants – three groups
Accumulate GA and mostly unresponsive to applied GA (e.g. gai in arabidopsis)
Reduced GA response but attain full responses with high doses of exogenous (added) GA (e.g. lgr in pea)
Reduced GA response but do not respond to the application of high doses of GA (e.g. lk and lkb in pea)
Ross et al. 1997

17. Dwarf mutants
Normal rice (right) and the GA-deficient “superdwarf” mutant.
A comparison of 28 day-old plant of the normal and Dwarf-1 mutant in bean
Praona and Green, 1967

18. Significance of GA mutants
Insight into GA biosynthesis and regulation
Help us understand plant growth and development
Sex determination in maize – creation of double mutants
Suitable for production in space…?

19. GA-deficient “superdwarf” rice and normal wildtype plants – both types are 21 days old

20. The superdwarf mutation occurs in the late steps of GA synthesis
The superdwarf mutation occurs in the late steps of GA synthesis. Other mutations occur at other steps, such as in a hormone receptor. These mutants are valuable tools in studying GA’s role in plant biology.
What experiment could we do to distinguish between these two types of mutation, i.e., synthesis or response?
Hormone synthesis
Physiological
response
Transport of hormone

21. GA exerts its effects on wild type plants – seen here 6 days after treatment with 0 mg/ml or 10 mg/ml GA
0 mg/ml
10 mg/ml

22. Superdwarf rice, 6 days after treatment with 10 microliters of
0, 1, and 10 mg/ml Gibberellic Acid (GA)
0 mg/ml
1 mg/ml
10 mg/ml

23. GA20 GA1 GA-3-beta hydroxylase H inactive active
A hydroxylase is an enzyme that adds a hydroxyl group (-OH) to a substrate
GA1
active

24. The rice superdwarf mutant (also called Hosetsu-waisei dwarf from the original Japanese description) is caused by a change in the gene encoding a GA-3b-hydroxylase.
NORMAL (wildtype) Superdwarf mutant
Deletion of a G residue 5’
DNA
messenger
RNA 5’
transcription 5’ 5’
translation
protein
Normal, functional protein
The deletion of a G causes a premature “stop” codon – so translation ends early and a full normal protein is not made

25. V P G L Q L F R R G P D R W V A V P A V
Shown is a small portion of the rice GA 3b-hydroxylase gene. The G at nucleotide #750 of the gene is deleted in the superdwarf mutant. This causes a change in the amino acids that are encoded after that point. The letters above the DNA sequence are the one-letter abbreviations for the amino acids that are encoded by each triplet codon.
absent in mutant
Wildtype sequence
V P G L Q L F R R G P D R W V A V P A V
721 gtcccggggctgcagctgttccgtcgagggcccgaccggtgggtggcggtgccggcggtg 780
A G A F V V N V G D L F H I L T N G R F
781 gcgggggccttcgtcgtcaacgtcggcgacctcttccacatcctcaccaacggccgcttc 840
H S V Y H R A V V N R D R D R V S L G Y
841 cacagcgtctaccaccgcgccgtcgtgaaccgcgaccgcgaccgggtctcgctcggctac 900
Mutant sequence
V P G L Q L F R R G P T G G W R C R R W
721 gtcccggggctgcagctgttccgtcgaggcccgaccggtgggtggcggtgccggcggtgg 780
R G P S S S T S A T S S T S S P T A A S
781 cgggggccttcgtcgtcaacgtcggcgacctcttccacatcctcaccaacggccgcttcc 840
T A S T T A P S * T A T A T G S R S A T
841 acagcgtctaccaccgcgccgtcgtgaaccgcgaccgcgaccgggtctcgctcggctact 900
Premature “STOP” codon

26. Using codon tables to determine amino acid sequences encoded by DNA
736 c t g t t c c g t c g a g g g c c c g a c c g g t g g
mRNA c u g u u c c g u c g a _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
A.A. L F R R _ _ _ _ _ C F L I Y S P T A W H Q R S R N K M V D E G

27. The experiment:
Plant rice seeds – mutant and wildtype.
Allow to germinate and then treat with GA at a young age.
Can alter volume, concentration, location, plant age, plant species...

28. What properties make these plants potentially useful for space travel?
These mutants, and others like them, have been considered for use in sustaining space travelers
(see .
What properties make these plants potentially useful for space travel?
Mutant wildtype
Plants of the same age