Transcript Plant hormones and rice dwarf mutants

Plant Hormones
Anjali More
University of Arkansas
Why do plants need hormones?
Hormones enable plants to:
• Respond to environmental factors and changes
• Direct developmental processes
Why do plants need hormones?
Parasites
Light
Temperature
Pathogens
Humidity
Insects
Stress
Toxins
Oxygen
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
What are plant hormones?
1.
2.
3.
4.
5.
6.
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
Plant hormones are chemical
messengers
Hormone synthesis
Message
Physiological
response
General Plant Hormones
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Auxins
Cytokinins
Abscisic acid
Ethylene
Gibberellins
• Jasmonic acid
• Salicylic acid
Classical
phytohormones
Auxins
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
Cytokinins
• Stimulates cell division
• Lateral bud development
• Delays senescence and
promotes nutrient uptake
Effect of cytokinin application on leaf
senescence
Rost et al., 1998
Abscisic acid
ABA – The stress hormone
• ‘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
www.ars.usda.gov/.../ jan01/acid0101.htm?pf=1
Ethylene
• Fruit ripening
• Opening of flowers
• Induces seed
germination
• Initiation of stem
elongation and bud
development
Tomato
Ethylene treated
Banana
Gibberellins
• Ubiquitous in both flowering (angiosperms) and nonflowering (gymnosperms) plants as well as ferns
• Many forms exist, named GA …GAn in the order of
discovery
Gibberellins
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
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
www.school.net.th/.../ science/10000-6600.html
Functions of gibberellins
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•
•
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Cell elongation
Seed germination
Flower induction
Breaking dormancy
- 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
Fruit growth – seedless
grapes
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
1.
Accumulate GA and mostly unresponsive to applied GA
(e.g. gai in arabidopsis)
2.
3.
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
Dwarf mutants
Normal rice (right) and the GAdeficient “superdwarf” mutant.
A comparison of 28 day-old
plant of the normal and
Dwarf-1 mutant in bean
Praona and Green, 1967
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…?
GA-deficient “superdwarf” rice and normal wildtype
plants – both types are 21 days old
Superdwarf
Wild type
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
Transport of hormone
Physiological
response
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
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
H
GA20
inactive
A hydroxylase is an
enzyme that adds a
hydroxyl group (-OH) to
a substrate
GA1
active
The rice superdwarf mutant (also called Hosetsuwaisei dwarf from the original Japanese description)
is caused by a change in the gene encoding a GA3b-hydroxylase.
NORMAL (wildtype)
Superdwarf mutant
Deletion of a G residue
5’
DNA
5’
transcription
5’
translation
messenger
RNA
5’
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
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
721
781
841
V P G L Q L F R R G P D R W V A V P A V
gtcccggggctgcagctgttccgtcgagggcccgaccggtgggtggcggtgccggcggtg
A G A F V V N V G D L F H I L T N G R F
gcgggggccttcgtcgtcaacgtcggcgacctcttccacatcctcaccaacggccgcttc
H S V Y H R A V V N R D R D R V S L G Y
cacagcgtctaccaccgcgccgtcgtgaaccgcgaccgcgaccgggtctcgctcggctac
780
840
900
Mutant sequence
721
781
841
V P G L Q L F R R G P T G G W R C R R W
gtcccggggctgcagctgttccgtcgaggcccgaccggtgggtggcggtgccggcggtgg
R G P S S S T S A T S S T S S P T A A S
cgggggccttcgtcgtcaacgtcggcgacctcttccacatcctcaccaacggccgcttcc
T A S T T A P S * T A T A T G S R S A T
acagcgtctaccaccgcgccgtcgtgaaccgcgaccgcgaccgggtctcgctcggctact
Premature “STOP” codon
780
840
900
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
F
L
R
_
S
_
_
Y
_
_
C
W
L
P
H
R
Q
I
T
M
V
N
S
K
D
R
G
A
E
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...
These mutants, and others like them, have been
considered for use in sustaining space travelers
(see www.usu.edu/cpl) .
What properties make these plants potentially useful
for space travel?
Mutant
wildtype
Plants of the same age