Plant HormonesPlant HormonesAnjali 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?

Pathogens

Parasites

Humidity

Temperature

Light

Toxins

Insects

Oxygen

Stress

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. Occur in small amounts2. Organic compounds3. Synthesized by plants4. Active at low concentrations5. Promote or inhibit growth and developmental responses6. 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 MessagePhysiological response

General Plant Hormones

• 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

Effect of cytokinin application on leaf senescence

• Stimulates cell division• Lateral bud development• Delays senescence and

promotes nutrient uptake

Rost et al., 1998

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

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

Banana

Ethylene treated

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

Discovered in association with foolish seedling disease of rice Discovered in association with foolish seedling disease of rice (“Bakanae”) caused by the fungus, (“Bakanae”) caused by the fungus, Gibberella fujikuroi. Gibberella fujikuroi. The fungus produces GA.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

Gibberellins

www.school.net.th/.../ science/10000-6600.html

• Cell elongation

• Seed germination

• Flower induction

• Breaking dormancy

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

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. Reduced GA response but attain full responses with high doses of exogenous (added) GA (e.g. lgr in pea)

3. 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

A comparison of 28 day-old plant of the normal and Dwarf-1 mutant in bean

Praona and Green, 1967

Normal rice (right) and the GA-deficient “superdwarf” mutant.

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…?

Superdwarf Wild type

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

Hormone synthesis Transport of hormonePhysiological response

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?

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 of0, 1, and 10 mg/ml Gibberellic Acid (GA)

0 mg/ml 1 mg/ml 10 mg/ml

GA20

GA1

GA-3-beta hydroxylase

H

inactive

active

A hydroxylase is an enzyme that adds a hydroxyl group (-OH) to a substrate

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-3hydroxylase.

DNA

messengerRNA

5’

5’

5’

5’

Deletion of a G residue

NORMAL (wildtype) Superdwarf mutant

transcription

translation

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

protein

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

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

Premature “STOP” codon

Shown is a small portion of the rice GA 3-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

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 _ _ _ _ _

FL

L

I

M

V

S

P

T

A

Y

H

Q

N

K

D

E

C

W

R

S

R

G

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 wildtypePlants of the same age