Plant Hormones

Question 1

Introduction

Answer 1

• As a plant grows its phenotype, is the outcome of an interaction between its genetic instructions, or genotype, and the external environment.
• The growth and differentiation of cells in different parts of the plant are coordinated in response to these inputs.
• There must be communication between these levels.
• How does the plant receive and respond to environmental
inputs or “signals”?
• What communication is inside the plant to adjust growth and
development in response to the environment?
• Part of the answer lies in an understanding of plant hormones.

Question 2

What is a plant hormone?

Answer 2

An organic substance other than a nutrient that in minute amounts modifies a plant physiological process.

Question 3

What are the characteristics of a plant hormone?

Answer 3

Plant sense and respond to external & internal signals
• Characteristic of a hormone
– Must have a set target outcome
– Must be transportable from site of synthesis to site of action
– Must be able to be activated (when needed) and deactivated (when no longer needed)
– Must be present in very low levels when not needed.
– Must have a specific binding site (receptor)
Hormone from the Greek word meaning “to excite”

Question 4

Name 5 Plant Hormones

Answer 4

• Auxins (group of compounds) eg Indole acetic acid (IAA), Indole Butyric acid (IBA)
• Gibberellins (more than 100 forms)
• Abscisic acid (only one)
• Cytokinins (multiple forms)
• Ethylene (only one)

Question 5

What is a Bioassy?

Answer 5

• The actions or effect of plant hormones was known long before the compound was identified.
• Bioassays were developed to quantify a response before we could measure them
• Bioassay – using a biological response to a compound as a way of quantification

Question 6

What are Auxins?

Answer 6

Auxin (IAA)
• Auxins are produced mainly in
actively growing tissue
e.g. shoot tips (shoot apical meristem) and growing leaves and fruits.
• Apical dominance – auxin inhibits lateral bud development
• Delay fruit development and ripening
• Promote cell enlargement and cell division

Question 7

What do Auxins do?

Answer 7

Auxin (IAA)
• Promotes coleoptile bending towards light
• Auxins regulate two important processes in plant growth: phototropism (response to light) and gravitropism (response to gravity)
• Auxins promote stem growth by stimulating cell elongation
• Auxins control vascular differentiation of xylem and phloem
• Auxins stimulate lateral root growth and root initiation on stem cuttings

Question 8

What does Apical Dominance mean?

Answer 8

Apical Dominance

 The growing tip of a plant produces auxin which inhibits the lateral buds

Question 9

What does Apical Dominance do?

Answer 9

Apical Dominance
 The growing tip of a plant produces auxin which inhibits the lateral buds
 Removing the apical meristem stops apical dominance and lateral buds grow
 Used to create “BUSHY” appearance

Question 10

What are the advantages of Apical Dominance?

Answer 10

Advantages of Apical Dominance
• Ensures growth upward (toward the light)
• Suppresses competition
• Ensures a set of backup buds in case the apical bud is damaged

Question 11

What do Gibberellins do (GA)?

Answer 11

Gibberellins (growth promoters)
• Stimulate stem elongation by stimulating cell division and elongation.
• Stimulates bolting/flowering in response to long days.
• Breaks seed dormancy in some plants which require
stratification or light to induce germination.
• Stimulates enzyme production(alpha-amylase)in germinating cereal grains for mobilization of seed
reserves.
• Can cause parthenocarpic(seedless)fruit development.
• Can delay senescence in leaves and citrus fruits.

Question 12

Where are Gibberellins produced and what is their role?

Answer 12

Gibberellins
• Produced in embryonic tissues(meristems)
• Promotes stem elongation
• Many”dwarf”plants are genetic mutants deficient in
gibberellin synthesis ( Pea mutants from pracs)
• Dwarf GA mutant crops – wheat, barley & rice, less
prone to storm damage and are more productive
• Largergrapes–sprayedwithGA
• Enhancestheeffectsofauxin
• Stimulates germination in buds and seeds
• Used commercially to break dormancy
Gibberellin(GA)- stimulates release of seed dormancy
( no GA no germination!)
GA activates
alpha -amylase enzyme in aleurone layer to break down endosperm starch to sugars
ABA (abscisic acid) is an antagonist of GA - it inhibits germination. The balance of ABA and GA determines the ease of germination.

Question 13

What are Cytokinins?

Answer 13

• Isolated from the “liquid endosperm” of coconut (milk)
• Increased cell division
• Plant disease “crown gall” caused by bacterium (Agrobacterium tumefaciens) that produces Cytokinins and auxin
(A. tumefaciens is now used by molecular biologists to insert novel genes into plants)
• Produces a tumour (undifferentiated cells)

Question 14

What role do Cytokinins play?

Answer 14

Cytokinins
• Stimulates cell division.
• Stimulates morphogenesis (shoot initiation/bud formation) in
tissue culture.
• Stimulates the growth of lateral buds-release of apical
dominance.
• Stimulates leaf expansion resulting from cell enlargement.
• May enhance stomatal opening in some species.
• Promotes the conversion of etioplasts into chloroplasts
• Slows up the process of senescence

Question 15

What is Abscisic Acid (ABA)?

Answer 15

Abscisic Acid (ABA)
• Two compounds isolated, one thought to cause abscission of leaves the other promoted dormancy of buds
• Two names - Abscisic acid and Dormin were Found to be the same compound!
• LaterfoundthatABAisnotinvolvedinleaf
abscission (this is caused by ethylene)
• But, the name stuck – NB -Abscisic acid is NOT
involved in leaf abscission!!!
• Stress hormone = anti-GA = growth inhibitor

Question 16

What does Abscisic Acid(ABA) do?

Answer 16

Abscisic Acid (ABA)
• Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
• Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
• Induces seeds to synthesize storage proteins.
• Inhibits the affect of gibberellins on synthesis of alpha-
amylase.
• Effect on induction and maintenance of dormancy.
• Induces wound response in pathogen defenc

Question 17

What is the key Effect of ABA?

Answer 17

Key Effect of ABA
 Protective mechanism in water stress
 ABA is produced in leaves when the water potential of the leaf becomes more negative
• Protective mechanism in water stress
• ABA is produced in leaves when the water potential of the leaf
becomes more negative
• All plants do the above
• NOT ALL PLANTS DO THE FOLLOWING
• ABA precursors (subunits) are produced in the leaves and
move to the root
• Root uses these to produced ABA which is then transported to
the leaves
• ABA triggers guard cells to pump potassium into neighbouring
cells. Water follows by osmosis, guard cells become less turgid
and close.
• Cause the closure of stomata as an “early warning system”

Question 18

What is Ethylene H2C=CH2?

Answer 18

Ethylene H2C=CH2
• Onlygaseoushormone
• Onerottenapplespoilsthebarrel-ethylene
• Ethephon–isthemostwidelyusedgrowth
regulator, when metabolised by plants it is converted to ethylene
• Gasheatersinglasshouses–notgoodasplants senesced – ethylene was part of gas mix.
• Problemswithcostcuttinginglasshouseirrigation systems. Polypropylene pipes were requested; polyethylene was installed- guess what happened to plants!
• IntraplantandInterplantsignalling
• Usedtosynchronizeripening–bananas,rice, cotton, wheat, coffee…

Question 19

What does Ethylene H2C=CH2 do?

Answer 19

Ethylene H2C=CH2
• Stimulates the release of dormancy(anti-ABA)
• Stimulates shoot and root growth and differentiation
• Tripleresponse,Shorter,thicker,slowergrowing
(response to physical damage or contact)
• May have a role in adventitious root formation.
• Stimulates leaf and fruit abscission.
• Stimulates flower and leaf senescence.
• Stimulates fruit ripening.