Regulating Plant Growth and Development
Characteristics of Plant Growth Regulators
Groups of Plant Growth Regulators
Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
Synthetic Growth Regulators
Signals leading to flowering
Plants receive external signals
temp
light
attack from pathogens
man-made sources
heavy metal contamination
air pollution
translated to internal responses using a complex signaling network
involves Plant growth regulators
Endogenous plant growth regulators
naturally occurring, organic substances, produced within the plant, that influence a number of growth and developmental responses
cell division
cell enlargement and differentiation
inducing and breaking dormancy
flowering and fruit development
not a part of any major metabolic pathway but can influence them.
opposed to exogenous externally applied plant growth regulators
PGR
The same conc of a PGR can have different effects in different plants
effects of PGR may vary depending on the maturity or sensitivity of plant tissue
number of hormone receptors
PGR molecules must interact with a receptor molecule
forms a hormone/receptor complex
sets off a chain of events
produces a response
the receptor is usually a protein
the exact nature is a chain of events is mostly unknown
2 possible modes of action
Stimulation/repression of gene transcription
leading to the promotion/suppression of the production of mRNA
example
increase in alpha amylase activity in aleurone cells
in seeds
in response to gibberelic acid
increase in gene transcription leading to greater production of alpha amylase
action on a membrane
allowing the release of a chemical produced for the acid-growth mode of action of auxins
5 main groups
Auxins
Gibberellins
Cytokinins
Abscisic acid
Ethylene
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discovered from some physiological event
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PGR can be single molecules or groups of similar molecules
simplest structure
Ethylene
Abscisic acid
more complex structure
Auxins
Gibberellins
Cytokinins
recently discovered PGR
Jasmonates
involved in regulating plant responses to abiotic and biotic stresses
Brassinosteriods
found to promote stem elongation and cell division
works with auxins to do this
Plant responses to PGR
promotion of plant growth
auxins, gibberellins and cytokinins have a similar range of effects in promoting plant growth and development
for a particular species or response only one of the 3 may be active
inhibition of growth
fruit set in tomatoes is stimulated by auxins but auxins are ineffective in citrus fruits
all tend to promote cell elongation and division
auxins
delay leaf senescence
inhibit leaf abscission
promote parthenocarpy
Cytokinins and gibberellins
can break dormancy
stimulate long day plants to flower
responses may depend on
interactions with other PGR
absolute levels
Abscisic acid and ethylene are associated with
inhibiting growth
dormancy
senescence
fruit rippening
Ethylene
inhibit cell elongation
accelerates senescence of leaves
promotes fruit ripening
Abscisic acid
involved in drought tolerance
bud and seed dormancy
different hormones can be associated with different aspects of development
ABA
Survival
Ethylene
Terminal events
Cytokinin
Cell division
Auxin/GA
Growth
PGR Vs Plant Hormone
Hormones
animal hormones are synthesised in specific organs and transported to other organs to produce a response
PGR
the same PGR can be produced in different tissues and may not need to be transported as the site of action may be close to the site of synthesis
Phototropism
(Went, 1926) discovered that a compound caused curvature of oat coleptiles towards light
experiment
1) Went grew oat seedlings in the light and cut coleoptile tips from them
2) He put the excised tips onto agar
3) He cut the agar into small blocks and put them onto one side of the decapitated shoots in the dark
this caused the coleoptiles to bend in the opposite direction to the agar
the compound was later purified and identified as IAA
indole acetic acid
often equated with auxin
there are other structurally similar compounds which should be classified as auxins
4-chloroindoleacetic acid
widespread but less active than IAA
Indolebutyric acid IBA
was thought to be a synthetic auxin but has now been documented in various dicots
Synthesised in a plant from the amino acid Tryptophan
all have a similar structure
effects of auxins
cell division + differentiation
activation of the vascular cambium in spring
due to auxins and the involvement of gibberellins
high auxin/gibberellins ratios
produce more xylem
low auxin/gibberellins ratios
produce more phloem
elongation growth
different concs of auxins are effective for producing cell elongation in stems and roots
stem growth is promoted at 10-2
to 10-4 g/l
Root elongation is
stimulated at 10
-9 to 10-10 g/l
root growth is inhibited at this conc
shoots grow towards the light, roots grow away
Fruit development
seeds in fruit produce auxin
stimulates fruit development
Apical dominance
Auxin from the shoot apex inhibits growth of lateral buds
if the shoot tip is removed, lateral buds will start to grow within hours
certain levels of IAA stimulate production of ethylene which inhibits bud growth
but applying auxins to buds can promote growth
senescence and abscission
Auxin inhibits senescence and abscission of mature leaves, fruit and flowers
promotes leaf fall at later stages in abscission by stimulating
ethylene production
Transport of auxins
moves primarily
through parenchyma cells in contact with vascular bundles
movement in roots and stems is slow
movement is polar
in roots moving towards the root apex
in stems moving towards the base
requires metabolic energy
In the leaves non-polar transport of
auxin occurs in the phloem
where it is ten times faster than polar transport.