Download Growth and development

Growth and development
Early experiments on bending
responses to light (phototropism)
led to discovery of hormones
Light
Control
Opaque
cap
Tip
removed
Opaque
collar
Transparent
cap
Gelatin
Block
Mica
sheet
Tropisms - long term bending
responses involving plant growth
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Phototropism - bending towards light
Heliotropism - bending towards the sun
Hydrotropism - bending towards water
Gravitropism - bending under the influence
of gravity.
• Thigmotropism - bending as a response to
touch
Nastic movements
• Short lived responses involving turgor
pressure of cells
• examples include leaf movements following
the sun (e.g. Bean) and sensitivity to touch
e.g. Mimosa
Plant ‘hormones’
• Plant need to coordinate their growth and
development and tune their development to
that of their environment.
• In order to do this, they produce chemicals
which are effective at low concentration
which are moved around the plant.
• This definition is characteristic of animal
hormones. A better phrase is Plant Growth
Regulator.
Auxin
• Discovered as a result
of work on tropisms
• One main compound,
Indole -3- acetic acid
(IAA)
• Produced in growing
points (shoot and root
apical meristems and
young leaves)
• Involved in
– Cell division (with
cytokinin)
– Stem elongation (with
Gibberellins)
– Differentiation of
xylem and phloem
– Branching (with
cytokinins)
– Fruit development
– Tropisms
Cytokinin
• Discovered by trial
and error as a
component of
degraded DNA.
• One compound Zeatin
• Produced in roots
• Involved in:
– Cell division (with
auxin)
– Apical dominance
(with auxin)
Gibberellins
• Discovered because of • Involved in:
‘foolish seedling’
– Stem elongation (with
auxins)
disease of rice (long,
– Fruit growth (with
spindly growth) auxins)
caused by a fungus,
– Germination
Gibberella fujikori that
secretes Gibberellins
• Produced in growing
points (shoot and root
• Many gibberellins
apical meristems and
known (>80)
young leaves)
Abscisic Acid ( ABA)
• Discovered because of
work on leaf
abscission and
dormancy
• One compound (ABA)
• Produced all over the
plant, especially in
green tissues.
• Involved in:
– Leaf and fruit
abscission
– Seed dormancy
– Embryo development
– plant responses to
water stress
Ethylene
• Discovered because of • Involved in:
work on ripening of
– Senescence
fruits (Citrus and
– abscission
pineapple)
– Fruit ripening
• One compound, the
gas ethylene.
• Produced in all tissues,
especially in response
to stress.
Agricultural uses of plant
hormones
• A knowledge of the interactions of plant
hormones in plant development has resulted
in many agricultural applications of plant
hormones, either by their direct application,
or by inhibition of their action.
Agricultural uses of auxins
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Hormone rooting powders
Prevention of fruit drop
Herbicides e.g. 2,4-D
Development of seedless fruits
Use in plant tissue culture for cell division,
somatic embryogenesis and rooting
Agricultural uses of cytokinins
• Use as an ‘anti-ageing’ spray to retard
senescence of cut flowers
• Use in plant tissue culture for cell division,
increasing branching, production of somatic
embryos, and for adventitious shoots (2
routes to get a whole plant back from a
single cell)
Agricultural applications of
Gibberellins
• Dwarf plants have increased agricultural
productivity
• Dwarf apple by treatment with inhibitors
• Enhanced stem elongation e.g. sugar cane
• Production of seedless grapes.
• Used to break dormancy of seeds
Agricultural uses of ABA
• ‘Stress hormone’ causing stomata to close
under water stress
• Used in plant tissue culture to ensure that
somatic embryos develop normally.
Agricultural uses of Ethylene
• Ethylene inhibitors (e.g silver ions) used in
the cut flower trade to increase shelf life.
• Promotes ripening of fruits (bananas are
picked green and ripen on board ship
controlled by ethylene. Controlled ripening
of tomatoes by use of anti-sense to ethylene
• Abscission of fruits to synchronise
harvesting.
Circadian Rhythms
• Rhythms with a cycle of about a day (exact
rhythm will vary, but resynchronised every
day)
• Used to synchronise the plant responses to
the day/night cycle and the seasons,
especially flowering.
Flowering
• A critical daylength is often needed to
induce flowering. 3 responses:
– Short day. - plants will only flower when the
day length is less than a certain length.
– Long day - plants will only flower when the
day length is more than a certain length.
– Day neutral. Flower all year round.
Photoperiodism
‘Short day’ plants
24
h
‘Long day’ plants
Night
Day
Phytochrome
Pr
Red
Pfr
Far red
Slow conversion in
darkness