Transcript Ch. 5 Plant Hormones

Hormonal regulation in plants
Plants do not have a nervous system or
endocrine system
However they do produce hormones
These hormones are produced by
specialised tissues
Hormone
Produced in
Transport in Effects
plant
Auxins
Apical bud,
young leaves
Moves from Growth through elongation of
tip to base
cells, tropisms, apical
through cells dominance, growth of fruit
and delay of ripening
Gibberellins
Young leaves,
roots, shoots
and embryo in
seed
Xylem and
Phloem
Growth through cell division
and cell elongation.
Stimulates germination and
flowering
Cytokinins
Roots
Xylem
Growth through cell division
and differentiation. Breaks
dormancy.
Abscisic Acid
Older leaves,
root caps and
stem
Xylem and
Phloem
Dormancy, shedding leaves
and fruit, closure of stomata
Ethylene
Stems,
ripening fruit,
damaged
tissue, seeds
Diffusion
through
intercellular
spaces
Fruit ripening, aging and
shedding of leaves and
flowers, germination
Auxins and tropisms
Growth of a plant in response to a
stimulus:
– Light (phototropism)
– Gravity (geotropism)
Two types:
– Positive – growing towards the stimulus
– Negative – growing away from the
stimulus
Phototropism
When a plant moves or grows in
response to light, it shows
phototropism
If it moves/grows towards the light, it
shows positive phototropism
This behaviour is controlled by a plant
hormone called auxin
Phototropism
• The tip of the shoot
shown in the
diagram is called a
coleoptile
• This is where auxin
is produced
• The distribution of
auxin within the
shoot changes
depending on the
position of the light
source
Phototropism
If the light
source is
even, the
shoot grows
straight up
Auxin causes the plant to bend towards
the light source because:
– It moves away from the light source,
accumulating on the darker side
– causing growth to occur more quickly at
the darkest side
– because of the difference in growth rates
on the dark side and illuminated side of the
shoot, it starts to bend
Page 154-5,
Fig 5.22
Phototropism
Light source
evenly
distributed over
shoot
Light source
not evenly
distributed over
shoot
Due to even light distribution,
auxin is also evenly distributed
in the shoot.
 Growth rates equal on either
side of the shoot and grows
straight up
Auxin concentrates on the
dark/shaded side of the shoot
causing growth to occur more
quickly there
Uneven growth causes the
shoot to bend towards the light
Phototropism experiments
Geotropism
When a plant moves or grows in
response to gravity, it shows geotropism
If it moves/grows towards the gravity
(like plant roots) it shows positive
geotropism
If it moves/grows away from gravity (like
shoots) it shows negative geotropism
This behaviour is also controlled by a
plant hormone called auxin
Geotropism
Auxin is produced in growing roots and
shoots of a plant
Auxin moves towards gravity, causing
growth to occur more quickly in regions
closer to the source of the gravity
Geotropism
Auxin accumulates here so that
most growth occurs here, shoot
grows upwards
Auxin accumulates here
so that most growth
occurs here, root grows
downwards
Florigen?
The hormonal control of flowering in
plants is not well understood
Some scientists suspect that a hormone
which they have named florigen may be
involved
However florigen has yet to be isolated
(extracted from plants)
It’s existence is disputed
Signal Transduction
Because plant hormones are generally
water-soluble, plant hormones trigger
signal transduction in order to carry out
their effect on target cells
Similar to signal transduction in animal
cells