Plant responses to internal and external signals
Functions of the 5 classical plant hormones
How cells respond to stimuli:
Signal transduction pathways link signal
perception to response. Plants respond to
biotic and abiotic signals
Plants cannot move and thus unlike animals
have to deal with the environment they find
themselves in
Examples of second messenger is Ca2+ and
cGMP
Relay proteins often undergo post-translational modifications e.g. phosphorylation
Plant hormones:
Plant hormones - Chemical signals that modify or control physiological processes. Plant growth
regulator. Coordinates every aspect of plant growth and development and response to stimuli.
Affect the division, elongation and differentiation of cells.
Preferred term is plant growth regulator as, for example, sucrose can be a signal that acts to bring
about physiological and developmental change (but at much greater concentration). Hormones are
generally considered to act at very low concentrations.
Classical experiments involving grass coleoptiles:
The discovery of plant hormones:
Charles Darwin and his son (late 1800s) conducted
some of the earliest experiments on how plant deal
with the light ‘signal’ – concluded only the tip senses
the light Bosen-Jen > production of mobile signal.
Curving is some way away from the tip –
transmissible signal.
• Agar-coleoptile
hormone experiments:
Frits Went (1926)
experiment
Active compound is derived
from the coleoptile tip.
Active compound is
diffusible. Concluded that
active compound is present
at a higher concentration on
the dark side of a shoot.
Went named the active
, molecule auxin. Greek auxein – means to increase. In other species growth inhibitors may also
work on the ‘light side’.
Overview of plant
hormones:
Plant hormones act at
very low
concentrations >
profound effects on
plant growth and
development (by
division, elongation
and differentiation).
Signal amplification
occurs in signal
transduction
pathways.
Interactions between
different hormones
rather than individual
hormones that bring
about response.
1. AUXIN:
Indole acetic acid (IAA) is a common auxin in plants
Auxin is produced in shoot tips and is transported down the
stem (tip to base polar transport) – not movement in the
other direction
Rate of transport typically 1cm per hour
Auxins – any substance that promotes elongation of
coleoptiles. IAA is the major naturally occurring auxin
Polar auxin transport – mediated by auxin transporters at the base of cells
2,4-D component of Agent Orange – defoliant
• Cell elongation: Stimulates elongation of cells in young developing shoots by binding to a
receptor in the plasma membrane. Auxin can also inhibit growth at high concentrations by
inducing the production of other hormones e.g ethylene which hinders growth: Acid growth
hypothesis.
Auxin binds to a receptor in the PM which activates proton pumps. Expansins become
activated by lower pH allowing microfibrils and cross-linking polysaccharides to slide over
one another. This also encourages uptake of ions into cell which then leads to water uptake
by osmosis > expansion. Auxin also acts to bring about changes in gene transcription >
transcription factors > sustained changes in growth involving new proteins and cell division.
• Plant development: Polar transport of auxin plays a role in pattern formation of the
developing plant. E.g. – reducing auxin flow from the shoot of a branch stimulates growth in
lower branches. Also plays a role in phyllotaxy. Polar transport of auxin from leaf margins
directs leaf venation pattern. Important in the activity of the vascular cambium.
Functions of the 5 classical plant hormones
How cells respond to stimuli:
Signal transduction pathways link signal
perception to response. Plants respond to
biotic and abiotic signals
Plants cannot move and thus unlike animals
have to deal with the environment they find
themselves in
Examples of second messenger is Ca2+ and
cGMP
Relay proteins often undergo post-translational modifications e.g. phosphorylation
Plant hormones:
Plant hormones - Chemical signals that modify or control physiological processes. Plant growth
regulator. Coordinates every aspect of plant growth and development and response to stimuli.
Affect the division, elongation and differentiation of cells.
Preferred term is plant growth regulator as, for example, sucrose can be a signal that acts to bring
about physiological and developmental change (but at much greater concentration). Hormones are
generally considered to act at very low concentrations.
Classical experiments involving grass coleoptiles:
The discovery of plant hormones:
Charles Darwin and his son (late 1800s) conducted
some of the earliest experiments on how plant deal
with the light ‘signal’ – concluded only the tip senses
the light Bosen-Jen > production of mobile signal.
Curving is some way away from the tip –
transmissible signal.
• Agar-coleoptile
hormone experiments:
Frits Went (1926)
experiment
Active compound is derived
from the coleoptile tip.
Active compound is
diffusible. Concluded that
active compound is present
at a higher concentration on
the dark side of a shoot.
Went named the active
, molecule auxin. Greek auxein – means to increase. In other species growth inhibitors may also
work on the ‘light side’.
Overview of plant
hormones:
Plant hormones act at
very low
concentrations >
profound effects on
plant growth and
development (by
division, elongation
and differentiation).
Signal amplification
occurs in signal
transduction
pathways.
Interactions between
different hormones
rather than individual
hormones that bring
about response.
1. AUXIN:
Indole acetic acid (IAA) is a common auxin in plants
Auxin is produced in shoot tips and is transported down the
stem (tip to base polar transport) – not movement in the
other direction
Rate of transport typically 1cm per hour
Auxins – any substance that promotes elongation of
coleoptiles. IAA is the major naturally occurring auxin
Polar auxin transport – mediated by auxin transporters at the base of cells
2,4-D component of Agent Orange – defoliant
• Cell elongation: Stimulates elongation of cells in young developing shoots by binding to a
receptor in the plasma membrane. Auxin can also inhibit growth at high concentrations by
inducing the production of other hormones e.g ethylene which hinders growth: Acid growth
hypothesis.
Auxin binds to a receptor in the PM which activates proton pumps. Expansins become
activated by lower pH allowing microfibrils and cross-linking polysaccharides to slide over
one another. This also encourages uptake of ions into cell which then leads to water uptake
by osmosis > expansion. Auxin also acts to bring about changes in gene transcription >
transcription factors > sustained changes in growth involving new proteins and cell division.
• Plant development: Polar transport of auxin plays a role in pattern formation of the
developing plant. E.g. – reducing auxin flow from the shoot of a branch stimulates growth in
lower branches. Also plays a role in phyllotaxy. Polar transport of auxin from leaf margins
directs leaf venation pattern. Important in the activity of the vascular cambium.
