Transcript Plant responses to internal and external signals

Plant responses to internal and
external signals
Chapter 39
I) Plant responses to hormones
*hormones are the major way plants can have a
long term response to stimuli.
A) detection of first
hormone
1) experiments by
Darwin and Darwin
2) experiment by BoysenJenson
B) Auxins
1) any hormone that
causes elongation and
growth
a) mainly found in
apical meristem of
shoot or root tip
b) moves by active
transport to dark side
of plant. Those cells
elongate and plant
grows toward light( or
inhibit growth in root)
c) can be added to
cuttings to promote
adventitous root
formation
C) Cytokinins
1) made in the roots
2) promotes cell
division, root growth,
and germination
3) in equal amounts
auxins and cytokinins
cause cell growth but
not differentiation
4) if cytokinin levels rise,
cells turn into shoot
buds, but if auxin levels
rise roots form
D) Gibberellins
1) Found in meristems
of apical buds and
roots, young leaves,
embryo
2) seed/bud
germination, stem and
leaf growth
3) must be present for a
fruit to set
4) when water is
absorbed by seed
gibberellins are
released to break
dormancy
E) Abscisic Acid(ABA)
1) high levels of ABA in
developing seeds
prevent germination
2) if leaf begins to wilt
from loss of water, ABA
buildup closes stomata
F) Ethylene **
1) auxin levels fall, and
ethylene triggers leaves
to fall off in the fall
2) Allow fruit to ripen
a) acts as positive
feedback
b) “one bad apple
can spoil the whole
bunch”
3) triple response
a) if seed germinates
under something the
mechanical stress on
the tip triggers
ethylene production,
which:
a1) slows growth
a2) thickens stem
a3) causes it to
bend
II) Plant responses to light
1) photomorphogenesis:
effects of light on plant
morphology
2) action spectrum: graph
that relates response to
wavelenth
A) Blue-light photoreceptors
1) cryptochromes:
pigments that inhibit
hypocotyl elongation
2) phototropin: pigment
that allows
phototropism
B) Phytochromes and seed
germination
1) phytochrome: a
protein with a nonprotein section that
responds to light
2) some seeds germinate
when provided with red
light
a) phytochrome response
is reversible and is
determined by the last
flash of light received.
b) Pr(P660) recognizes
red light
Pfr(P730) recognizes far
red
c) as Pr is hit by red light,
it changes to Pfr. Seeds
in the dark have just Pr,
but the appearance of
Pfr triggers
germination.
C) Shade avoidance
1) in full light amount of
Pr and Pfr is equal.
2) in shady areas, more
red light is absorbed
during photosynthesis
from the canopy, so the
presence of more far
red causes a shift to the
production of Pr
3) the increase of Pr
triggers growth of the
stem to try and reach
sunlight so the tree isn’t
shaded out.
III) Biological clocks and circadian
rhythms.
1) circadian rhythm: an
internal “clock” that
keep time for an
organism
A) Light sets the clock
1) Pr is made by plants,
and is converted to Pfr
during the day, which is
the active form.
a) a flash of red light
during the night resets
the clock and shortens
the night for the plant
2) Pr accumulates during
the night as the plant
makes it and it isn’t
converted to Pfr
because there is no red
light.
3) this is what triggers
flowering.
a) long-day plants
a1) flash of red
during the night
“shortens” the night
and can trigger
flowering
a2) if you flash red-far
red, the initial red flash
is cancelled out.
b) Short day plants
b1) are also affected
this way
B) Photoperiodism
1) the response of plants
to length of day and
night
2) Control of flowering
a) short-day plant: will
only flower if day is
shorter than a critical
time( 14 hours, for
example)
a1) can also say they
flower when day length is
decreasing
b) long-day: flower if day
is longer than a critical
period(14 hours for
example)
b11) flower when day
length is increasing
c) Day neutral: flower
plants reaches a certain
point of maturity,
regardless of day length
d) all of these trigger the
hormone florigen in
leaves, which then
travels to shoot tips to
make the flower
e) night length is what
controls this
IV) Responses to things other than
light
A) Response to gravity
1) gravitropism
a) gravity causes
statoliths to settle at
bottom of horizontal
root. Auxins then
congregate here and
inhibit growth, so
opposite side grows
faster to turn root
b) if stem is horizontal,
same thing happen, but
the auxins stimulate
growth to turn stem up
towards sun.
2) thigmotropism
a) respond to touch to
grow around an object