Photomorphogenesis

1
Photomorphogenesis 1) phytochrome-mediated
development phytochromes - absorb red and
far-red light (appear blue) may trigger
seed germination cause deetiolation of
seedlings inhibit flowering
2
deetiolation characteristics of etiolated
seedlings events during deetiolation
3
phytochrome can exist in two forms Pr and
Pfr characteristics of Pr characteristics
of Pfr - phytochrome is synthesized as Pr -
Pfr undergoes dark reversion to Pfr
4
selective degradation - ubiquitination -
degradation Pfr is ubiquitinated selectively
degraded Pr is not ubiquitinated not selectively
degraded relative amount of Pr vs. Pfr is
influenced by many factors
5
Additional factors a) Pr does absorb some
far-red light - plant in far-red light will
have about 3 Pfr (97 Pr) b) Pfr does
absorb some red light, - plant in red light
will have about 85 Pfr (15 Pr)
6
- sunlight usually acts like a red light
source - however, proportion of red vs. far-red
wavelengths varies widely with time, and with
placement of the plant - in some cases, sunlight
may act like a far-red light source
7
Seed germination and seedling growth under a leaf
canopy 1) seed germination for many species,
germination will not occur under a deep
canopy 2) seedling growth deetiolation will
not occur under a deep canopy
8
three categories of phytochrome-mediated
responses 1) low fluence responses - occur at
fluence of 1 1000 mmole of photons/m2 - law
of reciprocity - are photoreversible
9
2) very low fluence responses - occur at
fluence as low as 0.0001 mmole of
photons/m2 - responses are not
photoreversible 3) high irradiance
responses - require high fluence rate - are
not photoreversible
10
Phytochrome and circadian rhythms circadian
rhythms - include many plant processes (oxygen
evolution, stomatal opening, etc.) - the
period of a rhythm
11
also called endogenous rhythms 1) light is
responsible for entrainment of circadian
rhythms - entrainment resets the period 2)
light can directly influence some rhythms -
blue or red light may trigger some processes
12
nyctinasty sleep movements - leaves fold
vertically at night unfold during the
day special structure at the base of the leaf
(or leaflet) - the pulvinus motor cells
undergo cyclical changes in turgor pressure -
ventral motor cells dorsal motor cells
13
during the day, K and Cl- enter the ventral
motor cells and exit the dorsal motor
cells at night, the opposite occurs
14
mechanisms of action of phytochrome signal
transduction standard signal transduction
mechanisms 1) external stimulus causes opening
of membrane channels
15
2) G-protein systems - external stimulus
activates a membrane-bound G-protein - G-protein
activates another protein, e.g. a)
guanylylate cyclase - cyclic GMP - protein
kinases - gene regulatory proteins - effects
on gene transcription b) phospholipase C -
catalyzes the breakdown of a specific
phospholipid - products cause - opening of
calcium channels - activation of another
protein kinase - effects on gene expression
16
information known about phytochrome mechanism of
action 1) can have rapid effects on ion
movements and membrane potential 2) has
effects on gene expression
17
Photomorphogenesis 2) cryptochrome-mediated
development cryptochrome is a blue-light
photoreceptor - the three finger absorption
pattern and blue-light responses
18
cryptochrome-mediated photomorphogenetic changes
- many are similar to those mediated by
phytochrome, but occur more quickly
19
other blue-light photoreceptors, and effects A.
Phototropism and phototropins phototropism
positive phototropism - most studies carried
out on coleoptile tips early studies by
Darwins 1) light perception occurs at the tip
of shoot or coleoptile 2) unequal growth occurs
on the dark side vs. the lighted side of the
shoot
20
- light absorption at tip produces signal sent
downward into lower portions, causing unequal
growth - signal is a chemical signal
21
auxin (stimulates cell elongation) -
synthesized at shoot tips and transported
down - equal distribution produces straight
growth - during phototropism, - light is
absorbed by phototropin - phototropin is a
protein kinase, activated by light
absorption - influences distribution/transpor
t of auxin
22
(No Transcript)
23
B. Stomatal movements and zeaxanthin -
previously discussed stomates open in response
to blue light zeaxanthin - blue-light
photoreceptor for this response hypothesis -
zeaxanthin absorbs blue light triggers protein
kinase signal - activates H-ATPase in guard
cell membrane
24
Photoperiodism - plant responds to the length of
dark vs. light in each daily cycle -
synchronization with seasonal changes can
include control of seed germination stem
elongation anthocyanin synthesis flowering flo
wering is most well-studied of these phenomena
25
photoperiodic control of flowering a) plants
that flower in early fall b) plants that
flower in early spring synchronous
flowering and cross-fertilization
26
plants respond specifically to night length not
to day length response occurs depends on night
length as compared to critical night length
(CNL) 1) short day plants (SDP) response
induced when night length is gt CNL - often
late summer or fall-flowering plants
27
2) long day plants (LDP) response inhibited
when night length gt CNL (flowering occurs when
night length lt CNL) - often spring or early
summer-flowering plants
28
other variations 3) day neutral plants 4)
long-short day plants 5) short-long day
plants
29
a SDP and a LDP could flower in the same
photoperiod since the CNL varies with
species inductive cycle - some species will
respond following a single inductive cycle some
require several inductive cycles response can
be absolute or facultative
30
experimental evidence of CNL 1) photoperiods of
more than, or less than, 24 hours e.g., use SDP
that flowers in a photoperiod of 10 hr. L 14 hr.
D subject plant to 10 hr. L 10 hr. D
photoperiod - results? subject plant to 14
hr. L 14 hr. D photoperiod - results?
31
use LDP that flowers in photoperiod of
13 hr. L 11 hr. D subject to 13 hr. L
13 hr. D photoperiod - results? subject
to 11 hr. L 11 hr. D photoperiod - results?
32
2) night break experiments brief exposure to
light during the dark period e.g., SDP that
flowers in a photoperiod with 9 hr. L 15 hr. D
- interrupt "night" after 6 hours -
results? LDP that does not flower in
photoperiod of 12 hr. L 12 hr. D - interrupt
"night" after 5 hours - results?
33
"day breaks" have no affect on photoperiodic
response
34
photoreceptor appears to be phytochrome - flash
of red light causes a "night break" effect
far-red does not - a flash of red, followed
by far-red, cancels red light effect
35
detection of light in photoperiodism occurs in
leaves photoperiodic induction grafting of
induced leaves - induction produces chemical
stimulus (mRNA?) - transported to shoot
meristem through phloem - at shoot meristem,
causes conversion to floral meristem
36
cold-induced promotion of flowering
vernalization - of imbibed seeds (in annuals),
or of growing plants (in biennials) -
requires temperatures of slightly lt 0 C to 10
C - usually requires several weeks of
exposure chemical flowering stimulus -
grafting of vernalized leaf
37
- grafting can carry chemical flowering
stimulus (florigen) between different species