Transcript P fr - jan.ucc.nau.edu

Light and plant development
Photomorphogenesis - a change in plant
development induced by specific kinds of light
and not dependent on photosynthesis.
Photomorphogenesis involves special photoreceptors that
initiate developmental changes.
Photoreceptors transduce information in the environment
into appropriate developmental patterns.
Information examples
• position in a layered plant canopy
• seed depth in soil
• presence of competitors
• approach of sunrise
• day length
Two main photoreceptors
1. Phytochrome (Chapter 17) - a protein pigment that
absorbs red and far-red light and interconverts
between two forms.
involved with many and varied responses
2. Blue-light photoreceptors (Chapter 18)
guard cell responses
phototropisms
Phytochrome
Detects red and far-red light
Provides information the environment
Answers questions for plants:
Am I in the light?
Do I have competitors?
Is it time to flower?
Many phytochrome responses are reversible
Absorption of red light will cause a response that is
reversed by far red light.
Red ≈ 650 - 680 nm
Far red ≈ 710 - 740 nm
“photoreversible”
"But seeds are invisible. They sleep deep in the
heart of the earth's darkness, until some one
among them is seized with the desire to awaken."
—Antoine de Saint Exupery, The Little Prince
(Harcourt, Brace & World Inc.)
The classic phytochrome system - lettuce seed germination
Dark = very little germination
Brief red light causes germination
Red followed by far-red = no germination
first
second
Red then far-red then red again = germination
first
second
third
Photoreversibility
Red then far-red then red then far-red
first
second
third
fourth
Lettuce seed germination
Red and far-red have opposing effects
Each can reverse the effect of the other
How does this work?
1. Two antagonistic receptors?
2. One receptor with two forms?
Lettuce seed germination
Red and far-red have opposing effects
Each can reverse the effect of the other
How does this work?
1. Two antagonistic receptors
2. One receptor with two forms?
Two forms of phytochrome
“Pr” is red form, peak absorption in red
“Pfr” is far red form, peak absorption
in far-red
Red light converts Pr to Pfr
Far red light converts Pfr to Pr
Pr
Red light
Pfr
Far-red light
This is the basis of the
photoreversible responses
Pr
Pfr
Physiologically active
form of phytochrome
Pr
Red light
Pfr
Far-red light
Chlorophyll
production
De-etiolation
germination
Phytochrome location
In meristematic regions of etiolated seedlings,
areas of active cell division and expansion.
Pfr
de-etiolated
Red light
Pr
etiolated
General effect of Pfr is to reduce internode elongation.
Sun plants tend to be sensitive to amount of Pfr, while
shade plants are often insensitive.
Shaded environments
have lower red light to
far-red light ratio.
What does this do to
Pfr/Ptotal ratio?
Phytochrome responses vary in timing.
Slow vs. rapid phytochrome responses
Slow
Morphological responses
Effects on gene
expression
Rapid
Biochemical responses
Effects on ion fluxes
and turgor
Nyctinastic leaf movements involve
a rapid phytochrome response.
Day
Night
Folding and opening of leaflets involves changes in turgor
of two sets of ”motor” cells, the turgor changes being driven
by fluxes of K+ and Cl-. Changes in the PMF also involved.
Fig. 17.14
Opening and closing of leaflets can be entrained
as a circadian rhythm.
Phytochrome is involved with effects on leaf movement.
1. Red light followed by darkness causes leaflets to
close, and this effect can be reversed by far-red light.
Example of a rapid response based on an immediate
biochemical change.
2. Red/far red treatments influence gene expression of
light harvesting proteins, which in turn alters leaf
movement responses.
Example of a slower response requiring a change in
gene expression.
The mechanism of phytochrome-mediated leaf movements
Phytochrome (Pfr) regulates H+ pumps and K+ channels
of motor cells.
Fig. 17.14
Phytochrome and plant competition: how do plants
detect the presence of neighbors that compete
for sunlight?
Web Essay 17.2
With increased
density of neighbors,
the R/FR
ratio perceived by
QuickTime™ and
a
stems
decreases
TIFF (Uncompressed) decompressor
are needed to see this picture.
because of increased
FR reflection from
leaves and stems.
Plant neighbors?
Red
absorbed
Far red reflected
from other plants.
Far red enriched = neighbors
R/FR ratio
With increased density of neighbors,
the R/FR ratio perceived by stems decreases
i
Leaf area index, m2 m-2
A low R/FR ratio allows internode elongation,
a favorable response to potential light competition by
neighbors
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Physiologically active
form of phytochrome
Pr
Red light
Pfr
Far-red light
etiolation
De-etiolation
The effect of FR reflection by neighboring plants can
be simulated using mirrors that selectively reflect Red
or Far Red light.
As for reflection by plants, FR reflection increases
internode elongation. Recall that this is because little of
the Pfr form is then present to inhibit internode elongation
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Filtering out the FR light
received by the stem reduces
internode elongation at high
densities of neighboring plants.
Under other plants?
Red
absorbed
Far red
reflected
from other
plants or
transmitted.
Far red enriched = understory
Phytochrome and flowering.
When is the right time to flower?
• Unreliable indicators of time of year
– Temperature
– Moisture
– Light levels
• Reliable: length of day/night
– Varies with season
– Varies with latitude
• Detected by phytochrome
Sunlight
Mostly red
A little far red
In sunlight
Pfrr
Pfrr
Pfr
Pfrr
Pfr
Pfr
Pfrr
Pfr
Pr
Pfr
Pfrr
Pfrr
Prfr
Pfr
Pfr
Pfr
Pfrr
Prfr
Pfrr
Pfr
In sunlight most P gets converted to Pfr form.
Start of night
Most P in Pfr form.
Pfrr
Pfrr
Pfr
Pfrr
Pfr
Pfr
Pfrr
Pfr
Pr
Pfr
Pfrr
Pfrr
Prfr
Pfr
Pfr
Pfr
Pfrr
Prfr
Pfrr
Pfr
In the dark
Pfr form changes gradually to Pr form.
Pfrr
Pfr
Prfr
Pfr
Pfr
Pfrr
Prfr
Pfrr
Pr
Pfr
Pfrr
Pfrr
Prfr
Pfr
Pfr
Prfr
Pfrr
Prfr
Pfrr
Prfr
After a short night
Much Pfr still left.
Pfrr
Pfr
Prfr
Pfr
Pfr
Pfrr
Prfr
Pfrr
Pr
Pfr
Pfrr
Pfrr
Prfr
Pfr
Pfr
Prfr
Pfrr
Prfr
Pfrr
Prfr
Long day plant = Short night plants
• Needs short night to flower
• Needs Pfr still present at end of night
• Pfr promotes flowering for LDPs
Later in the night
More Pfr changes to Pr.
Pfrr
Prfr
Pfr
Pfr
Pfr
Pfrr
Pfr
Pfrr
Prfr
Pfrr
Prfr
Pfrr
Pfr
Pfr
Pr
Prfr
Pfrr
Prfr
Pfrr
Prfr
After a long night
All the Pfr is gone.
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Pr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Prfr
Day dawns
Pfrr
Pfrr
Pfr
Pfrr
Pfr
Pfr
Pfrr
Pfr
Pr
Pfr
Pfrr
Pfrr
Prfr
Pfr
Pfr
Pfr
Pfrr
Prfr
Pfrr
Pfr
Most P gets converted to Pfr form again.
Short day plant = Long night plant
• Needs long night
• Needs Pfr gone at end of night
• Pfr inhibits flowering for SDPs
Can trick a SDP into not flowering
with a brief flash of red light
during the long night, this resets
much of its phytochrome to Pfr
form.
LDP
SDP
Long day: Pfr left at end of short night.
Pfr promotes flowering for LDPs.
Pfr inhibits flowering for SDPs.
Short day: Pfr gone at end of long night.
No Pfr to promote flowering for LDPs.
No Pfr to inhibit flowering for SDPs.
Waiting for the right time
• Plants grow leaves until it is time to flower
• LDPs wait until the day is long enough
– Really night short enough
– Some time before June 21
• SPDs wait until the day is short enough
– Really night long enough
– Some time after June 21
• Flower opening happens later
Day neutral plants
• Flower when mature enough
• Maybe other environmental signals (temp?)
• Day length (dark length) doesn’t matter