light and plant development photomorphogenesis - a change in plant development induced by specific...
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Light and plant development
Photomorphogenesis - a change in plant development induced by specific kinds of light
and not dependent on photosynthesis.
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Photomorphogenesis involves special photoreceptors thatinitiate developmental changes.
Photoreceptors transduce information in the environmentinto appropriate developmental patterns.
Information examples • position in a layered plant canopy • seed depth in soil• presence of competitors• approach of sunrise• day length
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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 responsesphototropisms
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Phytochrome
Detects red and far-red lightProvides information the environmentAnswers questions for plants:
Am I in the light?Do I have competitors?Is it time to flower?
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Many phytochrome responses are reversible
Absorption of red light will cause a response that is reversed by far red light.
Red ≈ 650 - 680 nmFar red ≈ 710 - 740 nm
“photoreversible”
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"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.)
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The classic phytochrome system - lettuce seed germination
Dark = very little germination
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Brief red light causes germination
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second
Red followed by far-red = no germination
first
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secondfirst third
Red then far-red then red again = germination
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secondfirst third fourth
Red then far-red then red then far-redPhotoreversibility
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Lettuce seed germination
Red and far-red have opposing effectsEach can reverse the effect of the other
How does this work?
1. Two antagonistic receptors?
2. One receptor with two forms?
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Lettuce seed germination
Red and far-red have opposing effectsEach can reverse the effect of the other
How does this work?
1. Two antagonistic receptors
2. One receptor with two forms?
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Two forms of phytochrome
“Pr” is red form, peak absorption in red
“Pfr” is far red form, peak absorption in far-red
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Pr PfrRed light
Far-red light
This is the basis of the photoreversible responses
Red light converts Pr to PfrFar red light converts Pfr to Pr
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Pr
Pfr
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Pr PfrRed light
Far-red light
Physiologically active form of phytochrome
germination
De-etiolation
Chlorophyllproduction
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Phytochrome location
In meristematic regions of etiolated seedlings,areas of active cell division and expansion.
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Pfr PrRed light
etiolatedde-etiolated
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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 environmentshave lower red light to far-red light ratio.
What does this do toPfr/Ptotal
ratio?
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Slow vs. rapid phytochrome responses
Slow RapidMorphological responses Biochemical responses
Effects on gene Effects on ion fluxesexpression and turgor
Phytochrome responses vary in timing.
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Nyctinastic leaf movements involve a rapid phytochrome response.
Day Night
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Folding and opening of leaflets involves changes in turgorof two sets of ”motor” cells, the turgor changes being drivenby fluxes of K+ and Cl-. Changes in the PMF also involved.
Fig. 17.14
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Opening and closing of leaflets can be entrainedas a circadian rhythm.
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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 immediatebiochemical 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.
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The mechanism of phytochrome-mediated leaf movements
Phytochrome (Pfr) regulates H+ pumps and K+ channelsof motor cells.
Fig. 17.14
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Phytochrome and plant competition: how do plantsdetect the presence of neighbors that compete
for sunlight?Web Essay 17.2
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
With increased density of
neighbors, the R/FR ratio perceived by stems decreases
because of increased FR
reflection from leaves and stems.
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Plant neighbors?
Far red reflected from other plants.
Red absorbed
Far red enriched = neighbors
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R/F
R r
ati
o
Leaf area index, m2 m-2
i
With increased density of neighbors, the R/FR ratio perceived by stems decreases
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A low R/FR ratio allows internode elongation,a favorable response to potential light competition
by neighbors
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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Pr PfrRed light
Far-red light
Physiologically active form of phytochrome
De-etiolationetiolation
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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 ofthe Pfr form is then present to inhibit internode elongation
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QuickTime™ and aTIFF (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.
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Under other plants?
Red absorbed
Far red reflected from other plants or transmitted.
Far red enriched = understory
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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
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Sunlight
Mostly red
A little far red
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Pr
Pfr
In sunlight
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pr
Pfr
Pfr
In sunlight most P gets converted to Pfr form.
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Pr
Pfr
Start of night
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pr
Pfr
Pfr
Most P in Pfr form.
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Pfr
In the dark
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPr
Pfr
Pfr
Pfr form changes gradually to Pr form.
Pr
Pr
Pr
Pr
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Pfr
After a short night
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPr
Pfr
Pfr
Much Pfr still left.
Pr
Pr
Pr
Pr
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Long day plant = Short night plants
• Needs short night to flower• Needs Pfr still present at end of night• Pfr promotes flowering for LDPs
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Pfr
Later in the night
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pr
Pfr
Pfr
More Pfr changes to Pr.
Pr
Pr
Pr
Pr
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Pfr
After a long night
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPr
Pfr
Pfr
All the Pfr is gone.
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
PrPr
Pr
PrPr
Pr
Pr
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Pr
Pfr
Day dawns
Pfr
PrPr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pfr
Pfr
Pfr
Pfr
Pfr
PfrPfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pfr
Pr
Pfr
Pfr
Most P gets converted to Pfr form again.
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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 P fr form.
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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.
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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
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Day neutral plants
• Flower when mature enough• Maybe other environmental signals (temp?)• Day length (dark length) doesn’t matter