Light and plant development

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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 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

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• Two main photoreceptors
• 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

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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?
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Many phytochrome responses are reversible Absorpt
ion of red light will cause a response that is
reversed by far red light. Red 650 - 680
nm Far 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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Red followed by far-red no germination
second
first
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Red then far-red then red again germination
second
first
third
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Red then far-red then red then far-red
Photoreversibility
second
first
third
fourth
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• Lettuce seed germination
• Red and far-red have opposing effects
• Each can reverse the effect of the other
• How does this work?
• Two antagonistic receptors?
• 2. One receptor with two forms?

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• Lettuce seed germination
• Red and far-red have opposing effects
• Each can reverse the effect of the other
• How does this work?
• 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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Red light converts Pr to Pfr Far red light
converts Pfr to Pr
Red light
Pr Pfr
Far-red light
This is the basis of the photoreversible
responses
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Pr Pfr
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Physiologically active form of phytochrome
Red light
Pr Pfr
Chlorophyll production
Far-red light
De-etiolation
germination
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Phytochrome location In meristematic regions of
etiolated seedlings, areas of active cell
division and expansion.
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Red light
Pfr
Pr
etiolated
de-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 environments have lower red light to
far-red light ratio. What does this do
to Pfr/Ptotal ratio?
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Phytochrome responses vary in timing.
Slow vs. rapid phytochrome responses Slow Rap
id Morphological responses Biochemical
responses Effects on gene Effects on ion
fluxes expression and turgor
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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 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
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Opening and closing of leaflets can be
entrained as 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 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.
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The mechanism of phytochrome-mediated leaf
movements Phytochrome (Pfr) regulates H pumps
and K channels of motor cells.
Fig. 17.14
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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 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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With increased density of neighbors, the R/FR
ratio perceived by stems decreases
i
R/FR ratio
Leaf area index, m2 m-2
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A low R/FR ratio allows internode elongation, a
favorable response to potential light competition
by neighbors
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Physiologically active form of phytochrome
Red light
Pr Pfr
Far-red light
De-etiolation
etiolation
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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
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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?
Far red reflected from other plants or
transmitted.
Red absorbed
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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In sunlight
In sunlight most P gets converted to Pfr form.
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Start of night
Most P in Pfr form.
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In the dark
Pfr form changes gradually to Pr form.
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After a short night
Much Pfr still left.
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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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Later in the night
More Pfr changes to Pr.
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After a long night
All the Pfr is gone.
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Day dawns
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 Pfr 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) doesnt matter