Plant Biology MSc course Plants and their Environment 2007
I: Abiotic interactions
Thijs Pons and Marcel Proveniers
Role of Phytochromes
in Shade Avoidance
Ecophysiological and Molecular aspects
Light as source of:
• Energy - for photo-autotrophic growth
• Information – maximize/optimize resource acquisition
Shade Avoidance in (shade avoiding) plants:
Avoidance of shade by neighbors in time and space by
adjustment of growth and development that optimizes light capture
for photosynthetic utilization
(and may suppress light capture by neighbors)
Perception of the light climate
Aspects of light climate
Perception by pigments
(photoreceptors)
• Presence of light
• Spectral Photon Distribution
• Photon Flux Density (PFD)
• Directional component
• Photoperiod
-phytochromes
-phototropins
-cryptochromes
-chlorophylls
•Other physiological processes
indirectly associated with light
availability
Photoreceptors in the life cycle
Phytochromes,
involvement in shade avoidance during the life cycle
Life cycle
process
Seed
Dormancy enforced in:
• Darkness
• Continuous light, low R:FR
• High PFD
Seedling
De-etiolation when emerging:
• High R:FR
• Low R:FR
Vegetative plant
Response to low R:FR in (or under) a leaf canopy
• Petiole hyponasty
• Elongation of internodes, petioles and leaves
• Phototropy
• Leaf Senescence; Reallocation of Photosynthetic capacity,
• Increased apical dominance
• Allocation to stems, away from storage, roots, leaves
Mature plant
• Early flower induction at low R:FR
• Reduced seed set at low R:FR
• Photoperiod sensitivity
In this part of the course we will investigate the role of Phytochromes
in different aspects of shade avoidance
• Significance in a Plant’s life cycle
• Physiological characterization of the process
• Which phytochromes are involved in perception and signal transduction
• Interaction with other photoreceptors, other signaling
mechanisms and phytohormones
• Transcription under phytochrome control
• Downstream molecular processes
Life cycle
process
Seed
Dormancy and germination in soil and on the soil surface
under a leaf canopy or in the open
• exposure to light of short duration or very low intensity
• exposure to continuous canopy shade light of variable R:FR
Vegetative
plant
Response to low R:FR in leaf canopies
• petiole hyponasty
• elongation of internodes and petioles
Organization the course
MSc course Plants and their Environment
Part I: Abiotic interactions
Monday 23 April
• Short introduction by lecturers
• Distribution of topics for preparation of presentations
Private Study:
• Preparation of presentations
• Study of the subject of other presentations
• Write Essay
Tuesday 1 & Thursday 3 May
• Presentations of selected topics
• Discussions chaired by other students from the group
Tuesday 8 May
• Hand in Essay that describes in your own words the main topics that were
discussed.
Phytochrome spectra
r 660 nm
Pr
dark reversion
Pfr
fr 730 nm
degradation
synthesis
Physiological action
R:FR
PFD 655-665 nm
PFD 725-735 nm
•open: 1.2
•canopy: 0.2
Pfr/P:
•open: 0.6
•canopy: 0.1
•R: 0.8
•FR: 0.02
Labile phytochrome
phyA
type II
(seedling phytochrome)
Stable phytochrome
phyB (+C, D, E) type I
Smith 2000
(seed phytochrome)
Red – Farred reversibility
an indication that phytochroom is involved
80
60
40
20
Red Farred
- Red
RedFarred
Red
Farred
0
Dark
germination (%)
100
Light responses
in seeds
Relative importance
of the 3 response
types is very different
between species and
history of the seed
Lactuca sativa
(lettuce seeds)
The three response types
are also observed in seedlings
VLFR = Very Low Fluence Response (phyA).
a low concentration of Pfr breaks dormancy
LFR = Low Fluence Response (phyB)
a higher threshold value of Pfr is required for dormancy breaking
HIR = High Irradiance Response
irradiation of long duration inhibits germination;
effect increases with increasing irradiance
FR-HIR most effective at 710 – 720 nm and low R:FR (phyA)
R-HIR most effective in red and white light (phyB in seedlings)
Pons, 2000
Germination in the soil
and on the surface
VLFR
HIR
Exposure of the weed seed population
to light during soil cultivation
Scopel et al. 94
VLFR + LFR
VLFR + LFR
Canopy shade
R
FR
R
FR
LFR + HIR
Germination of Plantago major
Pons 2000
Shade avoidance in seeds
Avoidance of germination at a time when the resulting seedling
would be subject to competition from established plants
- Dormancy enforced by darkness in soil.
Breaking of dormancy by short exposure to light during disturbance.
Functional significance:
Germination is delayed until after a disturbance event.
- Dormancy enforced by canopy shade light (low R:FR)
Functional significance:
- Germination is delayed until an opening in the leaf canopy is created
typically by a disturbance event.
- Dormancy is enforced until the seed is buried in the soil and darkness
maintains dormancy further.
Shade avoidance in
vegetative plants
dense stand
50
height (cm)
Light environment in
leaf canopies
b
open stand
a
60
40
30
20
10
0
0
Lysimachia vulgaris
different canopy densities
20
40
60
irradiance (%)
80
100
0.0
0.2
0.4
0.6
R:FR
0.8
1.0
1.2
Nicotiana
Vegetative development
shade avoidance in leaf canopies
Arabidopsis
High Light
Spectral Shade
Low
High
canopy density
• Petiole hyponasty
• Petiole elongation
• Internode elongation
• Leaf Senescence
Boonman 06
Pierik et al. 2005
Spectral and neutral shade effects
Potentilla
internode length (mm)
reptans
recta
100
Arabidopsis
80
60
40
20
High Light
petiole length (mm)
0
250
200
150
100
Spectrally
Neutral Shade
50
0
PFD %
R:FR
C
N
100
1.1
24
1.1
erect
stem
Huber et al. 1996
S
C
N
S
24
0.2
100
1.1
24
1.1
24
0.2
creeping
stoloniferous
Low R:FR
Pierik et al. 2005
R:FR effects independent of PAR
Chenopodium a shade avoiding species
Adapted from H. Smith
Reflected light
FR absorbed by stems increases with
FR
increasing LAI before PAR (R & B)
incident on the leaves decreases.
This triggers internode elongation and
a phototropic response under
control of phytochrome.
R&B
Ballare 99
Use of phy mutants
Mutants lacking phyB have a constitutive
shade avoidance phenotype.
Long petioles, long internodes, low chlorophyll,
early flowering;
phyA resembles very much the wild type.
This means that the spectrally sensitive component
of shade avoidance is largely under control of phyB.
There are also other phytochromes involved.
Wt
phyB
Photomorphogenesis under control of
phytochrome in vegetative plants
•Shade avoidance
- largely under control of phyB
Functional significance: Maximize capture of light
and its photosynthetic utilization when
similar-sized plants compete for light.
(shade tolerant species do not show
a pronounced shade avoidance response)
Topics for literature discussions on
Involvement of Phytochrome in shade avoidance in seeds and vegetative plants
Seeds (tuesday 1 May)
1. Role of different phytochromes in Arabidopsis
Shinomura et al. (1994), Botto et al. (1996), Hennig et al. (2002)
2. Role of different phytochromes in Tomato
Sinchijo et al. (2001), Appenroth et al. (2006)
3. Interaction between light and temperature effects
Donohue et al. (2007), Penfield et al. (2005)
4. Downstream regulation
Oh et al. (2004), Oh et al. (2006)
Vegetative Plants (thursday 3 may)
5. Role of different phytochromes 1
Devlin et al. (1996), Devlin et al. (1998)
6. Role of different phytochromes 2
Devlin et al. (1999), Franklin et al. (2003)
7. Regulation of gene expression
Salter et al. (2003), Sessa et al. (2005)
8. Primary target genes
Devlin et al. (2003), Roig-Villanova et al. (2006)