1. 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

2. Energy - for photo-autotrophic growth
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)

3. 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

4. Photoreceptors in the life cycle

5. Phytochromes, involvement in shade avoidance during the 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

6. 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

7. 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.

8. Phytochrome spectra R:FR Pfr/P: PFD 655-665 nm PFD 725-735 nm
synthesis
Physiological action
R:FR
PFD nm
PFD 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 (seed phytochrome)
Smith 2000

9. Red – Farred reversibility an indication that phytochroom is involved

10. 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
Pons, 2000
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)

11. Germination in the soil and on the surface
VLFR
Exposure of the weed seed population
to light during soil cultivation
Scopel et al. 94

12. Canopy shade
Germination of Plantago major
Pons 2000

13. 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)
- 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.

14. Shade avoidance in vegetative plants
Light environment in leaf canopies
Lysimachia vulgaris
different canopy densities

15. Vegetative development shade avoidance in leaf canopies
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

16. Spectral and neutral shade effects
Arabidopsis
High Light
Spectrally
Neutral Shade
erect creeping
stem stoloniferous
Low R:FR
Huber et al. 1996
Pierik et al. 2005

17. R:FR effects independent of PAR Chenopodium a shade avoiding species
Adapted from H. Smith

18. Reflected light FR absorbed by stems increases with
increasing LAI before PAR (R & B)
incident on the leaves decreases.
This triggers internode elongation and
a phototropic response under
control of phytochrome. FR
R & B
Ballare 99

19. Use of phy mutants Wt phyB 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

20. 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)

21. Vegetative Plants (thursday 3 may)
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)