Role of Phytochromes in Shade Avoidance

Ecophysiological and Molecular aspects

Plant Biology MSc course Plants and their Environment 2007

I: Abiotic interactions

Thijs Pons and Marcel Proveniers

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

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

• 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

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

fr 730 nm

degradation

Pr Pfr dark reversion

Physiological action

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)

R:FRPFD 655-665 nmPFD 725-735 nm•open: 1.2•canopy: 0.2

Smith 2000

synthesis

Red – Farred reversibilityan indication that phytochroom is involved

0

20

40

60

80

100

Dar

k

Farr

ed Red

Red

-Fa

rred

Red

-Fa

rred

- Red

germ

inat

ion

(%)

Light responses in seeds

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 Responseirradiation 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)

The three response types are also observed in seedlings

Lactuca sativa(lettuce seeds)

Relative importance of the 3 response types is very different between species and history of the seed

Pons, 2000

Germination in the soil and on the surface

HIR VLFR + LFR VLFR + LFR

Exposure of the weed seed population to light during soil cultivation

Scopel et al. 94

VLFR

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

Lysimachia vulgarisdifferent canopy densities

0

10

20

30

40

50

60

0 20 40 60 80 100irradiance (%)

he

igh

t (c

m)

open stand

dense standa

0.0 0.2 0.4 0.6 0.8 1.0 1.2R:FR

b

Light environment in leaf canopies

Vegetative developmentshade avoidance in leaf canopies

Low Highcanopy density

Nicotiana

Arabidopsis

High Light Spectral Shade

• Petiole hyponasty

• Petiole elongation

• Internode elongation

• Leaf Senescence

Boonman 06 Pierik et al. 2005

Spectral and neutral shade effects

Arabidopsis

High Light

SpectrallyNeutral Shade

Low R:FR erect creeping stem stoloniferous

Potentilla

PFD % 100 24 24 100 24 24R:FR 1.1 1.1 0.2 1.1 1.1 0.2

recta

0

20

40

60

80

100

inte

rno

de

len

gth

(m

m)

reptans

0

50

100

150

200

250

C N S

pet

iole

len

gth

(m

m)

C N S

Huber et al. 1996 Pierik et al. 2005

R:FR effects independent of PARChenopodium a shade avoiding species

Adapted from H. Smith

Reflected light

FR

R & B

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.

Ballare 99

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.

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)