Plant Responses to Light Stress

Kristen LeachAdvisors:

Georgia Davis and Bob SharpOctober 13, 2005

Light Stress

• A plant is under light stress when it is unable to quench the light energy it is receiving either by way of photochemical or non-photochemical process.

• Leads to photoinhibition and possible free radical damage

Photoinhibition

• The reduction in capacity for photosynthesis

• Inhibition is primarily in photosystem II reaction center

• Occurs when the system becomes light-saturated

• Is reversible to some degree• Depends on how adapted the plant is to

varying light conditions

Light in Excess

Long S et al (1994) Annu. Rev. Plant Physiol. Plant Mol. Bio. 45: 633-662

Light Intensity (µmol m-2 s-1)

Ab

sorb

ed L

igh

t (µ

mo

l m

-2 s

-1)

Cloudy Full Sunlight

Free Radicals

• Also known as reactive oxygen species (ROS)

• Superoxide anion (O2-), singlet oxygen,

hydrogen peroxide (H2O2), and hydroxyl radical (OH-)

• Other ROS may also be formed from leakage of the electron transport system

• Reacts with proteins and may cause cellular damage

Evolution of Free Radicals

Taiz L and E Zeiger (2002) Photosynthesis: The light reactions. Plant Physiology 3 rd Edition. Sinauer Associates, Inc., Massachusetts p.137

Photosystem II

http://www.ahpcc.unm.edu/~aroberts/main/psii.GIF

Specific Effects of High Light

LeafMovement

seconds minutes hours weeksdays0

pH Δ

ChloroplastMovement

Developmental Changes

Time

• High light decreases– Leafy area– Seed size– Yield

Effect of High Light

Kasahara M et al (2002) Chloroplast avoidance movement reduces photodamage in plants. Nature420: 829-832.

Chloroplast Movement• Ideal light, chloroplasts line up along the

periclinal walls.

• Increased light intensities, chloroplasts move to the anticlinal walls.

Takagi, S. J Exp Biol 2003;206:1963-1969

Identification of Chloroplast Movement Mutants

Leaves of two week old Arabidopsis plants ethylmethane sulfonate (EMS)- mutagenesis or T-DNA tagged insertions were covered with a black plate with a 1mm slit cut in it, then exposed to strong cool white light for one and a half hour.

Oikawa et al. (2003) The Plant Cell, Vol. 15, 2805-2815

Phototrophins are involved in Light Sensing

• Light is sensed by Phototrophin 1 (Phot 1) and Phototrophin 2 (Phot 2).

• Both sense light in the blue region of the visible spectrum.

• Phot 1 is responsible for the accumulation response under high light.

• Phot 2 is responsible for both the avoidance response under high light and for accumulation response under low light.

Phot 2

Actin Responsible for Movement

• Chloroplast unusual positioning 1 (chup 1)

• Contains an actin filament binding domain

• Actin filaments have been shown to be involved in organelle movement

• Causes chloroplasts to accumulate at the bottom of the cell

Oikawa K et al. (2003) Plant Cell 15: 2805-2815.

Effect of High Light on chup1

Oikawa K et al. (2003) Plant Cell 15: 2805-2815.

Wild Type chup 1

Low Light

High Light

Chloroplast Movement Depends on the Size of the Chloroplasts

• Jeong et al (2002) looked at the effect of size on the movement of chloroplasts under high light conditions.

• Experiments used a transgenic tobacco line with antisense suppression/sense expression AtFstZ which causes larger and fewer chloroplasts because it lacks chloroplast cell division.

Jeong et al (2002) Plant Physiol 129:112-121

Chloroplast Movement and Size

Wild type

AtFstZ Mutant

• If I can find the information I would like to insert a table here talking about the chloroplast size of different species and their response to high light.

Chloroplast Future Studies

• Try to determine what the signaling pathway is.

• Look at chloroplast size in relation to the avoidance response in agronomically important crops.

Leaf Movement

• Heliotropism – movements of a leaf in response to the light environment

• Two types – – Diaheliotropism – a leaf follows the sun as it

crosses the sky. Also known as “solar tracking”.

– Paraheliotropism – a leaf orients itself parallel to the sun’s rays to avoid direct radiance.

Paraheliotropism

• Has been well documented in leguminous species.

• Generally occurs during solar mid-day.

• Protects the plant from high light damage, increased leaf temperatures, and excess moisture loss.

Soybean Paraheliotropism

Biological Factors

• Paraheliotropism is exaggerated in plants experiencing extreme biotic factors.

• Drought and temperature play an important role in the degree of leaf angle change.

Drought and its Effects on Paraheliotropism in Siratro

• Siratro (Macroptilium atropurpureum) is used as a forage crop

• Native to North and Central America

• Related to soybean

Drought and its Effects on Paraheliotropism in Siratro

• Examined the effects of water deficits and temperature on paraheliotropism.

• They exposed leaves of Siratro to elevated light regimes and either restrained the leaf or allowed it to move freely.

• They measured fluorescence-emission characteristics to determine the effect light was having on the photosynthetic appartus.

Ludlow M and O Bjorkman (1984) Planta 161: 505-518.

Drought, Paraheliotropism, and Photoinhibition

Water Stressed Siratro

FM, 6

92

Leaf Temperature (°C)

Heat, Paraheliotropism, and Photoinhibition

Water Stressed Siratro

FM, 6

92

Leaf Temperature (°C)

Restrained

Field-Grown Beans

• Pastenes et al revisited the subject in 2004 where he looked at the effect of water-stress on field-grown beans.

• In his study he included air temperature, humidity and measured leaf angle, D1 protein content, CO2 assimilation, and stomatal conductance.

Pastenes et al (2004) J Exp Bot 56:425-433.

Field-Grown Beans

• Restrained well-watered leaves– Increase in CO2 assimilation.

• Water stressed leaves– Leaf angles throughout the day were greater

compared to watered plants.

• Restrained water stressed leaves– Showed an increase in leaf temperature when

compared to its unrestrained and well-watered counterparts.

Field-Grown Beans

• D1 protein content– Well-watered restrained, water stressed and

water stressed restrained so significantly lower contents when compared to a well-watered plant.

– Three are not significantly different from each other.

Leaf Movement Future Research

• Take a closer look at the net carbon loss when a leaf movement is prohibited and how this will effect seed size, seed quantity, and seed quality

• Determine the molecular mechanisms involved in this response

Future Research

• Need to identify genotypes which can respond faster to high light conditions– Smaller but many chloroplasts– Change leaf angle

• Need to identify genotypes which can respond well to other environmental factors that also effect the light reaction process– Increased drought tolerance– Plants that can respond to a wider range of

temperatures

Summary

• Chloroplast and leaf movements are important avoidance mechanisms.

• They help avoid adverse effects caused from high light damage, increased leaf temperate, and moisture loss.

• There is still a long way to go in understanding the response pathway to both mechanisms.

Acknowledgements

• Dr. Georgia Davis

• Dr. Bob Sharp

• Members of the Davis Lab and Sharp Lab

• NSF Grant DBI-0211842 for my funding.