Post on 16-Dec-2015
transcript
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.