Transcript Slide 1

Plant Responses to Light
Stress
Kristen Leach
Advisors:
Georgia Davis and Bob Sharp
October 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 lightsaturated
• Is reversible to some degree
• Depends on how adapted the plant is to
varying light conditions
Absorbed Light (µmol m-2 s-1)
Light in Excess
Cloudy
Light Intensity (µmol m-2 s-1)
Long S et al (1994) Annu. Rev. Plant Physiol. Plant Mol. Bio. 45: 633-662
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
Chloroplast
Movement
Leaf
Movement
pH Δ
0
seconds
minutes
hours
Time
Developmental
Changes
days
weeks
Effect of High Light
• High light decreases
– Leafy area
– Seed size
– Yield
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
Wild Type
Low Light
High Light
Oikawa K et al. (2003) Plant Cell 15: 2805-2815.
chup 1
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
Heat, Paraheliotropism, and
Photoinhibition
Water Stressed Siratro
Water Stressed Siratro
FM, 692
FM, 692
Restrained
Leaf Temperature (°C)
Leaf Temperature (°C)
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 wellwatered counterparts.
Field-Grown Beans
• D1 protein content
– Well-watered restrained, water stressed and
water stressed restrained so significantly
lower contents when compared to a wellwatered 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
•
•
•
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Dr. Georgia Davis
Dr. Bob Sharp
Members of the Davis Lab and Sharp Lab
NSF Grant DBI-0211842 for my funding.