Plant Responses to Light Stress

1. Plant Responses to Light Stress Kristen Leach Advisors: Georgia Davis and Bob Sharp October 13, 2005

2. 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 Light Stress

3. 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 Photoinhibition

4. Light in Excess Absorbed Light (µmol m-2 s-1) Cloudy Full Sunlight Light Intensity (µmol m-2 s-1) Long S et al (1994) Annu. Rev. Plant Physiol. Plant Mol. Bio. 45: 633-662

5. 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 Free Radicals

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

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

8. Chloroplast Movement Leaf Movement pH Δ Developmental Changes 0 seconds hours days weeks minutes Specific Effects of High Light Time

9. 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.

10. Ideal light, chloroplasts line up along the periclinal walls. Increased light intensities, chloroplasts move to the anticlinal walls. Chloroplast Movement Takagi, S. J Exp Biol 2003;206:1963-1969

11. 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

12. 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. Phototrophins are involved in Light Sensing

13. Phot 2

14. 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 Actin Responsible for Movement Oikawa K et al. (2003) Plant Cell 15: 2805-2815.

15. Effect of High Light on chup1 chup 1 Wild Type Low Light High Light Oikawa K et al. (2003) Plant Cell 15: 2805-2815.

16. 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. Chloroplast Movement Depends on the Size of the Chloroplasts Jeong et al (2002) Plant Physiol 129:112-121

17. Chloroplast Movement and Size Wild type AtFstZ Mutant

18. 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.

19. Try to determine what the signaling pathway is. Look at chloroplast size in relation to the avoidance response in agronomically important crops. Chloroplast Future Studies

20. 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. Leaf Movement

21. 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. Paraheliotropism

22. Soybean Paraheliotropism

23. Paraheliotropism is exaggerated in plants experiencing extreme biotic factors. Drought and temperature play an important role in the degree of leaf angle change. Biological Factors

24. 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

25. 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. Drought and its Effects on Paraheliotropism in Siratro Ludlow M and O Bjorkman (1984) Planta 161: 505-518.

26. Drought, Paraheliotropism, and Photoinhibition

27. Heat, Paraheliotropism, and Photoinhibition Water Stressed Siratro Water Stressed Siratro Restrained FM, 692 FM, 692 Leaf Temperature (°C) Leaf Temperature (°C)

28. 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. Field-Grown Beans Pastenes et al (2004) J Exp Bot 56:425-433.

29. 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

30. 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. Field-Grown Beans

31. 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 Leaf Movement Future Research

32. 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 Future Research

33. 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. Summary

34. Dr. Georgia Davis Dr. Bob Sharp Members of the Davis Lab and Sharp Lab NSF Grant DBI-0211842 for my funding. Acknowledgements