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

2. 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

3. 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

4. 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 ) Absorbed Light (µmol m -2 s -1 ) CloudyFull Sunlight

5. Free Radicals Also known as reactive oxygen species (ROS) Superoxide anion (O 2 - ), singlet oxygen, hydrogen peroxide (H 2 O 2 ), 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

6. 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

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

8. Specific Effects of High Light Leaf Movement seconds minutes hoursweeksdays0 pH Δ Chloroplast Movement Developmental Changes Time

9. 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.

10. 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

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. 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.

13. Phot 2

14. 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.

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

16. 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

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. 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.

20. 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.

21. 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.

22. Soybean Paraheliotropism

23. 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.

24. 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

25. 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.

26. Drought, Paraheliotropism, and Photoinhibition

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

28. 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, CO 2 assimilation, and stomatal conductance. Pastenes et al (2004) J Exp Bot 56:425-433.

29. Field-Grown Beans Restrained well-watered leaves –Increase in CO 2 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.

30. 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.

31. 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

32. 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

33. 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.

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