27 Jun Tougher Plants Beating Stress by Protecting Photosynthesis in Genetically Modified Plants
Tougher Plants” by Pals-Rylaarsdam and Tischler
Page 1byRobin Pals-Rylaarsdam and Monica L. Tischler Department of Biological Science BenedictineUniversity, Lisle, ILPart I – StressAlice and Todd looked out over their 25-acre plot of tomatoes. No doubt about it, the plants lookedbad. This was their first harvest after graduating from the University of Florida and blowing alltheir graduation money on their local natural farm venture.“I thought the irrigation system was all we would need. We should have majored in agricultureinstead of graphic design,” said Todd. “We set up a beautiful website, but we don’t have anythingto sell!”With bills coming due and, worse yet, having to ask her parents for another loan, Alice decided tocall the chair of agronomy at her alma mater to see if he could help. His secretary quicklyreferred her to the local agriculture extension office, where a cheerful extension agent answeredthe phone and agreed to drive out to the tomato farm.Question1. What are the major stresses that agricultural plants face?NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
Tougher Plants: Beating Stress by Protecting Photosynthesis in Genetically Modified Plants
Part II – Glycine Betaine
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
“Your plants are facing three environmental stresses out in the field,” Florida State ExtensionAgent Dory told the would-be farmers. “Heat in the summer, frost even in our mild Florida winters,and salt from the irrigation that’s been going on in those fields for decades. Have you consideredplanting a genetically modified strain of tomatoes that can help the plants survive thesestresses?”“But we want to be organic!” said Todd, horrified.
“Although the U.S. Department of Agriculture doesn’t allow genetically modified crops to be labeledas organic, you can raise a crop without using pesticides or chemical fertilizers, and stillproduce pesticide and chemical-free fruits and vegetables,” advised Dory. “Most consumers worrymost about the chemicals. There’s been some really nice work done with using plants that make extra
H3C H3C
CH3 ON CCH2 OH
glycine betaine, a modified amino acid. I’ll show you some papers from the peer-reviewedliterature, where scientists communicate with each other after they make discoveries. Want to takea look?”
Figure 1. The structure of glycine betaine.
“Sure,” said Alice. “What can this glycine betaine thing do for our tomatoes?”Questions2. a. In Figure 2 below, what does wild type mean?b. How is L1 different from wild type?3. What does glycine betaine do for the leaves, flowers, and fruits in cold-exposed plants?
Figure 2. Susceptibility to cold stressat different plant stages. Nine week- a bold wildtype (a, c) and L1 transgenicplants (b, d), at stage when two or three flowers first opened fully, were incubated at 3C for 7days (16h daylight/8h dark), then transferred to a 25C greenhouse. Leaves were observed after oneday in the warm greenhouse (a, b), further flower development was observed after 2 weeks in thewarm greenhouse.c dCredit: Photos by Robin Pals-Rylaarsdam.This figure is intended to simulate Figure 6 in Park, E.-J., Jekni?, Z., Sakamoto, A., DeNoma, J.,Yuwansiri, R., Murata, N. and Chen, T. H. H. (2004), Genetic engineering of glycinebetainesynthesis in tomato protects seeds, plants, and flowers from chilling damage. The Plant Journal,40: 474–487.
“Tougher Plants” by Pals-Rylaarsdam and Tischler
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Part III – Photosynthesis
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
“Those genetically modified plants plants look great,” said Todd. “How does glycine betaine givethem that protection?”Dory showed them another figure from the paper. She asked them some questions so they couldunderstand what they were looking at.Questions
804. What is the X-axis measuring?
5. What is the Y-axis measuring?6. What does WT mean?7. What are L1 and L5?“Those are the kinds of questions you ask for any graph you see in the scientific literature,” saidDory. “But to understand this graph, we need to think a little more about photosynthesis. Do youknow anything about that?”Alice grinned. “We met and fell in love in Introductory Biology our first year of college,” shesaid. “I think Todd would have flunked the course if I hadn’t taught him all about photosynthesis.”Todd pulled a face. “That’s not quite how I remember it, but yes, we used to know aboutphotosynthesis pretty well,” he said. “The graph measures ion leakage. How can that relate tophotosynthesis?”Questions8. Which photosynthesis process is most affected by ion leakage?a. Whether the cells can capture lightb. Whether the Calvin cycle will functionc. Whether ATP synthase will functiond. Whether electrons will transfer from photosystem II
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WT30
L120L5
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00 1 3 5 7
Days after cold treatment
Figure 3. Effects of chilling on various growth parameters. Five-week-old greenhouse-grown wildtype and independent homozygous transgenic lines (L1, L5) were chilled (3°C) for 5 days, thenreturned to greenhouse.Credit: Redrawn and adapted from Figure 5(g) in Park, E.-J., Jekni?, Z., Sakamoto, A., DeNoma, J.,Yuwansiri, R., Murata, N. and Chen, T. H. H. (2004), Genetic engineering of glycinebetainesynthesis in tomato protects seeds, plants, and flowers from chilling damage. The Plant Journal,40: 474–487.
9. What trends can you observe in the graph? List at least two.
“Tougher Plants” by Pals-Rylaarsdam and Tischler
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Part IV – Heat Tolerance
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
“Wow, glycine betaine is great with potential frost damage. Maybe that late frost last spring iswhy our plants look so terrible,” said Todd.Dory nodded. “During the Florida growing season we have to worry about heat as well. Let me showyou a study where glycine betaine helped with heat tolerance in plants. This study is with tobaccoplants, but it should apply to your tomatoes too.”
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WT Genetically modified
020 25 30 35 40 45 50 55Temperature of treatment (oC)
Figure 4. Changes in the oxygen-producing activity of PSII determined with thylakoid membranesisolated from leaves after exposed to different temperatures 25, 30, 35, 40, 45, or 50°C in thechambers for 4 h, in wild type and transgenic plants. The values are mean + SE of three independentexperiments.Credit: Redrawn and adapted from Xinghong Yang, Xiaogang Wen, Hongmei Gong, Qingtao Lu and ZhipanYang, et al. (2007), Genetic engineering of the biosynthesis of glycinebetaine enhancesthermotolerance of photosystem II in tobacco plants, Planta, Volume 225, Number 3 (2007), 719–733.
Questions10. What is the X-axis measuring?11. What is the Y-axis measuring?12. What are are two types of plants studied in this experiment?13. Which process in photosynthesis produces oxygen?14. What trends can you observe in the graph? List at least two.
“Tougher Plants” by Pals-Rylaarsdam and Tischler
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Part V – Photosystem II“Why would heat change the activity of photosystem II?” asked Alice.
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
“Great question,” answered Dory. “Let me show you what photosystem II looks like.” Dory pulled upthe ribbon diagram of the structure of this huge protein complex. Cool, huh?” She asked them,15. “Do you know which part of the molecule goes through the thylakoid membrane?”a. Blue boxb. Red circlesc. Trick question—this is a big bunch of squiggles.“I remember,” said Alice. “Parts of a protein in the membrane are much more stable than thoseinside or outside of the membrane.”“Right,” said Dory,16. “So how does this explain why increased temperatures decrease photosystem II activity?”a. Thylakoid membranes become more permeable to ionsb. The chlorophyll breaks downc. The peripheral proteins lose their ability to bind to the transmembrane proteinsd. Water cannot bind to PSII to form oxygen.
Figure 5. Structure of Photosynthesis II, PDB 2AXT.Credit: Modified from image by Curtis Neveus, used in accordance with the Creative CommonsAttribution- Share Alike 3.0 License, from Wikipedia athttp://en.wikipedia.org/wiki/File:PhotosystemII.PNG.
“Tougher Plants” by Pals-Rylaarsdam and Tischler
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NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE“Tougher Plants” by Pals-Rylaarsdam and TischlerPage 62Licensed image in title block ©brozova|Fotolia, id #36303278. Case copyright held by the NationalCenter for Case Study Teaching in Science, University at Buffalo, State University of New York.Originally published October 23, 2012. Please see our usage guidelines, which outline our policyconcerning permissible reproduction of this work.Part VI – Salt“All right, now we know that the genetically engineered tomatoes can take heat and cold. We’reset!” crowed Todd.“Don’t be too hasty, Todd. There’s still one more potential stress on your tomatoes,” warned Dory.“You’ve planted on a field that had been irrigated for many years. The water evaporates in theheat and leaves behind salt. If you lived in a cold climate you’d have to worry about salt runofffrom the roads in the wintertime. At least that’s not an additional problem for you guys.”“So is there any evidence that glycine betaine can help with salty soil?” asked Alice.“Sure, let’s take a look at this third paper,” suggested Dory. “It’s looking at isolated thylakoidmembranes, but we can gain some good information from this one too.”Questions17. What is the X-axis measuring?18. What is the Y-axis measuring?19. What do lines a, b, and c refer to?20. What trends can you observe in the graph? List at least two.21. Which protein subunit of photosystem II is most stable and likely to be in the membrane?22. Which protein subunit is least stable and likely to be a peripheral membrane protein?23. What does glycine betaine do to protect photosystem II activity?Todd and Alice decided to give the genetically modified tomatoes a chance. They would still need toask her parents for a loan to tide them over, but they were hopeful that the next harvest would getthem on the right track.(c)
(b)(a)Betaine (M)0 1 2 3100020406080Figure 6. Effects of betaine on thedissociation of 18-(a), 23-(b) and 33-(c) kDa extrinsic proteins from PS2 particles by NaCl. PS2 particles were incubated in media thatcontained1.2 M NaCl, 0.3 M sucrose, 0.025 M MES/NaOH (pH 6.5) plus indicated concentrations of betaine.Credit: Redrawn from FEBS Letters, 296(2),N. Murata, P.S. Mohanty, H. Hayashi, G.C. Papageorgiou, Glycinebetaine stabilizes the associationof extrinsic proteins with the photosynthetic oxygen-evolving complex,
187–9. Copyright (1992), with permissionfrom Elsevier. http://www.febsletters.org/.
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