Guard Cells Tolerating Heat

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Gary Tallman. Photo Source: Willamette University Dept. of Biology

The professor that got me into plant biology, taught me plant physiology, and in a many ways helped me develop my technical writing ability is retiring this year. Gary Tallman, PhD is retiring from being the Taul Watanabe endowed Professor of Biology at Willamette University in Salem, OR, but has had a long career in science, but I’m not going to tell the full story and focus on what he started at UC Pepperdine and brought to Willamette. Not only was he a great teacher, but also became a good friend over the years as well.

If you’ve not heard of or been to Willamette, it’s a charming campus in across the street from the Capitol Building in Salem, OR. It was founded in 1842, making it the oldest university in the far western US. There is a lot of Northwest green, with evergreens in many places on campus, including the iconic star trees.

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The iconic Star trees (Sequoiadendron giganteum). Planted in 1942, the 100 year anniversary of Willamette University. And yes, these trees have guard cells too. Photo by Susan Ham
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Willamette University Mill Stream, Clock Tower, and Library

Professor Tallman’s research focus most recently has been on working out how guard cells of tree tobacco, Nicotiana glauca, behave in cell culture. However, his work goes back further that that, as you can see in a Pubmed search of his name.

Tiny Cells, Huge Impact

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Stoma (Stomata, plural) are the pores in plant leaves. Guard cells are the retractable covers of stomata. Photo courtesy of Gary Tallman

Guard cells are amongst the most specialized and environmentally attuned cell types virtually all plants have. Guard cells originated ~400 million years ago when plants colonized land and developed waxy cuticles over most of their surface to avoid drying out. But this raised the problem of how to get carbon dioxide— the key component in the synthesis part of photosynthesis that plants use to build their bodies.

He told me in a Facebook message how his lab came to work on solving how guard cells behave at various temperatures:

“The discovery that guard cells of Nicotiana glauca were heat tolerant was a fortunate discovery created when my research assistant accidentally incubated some cultured cells in a 37C incubator instead of the usual 30C incubator. A few days later we noticed that cell survival was actually quite high but that guard cells were not re-entering the cell cycle under the influence of an auxin and a cytokinin the way they normally would at 30C. We later showed that heat blocks auxin signaling for gene expression in these cells, probably by inhibiting nitric oxide synthesis required to activate the auxin receptor. Never underestimate the power of serendipity.”

Guard cells uncover a pore (singular stoma, plural stomata)  in leaves (& sometimes stems) by increasing their internal water pressure. As water is taken up (through osmosis), the pressure builds inside the cells causing them to bow into a bean shape, opening the leaf for gas exchange— and water loss. Guard cells largely govern how much plants can build their bodies from the carbon dioxide in the air. Yes, plants mostly build their bodies from carbon in the atmosphere. Closing the pore is the reverse of the process as water leaves the cells.  Plants can adjust the aperture for a range of given conditions. Because of their importance as gatekeepers, guard cells have to be sensitive to a lot of environmental inputs: light, humidity, pathogens (that can get into plants through the pore), temperature, and the plant’s water status.

I wrote about one study on how guard cells develop awhile ago. I concluded that post with the fact that guard cells are responsible for rain in tropical rain forests. Tiny cells, huge effect on Earth as a collective. They are a focus of study for many labs for many reasons. For instance, recently, some proposals about how guard cells evolved was reported.

An evolutionary innovation in grass guard cells may well be part of their success (grasses are the major grain staples humans rely on plus another 10,000 or so species of plants). Grasses have ‘subsidiary cells’ adjoining the guard cells that cover the stoma. These cells let grasses respond faster to changing conditions. A small change in one gene involved in specifying guard cell fate allowed that gene product, a protein, to move to neighboring cells and form the subsidiary cells was recently reported in Science.

Research in the Tallman lab fits into work on how temperature affects guard cell and by extension plant physiology. How plants recognize and respond to heat is an important research question in a world where climate change is heating up much of the Earth.

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Guard Cell Protoplasts. Panel A. Freshly isolated. Panel B 32 ºC, 1 week. Panel C 38ºC+ABA (a plant hormone responsible for many stress responses). Panel D. 38ºC. From Nathan N. Gushwa et al. Plant Physiol. 2003;132:1925-1940 (OA).

It’s no small thing to be able to separate pure guard cells from the leaves where they are embedded. And the number of guard cell protoplasts (GCPs) isolated in Dr. Tallman’s lab likely numbers in the tens of millions if not more (each preparation yields tens to hundreds of thousands of cells). It is also a rare case in plant biology of isolating pure cultures of a single cell type, something hard to achieve, even with modern cell sorting technologies that rely on tagging cells with reporters using genes expressed in only certain tissue types.

At Willamette the research in Dr. Tallman’s lab was almost all done by undergraduates and focused on trying to figure out why guard cells isolated in cell culture grown at high temperatures, 38 ºC (100 ºF), survived well, but didn’t grow much. They didn’t reenter the cell cycle either. A trick many plant cells can do is revert back to a stem cell-like state, start dividing, and under the right lab conditions regenerate the whole plant. This line of research answers how plant cells from a plant that grows in the tropics and adapted to high temperatures deal with heat stress. However, working out what is going on inside a single kind of cell is hard enough, let alone a whole plant.

As he noted in his comment above, over years, he and his students figured out that when nitrous oxide production is blocked by high temperatures, it prevents auxin (an important plant hormone) signaling and it’s working with cytokinin to enter the cell cycle. As noted in this 2012 Plant Physiology paper (OA), blocking Nitrous oxide with a chemical inhibitor mimics a lot of the high temperature responses in GCPs, and indeed intact plants. It may well be that chronic higher temperatures (as is happening as climate change continues) will make even the highly thermally tolerant N. glauca susceptible to heat stress. This may suggest a physiological limit for closely related plants. N. glauca belongs to the nightshade family of plants that includes crop plants like tomatoes, potatoes, and chili peppers.

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The model after decades of patient research. From Beard et al. Plant Physiol. 2012;159:1608-1623 (OA). Blocking NO or heat preventing NO leads to long term cell survival. Otherwise, GCPs, with auxin, cytokinin, and ethylene reenter the cell cycle. 

The behavior of GCPs in culture is also a great example of how plant hormones interact and act together to push, pull and ultimately work out a single output. Plants — life generally— is a complex network of interactions, feedback loops, and redundancy. The model of how GCPs respond to heat or nitrous oxide is an example of that.

My Reflections

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The Bistro, student run coffee shop on Willamette’s campus. Great Scones and coffee.

This is where I break from reporting on science to add a more personal note. Gary Tallman is a great teacher and mentor and his students have gone on to do many great things.  I hope some will show up in the comments here and tell their stories of his influence on their lives.Willamette got a good scientist, instructor, and administrator when they hired him from Pepperdine. Teaching is one of those things where impact is hard to measure. It can be immediate and exams can determine learning in a course, perhaps, but there are more subtle things showing up years later too.

I was part of his 1999 fall semester senior class in Plant Physiology. We had to adjust the time of the class on the first day for some reason and of course, the only time eight seniors and one junior had free was the 8am slot. And so that’s when we met, with coffee and trying to be awake that early working through lecture topics on Plant Physiology, reading primary literature as well as Taiz and Zeiger’s Plant Physiology textbook. Exams were hard and take home and open book– several of us pulled all nighters to type up answers and hand in the exams on time (I think I did get a good night’s sleep before turning  in one of the exams).

Professor Tallman also quickly made me realize just how quantitative and mathematical biological science was with water potential equations and more (a lesson I probably should have taken closer to heart– and this was before biology got even *more* mathematical and computer–based– I’m working to learn more now). We had labs too and an independent research project. I remember differentially sealing leaves on one side to see if the guard cells on the other opened more or less compared to a paired control leaf. Probably not a great experiment, but it did give me initial ideas into how to design experiments.

Of course, this class is what got me into plant science and this class along with the institution has made me want to be an educator. I’m sitting here now, a writer about plant science that hopes this blog and my writing serve as a source of learning because of Dr. Tallman and the liberal arts education from Willamette (where I did do more than my required share of ‘writing centered’ courses– Plant Physiology I think was one of them). So I can say Dr. Tallman has had a profound impact on my life and it has always been a pleasure when I’m in Salem, OR visiting campus to hang out with him again.

Again, I invite anyone with memories or stories of Professor Tallman, doing research in his lab, or how he’s influenced your subsequent career to share them in the comments.

 

 

 

 

 

 

 


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