'Road testing' plants reveals photosynthesis secrets

A heat map from the DEPI chamber. Courtesy of Kramer Lab

 

Imagine that you want to test drive a new car, but you’re not allowed to take it out of the parking lot. You may start the engine, but you wouldn’t know, for instance, how it handles on the freeway or winding roads, or if the tires grip on wet pavement. To really know how that car performs, it needs to be road tested and its components carefully monitored.

A similar problem has faced scientists studying plants, and in particular photosynthesis, the process by which plants capture solar energy to generate all of our food and some of our fuels.

For decades, these scientists have been studying photosynthesis in the lab with sophisticated instruments and under carefully controlled conditions. In fact, this “reductionist” approach has allowed researchers to systematically dissect complex processes like photosynthesis into component parts that are much simpler to study. The result has been that we have a pretty detailed picture of this wonderful biological machine that has powered life for over a billion years.

But scientists still don’t understand how this beautiful machine works “on the road,” that is, its natural environment. The problem is both highly complex and extremely important. Photosynthesis is highly sensitive to rapid changes in environmental conditions like light, temperature, humidity, the availability of water and nutrients etc. Even more critically, when the plant cannot properly control photosynthesis under these conditions, it can produce toxic side reactions that can damage or kill it, leading to loss of yield.

“We now suspect that many, if not most, of the genes in a plant are there to help it cope with these environmental changes and perils,” says Dr. David Kramer, Hannah Distinguished Professor in Photosynthesis and Bioenergetics at the MSU-DOE Plant Research Laboratory (PRL).“ And although we know a lot about the core machinery of photosynthesis, we have very little idea what these other genes do. Yet these are the very components that not only keep photosynthesis working efficiently but also keep it from killing the plant!”

 

Bringing nature to the lab

Part of the problem is that photosynthesis has not been ‘road tested,’ and laboratory conditions don’t properly engage plant genes. The Kramer lab (funded by the Department of Energy, Office of Science, Basic Energy Sciences) and the MSU Center for Advanced Algal and Plant Phenotyping (CAAPP) have both set out to solve this problem by bridging the gaps between the lab and the natural environment. To do this, the PRL assembled a broad team of scientists (Linda Savage, Dr. Jeffrey Cruz, Dr. Mio Cruz, Geoffry Davis), engineers (Robert Zegarac), and software developers (Dr. Jin Chen) to build new technologies.

In a new breakthrough, the team has developed a simulation chamber called Dynamic Environmental Photosynthetic Imaging, or DEPI for short, that captures the types of environmental conditions seen in the field and replays them in the lab. Among its many features, it can play with light intensities and durations or replay past weather patterns that have been recorded or new ones projected for the future. “The chambers are equipped with special cameras that can detect and quantify visible signals produced in real time by plants during photosynthesis, and we can see them as pictures and movies,” says Linda Savage, facilities manager at CAAPP. (The image at the very top is an example of a photosynthetic activity heat map of various plants.)

 

Image of the DEPI chamber, with lights and plants ready to be tested.
The DEPI chamber ready to test a new batch of plants.  

 

While traditional methods might rely on sensors applied to a single leaf at a single point in time, DEPI reveals what is happening in the whole plant, over an unlimited time period. As a result, plants are demonstrating a whole range of new processes, most notably varying behaviors under dynamic environmental conditions, such as when light changes rapidly as it might do on a windy day with partially cloudy skies. But, CAAPP director Jeff Cruz says, “Because these simulated conditions are reproducible in DEPI and because of our sophisticated monitors, we can study these processes with high precision and in great detail.”

DEPI can also monitor hundreds of plants at the same time, which can be used to identify and compare specific genes that are involved in these processes. “Once we know the process and the genes, we can identify superior variants and potentially combine genes or traits into elite varieties that can lead to improved field (or road) performance,” Kramer says. In fact, photosynthesis is thought to be a major limitation for crop productivity, but because it is highly complex and has been difficult to study, it has not received the same attention from scientists as other traits.

That is precisely one of the problems PRL is addressing, according to Dr. Christoph Benning, PRL Director and MSU Foundation Professor. “Our scientists are dedicating considerable efforts to understanding basic mechanisms of photosynthesis and how the ‘solar panel,’ the photosynthetic membrane and ultimately the leaf, is built.  David Kramer and his team have taken photosynthesis research to the next level by studying plant photosynthesis in DEPI’s sophisticated environmental chambers.  This has led to new insights into what limits photosynthesis under conditions that ultimately matter to the practitioner, the farmer, or gardener.”

 

Image of the journal cover, including plant heat maps, indicating photosynthetic activity.
The journal cover, including plant heat maps, indicating photosynthetic activity. Source: Elsevier (CC BY 4.0) 

 

Towards a global system

The CAAPP has built an array of DEPI systems that are already being used on a wide range of projects at MSU to fuel discoveries into plant responses to environmental stresses like drought, extreme weather conditions, or pathogen infections. Dr. Gregg Howe, MSU Foundation Professor, specializes in plant defense against herbivores and insects. His lab used the DEPI system towards publishing a Plant Physiology article. “DEPI helped us understand how the stress responses in plants are connected with photosynthetic efficiency, and there are some data reported in that paper that clearly would have not been possible without the high frequency measurements we got from it.”

The CAAPP aims to expand DEPI’s potential. There are massive amounts of data on plant species, too much for any one institution to tackle. And with global warming challenging plants – and by extension the global food supply – to react to increasingly unexpected environmental conditions, humans need to find smart solutions to maintain and improve yields, and fast. That is why Kramer envisions the creation of a multi-institutional international network of DEPI chambers, allowing researchers around the world to collectively tackle these bigger questions and imagine real and impactful solutions to some of our most pressing problems. The DEPI at MSU is a first step into that larger vision.


The Kramer lab has published the article on DEPI in the Cell Systems journal (and made the cover). Primary author is Jeff Cruz. Additional contributors are David Kramer, Linda Savage, Robert Zegarac, Christopher Hall, Mio Cruz, Geoffry Davis, Wm. Kent Kovac, and Jin Chen. In addition, many people contributed to the building and running of the facility, including Nathan Galbreath, Jeremy Broderson, David Hal, and Oliver Tessmer.

The paper can be accessed here.