Molecular tuning: Evolution in fast forward

19 Apr 2021

Biologists led by Dario Leister want to give photosynthesis a helping hand. With the goal of increasing crop yields, they are using algae to accelerate the genetic adaptation of crop plants to changing environmental conditions.

Aufnahme vom Inneren des Labors von Prof. Dr. Dario Leister

Light shower for blue-green algae. | © LMU

Visitors to the laboratories of Dario Leister are met by an intense green glow. Vibrators on the floor swirl pale green liquids in bulbous flasks, while above, on illuminated shelves, dozens of Petri dishes teem with dark green patterns. Some of the shapes are square, others are round, while yet others resemble sketchy little fir trees such as a child might draw them. Green liquids in large flasks are also bathed in light at regular intervals in a device at the long end of the laboratory room in Martinsried. This creates colorful effects not unlike the marks made by a fistful of thick colored pencils in every shade of green. “We grow cyanobacteria here,” says Leister.

In his laboratories at Martinsried, Professor Leister, Chair of Plant Molecular Biology, tests how cyanobacteria (sometimes called blue-green algae) and other model systems respond to different light and temperature conditions. Behind the experiments lies a bold, ambitious plan. Biologists worldwide are trying to make the molecular apparatus that enables plants to perform photosynthesis more efficient. As monumental as this challenge is, the necessity is just as great. The human population is growing, and by 2050 there could be as many as 9.7 billion people on the planet. To feed all these mouths, agricultural productivity needs to increase — by more than 50 percent, says Dario Leister. But climate change is driving the loss of more and more farmland. Moreover, the soils in many places are exhausted from intensive cultivation, and the demand for biofuels and foods of animal origin is increasing. Both of these issues are exacerbating the situation.

Consequently, Leister’s team wants to get plants to adapt better and more quickly to climate change. A key to this is photosynthesis, which the scientists plan to make not only more efficient, but also more robust against environmental changes. In photosynthesis, algae and plants use sunlight to split water and fix carbon dioxide, producing energy-rich carbon compounds — and oxygen as a side product. The foundation of life on Earth, photosynthesis furnishes the energy that biosystems and organisms need. To power this process, plants and algae absorb sunlight through photosystems, which are akin to molecular solar power plants. They possess two variants of these energy converters. “You could describe them as the engine of the plants,” says Leister.

The molecular biologist is fond of car analogies: “Photosynthesis is an extremely complex process. There’s no use in just increasing the engine power; it’s about all the components working together in harmony,” explains Leister. Even if some elements of photosynthetic light reactions may seem to be ideal, he adds, the overall efficiency of the conversion of light into biomass has not been optimized in the course of evolution. “Why would it be?” asks Leister. “Evolution means survival, which means producing fit offspring, and that is not synonymous with producing a lot of biomass.” This is precisely where his research comes in.

Leister is pursuing two approaches: One is synthetic biology, where the researchers are recreating the fundamental processes of photosynthesis as they seek to improve the effectiveness of the process. With this approach, the scientists have to simultaneously make the organism more robust, so that it can handle the improved photosynthesis. Or to stick with the car metaphors: “An engine with greater horsepower needs a suitably strong chassis.” Hence the second approach, where the researchers harness the power of evolution in the laboratory by getting organisms to adapt to high light intensities. For this combination of synthetic biology and laboratory evolution, Leister uses various model systems, but above all cyanobacteria, which are members of the Bacteria domain, and out of which the chloroplasts of green algae and plants originated. To make the research findings usable for crops, the researchers then use plants such as the thale cress, which molecular biologists are apt to call their ‘workhorse.’ Recently they have also taken to using camelina (also known as false flax or gold-of-pleasure) as a simple representative of the all-important crop plants.

Leister pours cold water on any overheated expectations: “The processes inside plants are complicated and there are no easy solutions,” he cautions. He cites the example of a much-publicized paper by American colleagues, who presented a supposedly simple solution for improving the effectiveness of photosynthesis. In a prestigious scientific journal, the researchers had reported about genetic modifications to tobacco plants that were supposed to make them more robust against natural fluctuations in the amount of sunlight they receive. The US scientists had deliberately overexpressed three proteins in tobacco plants that play a role in adaptation to light, so that the plants produced more of them. As a consequence, these ‘VPZ’ lines grew faster under field conditions. This result was then hailed as a universal solution for improving the yield of crop plants.

Complex problems require complex solutions

Prof. Dr. Dario Leister steht vor einigen Pfalnzen in einem Gewächshaus.

Prof. Dario Leister in the greenhouse in Martinsried. | © LMU

“To our minds, it all seemed too pat,” recalls Leister. “If better growth meant more offspring, then evolution would have already come up with that solution by itself.” He decided to reproduce the experiment himself in the thale cress model system. Recently published in Nature Plants, his results showed that simple and universally valid solutions for making plants such as tobacco or indeed thale cress more robust against stressful conditions do not actually exist. Although the three key proteins increased the flexibility of photosynthesis, as they had done in the tobacco experiment from the United States, meaning that the plants were able to adapt more quickly to rapidly changing light conditions, they did not grow any faster. In fact, the contrary was true. “The simplest explanation for the different behavior of the genetically modified tobacco and thale cress plants is that the latter do not cope with the higher effectiveness of photosynthesis. This means that other components also have to be modified in the thale cress to ultimately produce added value,” says Leister. “The tough competition for research funding and grants induces people to jump the gun and publish apparent sensations before all the necessary checks have been carried out; in this case, testing the approach directly in crop plants,” he emphasizes. “As researchers, however, we shouldn’t encourage any false expectations, but communicate the sober truth that complex problems usually require complex solutions.”

Leister sees his work as underscoring the complexity of the adaptation process of plants to changing climate conditions. Specifically, he observes, it is not enough to just pull one lever and presto!, crops will be able to cope with increasing drought or fluctuating light intensities, as such attempts had thrown up unforeseeable results. In his research group, therefore, the LMU biologist takes a systems biology approach, which he describes as more ‘holistic’ in nature. “We have to learn to understand these complex networks.”

Step by step, the researchers are learning more about the key components and are developing models to identify the main elements of adaptation. Recently, for example, they discovered that the efficiency of a little protein that transports electrons between the two photosystems of a plant is crucially dependent on the specific architecture of the systems. Now the researchers are trying to establish what the optimal design would look like by varying individual components in a sort of building block approach. Actually deriving a bigger picture from such experiments, however, can only succeed through cooperation with other research groups. Consequently, Leister’s work is also embedded in initiatives such as the Collaborative Research Center TRR 175, which is funded by the German Research Foundation. Leister is the speaker of the research center.

An important first step in the researchers’ systems biology approach is to systematically ascertain how plants react when light and temperature change or they suddenly have to survive with less water, by investigating all molecules that are identifiable in a plant and their concentrations. To this end, the researchers are carrying out experiments with thale cress in Leister’s laboratories. In a targeted manner, they are simulating changes, both fast and slow, now varying the light irradiation, now the water supply, now the temperature.

Next, they analyze at the molecular level which proteins and metabolites have changed in their concentrations following certain alterations, or have changed their position in the cell. In this way, they can identify candidates that could make the plant more robust against high levels of light irradiation, cold, or heat. “At these data volumes, bioinformatics plays an important role, and AI approaches will help systems biology studies make great strides forward,” states Leister.

Nevertheless, Leister sees a fundamental limitation in the systems biology approach. “Say we find proteins or metabolites that increase or decrease under stress, and then try to improve plants by simply producing more or less of them, we run the risk of ending up in the same place as the tobacco researchers from the US,” he explains. “Moreover, we would be neglecting a major aspect of evolution and plant breeding, namely that it’s not just the quantity of a biomolecule that can be decisive, but also its quality — in the case of proteins, that means the change in the amino acid sequence.”

As such, Leister is also working on adaptations at the gene level in Munich. He mainly uses cyanobacteria and green algae for this purpose, because these organisms not only allow him to investigate systems biology adaptation processes that take place at the metabolic level over a few hours and days, but also to produce genetic modifications within weeks and months which adapt these organisms permanently to changing environmental conditions, such as high levels of light, cold, or heat. Cyanobacteria and green algae are attractive for researchers because they adapt very quickly due to their short generation time. “We let nature do the work. It’s evolution in fast forward.”

The molecular biologist points at the bulbous flasks in the vibrators and the arrays of Petri dishes in the basement lab. “In a few months, we managed to produce cyanobacteria which could tolerate many times the light quantity that would normally kill them,” explains Leister. Nowadays, finding the underlying genetic modifications is child’s play. When the same genes are also present in plants, it’s possible to transfer the modified genes to plants such as thale cress and camelina so as to improve their robustness against light stress.

It is only logical, then, that the LMU researchers now want to test the results of the laboratory evolution they had obtained with algae to see how they work in camelina. After all, there remains the task of investigating what boosting photosynthesis means specifically for plants that are important for nutrition. In principle, better photosynthesis provides more energy and more metabolites. But crucially such increases in biomass do not necessarily mean bigger yields — for instance, if the seeds are the parts of the plant that are utilized, as is the case with camelina. So the question becomes: Where does the plant put the energy gains from more efficient photosynthesis? “With camelina as a model for a typical crop plant, we see once again that it’s ultimately the overall system that counts,” says Leister. Researchers only obtain certainty when they carry out experiments in greenhouses.

“There’s still a long way to go before we have crop plants with improved yields through more effective photosynthesis or more robustness against environmental changes,” says Leister. “Only when we systematically tackle the adaptation of plants to changing environmental conditions is there the realistic prospect that we can offer sustainable solutions.” Evolution in fast forward can make an important contribution, but it can only be studied effectively in cyanobacteria and green algae. “Of course, algae and especially cyanobacteria are hardly the same as crop plants, and yet they are so closely related that fundamental metabolic processes are very similar.” The prospect that genetic modifications making the algae more robust against certain environmental changes could have a similar effect in crop plants is clear and obvious. How the manipulated plants will react to heat, drought, or strongly fluctuating light is what the LMU researchers intend to find out. For these experiments, they are using green algae, which are more closely related to crops than cyanobacteria, along with combinations of environmental stressors such as most commonly arise in nature.

With their short generation time and good genetic manipulability, algae are therefore study objects that can be used universally in systems and evolutionary biology investigations, thus bridging the gap to synthetic biology. For Leister, the ideal situation is when both research areas complement each other: “When we know the most important components and their behavior, then we can start to build artificial systems.” Nonetheless, he is quick to add that this research is still in its infancy. Last year, Leister received a generously funded Synergy Grant from the European Research Council for the project “PhotoRedesign: Redesigning the Photosynthetic Light Reactions”.

Together with specialists from the United Kingdom and the Czech Republic, he wants to develop concepts for making even better use of the photochemical potential of sunlight. “We know the photosynthetic apparatus in every detail: every gene, every protein, the exact structure,” says Leister, before reverting to the idiom of the DIY auto mechanic: “Now we could take apart three different engines and try to make a new, better one by recombining the components.” Surprising left-field combinations are conceivable here, he adds, paths that nature never took or did not further pursue in the course of evolution. First, Leister plans to incorporate these systems in cyanobacteria and see what happens. “If we can tune photosynthesis in cyanobacteria, then we will transfer the results to crops in order to improve their yields.”

Text: Hubert Filser

Prof. Dr. Dario Leister is Chair of Plant Molecular Biology at LMU. Born in 1967, Leister studied biochemistry at the University of Tübingen and earned his PhD at the Max Planck Institute for Plant Breeding Research in Cologne. He pursued postdoctoral studies at the John Innes Center, The Sainsbury Laboratory, Norwich, England, and at the MPI for Plant Breeding Research, where he headed up an independent team of junior scientists. He received a Habilitation (formal post-doctorate qualification to become a professor in German universities) in genetics at the University of Tübingen, and in 2005 he was appointed professor at Munich. Leister is speaker for the Transregional Collaborative Research Center “The Chloroplast as a Central Node in the Acclimation of Plants” (TRR 175). Last year, the European Research Council (ERC) awarded him a transnational Synergy Grant.

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