Biologists from Ludwig-Maximilians-Universitaet (LMU in Munich) significantly improved the tolerance of blue-green algae to high light levels – using artificial evolution in the laboratory.
Sunlight, air, and water are all that cyanobacteria (more commonly known as blue-green algae), true algae, and plants need for the production of organic compounds (i.e. based on carbon) and molecular oxygen through photosynthesis. Photosynthesis is the main source of the building blocks for organisms on Earth. However, too much sunlight reduces the efficiency of photosynthesis because it damages “solar panels”, ie the photosynthetic mechanisms of cyanobacteria, algae and plants. A team of researchers led by LMU biologist Dario Leister have now used “artificial evolution in the lab” to identify mutations that allow single-celled cyanobacteria to tolerate high levels of light. The long-term objective of the project is to find ways to equip cultivated plants with the capacity to cope with the effects of climate change.
The cyanobacteria used in the study were derived from a strain of cells that were used to grow at low levels of light. “To get them out of the shadows, so to speak, we exposed these cells to successively higher light intensities,” Leister explains. In an evolutionary process based on mutation and selection, cells have adapted to the gradual alteration of lighting conditions – and because each cell divides every few hours, the adaptation process took place at a much higher rate than what would have been possible with green plants. To aid the process, the researchers increased the rate of natural mutation by treating cells with mutagenic chemicals and irradiating them with UV light. At the end of the experiment, the surviving blue-green algae were able to tolerate light intensities above the maximum levels that can occur on Earth under natural conditions.
To the team’s surprise, most of the more than 100 mutations that could be linked to increased tolerance to bright light resulted in localized changes in the structures of single proteins. “In other words, the mutations involved primarily affect the properties of specific proteins rather than altering the regulatory mechanisms that determine the amount of a given protein produced,” explains Leister. As a control, the team then introduced the genes for two of the modified proteins, which affect photosynthesis in different ways, into unadapted strains. – And in each case, they found that the change did indeed allow the modified cells to tolerate higher light intensities than the progenitor strain.
Improving the tolerance of crops grown to higher or fluctuating light intensities potentially provides a means of increasing productivity and is of particular interest in the context of ongoing global climate change. “The application of genetic engineering techniques to plant breeding has so far focused on quantitative change – on making more or less of a specific protein,” Leister explains. “Our strategy makes qualitative change possible, allowing us to identify new protein variants with new functions. As long as these variants retain their function in multicellular organisms, it should be possible to introduce them into plants.
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