How could green algae improve our crop plants?
Photosynthesis is the most important biochemical process on our planet, supporting nearly all life on earth. Atmospheric carbon dioxide (CO2) gets “fixed” by plants into carbon-based sugar, an energy-rich compound, using sunlight as a source of energy. All other life forms including animals and humans are ultimately dependent on plants as a source of food and energy. Further, photosynthesis releases oxygen. Over the last three billion years this has generated our present atmosphere, allowing us to breath. But how does photosynthesis work in plants and other photosynthetic organisms and what determines how efficient it is? A question that many researchers try to investigate worldwide.
During evolution, different possibilities have been developed to improve photosynthesis. The basic mechanism of the photosynthesis is always the same, but the process has been adapted or even improved in different groups of photosynthetic organisms. The research team including Prof. Mark Stitt of the Max Planck Institute of Molecular Plant Physiology and scientists from Stanford, Cambridge, Missouri and Bayreuth, analysed how green algae have adapted their photosynthesis to become more efficient.
It all starts with the world’s most abundant enzyme, Rubisco. This protein fixes the atmospheric CO2. But there is a problem, which reduces the photosynthesis efficiency dramatically: Rubisco not only reacts with CO2, but also with oxygen. Photosynthesis first evolved about 3 billion years ago, a time when the Earth’s atmosphere had more abundant CO2 than today, while oxygen was just a trace gas. In these conditions, the reaction with oxygen was negligible. However, as photosynthetic organisms became more and more populous in the ancient Earth, they changed the atmosphere's composition. Nowadays, CO2 makes up only about 0.04 percent of molecules in the atmosphere and oxygen about 21%. The low level of CO2 and the side-reaction with oxygen decreases the rate of photosynthesis and the growth rates of plants, including crops. Oxygen not only competes with CO2, it also leads to the formation of toxic compounds whose elimination requires energy. To avoid this inhibitory effect of oxygen decrease in photosynthetic efficiency, several groups of organisms have evolved mechanisms to accumulate CO2. This excludes oxygen from the binding site of Rubisco and increases photosynthetic efficiency.
In addition to some plants like corn and sugar cane, green algae belong to these photosynthetic organisms that accumulate CO2. While this process can be quite complex in plants, it seems that algae found a relatively easy way. They evolved a microstructure, called the pyrenoid, which contains Rubisco and in which CO2 is concentrated. A pyrenoid provides such a tremendous growth advantage that nearly all algae in the oceans have them. About a third of the planet's carbon fixation is thought to happen in pyrenoids, yet we know almost nothing about how these structures are formed at a molecular level. The international research team analyzed these mechanisms in the model organism Chlamydomonas reinhardtii, a fresh water alga. They wanted to understand how the protein Rubisco gets packed into the pyrenoid and how it gets into contact with the accumulated CO2. The research team in Potsdam developed a method to isolate and analyze pyrenoids and determined which proteins it contained. They found Rubisco, as expected, and a protein that helps Rubisco to fold into its active structure. In addition, a third, so far unknown, protein was identified as present in large amounts in pyrenoids.
The new protein was studied in detail. “When we reduced the level of CO2 in our experiments, the unknown protein was strongly accumulated. Moreover, the size of the pyrenoid increased”, explains Tabea Mettler-Altmann, a former PhD student in this project. To determine the function of this protein, the international team used a mutant of Chlamydomonas reinhardtii, which is not able to produce this protein. “This mutant had strong deficiencies. The pyrenoids were smaller and Rubisco was not assembled into them”, describes Prof. Mark Stitt. The mutants were also severely impaired in their ability to carry out photosynthesis in low CO2. In this way, the research team demonstrated the important structural role of the unknown protein in the pyrenoid biosynthesis. Due to that they called it EPYC1, for Essential Pyrenoid Component 1.
What’s more, proteins similar to EPYC1 are found in most pyrenoid-containing algae, but are not found in algae that lack these microstructures. This suggests a parallel development of the mechanism at several time points of evolution. The question rises whether this simple process could be engineered into crop plants. Should this be possible, it is expected to enhance crop yields by as much as 60 percent. This could improve future yields of different crop plants as rice, wheat or potato by just improving the photosynthesis efficiency. This is good news, in a growing world with a declining area of land that can be used for agriculture. By engineering crop plants with more efficient mechanisms of algae, we could grow more food in less time using less water and less nitrogen fertilizer.