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Biochemical and paleocyte-based modeling revealed that plants with a new photosynthesis pathway, known as C4, today found in several important crops, appeared when atmospheric carbon dioxide was quite high, about 30 million years ago. According to a team run by biologists from Penn, including a postgraduate student Haoran Zhou, water limits, not Co2, caused an initial spread. Credit: Eric Sucar / Pennsylvania University
In today's Earth's dramatically changing environment, organisms needed to develop new strategies to keep pace. From the midpoint of oligocene, approximately 30 million years ago, the average expected myocenum, about 5 million years ago, reduced carbon dioxide concentrations in the atmosphere by about a third. In the same period, a new form of photosynthesis appeared in the subset of plants – C4 path. In the subset of plants, the C4 pathway supplemented the earlier F3 photosynthesis pathway, which means that these species now use solar energy using two different strategies.
Researchers have long believed that by reducing carbon dioxide levels, plants were diverted to this innovation, but it was a new study. National Academy of Sciences works, based on biochemical modeling conducted by Pennsylvania University biologists and modeling of Paleilkilimus, conducted by the Purdue University group, indicates that water availability can be a decisive factor in the emergence of C4 plants.
"The origins of the original C4 that took place when atmospheric carbon dioxide remained very high seems to depend on water constraints," says Haoran Joe, a graduate of the School of Biology at Art and Science School and the first author's work. "Then, about 5 to 8 million years ago, there is a significant spread of C4 meadows as carbon dioxide becomes lower and lower, and carbon dioxide and light intensity are indeed the limiting factors that contributed to C4 at the time."
"What we are seeing," says Erol Akçay, Penn Professor of Biology, "is that the high-water efficiency of the C4 road is sufficient to create an initial ecological advantage in relatively dry environments. This is the benefit of this kind of physiological modeling. If you were looking for only temperature and carbon dioxide, you could miss that water and light role. "
The work of researchers also suggests that C4 plants could have a competitive advantage over C3 plants, even if the level of carbon dioxide in the atmosphere was still relatively high – the late oligocene.
"The conclusion is that C4 could have evolved fairly quickly than we previously thought," says Penn Brent Helliker, associate professor of biology, who, along with Akçay, serves as an advisor to Zhou. "It supports some molecular clock calculations when C4 evolved."
The first stable compound of the plant with C3 photosynthesis, obtained from photosynthesis, contains three carbon atoms; C4 plants in the first compound have four carbon atoms. First, the C3 pathway developed, which functioned efficiently when the atmosphere was rich in carbon dioxide. However, C4 plants developed dozens of times independently of the C3 plants that were capable of effective photosynthesis despite lower carbon levels, due to the additional process step, which serves to deliver carbon from air to the inner cell layer, where the rest of the cycle proceeds. Using this "closed" system, where photosynthetic machines do not interact directly with external air, C4 photosynthesis allows plants to increase food with less leakage of water than the C3 path.
Today, about a quarter of the planet's vegetative cover consists of C4 plants. C4 has several important crop species, including maize and sugar cane. The results of fossil recordings and isotopic studies have helped scientists to evaluate when this path evolved, although these calculations were later than those provided by molecular clocks from phylogenetic analyzes of different plant species, resulting in some uncertainty as to when the path appeared and when it dominated some ecosystems.
Zhou, Akçay and Helliker developed a multilayer model to find out the factors that could contribute to the C4 photosynthesis pathway. They considered variables that affect photosynthesis as well as those that influence the hydraulic system in which the plants "decide" either to allocate more energy to the growth roots to soak up water or to make more leafy material that could help obtain light and carbon dioxide, but also expose them to more water loss. In addition, plants can determine the optimal balance between carbon growth and water losses. In relation to these two systems, the researchers' model contained four factors that could either favor C3 or C4 lines: carbon dioxide concentration, light, temperature and water availability.
According to model C4, the evolution turned out to be two phases. When carbon dioxide was still high, the C4 appeared on the globe that had become warmer and smarter. But it failed to gain a competitive advantage over C3 factories up to several million years later, when carbon dioxide was very low, and the meadow areas provided open-air habitats with many lights. In these regions, the C4 meadows expanded and replaced the C3 meadows.
To see how this model interacts with the days of paleoclimate C4 plants, the Penn team collaborated with Purdue University's Matthew Huber, Pauleclimate model funded by the National Science Foundation to simulate the myocenic climate, and graduate Ashley Dicks. Using climate model output and Paleoclot data, including carbon dioxide levels, temperatures and rainfall, researchers predicted the probable geographical distribution of C3 to C4 plants over the period from late oligocene to early myocen in about 30 to 5 million years ago. They found two regions that were not previously identified, since most likely after the first development, C4 plants could dominate due to water efficiency: Northwest Africa and Australia.
"These are two previously unrecognized world pockets, where C4 plants could have had an ecological advantage and are really taken over," says Akçay.
"It was a really exciting opportunity," says Huber, "when the Penn band approached us, because it is a truly new application in the production of the paleoclimate model, helping to establish a link between what climate models show us about the past and future climate and verifiable models from geological record. "
Although the study did not investigate what might happen in the future as atmospheric carbon levels increase again, it can help to understand why plants are distributed as they are today and how they can respond to future conditions.
"The climatic conditions that were present when C4 evolved today may still be important," says Hellicker. "If the origin of C4 plants originated mainly due to water restrictions when carbon dioxide was high, then these plants can be found today in a dry environment, while if it is more carbon dioxide, leading to their evolution and dominant position, these plants can be found today in damp places. "
In addition, some scientists believe that the use of other crops, such as rice, in C4 photosynthesis can help increase food production, so the model could help predict where such plants could grow optimally.
Continue research:
As climate change, plants may not be able to pump carbon quickly out of the air
More information:
Haoran Zhou et al., C4 photosynthesis and climate with optimal dissipation National Academy of Sciences works (2018). DOI: 10.1073 / pnas.1718988115
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