Dry conditions may have helped a new type of plant gain ground on Earth

Researchers have long believed that lowering carbon dioxide levels was at the root of plant creation thanks to this innovation, but a new study in the Proceedings of the National Academy of Sciences, based on biochemical modeling by a group led by biologists from the University of Pennsylvania and paleoclimate modeling by a group from Purdue University, indicates that water availability may have been the determining factor in emergence of C4 plants.

"The origin of C4, which occurred while atmospheric carbon dioxide was still very high, seemed to be dictated by the limitation of water consumption," says Haoran Zhou, a graduate student in the Department of Biology. Faculty of Arts and Sciences and lead author of the journal. "Then, about 5 to 8 million years ago, the C4 grbadlands expanded, and that's because carbon dioxide was being reduced more and more. 39, light intensity were the limiting factors favoring the C4 at that time. "

Erol Akçay, an badistant professor of biology at Penn, explains, "The increased water consumption of the C4 pathway is enough to give it an initial ecological benefit in relatively arid environments. physiological modeling. If you only consider temperature and carbon dioxide, you might miss that role of water and light. "

The researchers' work also suggests that C4 plants could have a competitive advantage over C3 plants, even when carbon dioxide levels in the atmosphere were still relatively high at the end of the Oligocene.

"The conclusion is that C4 could have evolved a little earlier than we previously thought," says Penn's Brent Helliker, an badociate professor of biology, who, along with Akçay, plays the role of Zhou's advisor. "This corroborates some molecular clock estimates for the evolution of C4."

In plants with a C3 photosynthesis pathway, the first stable compound produced during photosynthesis contains three carbon atoms; in C4 plants, the first compound has four carbon atoms. The C3 route evolved first, operating efficiently when the atmosphere was rich in carbon dioxide. However, C4 plants evolved independently of C3 plants dozens of times, capable of photosynthesis effectively despite lower carbon levels, thanks to an additional step of the process that is used to pump carbon from the air into a layer internal cell where the rest of the cycle works. By running this "closed" system, where photosynthetic machinery does not interact directly with outside air, photosynthesis C4 allows plants to produce more food with less water loss than the pathway. C3.

Today, about a quarter of the planet's plant cover is composed of C4 plants. Several important crop species, including corn and sugar cane, have the C4 pathway. The results of the fossil record and isotope studies have helped scientists estimate when this pathway has evolved, although these estimates are more recent than those suggested by the molecular clock data from the badyzes. phylogenetics of various plant species, which has created some confusion as to when the path appeared. dominated in some ecosystems.

To examine more closely the factors that could have favored the propagation of the C4 photosynthesis pathway, Zhou, Akçay and Helliker have created a multi-layered model. They considered variables that affect photosynthesis as well as those that influence the hydraulic system, in which plants "decide" to devote more energy to growing roots to absorb water, or to construction. more leaves that can help absorb light and carbon dioxide, but also exposes them to greater water loss. In addition, factories can determine the best balance between carbon gains and water losses. Combining these two systems, the scientists' model included four factors that could favor C3 or C4 lineages: carbon dioxide concentration, light, temperature, and water availability.

According to their model, the evolution of C4 seemed to unfold in two phases. When the level of carbon dioxide was still high, C4 emerged in parts of the world that became warmer and drier. But it did not take advantage of its competitive advantage over C3 plants until several million years later, when carbon dioxide levels were very low and grbadland expansion provided sufficient light for open habitats. In these areas, C4 grbadlands expanded and replaced C3 grbadlands.

To see how this model interacted with the paleoclimate at the start of the C4 plants, the Penn team collaborated with Matthew Huber of Purdue University, a paleoclimate modeller funded by the National Science Foundation to model the Miocene climate. , and Ashley Dicks, graduate student. Using climate model results and palaeoclimatic data, including carbon dioxide, temperature, and precipitation, researchers predicted the probable geographic distribution of C3 plants relative to C4 plants for the late Oligocene period. in the first Miocene, about 30 to 5 million years ago. They discovered two areas that had not been identified before and where C4 plants would probably have dominated after having evolved for the first time, thanks to the rational use of their water: North West Africa and Australia .

"These are two regions of the world until then unknown where C4 plants could have had an ecological advantage and taken over," said Akçay.

"It was a really exciting opportunity," says Huber, "when the Penn group contacted us because it's about a really new application of the paleoclimatic model output. link between what climate models tell us about past and future climates and verifiable models of geological recording ".

Although the study did not study what could happen in the future with the new increase in atmospheric carbon levels, it may help to better understand why plants are distributed as they are today. And how they might react to future conditions.

"The weather conditions that were present when the C4 evolved may still be important today," Helliker said. "If a C4 plant line evolves mainly because of the limitation of carbon dioxide content, these plants can be found in dry environments nowadays, whereas if carbon dioxide was at the origin of their evolution and dominance, they could be found in wetter places today. "

In addition, some scientists believe that taking photosynthesis of C4 by other agriculturally important species, such as rice, could help increase food production. The model could therefore help predict where such plants might grow optimally.

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More information:
Haoran Zhou et al, C4 photosynthesis and climate through optics of optimality, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073 / pnas.1718988115