Photosynthesis, despite being a broadly understood process, is actually a complex series of light-dependent reactions that scientists have been trying to fully unravel for hundreds of years. A research team led by EESA scientists has discovered a new carbon “pathway,” stemming from what is known as the C1 photosynthesis reaction, in which plants use carbon dioxide (CO2), water, and sunlight to make oxygen and organic compounds needed for growth. This pathway occurs during photosynthesis to help plants transform carbon into molecules that help build cells, carbohydrates, proteins, DNA molecules, and more.
This research, which could help scientists better understand how plants and forests might respond to climate change, was conducted as part of a Department of Energy (DOE) Early Career project named PECTIN (Poplar Cell wall Transformations and metabolic INtegration) and is described in an article published in the journal Communications Biology on Friday.
Carbon’s ‘Journey’ Throughout Photosynthesis
When CO2 is first absorbed by plants, the carbon atoms need to be manufactured into various molecules that each help plant development in different ways, known as “pathways.”
After photosynthesis occurs, carbon atoms in CO2 molecules go through a series of steps in what’s known as the Calvin-Benson cycle. In this cycle, carbon atoms are sent through various pathways that create larger molecules essential to plant growth and functioning. One pathway, for example, takes carbon atoms from CO2 molecules and uses them to create carbohydrates that give the plant energy. Another pathway uses the carbon atom to build proteins that support a plant’s cell growth and structure.
The ‘photosynthetic C1 pathway’ could be crucial for understanding oxygenic photosynthesis and the formation of climate-relevant gasses that are related to this process, like methane and carbon dioxide.
The New Carbon Path
Diagram of the newly discovered C1 pathway. Photo Credit: Kolby Jardine
The new pathway, discovered in collaboration with Frank Keppler from Heidelberg University, is connected to the photosynthetic-C1 pathway–a series of methyl transfer reactions that occur during photosynthesis.
“Think of carbon pathways like a bunch of soccer balls, representing carbon atoms, being kicked at a net,” explained Kolby Jardine, Berkeley Lab staff scientist and lead author of the study. “Some of them go into the net–that’s one pathway–some of them go to the left, the right, above. These are all different pathways of carbon that add one carbon atom at a time to growing molecules.
The pathway discovered, however, works while the soccer balls, or carbon atoms, are in motion trying to get to their destination. It’s helping to transfer the carbon atoms to all these different locations, where, when the atoms get to their final destination, they are used to build all these different things.”
While the pathway is quickly transferring single carbon atoms to destinations where they are used to build carbohydrates, proteins, and other molecules needed for plant growth, there are different “soccer players,” called constituents, also hard at work. These constituents help transfer carbon to where it needs to go, passing it off from one “player” to the next. Because this happens so quickly, though, there is a low concentration of carbon in the constituents themselves, which is why the pathway has gone undetected–until now. Click here to visualize the C1 pathway.
The Journey to Discovery
The scientists studied how California poplar tree leaves use and allocate carbon during photosynthesis at the Oxford Tract Experimental Facility in Berkeley, California. They also monitored CO2 exchange between the leaves and the atmosphere to understand how other environmental factors like temperature and precipitation affected photosynthesis rates. The team, which includes collaborators Trent Northen, Suzanne Kosina, and Aymerick Eudes from Berkeley Lab’s Environmental Genomics and Systems Biology Division, then used phylogenetic analysis, a technique that identifies evolutionary relationships between organisms based on their genetics, and mass spectrometry, a tool capable of telling scientists about the elemental composition of plant leaves for example.
The research team set out to understand how carbon atoms were being transferred throughout the photosynthetic process, and tested this using a technique called CO2 isotopic labeling–a method that tracks how carbon is used throughout a plant. This technique could show, for example, how much carbon plants use for building carbohydrates compared to how much carbon plants use for tissue growth.
These methods allowed the team to investigate how carbon is incorporated into the molecules of the plants during and immediately after photosynthesis, and also track how carbon molecules were used.
The Bigger Picture of Photosynthesis
Scientists have been unsure of how plant growth and function might be affected by the increase of CO2 levels in the atmosphere from human activity.
“The pathway doesn’t need to be activated with energy, so we’re comparing it to the rates of the Calvin Cycle, another process related to photosynthesis that does require energy,” explained Jardine. “We’ve seen that the Calvin Cycle has sped up with more CO2 in the atmosphere. We think it will be similar for this new pathway as well.”
Keppler added, “Our findings indicate that the photosynthetic C1 pathway might speed up, acting as a ‘missing link’ between boosted photosynthesis and faster plant growth under rising CO2 levels as our climate changes.”
The team expects that plant growth rates will increase because the new pathway helps to build molecules that plants need to grow, and this process is likely to speed up with more CO2 available. With this newfound understanding of how plants might respond to climate change and enhanced CO2 levels on a micro-level, scientists can use this information to understand how forests might respond to climate change on a larger scale, enabling better prediction of how well one of the planet’s most effective carbon sinks will continue to function.
