Plants appear to be breaking biochemical rules by making “secret decisions.”

Researchers have just uncovered a previously unknown process that explains plants’ “secret decisions” in releasing carbon back into the atmosphere.

“We found that plants control their breathing in ways we didn’t expect. They control how much carbon from photosynthesis they keep for building biomass using a metabolic channel,” University of Western Australia plant biochemist Harvey Millar told ScienceAlert.

“This happens right as a step before they decide to burn off a compound called pyruvate to make and release CO2 back to atmosphere.”

If you think back to high school biology, you might remember that plants produce sugars, or sucrose, during photosynthesis. The plant typically produces an excess of sucrose; some is saved, some is dismantled. This is called the citric acid (or tricarboxylic acid) cycle and is just as important to life.

As part of this cycle, sucrose, which has twelve carbon atoms, is broken down into glucose, which has six carbon atoms. Then glucose is broken down into pyruvate, which has three carbons. Using pyruvate for energy produces carbon as a waste product, so this is where the “decision” is made in the plant.

“Pyruvate is the final point for a decision,” Millar told ScienceAlert.

“You can burn it and release CO2or you can use it to build phospholipids, stored vegetable oils, amino acids and other things you need to make biomass.

The discovery came while working on a classic plant model organism called thale cress (Arabidopsis Thaliana). The researchers, led by University of Western Australia plant molecular scientist Xuyen Le, labeled pyruvate with C13 (an isotope of carbon) to track where it was shifted during the citric acid cycle and found that pyruvate from different sources was used differently.

This means that the plant can actually track the source of the pyruvate and act accordingly, either releasing it or holding on to it for other purposes.

“We found that a transporter on mitochondria directs pyruvate to respiration to release CO2but pyruvate made in other ways is retained by plant cells to build biomass — when the transporter is blocked, plants then use pyruvate from other pathways for respiration,” Le said.

“Imported pyruvate was the preferred source for citrate production.”

This ability to make decisions, the team suggests, breaks the normal rules of biochemistry, where normally every reaction is a competition and the processes don’t control where the product goes.

“Metabolic channeling breaks those rules by uncovering responses that don’t behave that way but are fixed decisions in metabolic processes that are shielded from other responses,” says Millar.

“This is not the first metabolic channel ever found, but they are relatively rare and this is the first evidence that one controls this process in respiration.”

Although plants are wonderful CO stores2 – Forests alone store around 400 gigatonnes of carbon – not every molecule of CO2 which is taken up by plants is then preserved. About half of the carbon dioxide that plants absorb is released back into the atmosphere.

Being able to get plants to store a little more carbon dioxide in the process could be an intriguing way to help our climate change problems.

“When we think about building and breeding plants for the future, we should not only think about how they can be good food and food for our health, but also whether they can be good carbon stores for atmospheric health, that we all have to depend on,” Millar told ScienceAlert.

This future security is still pending, because the researchers have only just left this biochemical process behind. But if we can hijack the way plants make decisions about carbon storage, it could be part of the larger puzzle of mitigating climate change.

The research was published in nature plants.