Plants Seem to Break the Rules of Biochemistry by Making ‘Secret Decisions’

Researchers have just discovered a previously unknown process that gives meaning to the “secret decisions” plants make when they release carbon into the atmosphere.

“We found that plants control their respiration in a way that we didn’t expect, they control how much carbon from photosynthesis they save to build biomass using a metabolic channel,” said the plant biochemist from the University of Western Australia. Harvey Millar says ScienceAlert.

“It happens right before they decide to burn a compound called pyruvate to make and release CO2 return to the atmosphere.”

If you think back to high school biology, you might remember that during photosynthesis, plants make sugar or sucrose. The plant generally produces an excess of sucrose; some are stored, some are degraded. This is called the citric acid cycle (or tricarboxylic acid), and it is just as important for life.

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

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

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

The discovery came about while I was working on a classic plant model organism called Thale’s watercress (Arabidopsis Thaliana). The researchers, led by University of Western Australia plant molecular scientist Xuyen Le, tagged pyruvate with C13 (an isotope of carbon) to track where it was moved 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 on it, choosing to either release it or save 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 – if the transporter is blocked then plants use pyruvate from other pathways for respiration”, The 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 typically every reaction is a competition and the processes don’t control where the product goes.

“Metabolic channeling breaks these rules by revealing reactions that don’t behave like that, but are definite decisions in metabolic processes that are immune to other reactions,” says Millar.

“This is not the first metabolic channel to be found, but they are relatively rare, and this is the first evidence that a channel governs this process in respiration.”

Although plants are wonderful CO sinks2 – forests alone store about 400 gigatonnes of carbon – not all CO molecules2 absorbed by plants is then retained. About half of the carbon dioxide absorbed by plants is released into the atmosphere.

Being able to get plants to store a little more carbon dioxide in this process could be a fascinating way to help our climate change misfortunes.

“As we consider building and raising 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 reservoirs for the health of the atmosphere that we all depend on,” Millar told ScienceAlert.

Such durability is yet to come, as researchers have only just discovered this late biochemical process. But if we can hijack the way plants make decisions about storing carbon, it could be a piece of the bigger puzzle of climate change mitigation.

The research has been published in natural plants.

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