Plants Appear to Be Breaking Biochemistry Rules by Making 'Secret Decisions'

Researchers have now found a previously unknown method that explains why plants make 'secret decisions' while releasing carbon into the environment.

"We found that plants control their respiration in a way we did not expect, they control how much of the carbon from photosynthesis they keep to build biomass by using a metabolic channel," said Harvey Millar, a plant scientist at the University of Western Australia.

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

You probably recall from high school biology that plants produce sugar or sucrose during photosynthesis. Sucrose is produced in excess by the plant, some of which is stored and part of which is degraded. This is known as the citric acid (or tricarboxylic acid) cycle, and it is vital to life.

Sucrose, which contains twelve carbon atoms, is broken down into glucose, which has six carbon atoms, as part of this cycle. The glucose is then broken down into pyruvate, a three-carbon compound. The 'choice' in the plant is made at this moment since using pyruvate for energy creates carbon as a waste product.

"Pyruvate is the last point for a decision," Millar explained.

"You can burn it and release CO2, or you can use it to build phospholipids, stored plant oils, amino acids and other things you need to make biomass." 

The finding was made while researching on thale cress, a well-known plant model organism (Arabidopsis thaliana). The researchers, lead by Xuyen Le, a plant molecular biologist at the University of Western Australia, utilized C13 (a carbon isotope) to trace where pyruvate was being transferred during the citric acid cycle and discovered that pyruvate from various sources was used differently.

This means the plant can identify the source of the pyruvate and respond appropriately, either releasing it or storing it for later use.

"We found that a transporter on mitochondria directs pyruvate to respiration to release CO2, but pyruvate made in other ways is kept by plant cells to build biomass – if the transporter is blocked, plants then use pyruvate from other pathways for respiration," Le explained.

"Imported pyruvate was the preferred source for citrate production."

The capacity to make judgments, according to the researchers, defies the usual laws of biochemistry, in which every reaction is a competition and the processes have no influence over where the result ends up.

"Metabolic channeling breaks these rules by revealing reactions that don't behave like this, but are set decisions in metabolic processes that are shielded from other reactions," Millar explains.

"This is not the first metabolic channel to ever be found, but they are relatively rare, and this is the first evidence of one governing this process in respiration."

Despite the fact that plants are excellent CO2 storage systems (forests alone store roughly 400 gigatonnes of CO2), not every molecule of CO2 taken in by plants is retained. Plants emit around half of the carbon dioxide they absorb back into the atmosphere.

In this process, getting plants to store a bit extra carbon dioxide might be an interesting method to help with our climate change problems.

"As we consider building and breeding plants for the future – we shouldn't just be thinking about how they can be good food and food for our health, but also if they can be good carbon storers for the health of the atmosphere that we all depend on," Millar said.

This type of futureproofing is still to come, as the researchers have just recently uncovered this biological mechanism. However, if we can influence how plants make carbon storage decisions, it might be one component of the larger climate change mitigation jigsaw.