Protecting photosynthesis from stalling: a 24-hr molecular hotline

  • May 20, 2019
  • fundamental research, Sharkey lab
  • Department of Energy Projects, Feature News Story, Fundamental Research, Nonfeature News Story
  • By Igor Houwat, Alyssa Preiser

Photosynthesis is the process that powers life on earth. Photosynthetic organisms capture energy from sunlight, which they use to tear carbon from atmospheric carbon dioxide. The carbon ends up in sugars and starches that sustain these organisms and the food chain above them.

The reactions that add carbon to the diet are the expand iconCalvin-Benson cycle. Nowadays, scientists know its main workings, but they continue to explore its details. Those include sub-cycles that support it and evidence that surrounding conditions – like light quality or temperature – affect its performance.

A new study from the Sharkey lab looks at a pivotal sugar molecule, called glucose 6-phosphate (G6P), that is the first step towards making starch. Starch in leaves breaks down at night to feed the plant when the sun is not shining. The G6P sugar molecule comes at a crossroads where it is either processed or transported by one of four different expand iconenzymes.

Calvin-Benson cycle produces starch
The pathway taken in the Calvin-Benson cycle to produce starch that feeds plants at night. This journey is full of chemical reactions and intermediary molecules.
By MSU-DOE Plant Research Laboratory, 2019

Here, the researchers analyze the enzyme, G6PDH, that diverts G6P away from starch synthesis. They also show it can work during the day. Previous studies, dating back to the 60s and 70s, claimed it is only active at night when its products help the plant. The study is published in Biochemical Journal.

Protecting the cycle through side reactions

“The G6P sugar molecule is very flexible,” says Alyssa Preiser, a student in the Department Biochemistry & Molecular Biology. “It can turn into precursors for sucrose and starch that go on to feed the plant. Or it can re-enter the Calvin-Benson cycle through a couple different ways,”

One of these ways is a series of side reactions that protect the cycle. The Calvin-Benson cycle is fragile and will stall if no carbon molecules are pumped into it. The side reactions pump a low flow of carbon into the main cycle to keep it running.

Enter the enzyme G6PDH. It diverts G6P from turning into starch and into the protective side reactions. Importantly, the G6P sugar fuels the first step of these reactions.

Pumping back resources into the cycle
G6PDH diverts G6P from turning into starch and into the protective side reactions (red arrows). That is how resources are pumped back into the cycle to keep it active.
By MSU-DOE Plant Research Laboratory, 2019

“We also looked into an earlier assumption that the enzyme just works at night,” Alyssa adds. “We found it active throughout the day. It’s usually at 10 to 20% of its maximum capacity. But under certain conditions, performance can increase.”

“During the day, there are times that expand iconphotosynthesis is so active, the Calvin-Benson cycle can’t keep up with the inflow of carbon. And again, it is also prone to breaking down,” Alyssa says. “The side reactions probably take excess carbon away from the cycle to reduce the pressure. Then, they feed the carbon back into the cycle when the time is right.”

From a practical point of view, Alyssa, adds, it is important to understand how the Calvin-Benson cycle works. The cycle is the key to organic life on Earth. We could someday improve it so plants increase their sugar and starch production. These enhancements would help farmers get higher crop yields.

This work was primarily funded by the US Department of Energy, Office of Basic Energy Sciences.