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Plants can crash when photosynthesis rates are high. This is one way they slow down.

(Cheesy poem warning)

With the need for speed;
a car goes far.
We avoid deadly mistakes,
With the one thing slowing us down: the brakes

Just as brakes are essential to whiz down a highway, plants rely on special expand iconproteins to maintain high rates of photosynthesis without crashing.

The big picture is this. expand iconPhotosynthesis is how plants ‘make food' for us all. Improving it is one of the last scientific barriers to increase crop yields and feed the world’s growing population. But researchers can’t simply force plants to put the pedal to the metal. That would be disastrous, because their ‘brakes’ are more complex than a car’s.

In a new study, the lab of Michigan State University’s Thomas D. Sharkey delves into a ‘brake’ protein, called GPT2. It helps manage photosynthesis in the presence of high levels of light or carbon dioxide, which can push photosynthesis into overdrive mode. The study is in the journal, Frontiers in Plant Science.

A molecule to manage starch production

GPT2 is part of the photosynthetic processes that introduce carbon into our diets. It is carbon that becomes the sugars and starches that fuel life.

“Starch is a plant’s backup battery,” says Sean Weise, Research Assistant Professor in the Sharkey lab. “A plant builds it up during the day when it can do photosynthesis. It uses the starch at night when it can’t, because the sun is gone.”

GPT2 sits in the chloroplast membrane and helps manage that starch production by allowing sugars to move into the expand iconchloroplast.

When photosynthesis increases rapidly, like when the sun peaks out from a cloudy day, GPT2 is quickly turned on.

“We think this recycles sugars back into the chloroplast for starch production,” says Sean. “Other genes responsible for starch synthesis also get turned on rapidly. However, genes involved in almost every other aspect of carbon dioxide metabolism, including sucrose synthesis, are turned down. That last part was a surprise to us, and we’re looking into it.”

Figure of the chloroplast
When photosynthetic activity is high, genes responsible for starch synthesis get turned on rapidly (red boxes) while genes in almost every other aspect of carbon dioxide metabolism are turned down (dark blue boxes). GPT2 and GAPN are highlighted (light blue boxes)
By Sean Weise, as found in Frontiers in Plant Science, CC By 4.0, 2019

The team thinks plants activate GPT2 as a sort of brake to allow the plant’s cells to keep up with increased photosynthetic activity and to increase starch stores for use at night.

What triggers this photosynthesis slowdown?

When photosynthesis activity is high, a lot of energy compounds are pumped from the chloroplast into the rest of the plant cell. Most of the time, this is desired, but too many of these compounds can cause chemical reactions that harm the plant cell.

That’s one situation where plants ‘step on the brakes.’

“When GPT2 is active, another gene, called GAPN, that processes the high energy compounds that end up in the expand iconcytosol, is muzzled,” Sean says. “That is how plants recycle compounds back into the chloroplast to keep them away from the rest of the plant cell.”

Another trigger that activates GPT2 is a expand icontranscription factor known as RRTF1.

“Transcription factors influence large numbers of genes at once,” Sean says. “If you control one transcription factor, you can affect dozens or even hundreds of other genes. That is why RRTF1 could potentially be an exciting target for improving photosynthesis through genetic engineering.”

Stepping back, Sean considers the big picture: improving photosynthesis.

“At first blush, GPT2 seems counterproductive. High rates of photosynthesis are good.” Sean says. “Why would a plant reduce its export of high energy compounds? Why put the brakes on productivity gains and instead recycle resources? However, when we look at plant metabolism more closely, we realize that there really can be too much of a good thing, and this could damage the plant cell.”

“Understanding the ‘brakes’ only gives us a more complete view of photosynthetic plant metabolism. It also identifies new targets for improvement. Perhaps, in the future, we can engineer plants that don’t need to put on the brakes as often.”

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

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