Published: April 3, 2019

Keeping plants nourished: the workings of a photosynthesis backup system

Contact(s): Igor Houwat MSU-DOE Plant Research Laboratory office: (517) 353-2223

Researchers at the Michigan State University-Department of Energy Plant Research Laboratory shed more light on one of the backup systems that support photosynthesis through difficult conditions. The study is published in Plant Physiology.

Plants introduce carbon into their diet through a photosynthetic process known as the Calvin-Benson cycle. This series of reactions mixes carbon with other chemicals to make new compounds, like starch or sugars, that sustain the plants and the rest of the food chain.

However, two out of five times, the cycle picks up oxygen instead of carbon dioxide. That hiccup, called oxygenation, creates compounds that plants can't use to grow. Even worse, it grinds the cycle to a halt. 

Plants have to clean those compounds and re-introduce them into the cycle so it can restart. The effort costs time and energy and requires moving the compounds to special cleaning sites elsewhere in the cell.

“The Calvin-Benson cycle has built-in backups to quickly restart the process whenever it slows down,” said Tom Sharkey, University Distinguished Professor at the Plant Research Laboratory. “The best way is a shunt, a series of side reactions that keeps a low flow of carbon products in the cycle. That makes sure the cycle restarts as fast as possible.”

By using plants that can’t clean up compounds made by oxygenation, the Sharkey lab has learned how the shunt works and that it needs extra photosynthetic energy to function.

An analogy for the shunt is the pilot light found in older gas appliances. This small flow of gas keeps the flame lit at a minimal level, so that when gas is supplied, the furnace, hot water heater or stove turns on quickly.

“The pilot light could seem to be a waste of gas,” Sharkey said. “But it serves an important function by keeping the system ready to come on rapidly, without the user having to find a match to light the flame.”

Sharkey and two other labs at the Plant Research Laboratory, Kramer and Hu, screened for mutant plants with defects in cleaning compounds mixed with oxygen. One mutant had a flaw at one of the special cleaning sites, the peroxisome.

The mutation slowed down the cleaning process, which led the plant to accumulate bad compounds at much higher levels compared to healthy plants.

That accumulation stopped the Calvin-Benson cycle. Since the mutant plant couldn’t properly restart the cycle, it found a workaround:

  1. The plant moved carbon outside the cycle and into the plant cell;
  2. It partially processed the carbon in a way similar to what goes on in the cycle;
  3. It reinserted the carbon into the cycle through a backdoor that opened up for this situation.
  4. The shunt grabbed and then pumped some of that carbon back into the cycle to help reboot it.

“The shunt’s increased activity requires extra energy,” Sharkey said. “Photosynthesis compensates by cranking up production of ATP in order to feed the shunt and drive the Calvin-Benson cycle.”

Even though the mutant is an exception, it forced the plant to reveal workarounds that are hard to see in healthy plants.

“In healthy plants, the Calvin-Benson cycle only works when there is light,” Sharkey said. “But in nature, there can be wide changes, like moving clouds that make light flicker on and off. In those situations, it is easy to collapse the Calvin-Benson cycle. We think the shunt plays a role in restarting it.”

“Nowadays, electronics have made the pilot light obsolete,” Sharkey said. “Similarly, once we fully understand the shunt, maybe we will be able to replace it with a more efficient system.”