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Reviewing photosynthesis in dynamic conditions: Photosystem I as a guard

The basics of photosynthesis, the process that converts sunlight into food and energy to power life on Earth, is now common knowledge. But the subtleties of its inner workings remain mysterious to scientists.

Take this number: Plants store about 1% of the sunlight they absorb as biomass suitable for human use. The rest is shed, partially in response to environmental stresses like excessive light exposure or drought conditions.

Researchers think that bumping up that storage rate, even by a small percentage (1 or 2 percent), could dramatically increase crop yields. But until we understand how photosynthesis adapts to changes in the environment, that goal remains elusive.

In a new study published in the journal Photosynthesis Research, scientists at the MSU-DOE Plant Research Laboratory shed light on how a expand iconprotein complex, Photosystem I (PSI), plays important roles in guarding plants from excessive sunlight exposure and in helping them navigate old age.

This is a new view of photosynthesis that could change the way scientists study it.

The Science: PSI responds to dynamic changes

Eliezer Schwarz, co-author on the study, says, “PSI, one of the fundamental photosynthetic protein complexes, has generally been regarded as more stable and less involved in responding to changes in the environment that affect photosynthetic productivity.”

But Eliezer, alongside co-authors Stephanie Tietz and John Froehlich, now thinks otherwise.

PSI works with another protein complex called PSII. Both host expand iconlight-harvesting antennae that capture sunlight, a bit like how a car antenna captures radio waves. The antennae usually stick around their respective complexes, but some will hop around.

“PSII can have different amounts of an antenna protein complex, called LHCII. (It’s the one that makes plants look green.) In some cases, a small fraction of LHCII can leave PSII and associate with PSI. This has been understood as one of the ways a plant calibrates its light absorption, balancing the amount of energy absorbed by each photosystem.”

But the researchers report unusually massive amounts of LHCII migration to PSI when:

  • There is EXCESSIVE SUNLIGHT, which could damage PSII complexes. As a consequence, they release their light-harvesting antennae to lower light absorption rates. PSI “stores” these antennae as a safety net.
  • PLANTS AGE and start degrading various cellular proteins, including PSII. This leads to free-floating antennae that could significantly harm the plant cell if left alone. In response, PSI picks the antennas up to prevent any damage.
Journal figure showing how PSI dynamics
The dynamics of PSI, notably parts d and e. d) Transitioning to high light, PSI picks up light-harvesting antennas from PSII. e) As plants age and PSII are degraded, PSI picks up free-floating antenna proteins to prevent damage. 
By Eliezer Schwarz, Stephanie Tietz, John Froehlich. Photosynthesis Research, CC BY 4.0

 “This is surprising,” Eliezer says. “We found that some past studies had seen something similar, but these were isolated observations with no physiological context. It doesn’t seem like anyone has paid any attention to them yet."

Perhaps the reason for this lack of attention is that PSII has traditionally been the focus of research, specifically when it comes to light stress responses, Eliezer adds.

“Our study suggests that some basic photosynthesis mechanisms may have been misunderstood and that PSI may do more things, play more central roles, than was previously recognized.”

If scientists pay attention to this research – and recent studies are independently showing similar results – it could fundamentally change how photosynthesis is taught in our textbooks.

This work was primarily funded by the US Department of Energy, Office of Basic Energy Sciences. Banner image by planetMitch aunger on Unsplash

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