Singlet Oxygen Can Both Damage and Signal Repair in the Chloroplast

Plants love sunlight, but too much of a good thing can be harmful: excess light can disrupt photosynthesis and create damaging oxygen compounds called reactive oxygen species (ROS).

But despite the bad reputation of ROS, their production in a plant cell’s chloroplast might have a bright side. Research in the Apel lab at the Boyce Thompson Institute is shedding light on the important role of one type of ROS, called singlet oxygen, in signaling to the plant when environmental conditions go bad. His most recent paper in Proceedings of the National Academy of Sciences, describes the role—and location—of singlet oxygen in signaling the repair of enzymes required for photosynthesis.

The recent paper builds upon previous research showing that when young plants bleach and die in response to excess light, it isn’t the singlet oxygen that causes the damage, but the signaling pathway that it sets off, which tells the plant to shut down in the face of stress.

Apel’s lab uses a mutant variety of Arabidopsis, a common model plant, to study the effects of singlet oxygen. In 1998 they identified a mutant plant that they later discovered had a mutation in the FLU gene, which codes for a regulator that tells the plant when to stop making a precursor compound that in the light, is converted into chlorophyll. flu mutant plants continue to accumulate the precursor in the dark, but when exposed to light again, these precursors generate a burst of singlet oxygen. The young flu mutants bleach and die, but older plants slow their growth and turn on their stress responses—not because of damage from ROS, but because another signaling molecule called EX1 detects the singlet oxygen and tells the plant to shut down.

By controlling how long these mutant plants spend in the dark, the researchers can create a range of singlet oxygen exposures and study the effects. Since discovering this plant, Apel and his colleagues have used this and similar mutants to trace the signaling pathway that occurs in response to singlet oxygen.

In the new paper, the researchers demonstrate where the singlet oxygen and EX1 molecules are active: in the margins of the grana stacks in the chloroplast inside plant cells. The membranes of the grana, which look like stacks of coins, hold the pigments and proteins that perform photosynthesis, including Photosystem II (PSII), a protein complex containing chlorophyll that splits water to provide electrons for the process. When the light energy it receives is too intense, parts of PSII become damaged and thus it needs constant repair.

Previously, scientists have proposed that singlet oxygen both causes this damage and independently signals its repair, and that the singlet oxygen is generated inside the core of the grana.

But the present study suggests that there is a second source of singlet oxygen out at the margins of the grana. This is where the researchers located EX1 and is also where the cell synthesizes chlorophyll.

“Previously, people believed singlet oxygen formation is only in the grana core. Now we have to modify that idea,” said lead author Liangsheng Wang, a postdoctoral researcher at BTI.

Additionally, the researchers showed that plants also need the FtsH protease to cleave the EX1 protein and to pass along the message from EX1 that excess light is generating singlet oxygen. FtsH has been known already to help remove the damaged PSII. Taken together, these findings link the signaling role of singlet oxygen with the repair of a key protein complex for photosynthesis.

“This means that the FtsH protein is involved in transferring the signal from EX1 to a downstream component. But what is the downstream component? We don’t know,” said Wang.

In future work, Apel, Wang and colleagues will continue to explore this stress signaling pathway and will look for the downstream molecules.

This work could help us to better understand and perhaps mitigate the damaging effects of excess sunlight and other environmental stresses on crop plants—an issue that may become more pressing as the earth continues to warm.

Other researchers on the project included Chanhong Kim and Urszula Piskurewicz, former postdoctoral researchers in Apel’s lab, who now work at the Chinese Academy of Sciences in Shanghai and the University of Geneva in Switzerland, respectively.