Understanding Plant Repair of Light-Damaged Photosystems

More than two billion years ago, cyanobacteria began producing oxygen in Earth’s primarily toxic atmosphere. Since then, the photosystem II protein complex—now shared by land plants, algae, and cyanobacteria—has played a central role in oxygen production.

Paradoxically, excessive light exposure can damage photosystem II, reducing a plant’s photosynthetic efficiency. Biochemists Steven McKenzie and Sujith Puthiyaveetil of Purdue University have uncovered long-lost details about how photosystem II self-repairs. Their findings, recently published in Plant Communications, shed new light on this essential process.

Photosystem II splits water molecules, extracting electrons and protons while releasing oxygen as a by-product—effectively powering life on Earth. However, the mechanisms that allow this massive protein complex to maintain and repair itself across various plant lineages remain only partially understood.

“The long-term goal of this research is to understand how we can engineer plants for improved photosynthetic efficiency,”

McKenzie, a postdoctoral scholar in biochemistry at Purdue.

Repairing photosystem II is energetically costly, requiring the complex to be disassembled, damaged proteins to be degraded, and new proteins to be synthesized and reassembled. “That’s a significant energy investment for the chloroplast,” McKenzie noted.

While photosystem II repair in chloroplasts is already efficient, McKenzie believes it can be optimized further. “By accelerating the repair process or reducing its energy demands, we could improve overall photosynthetic efficiency,” he explained.

Recent advancements in modifying photoprotective pathways have led to increased photosynthetic efficiency in crops. Similarly, targeting the photosystem II repair cycle through genetic engineering offers promising opportunities for further improvement.

“Inhibiting this repair cycle significantly reduces photosynthetic efficiency,” Puthiyaveetil emphasized. “This process is constantly at work—even in low light, photosystem II is undergoing turnover. In high light, the rate of damage and repair increases dramatically. But sometimes, repair can’t keep up, especially under additional stressors like drought, salinity, or high temperatures. This leads to a light-induced decline in photosynthesis.”

As photosystem II uses sunlight to split water, it inevitably experiences photodamage. For every 10 million photons absorbed by leaves, one photosystem II complex is damaged. On a sunny day, a single leaf can intercept up to 10 quadrillion photons per second.

The question of how this protein complex efficiently disassembles and replaces damaged proteins has long been a mystery. Photosystem II is a large molecular assembly, composed of around 25 protein subunits, multiple metal centers, chlorophyll molecules, and other pigments.

The new study in Plant Communications reveals that the chemical process of adding phosphate groups to proteins—known as phosphorylation—drives certain steps of photosystem II disassembly in Arabidopsis plants. Although scientists have known about photosystem II phosphorylation since 1977, its specific role in the repair cycle has remained unclear.

Initially, the Purdue researchers hypothesized that phosphorylation alone facilitated photosystem II disassembly. However, McKenzie suggested that oxidative protein modification might also play a role.

“Steve proposed that oxidative protein damage could serve as a disassembly mechanism,” Puthiyaveetil said. Further experiments confirmed that oxidative protein damage is indeed a key driver of photosystem II disassembly, especially in later stages. “We were quite surprised by the extent of its impact—full credit to Steve.”

Cyanobacteria, red and brown algae, and land plants all share the photosystem II repair mechanism. McKenzie’s hypothesis stemmed from the observation that cyanobacteria and non-green algae lack photosystem II phosphorylation but still manage to disassemble and repair their photosystems.

“We wanted to investigate whether an alternative mechanism could drive photosystem II disassembly,” McKenzie said. “That’s why we considered the possibility that the damage itself could initiate the process.”

Phosphorylation appears to serve two functions. “It can drive disassembly, but it may also act as a quality control mechanism for the repair process,” Puthiyaveetil explained. “Once the complex is disassembled, it must be repaired, but plants don’t repair their photosystems under prolonged high light conditions.”

Instead, they wait for light levels to subside. “There seems to be a molecular mechanism behind this delay between damage and repair,” Puthiyaveetil noted. Once light levels return to normal, the repair and reassembly process begins.

“That’s the quality control,” he continued. “Phosphorylation may prevent the degradation of damaged proteins until they have been dephosphorylated, as dephosphorylation has been shown to be a prerequisite for protein degradation.”

In his experiments, McKenzie worked with genetically modified plants that had varying levels of photosystem II phosphorylation. He also manipulated phosphorylation levels by altering light conditions and phosphate sources. “This approach allowed us to analyze how different phosphorylation levels influence photosystem II disassembly and repair,” he explained.

Their findings provide a clearer understanding of photosystem II’s repair mechanisms, opening the door to potential strategies for improving photosynthetic efficiency in plants and crops.