Plants have a mechanism that protects them from damage by light

The researchers have discovered that the key enzyme in photosynthesis can tune its activity to avoid being damaged by light and oxygen.

Knowing how photosynthesis is regulated and protected could allow scientists to improve the process, potentially making agriculture and food production more efficient.

For example, understanding how this regulatory mechanism works could help researchers to identify the factors that are beneficial for plant growth and to define how to adjust these in order to optimise growth in controlled cultivation.

Photosystem II, the central enzyme in photosynthesis, uses solar energy to remove electrons from water. The electrons are used to fix carbon dioxide from the atmosphere, creating a form of carbon that constitutes the fuel and building blocks for life. Photosystem II changed the planet by putting most of the energy into the biosphere and all of the oxygen into the atmosphere.

When leaves close their pores to prevent water loss, this also prevents air exchange so that carbon dioxide cannot enter the system. As the carbon dioxide inside the leaf is used up, the electrons have nothing left to react with and so they build up.

Although carbon dioxide is not entering the system, light still is, generating excess electrons. As the electrons have nowhere to go, they instead engage in ‘back-reactions’ that form a ‘killer molecule’ called singlet oxygen. This killer molecule can damage the photosystem II enzyme.

Now, by using a technique called spectro-electrochemistry, researchers have discovered a mechanism that protects the enzyme from this damage. The trapped electrons trigger the release of a bicarbonate molecule from the enzyme, which was previously thought to be constantly bound to it.

The new study, published today in Proceedings of the National Academy of Sciences, shows that this bicarbonate release not only slows down the water-splitting reaction but crucially also protects the enzyme from light damage due to the harmful back-reactions.

Bicarbonate is formed when carbon dioxide dissolves in water, so its concentration is related to the amount of carbon dioxide in the local environment. As well as low carbon dioxide levels causing electrons to build up and trigger the release of bicarbonate, the study also suggests the possibility that the level of carbon dioxide itself in the local leaf environment could impact on the bicarbonate binding.

“This is such an intuitive feedback mechanism at the heart of biology that I think it will go into school textbooks,” said lead author, Professor Bill Rutherford FRS from the Department of Life Sciences at Imperial.

“Now that we understand this new mechanism in the lab, the next step is to define when it kicks in out there in the field – not to mention the forest, greenhouse, plant pot, sea, lake and pond.”

Dr Andrea Fantuzzi, co-lead author also from the Department of Life Sciences at Imperial, added: “The role of bicarbonate has long been a mystery. Otto Warburg, Nobel laureate, friend of Einstein and one of the twentieth century’s leading biochemists, puzzled over this problem in the 1950s.

“Now the mystery is solved and a new regulatory mechanism defined. Not only is the question solved, but it could have real implications for understanding limitations to plant growth.”