The blueprint of a protein machine crucial for photosynthesis has been revealed in spinach leaves – an advance that could help researchers re-engineer the process to generate higher yields in crop plants.
The machinery is cytochrome b6f, a protein complex that provides the electrical connection between two other critical systems in photosynthesis – photosystem I (PSI) and II (PSII). PS II pulls electrons from water, which go to PS I via the cytochrome. Cytochrome b6f uses this electrical current to power a proton gradient, which is used to make the energy compound ATP that drives the conversion of carbon dioxide into sugars and biomass to power plant growth.
‘During photosynthesis, chlorophyll proteins capture light energy and use it to generate an electron current in the chloroplast,’ explains Matt Johnson, plant biochemist at the University of Sheffield, UK. Cytochrome b6f catalyses the slowest step in photosynthesis. ‘It’s a bottleneck,’ says Johnson. ‘Finding a way to improve the flow of electrons through the protein could therefore play a part in creating crop plants that grow faster.’
Researchers in the US and Australia have already shown that, by editing a single gene, plants can be made to produce more cytochrome and grow bigger as a result.
‘We’d like to find out exactly how each of the individual steps in converting the electron current into the proton battery occur and understand how cytochrome b6f senses the size of this current and uses that information to control which genes to switch on,’ explains Johnson, ‘to ensure it performs photosynthesis and grows optimally.’
The group’s research offers a detailed view of the molecular structure of the phytochrome complex (Nature, 2019, 575, 535). This was achieved by purifying supermarket spinach leaves with gentle detergent and freezing them in liquid ethane to allow examination under an electron microscope.
The cytochrome structure is not a surprise, says William Cramer, a plant scientist at Purdue University in Indiana, ‘but it’s a great advance to do it in plants’. Cramer showed the structure of this complex in cyanobacteria in 2014. He recently reported a gene edit that could slow the speed of electrons flowing through the cytochrome.
‘We now have a strategy for, in theory, making the system go faster,’ says Cramer, though he notes that photosynthesis has been operating for approximately 2.7bn years, so ‘it may be possible that nature has arranged the structure in the best possible way, and we cannot do better’.
However, Johnson is more optimistic. ‘Evolution selects plants that are best at reproducing successfully and its strategy is usually pretty conservative,’ he notes, prioritising survival over yield.