Photosynthesis study delivers surprise result

Crops of wheat (on the left) growing as green plants in pots in a research greenhouse and (on the right) ripening as golden ears in the field

Plant scientists thought that they had found a way to boost grain yield in wheat by increasing the amount of an enzyme, ca1pase. But their experiments, published in Plant Physiology, didn’t turn out as expected.

In some plants, ca1pase plays an important role in the activity of Rubisco, the protein that facilitates photosynthesis, the process which turns carbon from the air into biomass. Without Rubisco, plants wouldn’t grow and we’d all starve.

“Rubisco is the most abundant protein on the planet,” said Dr Elizabete Carmo-Silva, from the Lancaster University Photosynthesis Group, who led the study. “It’s also very inefficient: at Lancaster, we have been doing a lot of research to find ways to improve its efficiency, and so increase crop yield.”

“Not all Rubisco in a plant is active, because some gets bound to inhibitors: ca1pase gets rid of these inhibitors. I liken it to whether the Rubisco has had its morning coffee or not.”  

“Our hypothesis was that increasing the amount of ca1pase in plants would mean fewer inhibitors and so increase the activity of Rubisco – more Rubisco would have had its coffee. This would lead to increased photosynthesis, and so bigger plants and more grain.”

To test the hypothesis, scientists at Rothamsted Research produced several lines of wheat plants with the ability to produce more ca1pase. These were then grown in glasshouses in the Lancaster Environment Centre, alongside a control group of wild type wheat.  

Elizabete and her colleagues Ana Karla Lobo, Doug Orr and Marta Gutierrez took samples of the top leaves of the wheat when the plants were green and photosynthesising. They did further measurements when the wheat was ready for harvest. They analysed plant size, grain yield, the level of ca1pase and Rubisco inhibitors, as well as measuring how much Rubisco was present and how active it was. Some of the results were a complete surprise.

While there was up to 30 times more ca1pase expression in the treated plants and fewer inhibitors, there was a big decrease in Rubisco – up to 60% less. Even more unexpectedly, the treated plants were smaller than the control group, and had up to 72% less grain – the opposite of what the scientists had predicted.

“Initially I asked myself how our hypothesis turned out to be so wrong, why are the plants doing this?” Elizabete said. “But the fun thing about Rubisco is that it’s a very complex, tightly regulated system that we don’t fully comprehend yet.

“This result has given us ideas about what we need to test next in order to understand Rubisco regulation. There was a study about 20 years ago which suggested that if the inhibitors are bound to Rubisco, they protect it from degradation: so indirectly ca1pase could be leading to more Rubisco being degraded. Our next step is to set up an experiment to investigate Rubisco degradation in the leaves.”

Is this a major setback in the quest to increase crop production through increasing the efficiency of Rubisco? “Not at all,” says Elizabete. “It’s now clear that we can get a 20% increase in yield by improving Rubisco regulation. What we don’t know yet is exactly how to do it and what to target but I am convinced that we will get there.

“While the ca1pase research might seem disappointing at first, it’s an important step in designing a successful approach to improve Rubisco, which is vital if we are going to feed a growing population.”

The study was supported by N8 Agrifood Seed Core Funding and involved scientists from Lancaster University, Rothamsted Research and the Federal University of Ceará in Brazil, who will also be involved in the follow up experiments, along with the University of Leeds.

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