With climate-resilient crops in mind, study tackles flaw in photosynthesis

Rising temperatures and increasing drought are shortening crop-growing seasons in many parts of the world. Now scientists in Sweden report new advances toward engineering more efficient photosynthesis, work that could eventually contribute to efforts to adapt crops for climate change.

A team at KTH Royal Institute of Technology created and tested about 15,000 genetically altered versions of bacterial Rubisco—an enzyme that helps turn carbon dioxide into material for growth. Paul Hudson, professor in systems biology at KTH, says the study focused on bacterial rather than plant Rubisco and searched for versions that support bacterial growth.

In their article published in Science Advances, the researchers report they found several promising variants—and identified numerous changes that didn't appear to work.

Rubisco's costly inefficiency

Arguably the most important enzyme for life on Earth, Rubisco's role in bacteria and plants goes hand in hand with photosynthesis. Where sunlight provides the power, Rubisco captures carbon dioxide and incorporates it into material the organism can use to make sugars and grow.

Nevertheless, the enzyme is surprisingly inefficient. It works slowly and sometimes captures oxygen instead of CO₂. Plants must then spend energy recycling the unwanted products.

While the study focused on a bacterial Rubisco rather than the plant enzyme, its large database of successful and unsuccessful mutations offers a possible map for designing better enzymes, which may help researchers improve computer models of how Rubisco responds to genetic changes. The lessons may eventually guide work on plant Rubisco, but producing faster-growing, climate-resilient crops will require further testing in plant cells and whole plants.

Screening thousands of bacterial variants

Using a specially developed high-throughput screening platform, with each variant carrying a short DNA label, the team could simultaneously track the efficiency of thousands of candidate variants, including cells with single and combined mutations. Hudson says the screening method might also be adapted to other metabolic enzymes.

The research focused on bacterial Rubisco because its simpler structure makes it easy to produce and test on a large scale, and by doing so identify engineering principles that can be tested later in plant enzymes and plant cells.

Also, Hudson says, some bacterial form II Rubiscos process CO₂ faster than the plant enzyme. "But they also catalyze the oxygenation reaction more readily, meaning that they are less selective for carbon dioxide. Our idea was to take these fast Rubiscos and make them even faster, or at least reduce their oxygenation rate while retaining a high carboxylation rate."

Promising variants reveal engineering trade-offs

Two leading variants reacted less with oxygen while retaining useful carbon dioxide turnover, although both showed trade-offs in their overall carbon dioxide efficiency. Beneficial mutations often stopped working when combined.

The researchers also evaluated several computer models designed to predict how mutations would affect Rubisco. The models were more reliable for single mutations than for combinations of mutations.

"This shows that the effects of Rubisco mutations are complex and difficult to predict without a large amount of experimental data."