Hacking Photosynthesis Yields Turbocharged Crops

| by Janet Fang

Photo credit: This tobacco plant depends on a cyanobacterial enzyme for carbon fixation that works faster than the plant counterpart. Incorporation of this enzyme could lead to increases in the rate of photosynthesis and agricultural yield / Rothamsted Research

Scientists working with plant enzymes have figured out a way to enhance photosynthesis, creating turbocharged crops that could one day lead to crazy high agricultural yields. The work was published in Nature this week. 

The world’s population is projected to pass nine billion by 2050. And across the planet, crop yield is limited by the efficiency of photosynthesis -- capturing sunlight to produce sugar and oxygen from carbon dioxide and water. The conversion of CO₂ to sugar is mediated by an enzyme called Rubisco. And in plants, it’s a somewhat inefficient, slow-working enzyme. To compensate, plants have to produce a lot of it: Rubisco is possibly the most abundant protein on Earth, Nature reports, accounting for up to half of all the leaf's soluble protein. 

But there might be another way. Photosynthetic microbes called cyanobacteria have a faster form of Rubisco that’s coupled with CO₂-concentrating mechanisms. However, attempts at replacing the CO₂-converting machinery in plants with that of cyanobacteria have been unsuccessful. 

Now, a team led by Maureen Hanson from Cornell University has announced a successful generation of tobacco plants (Nicotiana tabacum, a common model for genetic studies) with Rubisco from a blue-green algae, Synechococcus elongatus. By replacing the gene for the carbon-fixing enzyme in tobacco plants with two genes for the cyanobacterial version, their engineered plants (pictured above and below) perform photosynthesis and have higher rates of CO₂ turnover than plants with the native version of the enzyme -- when grown in an elevated CO₂ environment.

“This is the first time that a plant has been created through genetic engineering to fix all of its carbon by a cyanobacterial enzyme,” Hanson says in a news release. “It is an important first step in creating plants with more efficient photosynthesis.” So how’d they succeed where others have failed? A broad-stroke approach, Hanson tells Popular Mechanics: Not only did they swap in the cyanobacterial genes, they also made several other genetic substitutions to include proteins for manufacturing the enzyme.
One key trick was to discourage wasteful reactions: Sometimes Rubisco wants to react with oxygen instead of CO2, which is a waste of energy. Rubisco in plants is less reactive with oxygen and the tradeoff is that it slows down carbon fixing and photosynthesis. Fast-fixing Rubisco in cyanobacteria is way more reactive with oxygen. To cope with that, cyanobacteria protect the enzyme in special micro-compartments, called carboxysomes, that keep oxygen out and concentrate CO2 for efficient photosynthesis. 

Previously, the team inserted blue-green algae genes in tobacco to create carboxysomes in plant cells, and they’re now working on combining genes for cyanobacterial Rubisco with genes for carboxysomes in the tobacco’s chloroplast -- the organelle where photosynthesis actually takes place.

Plant engineered for more efficient photosynthesis

ByKrishna Ramanujan

Alessandro Occhialini, Rothamsted Research

This image shows a tobacco plant that has been genetically engineered for the first time so that all its organic material comes from carbon fixation by a cyanobacterial (blue-green algae) enzyme. It is an important first step in creating plants with more efficient photosynthesis.

A genetically engineered tobacco plant, developed with two genes from blue-green algae (cyanobacteria), holds promise for improving the yields of many food crops.

Plants photosynthesize – convert carbon dioxide, water and light into oxygen and sucrose, a sugar used for energy and for building new plant tissue ­– but cyanobacteria can perform photosynthesis significantly more quickly than many crops can.

“This is the first time that a plant has been created through genetic engineering to fix all of its carbon by a cyanobacterial enzyme,” said Maureen Hanson, a co-author of the study and Liberty Hyde Bailey Professor of Plant Molecular Biology at Cornell.

“It is an important first step in creating plants with more efficient photosynthesis,” Hanson said.

The study is published Sept. 17 in the journal Nature. Myat Lin, a postdoctoral fellow in Hanson’s lab, and Alessandro Occhialini, a scientist at the U.K.’s Rothamsted Research, are co-lead authors of the study.

Crops with cyanobacteria’s faster carbon fixation would produce more, according to a computer modeling study by Justin McGrath and Stephen Long at the University of Illinois. Producing more crops on finite arable land is a necessity as the world’s population is projected to pass nine billion by 2050.

Though others have tried and failed, the Cornell and Rothamsted researchers have successfully replaced the gene for a carbon-fixing enzyme called Rubisco in a tobacco plant with two genes for a cyanobacterial version of Rubisco, which works faster than the plant’s original enzyme.

All plants require Rubisco to fix carbon during photosynthesis. Rubisco reacts with both carbon dioxide and oxygen in the air, but when it reacts with oxygen, a plant’s rate of photosynthesis slows down, leading to lower yields.

In many crop plants, including tobacco, Rubisco is less reactive with oxygen, but a trade-off leads to slower carbon fixing and photosynthesis, and thus, smaller yields. The Rubisco in cyanobacteria fixes carbon faster, but it is more reactive with oxygen. As a result, in cyanobacteria, Rubisco is protected in special micro-compartments (called carboxysomes) that keep oxygen out and concentrate carbon dioxide for efficient photosynthesis.

In previous research, Lin, Hanson and colleagues inserted blue-green algae genes in tobacco to create carboxysomes in the plant cells. In future work, the researchers will need to combine genes for cyanobacterial Rubisco with genes for carboxysomes in the tobacco’s chloroplasts, the site in the cell where photosynthesis takes place.

Co-authors include Martin Parry, a professor of plant biology, and researcher John Andralojc, both at Rothamsted Research. The study was funded by the National Science Foundation, the Biotechnology and Biological Sciences Research Council, the National Institutes of Health and the 20:20 Wheat Institute Strategic Program.