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.
Images: Rothamsted Research