A new study led by Cornell University describes an important step towards engineering crops for higher yields by improving photosynthesis. It is estimated that by 2050, farmers will need to grow 50% more food for a population of 9 billion people, but on a limited amount of arable land.
Researchers are working to insert elements from cyanobacteria, which is known to photosynthesize more efficiently than most crops, into crop plants.
During photosynthesis plants convert CO2, water and light into oxygen and sucrose, a sugar used for energy and for building new tissues. Along the process, Rubisco, an enzyme found in all plants, takes inorganic carbon from the air and converts it to an organic form the plant uses to build tissues.
Rubisco reacts with CO2 and O2 in the air; the latter reaction creates toxic by-products, slows photosynthesis and lowers yields. In cyanobacteria, the Rubisco is confined within microcompartments called carboxysomes that protect the Rubisco from oxygen.
The carboxysome allows the cyanobacteria to concentrate CO2 so Rubisco can use it for faster carbon fixation, Prof. Maureen Hanson, the lead researcher said. “Crop plants don’t have carboxysomes, so the idea is to eventually put in the entire carbon-concentrating mechanism from cyanobacteria into crop plants,” she added.
In order to work in crop plants, scientists must remove carbonic anhydrase, a naturally occurring enzyme, whose role is to create a balance between CO2 and bicarbonate in plant cells. But for the carbon-concentrating mechanism from cyanobacteria to work in crops, bicarbonate in the system must reach levels many times higher than those found at equilibrium.
“So in this study,” Hanson said, “we did that step [of removing anhydrase] that’s going to be needed to make the carboxysome work.”
The researchers used CRISPR/Cas9 gene-editing technology to disable genes that express two carbonic anhydrase enzymes that are present in chloroplasts. Hanson and colleagues removed 100% of the enzyme’s activity, and the plants barely grew. “It showed that plants need this enzyme to make bicarbonate that is used in pathways to make components of leaf tissue,” Hanson said. When they put the plants into a high CO2 growth chamber, they continued normal growth, as the high amounts of CO2 resulted in a spontaneous reaction to form bicarbonate.
Experiments showed that the absence of carbonic anhydrase did not interfere with photosynthesis, contrary to previously held views. A potential problem is that carbonic anhydrase found in chloroplasts is known to be involved in the plant’s defense pathways. However, researchers in Hanson’s group discovered they could incorporate an enzymatically inactive version of the carbonic anhydrase and still maintain the plant’s defense.
“We now know we can make an inactive enzyme that won’t affect our carbon concentrating mechanism but will still allow the crop plants to be resistant to viruses,” Hanson said.
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