Credit: CC0 Public DomainFarmers must grow 50% more food with a smaller amount of land to feed the projected 9 billion people by 2050. Plant scientists are racing against the clock to improve photosynthesis and engineer crops that produce higher yields.Researchers are trying to incorporate elements of cyanobacteria in crop plants. Blue-green algae (cyanobacteria), is known to be more efficient at photosynthesizing than other crops.Cornell University has published a new study that describes a major step in achieving this goal. "Absence of Carbonic Anhydrase In Chloroplasts Affects c3 Plant Development, but Not Photosynthesis," was published in the Proceedings of the National Academy of Sciences on August 11.Maureen Hanson is a professor of plant molecular Biology and the senior author. Kevin Hines, who was a student in Hanson’s lab, is the paper's senior author. Vishal Chaudhari is a postdoctoral associate at Hanson's laboratory.Photosynthesizing plants converts carbon dioxide, light, and water into oxygen. This sugar is used to produce energy and build new tissues. Rubisco, an enzyme that is found in all plants, "fixes" inorganic carbon in the air and converts it into an organic form for the plant to use to build its tissues.Rubisco reacts with both oxygen in the atmosphere and carbon dioxide, which can cause problems in crop photosynthesis. The latter reaction can create toxic byproducts, slow down photosynthesis, and lower yields. The Rubisco is protected from oxygen in cyanobacteria by microcompartments known as carboxysomes.Hanson stated that the carboxysome also allows cyanobacteria carbon dioxide to be concentrated so Rubisco can use this for faster carbon fixation. She explained that crop plants don't possess carboxysomes so it is possible to incorporate the entire carbon-concentrating mechanism of cyanobacteria in crop plants.Scientists must remove carbonic acidase from the chloroplasts. This enzyme is naturally found in plants and is responsible for photosynthesis. Anhydrase is responsible for creating an equilibrium between bicarbonate (CO2) and bicarbonate (water) in plant cells by catalyzing reactions that convert CO2 and water into bicarbonate. However, the bicarbonate levels in the system must be at least twice the equilibrium level to allow the carbon-concentrating mechanism of cyanobacteria in crops to work.Hanson stated, "So in the study, we did that step [of removing Anhydrase] that was going to be required to make carboxysome work."The authors present a CRISPR/Cas9 gene editing technology that can disable two genes that produce carbonic anhydrase enzymes found in chloroplasts. Another research group used a different method to remove 99 percent of the anhydrase enzyme activity and the plants grew normal. Hanson and his colleagues took out 100% of the enzyme activity. The plants barely grew after Hanson and co. Hanson stated that it proved that plants require this enzyme to produce bicarbonate, which is needed in pathways to create components of leaf tissue.They put the plants in a high CO2 growth chamber and they experienced normal growth. The high levels of CO2 caused a spontaneous reaction that formed bicarbonate.They believe they have a way to remove anhydrase while still having enough bicarbonate. The team plans to place a bicarbonate transporter onto the chloroplast membrane to allow bicarbonate to be imported from other parts of cells into the chloroplasts. This was recently funded by a $318,000 National Science Foundation grant. The extra bicarbonate will not only make anhydrase redundant, but it will also improve photosynthesis before carboxysomes are engineered into chloroplasts.Contrary to previous beliefs, experiments showed that photosynthesis was not affected by the absence of carbonic acidase.One potential problem is the fact that chloroplasts contain carbonic anhydrase, which is implicated in plant defense pathways. Hanson's team discovered that they can incorporate an inactive form of carbonic anhydrase into the plant's defense system.Hanson stated, "We now know that we can make an active enzyme that won’t affect our carbon concentration mechanism but will still permit the crop plants resistance to viruses."Learn more Plants engineered to increase photosynthesis efficiencyMore information: Kevin M. Hines and colleagues, The absence of carbonic anhydrase chloroplasts in C3 plant development, but not photosynthesis, Proceedings of the National Academy of Sciences (2021). Journal information: Proceedings of the National Academy of Sciences Kevin M. Hines et al, Absence of carbonic anhydrase in chloroplasts affects C3 plant development but not photosynthesis,(2021). DOI: 10.1073/pnas.2107425118
