By Saloni Dhopte, Genetics and Genomics ‘23
If there’s one flowchart I remember from my middle school science textbook, it’s the one for photosynthesis.
Plants are autotrophs- they make their own food using sunlight and carbon dioxide from the air. This process, called photosynthesis, is responsible for supplying most of the oxygen in the Earth’s atmosphere and maintaining all life on Earth. Naturally, such an important job is highly complex and involves a lot of sub-processes, each with their own key players. One such sub-process is the Calvin cycle. One of the main stages of the Calvin cycle is the fixation of carbon by an enzyme known as RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), which converts atmospheric carbon into organic compounds, a process known as carboxylation. Turns out, this enzyme is more important than our middle school science textbooks originally let on.
In 2021, global carbon emissions reached a staggering 37.12 billion metric tons . Through photosynthesis, plants used to be able to sequester enough carbon dioxide from the atmosphere into the soil to balance our emissions. Now, with the advent of modern agriculture, industry, and fossil-fuel-heavy lifestyles, that balance has been lost. Among other things, an increase in the carbon dioxide concentration in the atmosphere will result in a further rise in temperature, proving to be detrimental to life on Earth . While recollections of the Calvin cycle and its components may trigger unpleasant memories for biology majors, some scientists sought to look at this process from a new perspective and apply it to fight global warming. Various researchers are working to enhance the Calvin cycle, particularly by engineering RuBisCO in order to ‘improve’ photosynthesis and increase plants’ carbon sequestration potential .
But why RuBisCO? Its job is the rate-limiting step in photosynthetic carbon fixation . In addition to carboxylation, it also is responsible for the addition of oxygen (oxygenation) to CO2 to produce one molecule of 3-phospoglyceric acid (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). Thus, researchers want to engineer RuBisCO such that its oxygenation activity is limited, so most of the plant’s resources are used to increase carbon fixation. Researchers from the Universitat de les Illes Balears-INAGEA in Spain engineered specific amino acids in the large and small subunits of the RuBisCO enzyme to increase its overall carbon dioxide fixation efficiency. They also engineered carbon dioxide concentrating mechanisms (CCMs) of C4 plants (which have a relatively faster fixation efficiency) into C3 plants (which have a relatively slower fixation efficiency) to promote maximal carboxylation activity. However, the plants that were transformed with this engineered enzyme were unable to grow as fast as wild-type plants .
Another interesting method of engineering photosynthesis involves the use of artificially manufactured nanoparticles. Current chloroplast synthetic biology techniques suffer from low rates of chloroplast transformation (introducing a foreign gene into the plant cell organelle), fewer species are compatible with chloroplast transformation, and labor-intensive phenotype screening. Nanotechnology, on the other hand, allows for faster delivery of biomolecules to plants, with minimum off-target effects. The use of this technology has supported plant science research in a broad range of areas including plant signaling, stress communication, and environmental monitoring. Buriak et al. used nanotechnology to engineer photosynthesis for increased carbon fixation . Nanoparticles were introduced into the plant via leaf infiltration, and they capture carbon dioxide in the form of bicarbonate, thus increasing the amount of carbon dioxide that interacts with RuBisCO . The researchers were successful in increasing the carboxylation efficiency by 20% in vitro. The in-planta effects still remain to be investigated.
Most recently, the Innovative Genomics Institute (IGI)– founded by Nobel Prize winner Dr. Jennifer Doudna– announced the next step in photosynthesis engineering. With the support of an $11 million grant from the Chan Zuckerberg Initiative (CZI), they have assembled a team of scientists from universities across the country including UC Berkeley and UC Davis . Each research group is set to tackle the problem of increasingly inadequate carbon fixation by plants relative to the growing carbon emissions. They aim to enhance the natural carbon sequestering abilities of plants to match climate change. IGI scientists will be focusing on CRISPR-Cas9 genome editing– a technique that utilizes the mechanism of bacteria’s natural immune system to target specific sequences in a host genome and incorporate desired edits .
Current methods of increasing the carbon fixation rate of plants often lead to carbon dioxide being released back into the atmosphere via the respiration of the soil microbes. Thus, along with working towards increasing the efficiency of carbon dioxide removal from the atmosphere, the IGI scientists aim to develop methods of retaining the carbon in the soils. This has many benefits because carbon is nourishing for the soil, and increasing water retention. Scientists such as Dr. David Savage, Dr. Krishna Niyogi, and Dr. Pamela Ronald plan on employing genome editing techniques on rice to improve the efficiency of carboxylation . Dr. Peggy Lemaux and Dr. Myeong-Je Cho are working on developing high-efficiency genome editing protocols for sorghum (a type of grass used in cereals and animal feed), in an effort to improve its ability to remove carbon dioxide from the atmosphere . Drs. Jill Banfield and Jennifer Pett-Ridge are developing methods to measure the carbon fixed by these edited plants and also studying the microbial communities in the soil that will be responsible for the long-term maintenance of carbon . Thus, with these three distinct project areas, IGI strives to fight global warming and create a better tomorrow. So if any freshmen or sophomores are reading, pay attention to the Calvin cycle in your introductory biology courses–, it is key to the future of engineering photosynthesis.