A new study published on 4 January in the journal Science has shown that by constructing new metabolic pathways in transgenic tobacco plants, plant growth can be improved by up to 40 per cent. The researchers from the University of Illinois and the U.S. Department of Agriculture Agricultural Research Service used a synthetic biology approach ― applying engineering principles to redesign biological processes based on existing biological components ―to develop plants that grow faster and taller.
Improving crop productivity will be crucial to meeting the food demands an ever-increasing global population, expected to reach 10 billion by 2050. Boosting photosynthesis is one promising way of increasing biomass per unit of land. All plants use photosynthesis to capture carbon dioxide from the atmosphere, however, the process is not always very efficient. During photosynthesis, Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and sunlight are used to turn carbon dioxide and water into the sugars that fuel plant growth. In October last year, scientists showed that increasing the production of Rubisco in maize (corn) plants can make photosynthesis more productive (2).
However, in staple crops such as rice and wheat, Rubisco often binds to oxygen instead of carbon dioxide resulting in the production of toxic compounds. Plants must then use an energy expensive process called photorespiration to deal with these toxic byproducts, which reduces their efficiency. As it turns out, Rubisco has even more difficulty identifying carbon dioxide from oxygen as temperatures increase, which could pose even more problems for plant growth as global temperatures continue to rise. According to the authors, photorespiration can reduce the photosynthetic efficiency of these so-called C3 crops by 20 to 50 per cent.
During the two-year study, biologists examined the performance of three alternative photorespiratory pathways in field-grown tobacco. The tobacco plant is easier to modify and test than food crop and is therefore often used as a model. The first pathway was based on five genes from the Escherichia coli glycolate oxidation pathway, the second pathway used glycolate oxidase and malate synthase from plants and catalase from E. coli, and the third pathway used plant malate synthase and a green algal glycolate dehydrogenase. By effectively simplifying the photorespiration pathway, the plants were able to save enough energy to boost growth. After testing 1,700 tobacco plants, the third pathway was found to achieve more than 25 per cent increase in biomass.
The study is part of an international research effort called Realizing Increased Photosynthetic Efficiency (RIPE), supported by the Bill & Melinda Gates Foundation, the Foundation for Food and Agriculture Research (FFAR), and the UK Government’s Department for International Development (DFID). The programme is aimed at engineering crops to sustainably increase worldwide food productivity by improving the efficiency of photosynthesis.
Although it may take many years, the researchers are hoping to translate these results into useful crops such as soybean, cowpea, rice, potato, tomato, and eggplant. Crops engineered with this photorespiratory shortcut could help meet growing global food demands.
(1) South, P.F. et al. Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field. Science (2019). DOI: 10.1126/science.aat9077
(2) Salesse-Smith, C.E. et al. Overexpression of Rubisco subunits with RAF1 increases Rubisco content in maize. Nature Plants (2018). DOI: 10.1038/s41477-018-0252-4