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Typically, plants rely on the Calvin-Benson-Bassham (CBB) cycle to convert carbon dioxide in the atmosphere to usable organic matter for growth. Although this cycle is the main pathway for carbon fixation in all plants on Earth, it is surprisingly inefficient—losing one third of carbon in the cycle when synthesizing the molecule acetyl–coenzyme A (CoA) to generate lipids, phytohormones, and metabolites. Plants also lose carbon during photorespiration, which limits their growth. This is largely due to the inefficiency of an enzyme called RuBisCO.

In efforts to increase carbon uptake and reduce carbon loss in plants to boost biomass and lipid production, scientists have experimented with ways to increase the efficiency of RuBisCO, overexpress CBB cycle enzymes, introduce carbon-concentrating mechanisms, and reduce photorespiration losses. But, a new study in Science, focuses on a novel approach—creating an altogether new pathway for carbon uptake.

The researchers involved in the study introduced a synthetic CO₂ uptake cycle into the plant Arabidopsis thaliana. They refer to the engineered cycle as the malyl-CoA-glycerate (McG) cycle, which works in conjunction with the CBB cycle to create a dual-cycle CO₂ fixation system. The new cycle increases efficiency by using previously wasted carbon.

"In the McG cycle, one additional carbon is fixed when 3PG is the input, or no carbon is lost when glycolate is the input. In both cases, acetyl-CoA is produced more efficiently, which is expected to enhance the production of lipids and other important plant metabolites, including phytohormones," the authors write.

To test out the McG cycle, the team expressed six heterologous enzymes in Arabidopsis chloroplasts to establish the McG cycle. The results were impressive. Plants with the established McG cycle grew larger—up to three times in dry weight—and increased in leaf and seed numbers, and showed higher lipid content than their wild relatives. Triglycerides in the McG plants were up to 100 times the normal amount. The lipid content was so high that the plants formed pockets within their cells to hold the extra fats.

The McG cycle was found to increase efficiency by both reducing photorespiratory CO₂ loss and enhancing acetyl-CoA production. In total, the CO₂ assimilation rates were approximately doubled.

While the results were promising, the impacts of such changes are still unclear. The researchers note that the effects of the McG cycle in this experiment are "not necessarily transferable to crop plants, and the overexpression of heterologous genes may be silenced over successive generations." It is also possible that the increased carbon uptake may only be temporary, since the carbon could potentially be released as soon as the plants die. Long-term stability and ecological impact of the McG modification are also unknown.

Still, with further research and testing, the results here have the potential to increase crop yields and oil production for food and biofuels, in addition to contributing to carbon sequestration and climate change mitigation with enhanced plant growth.