A possible avenue for crop improvement in the face of growing food security challenges?
Photosynthesis is the main pathway by which inorganic carbon is converted into sugars and therefore biomass. Crucially, carbon dioxide is fixed in a reaction that is catalysed by an ancient enzyme called ribulose-1,5- bisphosphate carboxylase/oxygenase (Rubisco). Rubisco is perhaps the most abundant enzyme on Earth and in plant leaves it can represent up to 50% of the soluble protein. The action of Rubisco pulls down trillions of tons of carbon dioxide out of the atmosphere every year and this carbon becomes the foundation of almost all ecosystems on the planet. Despite the impressive resume, Rubisco is an incredibly inefficient catalyst. Its reaction rate is slow at only around 5 carbon dioxide molecules fixed per second, much less than typical enzymes. The extraordinary abundance of Rubisco is required to compensate for it’s less-than-stellar turnover rate. Consequently, the production of Rubisco represents a significant metabolic burden on plants. On top of that, Rubisco lacks substrate specificity. It is capable of binding oxygen molecules instead of carbon dioxide and this results in an unproductive side-reaction called photorespiration which is not only a waste of energy but also produces toxic products that then need to be dealt with by the plant. This tendency to bind oxygen is also increased at higher temperatures, making it a bigger problem for plants in the tropics, many of which have adaptations to help prevent this process from happening.
The current global situation presents us with the need to increase crop yields to sustain the growing population while simultaneously battling climate change. The deficiencies of Rubisco and its absolutely crucial role have inevitably made it the focus of much research. A group at Cornell University recently published a paper in Science Advances detailing a breakthrough in their study of Rubisco. Their aim was to understand how Rubisco has evolved in response to past climatic shifts, with the hope that this knowledge may be used to engineer Rubisco to have improved kinetics, adjusting crops to attain high yields, even during the escalating climate crisis.
In this study they examined the sequences of the large and small subunit of Rubisco from species within the Solanaceae family, which includes tomato, potato, pepper and tobacco. Using a computational workflow, they performed a phylogenetic analysis comparing these sequences from different Solanaceae species. From this they could make predictions about what the sequence of these subunits was in the last common ancestor of these species, 20-30 million years ago. From these predictions they generated 98 candidate ancient Rubisco enzymes which they resurrected by expressing them in E. coli and their activity was tested. It was found that all of them were functional and some displayed enhanced efficiency compared with contemporary Rubisco. Some displayed a higher turnover rate and appeared to have a higher optimal temperature than modern Rubisco which may reflect the fact that ancient Rubiscos evolved in a climate that was warmer than today. The carbon dioxide specificity level of the ancient Rubiscos was also not impaired. All this is a promising indication that Rubisco can be improved upon and engineered into crops to produce accelerated rates of photosynthesis and generate higher yields. The next step that this group will undertake will be to test this hypothesis in actual plants. This is a much more challenging prospect but has been made possible with technologies such as CRISPR gene editing.
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