Global agricultural productivity is projected to require major increases within the next century to meet the needs of our increasing population. On a cellular level, crop yield is limited by the inefficiency of photosynthesis. Our project aims to improve this efficiency by implementing an alternative ribulose 1,5-bisphosphate (RuBP) regeneration portion of the Calvin-Benson-Bassham (CBB) cycle. Two alternative pathways, which use enzymes from other metabolic reactions acting on common metabolites used in the CBB cycle, were explored first via computational modeling. Impacts on growth and reaction fluxes in silico assessed the validity of these pathways in creating a more robust photosynthetic cycle. One of these pathways, which overexpresses the native enzyme transaldolase, was also assessed in vivo. Ultimately, we showed the validity of two alternative pathways in allowing a more efficient regeneration stage of the CBB cycle. These pathways could eventually be implemented into higher plants to allow more robust cycling and therefore higher crop yield.
Photo by Tom Fisk from Pexels By 2050, global demand for crop production is expected to increase by 100% from 2005 levels (1). We depend on high agricultural productivity for numerous needs; not only must crop production fulfill the direct requirements for human and livestock food consumption, but it must also fuel the needs of a growing bioeconomy that demands bioenergy and biopharmaceuticals (2). Despite the rising demand, production is threatened by a variety of factors. This problem is accentuated by land limitations, as available crop areas shrink and land demand clashes with environmental protection and sustainability goals (2).
Similarly inspired by the aims of the RIPE project, our project uses the application of synthetic biology tools to improve the efficiency of photosynthesis. By improving photosynthesis, yield improves at the level of individual crops, amounting to a greater harvest yield without increased land usage. Specifically, our project targets a major inefficiency within the RuBP regeneration portion of the CBB cycle. The CBB cycle is the portion of photosynthesis responsible for carbon fixation. Carbon fixation, which converts carbon dioxide into the usable organic forms that plants use to grow, is a key determinant of crop production at the cellular level.
Photo by Livier Garcia from Pexels The CBB cycle is dependent on the continuous cycling of its intermediates. Many of these intermediates, however, can be consumed by other metabolic reactions, depleting the CBB cycle of its required compounds. The CBB cycle stalls until the missing intermediates can be replenished. This constant pausing culminates into major growth loss over the course of the growing season for a plant. By limiting the number of intermediates, however, there is less potential for other reactions to ‘steal’ these metabolites and thus drain the available pool.
Our project aims to reduce the number of intermediates in the CBB cycle by implementing an alternative RuBP regeneration pathway. This results in a more continuous CBB cycle that allows for more carbon to be fixed in a given timeframe, therefore improving growth yield.
We proposed two alternative pathways. Our first alternative pathway involves the use of the Escherichia coli- derived enzyme fructose-6-phosphate aldolase to truncate the native eight-enzyme pathway to one only reliant on three enzymes, thereby decreasing intermediate loss and enzymatic cost. Our second alternative pathway utilizes the native enzyme transaldolase to circumvent the use of another photosynthetic enzyme, once again decreasing the need for specific intermediates hypothesized to limit the photosynthetic cycle. Both alternative pathways were first simulated via computational modeling to predict the viability of the pathway and explore potential wet lab considerations. Due to the transaldolase pathway overexpressing a native enzyme rather than a foreign set as observed with the first, this transaldolase pathway was evaluated in vivo using the cyanobacterial strain Synechococcus elongatus PCC 7942. Cyanobacteria are generally considered an ideal model organism for exploration of and genetic manipulation of photosynthesis due to their malleable genome, homogenous population when cultivated, and rapid growth rate (2).
Our work could be integrated into similar systems in crop plants to increase yields, contributing to global food sustainability for the growing population.
For our project, we also developed significant education and communication efforts with our community. We approached this outreach with two main goals. First, we wanted to address the concept of genetically modified organisms (GMOs) as a controversial topic, in which factual information is often obscured by paranoia and unscientific claims. Due to the vital role of GMO crops in future applications of our project, we aimed to educate groups who had limited opportunities to accurately learn about GMOs. We focused on two age groups we believed were most susceptible to impact by unreliable sources; elementary age children and older adults. Second, we wanted to produce accurate educational content on the complexities of photosynthesis. In many education systems, photosynthesis is limited to “light” and “dark” reactions, a massive oversimplification of the foundational source of biological energy in our world. With the limitations of photosynthesis becoming a rising research focus, we wanted to alleviate the early miseducation and misconceptions children are often exposed to regarding photosynthesis. Thus, we aimed to generate significant material to educate rising generations on photosynthesis, its limits, and its importance in research.
Description: Global agricultural productivity is projected to not meet the needs of increasing populations developing higher standards of living. On a cellular level, crop yield is limited by the inefficiency of photosynthesis. Our project aims to improve this efficiency by implementing an alternative RuBP regeneration portion of the Calvin-Benson-Bassham (CBB) cycle. Two alternative pathways, which use enzymes from other reactions that act on common metabolites used in the CBB cycle, were explored first via computational modeling. Impacts on growth and reaction fluxes in silico assessed the validity of these pathways in creating a more robust photosynthetic cycle. One of these pathways which overexpresses the native enzyme transaldolase, was also assessed in vivo. Ultimately, we showed the validity of two alternative pathways in allowing a more efficient regeneration stage of the CBB Cycle. These pathways could eventually be implemented into higher plants to allow more robust cycling and therefore higher crop yield.
1. Tilman D, Balzer C, Hill J, Befort BL. 2011. Global food demand and the sustainable intensification of agriculture. PNAS 108:20260–20264. (https://doi.org/10.1073/pnas.1116437108)
2. Ort DR, Merchant SS, Alric J, Barkan A, Blankenship RE, Bock R, Croce R, Hanson MR, Hibberd JM, Long SP, Moore TA, Moroney J, Niyogi KK, Parry MAJ, Peralta-Yahya PP, Prince RC, Redding KE, Spalding MH, Wijk KJ van, Vermaas WFJ, Caemmerer S von, Weber APM, Yeates TO, Yuan JS, Zhu XG. 2015. Redesigning photosynthesis to sustainably meet global food and bioenergy demand. PNAS 112:8529–8536. (https://doi.org/10.1073/pnas.1424031112)
With any project, thinking carefully about and managing any risks is a vital step. Between the inherit risks of our project as well as challenges created by the COVID-19 pandemic, our team had to be thoughtful. Learn how the Miami University team handled safety and risk here!
Our team looked into designing different pathways to enhance photosynthesis of crops in order to increase crop yield. Learn more about our designs!
The main point of contact on the Miami University iGEM team is the principal investigator Xin Wang. Xin Wang can be inquired below. For additional contact information on the team, please refer to the Team page.
This work was funded and made possible by the National Science Foundation!