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Ray, S. Engineering Photosynthetic Plants. Encyclopedia. Available online: https://encyclopedia.pub/entry/21553 (accessed on 01 December 2024).
Ray S. Engineering Photosynthetic Plants. Encyclopedia. Available at: https://encyclopedia.pub/entry/21553. Accessed December 01, 2024.
Ray, Supriyo. "Engineering Photosynthetic Plants" Encyclopedia, https://encyclopedia.pub/entry/21553 (accessed December 01, 2024).
Ray, S. (2022, April 11). Engineering Photosynthetic Plants. In Encyclopedia. https://encyclopedia.pub/entry/21553
Ray, Supriyo. "Engineering Photosynthetic Plants." Encyclopedia. Web. 11 April, 2022.
Engineering Photosynthetic Plants
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One of the main contributors to biological carbon fixation is plants, during the process of photosynthesis. Plants are divided into three main categories based on their photosynthetic pathways, i.e., C3, C4, and crassulacean acid metabolism (CAM). These pathways are differentiated by their respective methods of carbon fixation and their ability to combat photorespiration.

genetic engineering photosynthesis carbon fixation C3 cycle crassulacean acid metabolism (CAM) phosphoenolpyruvate carboxylase C4 photosynthesis RuBisCo Synthetic biology Calvin–Benson–Bassham cycle Kranz anatomy

1. Engineering Photosynthetic Plants

One of the main contributors to biological carbon fixation is plants, during the process of photosynthesis. Plants are divided into three main categories based on their photosynthetic pathways, i.e., C3, C4, and crassulacean acid metabolism (CAM) [1]. These pathways are differentiated by their respective methods of carbon fixation and their ability to combat photorespiration. Photorespiration is the process by which the enzyme RuBisCo reacts with oxygen when there is a lack of CO2, wasting necessary energy and creating more CO2 [2][3]. C3 plants account for approximately 85% of the plant population. Characterized by its production of a three-carbon compound, 3-Phosphoglycerate, C3 photosynthesis does not have a mechanism capable of reducing the occurrence of photorespiration [1].
The C3 cycle, also known as the Calvin–Benson–Bassham cycle, has been determined to be the least efficient of the group. This is because in other pathways such as the reductive tricarboxylic acid cycles, the consumption of ATP is reserved only for inauspicious reactions that ultimately expedite carbon fixation such as RuBP-based carboxylation and carboxyl reduction. Additionally, to better conserve cellular resources, most of these unfavorable reactions are coupled with exergonic reactions, which, due to their spontaneous nature, diminish the need for the utilization of ATP [3]. Conversely, the Calvin cycle has no such functionality and frivolously uses ATP for these processes that have been optimized in other pathways.
C4 and CAM plants differ from C3 in their internal mechanisms to combat the occurrence of photorespiration. C4 plants are the least prevalent, accounting for approximately 5% of the plant population. In C4 plants, photosynthesis occurs in two different types of cells, mesophyll cells and bundle sheath cells. During C4 photosynthesis, CO2 reacts with the enzyme phosphoenolpyruvate carboxylase (PEPC), creating oxaloacetate. Oxaloacetate is then reduced into malate and transported to the bundle sheath cell, where decarboxylation occurs, allowing the CO2 molecule to enter the Calvin cycle [1]. By carbon fixation and the Calvin cycle occurring in two separate cells, RuBisCo is surrounded by a higher concentration of CO2, greatly reducing the chance of photorespiration. Approximately 10% of the plant population are CAM plants. Instead of photosynthesis occurring in two separate cells, like C4 plants, CAM plants complete photosynthesis in two separate parts of the day. During the night, CAM plants open their stomata to accumulate CO2, create oxaloacetate, and malate through the same mechanism as C4 plants, but during the day, their stomata are closed, and the CO2 proceeds to enter the Calvin cycle [1].

2. Engineering C4 Cycle into C3 Plants

With C3 photosynthesis being the least effective method of carbon fixation, genetically engineering effective photorespiration evasion mechanisms is a primary goal. The introduction of the C4 photosynthetic processes into C3 plants would help to minimize the occurrence of photorespiration. C4 plants evolved from ancestral C3 plants due to environmental conditions, such as decreased CO2 availability. C3 plants already have the necessary enzymes required to complete the processes of C4 photosynthesis; the main hindrance is the location where the photosynthesis takes place [4][5]. For C3 plants to exhibit C4 photosynthesis, the plants would need to incorporate bundle sheath cells into their processes, since C3 photosynthesis only takes place in mesophyll cells. Modification necessary to initiate this process includes increasing vein density, increasing the size and chlorophyll richness of bundle sheath cells, changing the organization of bundle sheath cells and mesophyll cells, engineering different forms of chloroplast for the respective cells, and compartmentalizing necessary enzymes into the respective cells [5]. Attempts to engineer single-cell and two-cell C4 photosynthesis mechanisms have occurred, to no avail. Since C3 plants already have the enzymes necessary to carry out C4 photosynthetic mechanisms, once the C4 mechanisms were put in place, overexpression and misexpression become an issue. It is hypothesized that the causes of failure revolve around the inability to control diffusion between the chloroplast and cytosol, affecting the necessarily high concentration of CO2 [5]. In order to effectively induce C4 photosynthetic mechanisms in C3 plants, extensive gene modifying technologies will be necessary, but these technologies are still in development. The technology would need the ability to insert genes necessary for the C4 cycle and possibly to remove genes that may suppress the cycle. The use of CRISPR/Cas9 is likely to be useful in knocking out unnecessary C3 functions (Schuler et al., 2016). Recent developments in next-generation sequencing (NGS) have led to breakthroughs in C4 research. Through NGS, five C4 plant species have been sequenced. Following this development, previously unknown components of C4 cycle biochemistry were discovered, and molecular insight regarding Kranz leaf anatomy became available [5].

3. Engineering CAM into C3 Plants

Another method to increase the effectiveness of carbon fixation in C3 plants would be to genetically engineer the photosynthetic mechanisms of CAM into the plants. Similar to C4 plants, CAM organisms also evolved from C3 plants, but their mechanisms for evolution are widely unknown. While CAM photosynthetic mechanisms and C4 plants share their regulatory elements with C3 plants, CAM seems to be a more likely candidate for genetically engineering C3 plants, based on its single-cell structure [6]. CAM plants were likely to evolve to increase the plants’ ability to survive in arid conditions, such as those of desert cacti. For CAM to be engineered into C3 plants, CAM-specific genes would need to be transferred into the plant, some pre-existing genes would require modifications, and traits specific to leaf structure might need to be modified. However, it is unknown how many genes would need to be modified for this process to be successful [7]. A few known modifications that C3 plants would need are a mechanism for nocturnal opening and diurnal closing of stomata, a mechanism for the decarboxylation of malate, and an anatomical modification to increase leaf succulence [6][7]. CAM engineering is still out of reach for modern plant biotechnology, since it only has the capability of modifying a few genes at a time. Synthetic biological approaches to CAM engineering prove to offer more potential. For synthetic approaches to CAM engineering, it is necessary to compile a library containing the different parts of CAM genes; design a circuit that includes carboxylation, decarboxylation, stomatal control, and anatomical structures; assemble multi-gene constructs; transfer the constructs into the chromosomes of C3 plants; and evaluate the newly engineered plants’ functionality. A few of the necessary parts for library construction include the promoter, terminator, protein-coding sequence, insulator sequence, recombination site, and linker sequences. After the library assembly, the previously mentioned steps would repeat in a cycle until the performances of the engineered plants are optimized [7].

References

  1. Kumar, V.; Sharma, A.; Soni, J.; Pawar, N. Physiological response of C3, C4 and CAM plants in changeable climate. India Pharma Innov. Int. J. 2017, 6, 70–79.
  2. Driever, S.M.; Kromdijk, J. Will C3 crops enhanced with the C4 CO2-concentrating mechanism live up to their full potential (yield)? J. Exp. Bot. 2013, 64, 3925–3935.
  3. Bar-Even, A. Daring metabolic designs for enhanced plant carbon fixation. Plant Sci. 2018, 273, 71–83.
  4. Aubry, S.; Brown, N.J.; Hibberd, J.M. The role of proteins in C3 plants prior to their recruitment into the C4 pathway. J. Exp. Bot. 2011, 62, 3049–3059.
  5. Schuler, M.L.; Mantegazza, O.; Weber, A.P.M. Engineering C4 photosynthesis into C3 chassis in the synthetic biology age. Plant J. 2016, 87, 51–65.
  6. Borland, A.M.; Hartwell, J.; Weston, D.J.; Schlauch, K.A.; Tschaplinski, T.J.; Tuskan, G.A.; Yang, X.; Cushman, J.C. Engineering crassulacean acid metabolism to improve water-use efficiency. Trends Plant Sci. 2014, 19, 327–338.
  7. Yang, X.; Cushman, J.C.; Borland, A.M.; Edwards, E.J.; Wullschleger, S.D.; Tuskan, G.A.; Owen, N.A.; Griffiths, H.; Smith, J.A.C.; De Paoli, H.C.; et al. A roadmap for research on crassulacean acid metabolism (CAM) to enhance sustainable food and bioenergy production in a hotter, drier world. New Phytol. 2015, 207, 491–504.
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