Credit: Living.org

Technical Analysis: Crop Based Carbon Capture

An Immediate and Efficient Solution to Carbon Emissions and Global Climate Change

With the global temperatures changing at an alarming rate, climate catastrophes are becoming more and more frequent. Countless hurricanes and uninterrupted rains are flooding agricultural land, taking away their fertility, as other regions are rather stricken with eternal droughts. How to continue feeding humanity in these conditions? How to continue housing humanity in these conditions? Sea levels are rising, steadily, taking people’s homes as we watch and do little to help.

Does it have to be this way? Is there nothing we can put in place to stop this madness? There is, in fact, a solution not many have heard about that could help dramatically. One little-known innovation that could solve close to everything. This innovation is called the“ideal plants”, engineered super crops we could plant, eat, and watch grow as they suck a quarter of the excess CO2 out of our overcharged air.

In this article, I want to make a proposal to how we can potentially reduce global climate change significantly with genetically engineered crops. Here is the table of content:

• Section 1: Why are Plants a Good Vehicle For Carbon Capturing
• Section 2: Dominant Factors in Plant Carbon Capacity
• Section 3: Genetic Engineering Pathways, a technical analysis
• Section 4: Impact and Execution Plan

Section 1: Why are Plants a Good Vehicle For Carbon Capturing

The reasons to why plants makes an appealing choice for a carbon capturing technology

Plants are a fabulous candidate for a carbon capturing technology on the ground of two reason. Firstly, plants are mostly made up of carbon. In fact, a general estimation across all plant species across the globe reveals that plants (again, on average) have an elemental composition of about 50% carbon, 6% hydrogen, and 44% oxygen. Second to that, plants are also an extremely favourable choice not least because of their natural evolutionary tendency to fixate atmospheric carbon dioxide into organic carbon. Below I will dive deeper into both.

Section 1.1: Carbon composition and plants biomass

The newest studies reveal that the composition of elemental carbon within plants have to be measured separately between herbaceous and woody plants, with woody plants holding a generally higher carbon composition. With focus on the herbaceous species, the study also found that arithmetic means of carbon content for herbaceous plants stood at around 46%, with slight variations between different organs. (reproductive organ, root, leaf, and stem were 45.01%, 45.64%, 46.85%, and 47.88%, respectively) This is significantly higher than all other kingdoms and taxons, with examples such as animals on average having less than 25% carbon content within their body structure.

As a result of the comparatively high carbon content, plants are the dominant kingdom with regards to carbon biomass in the world, holding up to 80% of all the biological carbon across all species.

Distribution of biological carbon by taxon in Gigaton, credit by Yinon et al.

Further adding to the justification of why plants makean incredible basis for carbon capturing technology, one study found that optimally, plants biome around the world should be able to store up to 916 Gigaton of carbon, but currently it is only storing less than 450 Gigaton of carbon. From the same study, it is also named that agricultural crops are the culprit for at least 28% of this 466 Gigaton deviation, sequestering only 10 Gigaton of carbon compared to the optimal 140 Gigaton of carbon.

Potential Vs. Actual biomass stock, credit to Karl-Heinz et al.

Section 1.2: Natural tendency to sequester carbon

Plants are autotrophs that generate energy through a process known as oxygenic photosynthesis (eluded above in section 1). Oxygenic photosynthesis is the process by which plants use solar energy to fixate carbon dioxide (CO2) into carbohydrates and other biological carbon, releasing oxygen (O2) as a by-product through a series of reactions occurring in the thylakoid membranes (where the photochemical reactions occur) and in the chloroplast stroma (where carbon-fixing and reducing reactions occur).

Due to this natural tendency to fixate and remove carbon dioxide from the atmosphere, plants are naturally equipped with organs capable of performing carbon fixation. This resolves a significant barrier to engineering as it utilise a natural process of carbon fixation, and only requires the engineers to explore pathways to maximise its efficiency.

Section 2: Dominant Factors in Plant Carbon Capacity

Finding the determinant variables behind the plant carbon capacity

The general method of carbon storage by plants are in form of Soil organic carbon and plant biomass itself. There are several pathways the carbon can take entering and exiting the terrestrial environment:

• Photosynthesis: Fixation of atmospheric CO2 into plant biomass and organ structures. Soil organic carbon (SOC) input rates are primarily determined by plant biomass.
• Respiration: Mostly in form of the respiration of symbiotic organism with the plant in the root system (heterotrophs).
• Decomposition: Decomposition of biomass by soil microbes results in carbon loss as CO2 from the soil due to microbial respiration, while a small proportion of the original carbon is retained in the soil through the formation of humus, a product that often gives carbon-rich soils their characteristic dark color.

Image credit Polarpedia.au

Basing on the pathways above, there are several notable factors relating to the plants’ ability to fixate and store carbon. Below are the three main variables contributing to carbon sequestering ability in a plant:

• Photosynthesis rate and efficiency, as more efficient photosynthesis will allow the plant to more effectively sequester carbon in the soil.
• Root biomass of plants which stores the bulk of the carbon sequestered as a result of its relatively large biomass compared to other plant organs.
• Recalcitrance or the resistance against decomposition of the plant debris after its death. The more resistant the plant is to decomposition, the longer the carbon fixated will remain in the soil before released back into the air.

There are several other external and environmental factors that also contributes to the carbon sequestering ability of a plant:

• Presence of water in the environment, scarcity of water resulting in reduced photosynthetic capability.
• Nutrients presence. Infertile soils result in larger biomass allocated to roots reducing photosynthesis.
• Temperature increase result in increase soil heterotrophic respiration and decrease in soil carbon
• Age of plant: Carbon sequestration tends to decrease the lifespan of plants.
• Atmospheric gases concentration: elevated concentrations result in increased carbon sequestration.
• Method of tillage: results in the loss of SOC.

Section 3: Genetic Engineering Pathways, A Technical Analysis

The master plan to enhance the plants’ ability to better and more effectively sequester carbon.

Basing on the three identified factors that directly influence the plants’ ability to sequester carbon from the atmosphere, we have identified three potential pathways where genetic engineering could enhance the plants capacity:

1. Genetic engineering to augment the root biomass and root system architecture.
2. Genetic engineering to enhance the plant’s photosynthesis efficiency by insertion of a carbon concentration mechanism.
3. Increased plant recalcitrance through increased suberin production and decomposition resistance.

Editing schematic according to pathways

Here I will discuss the in-depth editing and technique required for the three genetic engineering pathways.

Section 3.1: Augmented Plants Roots Biomass through Auxin Transportation Moderated Extension and NAC over-expression

Auxin, the first scientifically discovered plant growth hormone, plays an crucial role in the process of plant growth. In two recent studies, researchers discovered that auxin moderated root growth isprimarily regulated by three different genetic pathways.

The expression of the evolutionarily conserved EXO70A3 gene located on the 5th chromosome are positive related to the vertical growth and development of the plant root system. Overexpression of the EXO70A3 gene was successfully induced through a SNP (Single Nucleotide Polymorphism, when one pair of nucleotide changes) on the AT5G52350 protein coding region of the EXO70A3 gene. To genetically engineer the plant to over express the EXO70A3 gene can take two different pathway. Firstly, a CRISPR-Cas9 based gene editing system that induces a substitution mutation in the responsible gene sequence. And secondly, a Cauliflower Mosaic Virus-based viral vector to deliver the sequence of the EXO70A3 gene promoter. In simpler terms, the editing required can either be a small substitution on the original EXO70A3 gene to make it hyperactive (CRISPR based) or a cut and paste of another sequence that lead the EXO70A3 pathway to become hyperactive. Overall, the engineering difficulty is generally quite low.

Schematic of EXO70A3 gene, credit to Ogura et al.

The repression of TOB1 gene is found positively related to the development and extension of the root system. TOB1 is the name scientist have given to the defective At1g72140 coding region of the IBA transport PS173 gene. IBA (indole-3-butyric acid) is an important auxin precursor. To have auxin-induced growth in root, the IBA must be chemically converted into IAA (indole-3-acetic acid). Activation of TOB1 will cause the IBA to be transported off from its initial location via the pen3–4 activity, thus resulting in the suppression of root growth. In simple terms, if you switch off or suppress the TOB1 gene, the plant will be able to convert IBA to IAA and use it to grow bigger roots. The genetic engineering is generally done by insertion of a defective At1g72140 sequence (TOB1) into the plant genome. Overall the engineering difficulty is also generally simple.

Schematic of the TOB1 gene, credit to Michniewicz et al.

The overexpression of the NAC protein transcriptional factor are found to be positively related to the root biomass. The NAC protein is expressed with the RNA strand TaRNAC1. Over-expression of the TaRNAC1 strand (NAC) is found to be a primary promoter of the root biomass in wheat and further experimentally found to be evolutionarily conserved. The engineering required for over-expression of TaRNAC1 is not as straightforward. The experimental method requires the insertion of the OsPR1L2TaRNAC1 or OsPR1L2GFP expression unit upstream. This act as a promoter for the expression of the NAC transcriptional factors. In simple terms, NAC protein help the plant grow larger biomass in its root. To over-express NAC protein we have to insert another gene sequence that forces the plant to over-produce the NAC protein.

Schematic and TaRNAC1 expression in association to genetic pathway, credit to Chen et al.

Impact

The genetic editing performed on all three pathway will cumulatively increase the carbon capacity of the plant by between 59%-68%, with some estimation performed yielding number as high as 80%. Further, the extension of the root system vertically is positively related with the plants’ drought resistance. This is a significant economic incentive for the adoption of the modification into agricultural practice.s

Section 3.2: Enhanced Photosynthesis Efficiency Through C4 Pathway Introduction

The moonshot components of the technology are mostly concentrated in the genetic engineering of photosynthesis. Most plant perform photosynthesis according to the C3 pathway, a simpler but less efficient method of photosynthesis (direct chemical conversion from CO2 to carbohydratse). C4 pathway photosynthesis is inherently more complicated, where CO2 is fixated into bicarbonate in intermediary M cell before undergoing the final C4 acid decarboxylation. This method is generally considered to be more efficient chemically compared to its C3 counterpart under the same circumstances.

As of 2021, no effort has succeeded in synthetically create plant species that under goes C4 photosynthesis. However, scientist did outline a stepwise process that is projected to eventually synthetically develop C4 species:

1. Alteration of plant tissue anatomy
2. Establishment of bundle sheath morphology
3. Ensuring cell type-specific enzyme expression

Currently, the most prominent efforts are effectively concentrated on crossing a C3 and a C4 plant to create a C3-C4 hybrid. This will ensure the species holds the higher metabolism rate of the C4 plant species. This approach is found to be the most feasible, as it is known to have emerged independently at least 66 times in different phylogenetic backgrounds. Certain engineering objective must be achieved in order for this crossing to be successfully performed: (The content is highly technical and difficult to dissect, please skip this sub section if you found it difficult)

1. Restriction of Rubisco and the Calvin–Benson cycle to the bundle sheath chloroplast.
2. Initial conversion of CO2 to bicarbonate in M cells.
3. CO2 fixation by PEP carboxylase activity should be restricted to M cells while C4 acid decarboxylation must occur in the cytosol or chloroplast of the BS cells.
4. Conversion of oxaloacetate in M cells into either malate or aspartate is essential prior to diffusion into the BS cells.
5. The three-carbon compound released during decarboxylation in BS cells must be returned to M cells.
6. To ensure sufficient substrate for the initial CO2 fixation step, PEP needs to be rapidly recovered for initial bicarbonate incorporation and decarboxylation into the organic acids oxaloacetate and subsequently Asp.

In simple(er) terms, the plant must be engineered to develop more concentrated photosynthesis-related organs (chloroplasts) to allow a higher concentration of carbon. This opens up another indirect chemical pathway that allow a higher yield of fixated carbon.

Impact

The theoretical result of C4 pathway photosynthesis will be a 10 times higher concentration of inorganic carbon in the vicinity of Rubsico. The exact impact this will have on carbon sequestration is yet unclear, but the efficiency will most likely also have a proportional increase as dictated by chemical dynamic (concentration increase in reactant shift equilibrium toward the product side).

Section 3.3: Increased decomposition recalcitrance through altered suberin production

Suberin is an evolutionary conserved acylglycerol lipid strongly suggested to be involved in the decomposition rate of plant tissues. It has not yet been studied extensively, but enough is known about this polymer to form conjectures about how it might be useful to us in the future in order to sequester more carbon.

Increased suberin, in association with augmented cutin and other polymers, has been found to lower root decomposition rates in multiple plant species. This would mean that the carbon dioxide a plant absorbs in its life would stay stored in the soil for longer. However, the study that arrived at this conclusion had not genetically engineered a superior suberin content, but rather merely observed decomposition rates of different plant roots, such as spruce and beech roots.

The first graph presented above displays the percentage of the initial plant mass remaining depending on the amount of time it had spent decomposing. The second graph displays the percentage of certain lipids remaining in relation to the length of incubation. In those graphs, suberin is represented by the “b”. Data about suberin may be scarce, but from the data we do have, it seems there exists a near 1:1 correlation between suberin initial amount and rate of decomposition.

In order to modify the root quantity of suberin, we would have to get a clearer picture of the exact genes involved in the mechanism of suberin production. Nevertheless, we do know a lot of the cellular components associated with it. Categorized, here are the pieces of the puzzles that we currently possess:

• Encoding enzymes, such as β-KETOACYL-CoA SYNTHASE and KCS20.
• Fatty acid cytochrome P450 oxidases, namely CYP86A1 and CYP86B1.
• FATTY ACYL-COA REDUCTASEs, a catalyst.
• GLYCEROL-3-PHOSPHATE ACYLTRANSFERASE5, still a catalyst.
• ATP BINDING CASSETTE G transporters, the role of which being transporting various substances (in this case, suberin) from one cell’s cytoplasm to another’s.
• MYB factor AtMYB41. It was shown to act as a positive regulator of suberin biosynthesis in Arabidopsis thaliana under stress conditions.
• β-ketoacyl-CoA synthases, for fatty acid elongation. KCS2 is one of them.
• Cytochrome P450 oxygenases, for fatty acid hydroxylation, more specifically CYP86A1 and CYP86B1.

Theoretical Impact

Increasing the suberin root content of crops would likely have an impact on microorganismal life, as it feeds on decomposition. The neat amount of organic carbon would not be altered, and so they would not be left with less food, however, the same amount of fuel would be provided over a longer period of time. It is likely that the decomposers could adapt to this modification, as most decomposers are protozoa and bacteria, which evolve at a very rapid pace. However, further studies will have to confirm this.

Additionally, a study on naturally occurring enhanced suberin Arabidopsis mutants found that a two-fold increase in suberin resulted in increased root water and solute permeability. In other words, if we could artificially recreate such a mutant, it would be more resistant to droughts than its wild-type counterparts, which is where the economic incentive lies.

As for the impact on the climate, it is difficult to quantify with the current data. However, Joanne Chory, the most prominent researcher in this field, states that by planting crops with increased suberin on just 5% of the land currently used for agriculture, we could capture 50% of human’s yearly global CO2 emissions. That is a very exciting number, and it is why we chose to pursue this suberin pathway. However, until more data is available, this stays a conjecture. This is why the possible impact of suberin modification is not included in the calculations of the impact of ideal plants on the climate crisis. However, if this hypothesis were to be confirmed in the next few years, the impact would indeed be tremendous.

Section 4: Impact and Execution Plan

The execution master plan

Most of the identified engineering pathways are evolutionarily conserved within angiosperms if not all plants. This opens up the choice of crops we may perform the genetic editing on. Our proposed primary crop choice that these edits are appropriate for will be potato, followed by wheat, rice and other common grains. A full implementation in potato crops with these edits has the potential of fixating 114 million tones of carbon every year. If similar edits can be adopted on just 10% of all crops, it may erase 5% of global annual emission every year. A implementation on 50% of all crop will sequester more than 25% of global annual emission every year. The project will be divided into phases detailed below.

Section 4.1: Potato, The Initial Stage

Potato is chosen as the primary crop to perform the identified gene editing on for two reasons. Firstly, potato tuber (the part we eat) size and yield is identified as proportional to the biomass of the entire root system given all external conditions are controlled. This correlation means augmenting procedure produced on the root system of potato will go into increase the agricultural yield of the crop. Larger yield crop is considered the primary economic incentive for agricultural adoption of the technology. Secondly, potato production is large around the world, offering high potential for carbon sequestration and impact.

In the initial stage, the crop will be introduced primarily to countries under the jurisdiction of the FDA and EU commission of Food Safety. This is on the ground that these two regulation frameworks are generally considered to be the most mature of their type while both also govern relatively sizeable proportion of the global potato production.

Table break down of impact and region of implication

The initial process of development of the genetically modified crop will take around 2–4 years depending on factors such as success in genetic engineering and initial safety trials. Passing the regulation inspection will generally require between 3–4 years in the FDA framework and slightly faster in the EU commission of food safety. The total cost of this process will be estimated between $1.3–$1.5 Million USD.

The cost of producing the seeds for one acreage in Canada is currently between $170–500 CAD. This justifies the original implementation of this project to be exclusively in economically developed countries (North America and Western Europe), as the price will generally become a barrier for agricultural entities in developing nations. The primary economic incentive will be the 55% increase in fruit yield per acreage and the increased drought resistance in the background of the current global climate change.

Section 4.2: Out and Beyond

As most genetic pathways identified for this increased carbon sequestration capacity are evolutionarily conserved, editing done in potato crops should be easily transferable to other popular crops. Meanwhile, the implementation of potato can extend beyond the countries identified in the initial stages. This is on the ground that after recovery of the initial development cost and wide adoption of the technology, the general cost of seed and infrastructure will be by magnitudes lower.

We have identified the primary countries this technology will benefit.

Table break down of impact and region of implication

Conclusion

We tend to think of future technology to be a mixture of grandiose sci-fi-like spaceships and super computer sthat can simulate the universe. However, solutions to some of our most prominent questions may lie within some of the most common, humble subjects. Crops, ones that have been cultivated by human for thousand of years, may hold with themselves one of the keys to climate change. It is truly is a future worth a look.

Thanks for reading! I’m Henry, a 17 year old tech and longevity enthusiast on a mission to help extend human health span.

This article is written in collaboration with Flavie Prévost. If you like the content please remember to follow both of us on Medium. You can also find me on LinkedIn or join my monthly newsletter.