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Photosynthesis Enhanced Trees Grow Faster and Capture More Carbon

Published
3.30.2022

Living Carbon was founded on the idea that frontier plant biotechnology can increase terrestrial carbon capture & storage in ways that would otherwise not be possible. To test this hypothesis, we incorporated a photosynthesis enhancement trait in hybrid poplar trees. By increasing the efficiency of photosynthesis, we can help trees grow faster and act as partners in capturing more carbon from the atmosphere. Thus far, our results have been encouraging. After multiple generations of vegetatively propagated tree seedlings studied in a controlled environment, our lead photosynthesis-enhanced poplar tree seedling showed a 53% increase in the production of above ground biomass. Data generated from our molecular, morphology, and physiology analyses also indicate that our design works as intended. We continue to study these photosynthesis-enhanced seedlings in field trials, alongside other mechanisms of action for photosynthesis enhancement and extending the duration of carbon storage in trees.

Introduction

We envision a future in which natural processes that have evolved throughout the plant kingdom can be used to maximize the potential of plants to sequester carbon and rebalance ecosystems.

Forest carbon drawdown is one of our greatest allies in the climate crisis, but the impact of forest carbon solutions has been constrained by land-use efficiency, suitability of land to support forest stands, the growth rate of trees, and the duration of carbon storage before it is released back into the atmosphere. That changes starting today. There are many strategies to enhance carbon capture in plants, including increasing resistance to disease and drought, salt tolerance, decomposition resistance, and photosynthesis enhancement. Our initial focus has been two-fold: (1) improve carbon capture in trees via more efficient photosynthesis, and (2) improve carbon storage through decay-resistant wood, which slows the release of carbon through decomposition.

These research findings serve as an example of the many research projects being worked on by the Living Carbon team. To read the full white paper, click here.

Left: hybrid poplar seedling with photosynthesis enhancement trait. Right: control hybrid poplar seedling.

Our Strategy

The results shared in this blog post focus on photosynthesis enhancement as an example mechanism of action to increase carbon capture in trees. For these experiments, we selected a photorespiration bypass pathway and tested its effectiveness on photosynthetic enhancement in hybrid poplar trees.

Photorespiration is a wasteful side process of photosynthesis that slows growth and releases already-fixed carbon back into the atmosphere as CO₂ (Learn more about photorespiration in our FAQs). To grow faster, our trees recycle a toxic byproduct of photosynthesis with less energy, capturing more CO₂ over time.

Previous research shows that photorespiration bypass pathways increase biomass and photosynthesis efficiency in other C3 plants. The photorespiration bypass pathway used allows more energy to go into growth of trees, thus increasing biomass accumulation and carbon assimilation. Usually, waste products of photorespiration are exported from the chloroplast to multiple organelles for metabolic cycling. Our biotechnology enables the chloroplast to break down these waste products internally and turn them into energy-rich glucose and cellulose.

Internal breakdown of waste products within the chloroplast (left) vs. export to other organelles for metabolic cycling (right)

This process is similar to the natural process that already exists in 15% of plants, called C4 carbon fixation, which have separately evolved special features to combat photorespiration and are more photosynthetically efficient and productive. Examples of C4 plants include corn, sorghum, and sugarcane. Most other plants are C3 plants which experience photorespiration. Our strategy achieves similar results to C4 carbon fixation in C3 plants, starting with trees.

C4 carbon fixation evolved when CO₂ partial pressure was low and there was a higher partial pressure of oxygen in the air, which is the opposite of the trajectory we see today. The anthropogenic increase in atmospheric CO₂ has reduced the selection pressure on C4 plants. Rather than try to go against this evolutionary process, we incorporated natural processes from other plants and algae to achieve the same effect of avoiding photorespiration. The result is the creation of C3 plants that convert CO₂ to sugar more efficiently.

Switchgrass is an example of a C4 plant, compared to C3 hybrid poplar

Our Methods

We incorporated genes that drive natural processes from other plants and algae to create our desired trait in trees.

Plant biotechnology involves integration of one or several new pieces of DNA into the host genome. Often there is a degree of randomness involved. Each one of these DNA insertions is called an “event” and will exhibit different characteristics based on placement. We generated 41 independent events via transformation and tested these events for efficacy. After in-house-developed molecular analysis, we found 38 independent events that successfully expressed all intended genes.

Several of these young trees started to display faster growth early, as reflected by height and volume increase, as well as stem diameter measurements. We transferred these plants from tissue culture boxes into soil pots to grow in our greenhouse. We also propagated new plants from these original 38 to create more biological replicates (ramets) for testing. We moved forward with several events for further testing, two of which, named “A” and “B” for clarity, are shown in the data below.

We found that these photosynthesis-enhanced seedlings have reduced the expression of genes that would otherwise transport the toxic byproducts of photorespiration out of the chloroplast, and have exhibited an increase in plant height, stem volume growth, carbon assimilation rate, and biomass accumulation. These results demonstrate a promising strategy to increase biomass production through photosynthesis enhancement.

For reproducibility, we expanded and iterated on our experimental design using more ramets of the same plants. Ramets are biological replicates of the same plants, in this case taken from stem cuttings. We found similar results that support the efficacy of our design in our measurements of plant height, carbon assimilation rate, gene expression, and stem growth curve. This experiment with the second set of replicates is still in progress and we are in the process of collecting repeat biomass data. Based on projections, we expect the biomass accumulation results to be consistent as well.

Now, let’s dive into the data:

Increased Plant Height

After 17 weeks, seedlings (blue arrows) showed a visible height increase compared to controls (red arrows). Upon further analysis, we found these events demonstrate significantly taller height with p value less than 0.01.  

CO₂ Assimilation rate

We measured CO2 gas exchange on multiple events (A, B, C, D) from two separate experiments using the LI-6800 instrument and found that all leading photosynthesis-enhanced seedlings achieve a higher rate of carbon assimilation than controls when not limited by light or carbon dioxide.

Enhanced photosynthetic rate in our seedlings. Seedling A was included in both Experiment 1 (left) and 2 (right).

Gene Expression

We used qRT-PCR analysis to assess the effectiveness of our design in reducing expression of the target gene in engineered events.

Across our experiments, there was a significant reduction in the expression level of the glycolate glycerate transporter in leaf samples in all engineered events compared to controls. This reflects lower amounts of glycolate being transported out of the chloroplast, resulting in the inhibition of photorespiration. There was also successful expression of the genes showing the effectiveness of the shunt pathway to metabolize the retained glycolate back to CO₂ in the chloroplast for carbon fixation, rather than releasing it back into the atmosphere.

We have developed a reliable multiplex qRT-PCR protocol for gene expression analysis (left) and observed the desired reduction of expression in our seedlings (right).

Increased Biomass

At week 21 post potting, we harvested these plants to perform a biomass measurement. We looked at both fresh weight and dry weight for leaf, root, and stem tissues. As you can see below, event A has significantly higher biomass production in all tissue types, at both fresh weight and dry weight levels. Looking at above ground, dry weight, this event demonstrates a 53% increase in biomass over the control plants. Biomass accumulation is a strong indicator of carbon assimilation, with about half of biomass being stored carbon. In addition to genetics, biomass is also influenced by location and soil fertility.

Four different seedlings were included in a repeat experiment including seedling A and control. Another seedling “C” showed significant growth increase over the control.

What’s Next

While it was great to see efficacy verified in the lab, these photosynthesis-enhanced trees are meant to thrive in the real world. To further validate these trees in the ground, we partnered with Oregon State University to conduct a field trial in Corvallis where students and professors can learn from our processes and help to independently analyze the data. There are a total of 672 trees planted for this trial, including 468 with photosynthesis enhancement.

Living Carbon team planting seedlings for our first pilot project in Corvallis, OR.

Moving into 2022, we will be evaluating the photosynthetic performance and biomass accumulation of these trees with the OSU school of forestry staff and students. This year we will start to propagate our seedlings at scale by developing forest carbon projects with our land partners. So far, we have 3,200 acres of dedicated land for pilot planting projects.

Our launch window for carbon removal solutions is short, and the stakes couldn’t be higher, but it gives us hope to see these seedlings grow in our lab and beyond. We believe we can use the tools of biotechnology to benefit our ecosystems and create nature-based solutions to climate change.