Enhancing Photosynthesis: a Big Deal
Posted 10th July 2019 by Jane Williams
The PhotoSeed Technology
Enhancing photosynthesis is a critical step to increasing crop yields. This complex 156 step biochemical process has been the subject of many studies in multiple crops. While some step wise gains have been made, the true potential for increasing photosynthesis has not been realized potentially due to the negative feedback mechanisms that exist within plants to regulate this process.
ZeaKal is a plant science company formed in 2010 from the incubation pipeline of Kapyon Ventures in partnership with AgResearch Ltd. The company is focused on developing PhotoSeed™, a next-generation trait technology that has been proven to increase photosynthetic capacity and yield of several major crops.
PhotoSeed has been shown to enhance photosynthesis and plant growth rates in the model species Arabidopsis, the temperate grass forage perennial ryegrass and the legume crop soybean (Beechey-Gradwell et al., 2018; Bryan and Roberts 2016; Winichayakul et al 2013). Evidence comes from both growth room and field studies. PhotoSeed can enhance photosynthesis by as much as 24% and increase plant growth by up to 50%. In soybean, ZeaKal has also increased seed yield by up to 5% and oil yield by 16%.
In February 2019 Corteva Agriscience, Agriculture Division of DowDuPont, and ZeaKal announced a R&D collaboration to develop and test PhotoSeed. The collaboration will help accelerate the commercialisation pathway for this technology.
Following on from the Corteva Agriscience partnership, June 2019 saw another strategic collaboration announcement, this time between Canopy Rivers and ZeaKal. The PhotoSeed technology has the potential to translate into significant benefits for the cannabis and hemp industry. Due to prohibition, the cultivation of cannabis and hemp has lacked the agricultural research advancements that have significantly improved the cultivation of other crops.
Mechanism(s) for Enhanced Photosynthesis
Just how does PhotoSeed increase a plants intrinsic photosynthetic capacity? We have identified a number of changes in plants containing PhotoSeed that explain the enhanced photosynthesis.
In the leaves of plants that utilise C3 photosynthesis, the key enzyme responsible for photosynthetic carbon assimilation is Rubisco. Rubisco has an affinity for both O2 and CO2. Under ambient conditions, approximately one quarter of Rubisco enzymatic reactions are fixing oxygen (instead of CO2) to form glycolate. The recycling of glycolate is energy intensive and leads to a net loss of fixed carbon and nitrogen. In C3 photosynthetic plants under higher temperatures and/or water stress, the proportion of oxygen fixing cycles is considerably higher.
In PhotoSeed plants there is a constant demand for lipid production that forces the plant to continually generate two carbon unit precursors for condensation into the elongating fatty acid chain. In sourcing this precursor, the cell uses a three carbon molecule, 3-phosphoglyceric acid, from the Calvin cycle. Over a two-step process, the additional carbon atom is removed and recycled within the chloroplast in the form of CO2, thereby increasing the local CO2 concentration, which in turn increases carbon assimilation via Rubisco (Durrett et al., 2008; Winichayakul et al., 2013). Further support for CO2 recycling comes from research on the role of the inner pod wall in grain legume pods where a layer of cells refix respiratory CO2 from the embryo (Furbank et al, 2004; Schwender et al, 2004).
While we had proposed that CO2 recycling due to de novo fatty acid biosynthesis could explain the increased photosynthesis in PhotoSeed plants, additional CO2 recycling could also come from the oxidative pentose phosphate pathway as NADPH is regenerated.
Our initial working hypothesis suggested that PhotoSeed would only enhance photosynthesis in C3 plants and that C4 plants (e.g. corn) would not benefit as the plants already had a mechanism for enriching CO2. However, our thinking has evolved as we further study PhotoSeed plants and have evidence that the initial hypothesis only accounted for part of the increase in photosynthesis.
It has been shown that the overall carbon metabolite balance can regulate photosynthesis in plants. When plant sugars are depleted the genes involved in photosynthesis are activated to increase photosynthetic capacity. If sugar concentrations in the leaf increase beyond the plants capacity to utilize them, the genes involved in photosynthesis are repressed and photosynthesis is down regulated (Reviewed by Pego et al., 2000). Therefore, the carbon metabolite balance places an exquisite control over photosynthesis. This can be seen in plants that are placed in a carbon dioxide rich atmosphere. The increase in external CO2 leads to increased photosynthesis and increased leaf sugar concentrations. After a period of days photosynthetic rates begin to decline most likely due to the accumulation of sugars in the leaves.
If the original CO2 recycling hypothesis was the only mechanism for enhanced photosynthesis, when we grew PhotoSeed plants in an enriched CO2 atmosphere we would expect to see that non-PhotoSeed plants would have a greater benefit and effectively catch up to the PhotoSeed plants. However, this is not the case.
We have shown in experiments where PhotoSeed ryegrass was grown in ambient CO2 (400 ppm) or elevated CO2 (760 ppm) that PhotoSeed plants had elevated photosynthesis and higher growth rates compared to the control ryegrass in both ambient and elevated CO2. We also saw that the sugar balance is altered in PhotoSeed plants. PhotoSeed plants accumulate fatty acids in leaf micro organelles and there is a shift in carbon storage away from leaf sugars to leaf fatty acids. The PhotoSeed plants had higher leaf fatty acids and lower leaf sugars in both ambient and elevated CO2.
This would suggest that there has been a release of the constraint to photosynthesis placed by the negative feedback imposed by plant sugars. It is possible that by diverting 3-phosphoglyceric acid into fatty acids we are preventing the accumulation of plant sugars that would normally downregulate photosynthesis.
We have also performed a preliminary transcriptome analysis of PhotoSeed ryegrass and have supporting evidence to show upregulation of genes involved in the photosynthetic pathway and down regulation of some genes encoding sugar biosynthesis. We also think that the mechanism is more robust and that the enhanced photosynthesis will not be reduced by some of the negative feedback mechanisms that exist. Finally, although untested we think that plants with C4 photosynthesis will also benefit from PhotoSeed.
Greg Bryan, CTO, ZeaKal will be speaking at the upcoming Plant Genomics & Gene Editing Congress: USA on ‘Improving Crop Productivity and Sustainability by Enhancing Photosynthesis’.
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