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How To Make a Difference Through Plant Metabolism [Hackaday]
Generally when we consider the many plants around us, we imagine them efficiently using the electromagnetic radiation from the Sun via photosynthesis in their leaves — pulling carbon-dioxide from the air, as well as water from the soil via their roots, and grow as quickly as they reasonably can. In reality, the efficiency of this process is less than 10% of the input energy, and the different types of plant metabolisms that have formed over the course of evolution aren’t all the same.
Among the plant metabolisms in use today, some use significantly more efficient carbon fixation pathways, while others end up wasting a lot of the energy they obtained from photosynthesis with unnecessarily complicated processes, especially to deal with waste. How fast plants can grow if they had all evolved the most efficient carbon fixation pathway has been the subject of a number of studies these past decades, involving everything from crop plants to trees.
As these studies are showing us, more than a scientific and evolutionary biological curiosity, these genetically engineered plants offer real opportunities in everything from food production to reforestation.
Reinventing With Evolution
Over the course of billions of years on Earth, the process of evolution has led to the formation of fascinating biological structures, as well as many curious branching paths and incidences of reinventing the same structure differently. The vertebrate and cephalopod eyes for example, which appear to have formed independently, and are both very similar and wildly different. This process is called convergent evolution.
As striking as eyes and converging features such as wings among dinosaurs (birds), mammals and insects are, perhaps less obvious but no less important is the convergent evolution of photosynthesis. Over the course of millions of years, the rough versions of photosynthesis of early plants turned into a number of distinct photosynthesis pathways, all based around the RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) enzyme and associated Calvin cycle.
Most plants use so-called C3 carbon fixation, which uses a fairly basic Calvin cycle. This has an overall efficiency of at most 3.5% (relative to Sun radiation energy input), whereas the less common C4 carbon fixation cycle peaks at over 4%. C4 and CAM (crassulacean acid metabolism) are a form of convergent evolution, where both use phosphoenolpyruvate (PEP) to capture CO2 and thus create an increased concentration of CO2 around the RuBiscCO enzymes to reduce photorespiration.
A core issue with RuBisCO as can be seen in its above listed reactions is that it reacts with both CO2 and O2, whereas the latter reaction is obviously undesirable due to the lack of carbon atoms involved. The 2-phosphoglycolate (2-PG, or C2H2O6P3-) metabolic product that results from the reaction with oxygen is toxic to the plant as it inhibits some metabolic pathways and thus has to be dealt with. This is where for C3 plants photorespiration is essential, as it allows for the 2-PG to be converted to the desired PGA (3-Phosphoglyceric acid) that is used for the formation of sugars the plant needs to thrive, as captured in this graphic by Williams et al. (2013) of the metabolic pathways for C3 and C4 plants:
What this tells us is that many plants – including food crops and species of trees – which use the C3 carbon fixing cycle are spending a significant amount of the energy they gain from photosynthesis on breaking down this 2-PG that forms due to the interaction between RuBisCO and oxygen. Due to this photorespiration process, the loss of water via the stoma (pores) also increases.
Since RuBisCO binds more readily with oxygen rather than carbon-dioxide when temperatures increase, this puts natural limits on viable environmental conditions for C3 plants, and explains why C4 and especially CAM plants are found in warmer, more arid conditions. The logical conclusion thus is that if we were to transplant appropriate elements of the C4, CAM or other pathways as found in e.g. cyanobacteria into C3 plants, this could noticeably increase their growth rate by reducing the energy wasted on photorespiration.
Field Tests
After initial attempts at tweaking the RuBisCo enzyme directly to increase its affinity for carbon-dioxide were less than successful, focus during the 1990s shifted to understanding and optimizing. At this point in time, it’s generally acknowledged that engineering C4-style carbon fixation in C3 plants is a viable path forward, by using existing C4 plants as template. Relevant here is whether a C3 species also has a related C4 species to make the genetic engineering more straight-forward. Another active point of discussion here is whether to pursue a one or two cell strategy, as noted by Cui (2021).
Other researchers have sought to find novel ways to enhance photosynthesis, such as Nölke et al. (2014), who added the expression of a polyprotein (DEFp) taken from Escherichia coli glycolate dehydrogenase (GlcDH) to potato plants (Solanum tuberosum), with a resulting 2.3x increase in tuber yield. This same approach can potentially be applied to other plants as well, with likely a similar yield increase.
Wang et al. (2020) reported on a modified rice species using a similar approach as Nölke et al., albeit with mixed results. This study was followed by Nayak et al. (2022) who reported promising results that may lead to GE rice with these modifications being introduced in field trials. Related field trial data is available from South et al. (2019), who performed field trials using transgenic tobacco plants. These plants showed a roughly 40% boost in useful biomass production compared to the wild type.
Obviously, before any of these GE species would be distributed to farmers for next year’s crops a lot more experiments and field trials would have to be performed to ensure the effectiveness, long-term stability of these modifications and overall safety. Even so, these experiments provide a tempting glimpse at a future in which today’s agricultural output is increased by 150-200%, with zero need for additional nutrients, a decrease in water requirements and much better resistance to heatwaves, which are expected to occur much more regularly due to the ongoing climate change.
Which raises the question of whether a similar approach could be used to make regular trees much more efficient at fixing carbon from the atmosphere as well.
A Forest While You Wait
Conventional wisdom tells us that trees take a long time to grow. Perhaps unsurprisingly, most types of plants which are referred to as ‘trees’ (i.e. there is no biological definition of ‘tree’) use the C3 carbon fixation metabolism. In a recent preprint article by the Living Carbon Team et al. (2022), a modification akin to the previously discussed crop-based transgenic species is reported as having been applied to poplars. These hybrid poplars were subsequently planted in fields in Oregon, as detailed on the Living Carbon Team’s website. With the preprint article reporting a roughly 50% increase in biomass gain relative to standard poplars, this would lead credence to the lofty goals on the Living Carbon website
As explained on the FAQ page for the project, all the plants modified this way that are planted are female, ergo the genetic modifications will not spread to other, wild poplars via pollen, but will remain contained to just the planted trees. The project is done in partnership with Oregon State University (OSU), with over 600 of these hybrid poplars already planted. The goal is to get as many of them planted over the coming years as part of a carbon capture approach.
Together with the prospect of significantly boosting the output from crops, forests that grow 50% faster than conventional forests, this would seem to make for rather interesting future we can look forward to.
Genetic Engineering
One major elephant in the room when it comes to this topic is that of genetically modified organisms (GMOs), or “genetically engineered” (GE) as is the more correct term. Many countries have legislation that prohibits or severely limits the growing, import and sale of GE organisms, products, seeds, etc. Undoubtedly this will be the biggest hindrance in getting any of these photosynthesis-enhanced plants to be accepted.
Even though many arguments can be made for the inherent safety of these hybrid trees since neither humans nor cattle are likely to consume forests and trees in general, the divide between the logical world of science and the emotion-driven world of the average person and daily news cycle is stark indeed in this context.
Despite this, with the current course of the world towards one where droughts, famine and all other highly unpleasant symptoms of climate change will be felt by ever more people, it might be that the tools that science has provided us with will be our salvation here, allowing us to feed millions and make a sizable dent in the excess CO2 in the atmosphere, all by making plants better at hoarding carbon.