Redesigning photosynthesis to meet global food and energy needs of the 21st century. Review by SJ Taylor (PhD).

Redesigning photosynthesis in plants

An interesting review paper by Ort et al (2015) sketches out the possibilities for re-designing or  ‘re-evolving’ the complex photosynthetic mechanism of crop plants to increase harvestable yields. The authors cite the need for this kind of research is to address the growing limits on crop productivity measured against the projections for human population growth. While the main constraint on crop productivity at the moment is the availability of water and the lack of new lands for agricultural expansion in the developed countries, beyond this, many crops are also at their production limits, and not much more can be done to up their yields.  An important yield factor is that plants are not able to use all the photons they capture, and in fact, have to shut their stomata to conserve water, and so are not photosynthesizing maximally at midday when the light is intense. There are many other such constraints to maximum photosynthesis.

Private grain silos in South Africa, to store the maize harvest. Photograph SJ Taylor

Another key concern is that plants have evolved in an atmosphere of 240 ppm of carbon dioxide, and despite hopes about ‘carbon fertilization’ upping plant productivity, levels of 400 ppm and above will lead to kinetic limitations in the regeneration of the C02 acceptor molecule, ribulose-1,5 bisphosphate (RuBP), and could potentially stall photosynthesis in plants. This type of challenge may be eventually be tackled  through genetic engineering manipulations.

Who is carrying out this research?

Research has to have its own narrative, otherwise it will not attract funding, hence the northern hemisphere ‘shortage of land’ narrative of Ort et al., (2015). A book on African agricultural development makes a case for the 60% of Africa’s rich agricultural lands that have not been utilized effectively, and could become the breadbasket of the world, so no shortage of land in Africa, just a lack of basic research, basic investment in technology, basic provisions of market information and prices, access to markets and roads would make a huge different in crop production (Kirsten, Dorward, Poulton and Vink, 2009) without the need to re-engineer photosynthesis at great research cost.

What is interesting about the Ort et al (2015) paper is the sheer number of researchers involved in drafting this work (25) and the spread of research facilities across universities (USA, Amsterdam, UK, Germany, China, Australia), or mostly USA universities with a solid cohort of German collaborators, one Australian collaborator and one Chinese collaborating institute. Laboratories are mostly university based departments and institutes and are typically departments of biology, microbiology, crop sciences, genetics and cell biology, and computational biology. The lead author (Donald R Ort) is at the Global Change and Photosynthesis Research Unit at the University of Illinois, Urbana, while another author is at the Energy Institute for Genomics and Proteomics, University of California, Los Angeles. Another author is at the Centre for Bioenergy and Photosynthesis, at Arizonan State University. This attests to the initial paragraph in the publication which states that it is not just food shortages that this research could address, but the needs of a bio-economy in terms of bioenergy, chemical feed-stocks and biopharmaceuticals, all of which would require increased agricultural productivity, perhaps by as much at 60 – 120 % over 1960 levels of global food production.

One of the research centres conducting this photosynthesis research is the ExxonMobil Biomedical Sciences, Annandale, USA – which indicates that there are big business interests in the outcomes of this type of work.  ExxonMobil is an American multinational oil and gas company, and is one of the world’s largest international oil and gas companies (Wikipedia).

Plant breeding won’t be able to improve photosynthesis

Given the massive markets for food, bioenergy and novel plant chemicals, it  is now understood  that plant breeding alone will not be able to overcome any of the constraints to greater production of plants. No can photosynthetic up-scaling in plants be done through plant breeding, and that the only way forward is through thoroughly understanding the mechanics of photosynthesis (which is well known) and the genetic controls (not so well known), and applying novel genetic engineering mechanisms to the major crops.

There is now 60 years of research into the mechanism of photosynthesis, focusing on both green plants and photosynthetic microorganisms that are able to use energy from the sun to fix inorganic carbon from the atmosphere and create organic sugars through a complex pathway. These sugars are the building blocks for cellular products and process.  Research has investigated the ways in which plants and some microorganisms (cyanobacteria) capture light and C02and convert it into biomass,  all of which is under genetic control (Ort et al, 2015).  In plants, this is complicated by the way that plants partition some of this biomass into a harvestable product

Another complication is that more advanced plants (the C4 plants), with the first sugar formed from photosynthesis being a four carbon molecule, rather than the three carbon molecule of the C3 plants.  C4 plants include many of the monocotyledonous crops that are our main source of basic food (maize, sugarcane, but not rice).

Manipulating photosynthesis is no easy task

Manipulating photosynthesis is no easy task. There are many challenges in the genetic manipulation of the genes involved in photosynthesis in seed plants, including a lack of suitably characterised promoters, terminators and chloroplast transport signals, and a lack of mature technologies to stably transform very large DNA segments (say with 10 whole genes) into plants, and a lack of reliable technologies to do site-directed engineering of the plant or plastid genome  (Ort et al, 2015).  Even though we do understand the complex plant and microbial metabolic pathways by which photosynthesis takes place, we do not entirely understand how all these are controlled by their respective genes. In plants, the traits linked to photosynthesis include the anatomical features of higher plants as well as the biochemical components that all work together to increase the internal concentration of C02 in cells before it can be fixed by Rubisco (the ribulose-1,5-bisphospate carboxylase/oxygenase enzyme).  

There could also be ways to increase the light spectrum used by plants, and this is one of the aspects being investigated using novel photosystems from microbial organisms that use different parts of the visible spectrum.

Storage tanks, Graaf Reinett, storing agave spirits fermented from farm-grown agave plants, essentially also a potential way of producing biofuels from a range of wild-grown invasive plants. Photograph SJ Taylor

Genetic engineering of chloroplasts

Genetic manipulation of the plant chloroplast is not so simple, and although some work has been done successfully in the lab, this has not involved any of the plants we use as crops.  A major challenge in redesigning photosynthesis in plants is the complexity of the thylakoid membranes  inside the chloroplasts,, and that the chloroplasts and their genome is separate from the nuclear genome, and all are bounded by membranes. Finally, the plant cell has both membranes and a cell wall.  The leaf also has many layers of cells with different functions, and air spaces lined to the outside atmosphere through stomata.

Experimenting with photosynthesis in algae or photosynthetic bacteria (cyanobacteria) is much simpler, and there are model species for experimenting with photosynthesis in these single cell organisms.  Following much research on these organisms, it may be a viable possibility to replace the more complex mechanisms in advanced plants with some of the much simpler mechanisms of unicellular organisms.  Another challenge is understanding how changes made to genes in the chloroplast would scale up to the whole plant, and this would have to be assessed through countless lab and field trials. 

Creating synthetic genomes

It may also be possible to create synthetic genomes (currently only done in a model bacterial system) and to redesign the proteins in the photosynthetic process e.g. to redesign Rubisco. It is even potentially possible to redesign the plant canopy itself so that different parts of the plant have new mechanisms to optimise the capture of photons and C02 and perform the synthesis of those first sugar molecules. This concept is called the ‘Smart Canopy’ concept, and would involve understanding the inner working of plants and photosynthesis, replicating this synthetically, as well as modelling the way light falls on different parts of the plant.

Climate change and big business

One of the issues that climate activists worry about is big business benefiting from climate change, for example, big seed companies cornering the market in drought resistant cultivars of major crops, insurance  companies benefiting from insuring people at risk, and potentially, oil companies beginning to move into the biofuels market.  Although there will be vast impacts on global economic growth, say experts, there will be people and businesses that profit from the global climate crisis. In some cases, the line between getting rich and providing an essential adaptation service is not so clear. For example, the two fastest-growing occupations in the United State are solar panel installers and wind turbine technicians, and so the renewable energy sector benefits from climate change, while other businesses are beginning to calculate their risks from climate change (for example, coastal real estate) and what climate change will cost them.

Technical and ethical challenges of synthetic multi-GMO plants with manipulated photosynthesis

Many of the ideas expressed by Ort et al (2015) are ‘thought experiments’ – hypothetical schemas that could yield results, if only a massive amount of research funding could be found.  Much of the research will lead to a greater understanding of photosynthesis, and the final outcomes may not necessarily be better crops, but new ways of capturing light and turning it into energy, perhaps a new era of photovoltaics.

In general, creating new plants with up-scaled photosynthetic mechanisms would involve genetic engineering and the use of synthetic biology on a very large scale and pose a number of significant technical and ethical challenges, for example, in terms of GMOs (Ort et al, 2015). Ort et al., (2015) also comment that overcoming all the severe challenges will require major investment in long-term research programmes, and which is not being made yet, at least not in the public sector.

University field experiments, South Africa. Photograph by SJ Taylor.

Ort DR, Merchant SS and Alric J and 22 other authors (2015). Redesigning photosynthesis to sustainably meet global food and bioenergy demand. PNAS 112(28): 8529 – 8536.

Kirsten JF., Dorward AR., Poulton C., and Vink N (eds) (2009). Perspectives on African Agricultural Development. International Food Policy Research Institute.

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