Why Plants are Bad at Their Job

By Heiloi Yip

There is no question that photosynthesis has played a monumental role in the evolution of life as we know it. From single-celled cyanobacteria to 100-metre-tall redwoods, there are a vast diversity of photosynthetic organisms that convert the carbon dioxide in the atmosphere to the oxygen that we and other organisms require to survive. All photosynthesizers achieve this by a complex arrangement of metabolic pathways that convert sunlight energy into glucose for their own use. This process may seem to be meticulously optimised, but in reality, is actually very inefficient! It appears that even after hundreds of million years of evolution, the fundamental process of photosynthesis has serious flaws that greatly hinder its efficiency. 

Photosynthesis comprises of two different stages: the light reaction that absorbs energy from sunlight; and the dark reaction that fixes carbon dioxide into sugars. The most common pathway of the dark reaction is the C3 pathway, being the first pathway to have evolved among the origin of photosynthesis. RuBisCO is the enzyme directly responsible for fixing carbon dioxide, and as such plays a vital role in the dark reaction. Normally, RuBisCO catalyses a reaction between Ribulose Bisphosphate (RuBP for short) and carbon dioxide from the atmosphere to create phosphoglycerate, which is either converted into sugars or recycled back into RuBP. However, RuBisCO cannot differentiate very well between oxygen and carbon dioxide, and a reaction with the former molecule creates a waste product called phosphoglycolate. 

The phosphoglycolate can neither be processed into a sugar nor renewed into a RuBP molecule, and therefore must be broken down into simpler molecules. However, the recycling process is complicated and costly, requiring three different organelles and costing energy in the form of ATP. The end products are not very helpful either, releasing the fixed carbon in phosphoglycolate back into the atmosphere as carbon dioxide rather than being returned to RuBP. This process is called photorespiration, and it results in carbon being lost from the dark reaction pathways. For every 2 molecules of carbon dioxide fixed, 1 molecule of oxygen is also accidentally fixed. Combined with the energy requirement and loss of carbon, photorespiration is the reason why RuBisCO and the dark reaction are very inefficient at fixing carbon dioxide (Young, 2020). 

The evolutionary circumstances of photosynthesis can shed a light on how photorespiration came to be. Photosynthesis first arose in the common ancestor of cyanobacteria more than 2 billion years ago, in an environment which lacked oxygen at the time. Early cyanobacteria were anaerobic organisms, and thus the photosynthetic pathway was adapted to react with the abundant carbon dioxide without issue.  However, as the early cyanobacteria proliferated, the cumulative photosynthetic activities of each cell released a very significant amount of oxygen into the atmosphere (Akiko, 2006). As such, the photosynthetic pathways were never prepared to deal with an oxygen-rich environment, hence RuBisCO’s inability to differentiate carbon dioxide from oxygen and the affiliated inefficiencies of photorespiration. Yet, the C3 pathway in most organisms has not been touched by evolution, having remained relatively the same since its conception billions of years ago. It is theorised that any alterations to RuBisCO or the rest of the pathway will always lead to a less efficient metabolism, even if it helps RuBisCO in differentiating between carbon dioxide and oxygen. As such, a better pathway was simply never selected for, and no plants evolved to fully mitigate photorespiration (Ehleringer et al, 1991).  

Nonetheless, some lineages of photosynthesizers have developed adaptations to alleviate these issues. Some plants, like corn and sugarcane, use the C4 pathway with reduced photorespiration. This is achieved mainly by changes in anatomy: carbon dioxide is fixed from the atmosphere in the mesophyll cells, while RuBisCO is stashed away in neighbouring bundle sheath cells. The carbon dioxide is instead transported intercellularly to RuBisCO for fixation. This arrangement minimises RuBisCO’s contact with the atmosphere, and thus reduces accidental reactions with oxygen. Other plants opt for a strategy called the CAM pathway, exhibited in cacti and other arid desert plants. During the night, carbon dioxide is fixed from the atmosphere into a compound and stored inside vacuoles. When day arrives, the stomata shuts to prevent entry of atmospheric gases (including oxygen), and the carbon dioxide in the vacuole is consumed by RuBisCO as photosynthesis takes place (Sedelnikova, Hughes and Langdale, 2018). 

Only a handful of crops cultivated by humans use the C4 or CAM pathway, but genetic modification might be able to change that. The “C4 Rice” project aims to genetically modify rice, naturally a C3 plant, into utilizing a dark reaction pathway similar to C4 plants. Indeed, this is a difficult task with many challenges, mainly because such modification involves altering a magnitude of genes that result in both metabolic and anatomical changes. In short, a lot of carefully tuned modifications must be done to the rice genome before a C4 strain can be cultivated. However, if the project succeeds, this new strain of rice will be much more efficient at photosynthesis, thus producing a greater yield of food than its C3 counterpart. This in turn paves way for modification of other crops, increasing total food production to accommodate for the growing human population (C4 Rice, 2020). 

References:

C4 Rice (2020). The C4 Rice Project. [online] Available at: https://c4rice.com/ [Accessed 22 September 2020]

Ehleringer, J., Sage, R., Flanagan, L. and Pearcy, R. (1991). Climate change and the evolution of C4 photosynthesis. Trends in Ecology & Evolution, 6(3), pp.95-99. DOI: 10.1016/0169-5347(91)90183-X

Sedelnikova, O., Hughes, T. and Langdale, J. (2018). Understanding the Genetic Basis of C4 Kranz Anatomy with a View to Engineering C3 Crops. Annual Review of Genetics, 52(1), pp.249-270. DOI: 10.1146/annurev-genet-120417-031217

Tomitani, Akiko (2006). The evolutionary diversification of cyanobacteria: Molecular–phylogenetic and paleontological perspectivesPNAS, 103(14), pp.5442–5447. DOI: 10.1073/pnas.0600999103

Young, Sophie (2020). C4 photosynthesis. The Biologist, 67(3), pp.30-33. London: Think Publishing LTD

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