Photorespiration: a multifaceted process

Plants are famous for fixing carbon dioxide from the air into organic molecules through photosynthesis. However, did you know that plants can also “fix” oxygen? This alternative process is called ‘Photorespiration’. Photorespiration, like photosynthesis, is a light-dependent reaction. Also, it consumes oxygen to produce carbon dioxide, just like respiration. Historically, photorespiration has been considered a wasteful pathway because it reduces photosynthesis efficiency. But, there may be more than meets the eye.

If you want to learn more about this process, this article is for you!

What is photorespiration?

Photorespiration is a biological process that occurs in the presence of light and high oxygen (O₂) concentration in all photosynthetic organisms. During photorespiration, oxygen is consumed and as a result carbon dioxide (CO₂) is obtained. So, it is defined as the light-dependent consumption of oxygen coupled with the release of CO₂ from organic molecules.

Where does photorespiration come from? It originates from Rubisco's enzyme dual activity as it can either work with oxygen or CO₂. During photosynthesis, Rubisco works with CO₂ to fix it into sugar molecules. However, when it works with oxygen, the photorespiration pathway starts.

Since 1970, when this process was first discovered, it has been considered a wasteful pathway. Why is that? Because it reduces the efficiency of photosynthesis by about 25%. In other words, when photorespiration occurs, carbon fixation decreases as Rubisco will not work exclusively with CO_2. Therefore, crop productivity decreases.

Did you know that photorespiration is also considered crucial for photosynthetic organisms as a protective pathway? Yes, it is a recycling pathway for a specific toxic compound called phosphoglycolate (PG). In plants, PG inhibits various physiological processes by affecting the action of key enzymes.

How does photorespiration work?

Now, you may wonder, how exactly does photorespiration function? This controversial process is complex and takes place in three different organelles, as follows:

• In the chloroplast: Photorespiration starts in this small organelle responsible for photosynthesis. Inside it, Rubisco binds oxygen with a carbon molecule from photosynthesis to form two different products: PG and a 3-carbon molecule. The latter will enter the Calvin cycle while PG is converted into glycolate.

• In the peroxisome: Glycolate produced in the chloroplast is transported to the peroxisome. In case you do not know, peroxisomes are small organelles that contain enzymes crucial for different physiological processes. Glycolate is converted into a non-toxic compound called glycine, mainly used during protein formation.

• In the mitochondria: Mitochondria are the “powerhouse” of the cell as they produce the energy carrier ATP. Inside them, the glycine from the peroxisome produces ammonia and releases CO₂ as final products of the process.

Basically, photorespiration prevents the accumulation of PG inside the plant. Also, during this process 75% of the carbon used to produce PG is recovered. So, although photorespiration is not as efficient as photosynthesis in terms of carbon fixation, it still makes the best of a bad situation.

However, photorespiration is not a free process! It is important to note that photorespiration is “expensive” as it consumes ATP and NADPH (energy carriers produced during photosynthesis). Due to this, for crop improvement, photorespiration has been a target for engineering approaches in order to enhance crop yield.

Photorespiration vs photosynthesis

Rubisco normally prefers working with CO₂ rather than oxygen. So, photosynthesis is favored over photorespiration. However, oxygen concentration inside the cells tends to be high. Therefore, the oxygen concentration may be higher than CO₂ concentration, and as a result Rubisco favors photorespiration instead.

But it doesn't stop there. Did you know that environmental factors can favor photorespiration as well? Scientists have identified that high temperature, high light intensity, and dry conditions favor the reaction between Rubisco and oxygen. It is estimated that under these conditions, about 50% of the energy obtained from photosynthesis is used in photorespiration.

When photorespiration rates are high, plant growth reduces as the energy obtained from photosynthesis is not used to produce food for the plant! Plants have adapted to stress conditions and as a result of it, three different photosynthetic pathways exist:

• C3 photosynthesis: This is the most common photosynthetic pathway in land plants. Nearly 85% of plant species are C3 plants, like rice and wheat. This pathway is considered as the basic or “ancient” one and presents higher rates of photorespiration.
• C4 photosynthesis: This is the pathway present in grass crops such as maize, sorghum, and sugarcane. C4 plants are common in high-temperature conditions and have adapted to minimize photorespiration. How? They have physically separated the two stages of photosynthesis. You can read more about the different stages of photosynthesis here ‘Photosynthesis: the basis of life on Earth’.
• Crassulacean Acid Metabolism – CAM: This pathway is common in plants that live in dry, hot conditions such as cacti and pineapple. Unlike C4 plants, CAM plants separate in time the stages of photosynthesis. They concentrate CO₂ inside the cell at night, when temperatures are low, thus favoring the union of Rubisco with CO₂ instead of oxygen. 

The need for photorespiratory reactions

Photorespiration implies losses in carbon fixation and energy. So, you may wonder, why not try to eliminate this pathway instead of only minimizing it? The answer is that it is a process involved in overall plant physiology. Essential processes as basal metabolism, abiotic stress response, and nitrogen and sulfur assimilation depend on photorespiration. In addition, it has characteristics that are beneficial to plants such as:

• It could serve as an important energy sink through its consumption of energy carriers (ATP and NADPH).
• It protects plants from harmful effects of photoinhibition, a kind of stress in which photosynthesis is reduced by excessive light.
• It supports plant defense reactions, especially against drought or chilling.

Disrupting any photorespiratory reaction proves, because of the above, detrimental to a plant. Thus, the scientific research efforts are now focused on optimizing this process. It is important to set aside the view of photorespiration as a wasteful pathway and understand that it is a complex process connected to the overall health of the plant.

We hope this article gives you a better understanding of photorespiration. For more informational posts on different aspects of plant physiology, keep checking this space!

By Valeria Franco Franklin 24-January-2022

About the author

Valeria Franco is from Colombia, the land of orchids. She is a focused and passionate biologist who specializes in biotechnology and molecular biology. Valeria has prior laboratory and research experience. She is presently employed as a content creator at Lab Associates and is always looking for new challenges. Valeria is enthusiastic in plant science themes and reading as a tool for lifelong learning. Her hobbies include studying foreign languages, traveling, and archery.

References

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• Busch, F.A. (2020). Photorespiration in the context of Rubisco biochemistry, CO₂ diffusion and metabolism. Plant J, 101: 919-939. https://doi.org/10.1111/tpj.14674
• Peterhansel, C., Horst, I., Niessen, M., Blume, C., Kebeish, R., Kürkcüoglu, S., & Kreuzaler, F. (2010). Photorespiration. Arabidopsis Book, 8:e0130. doi: 10.1199/tab.0130
• Shi, X., & Bloom, A. (2021). Photorespiration: The Futile Cycle?. Plants, 10(5):908. https://doi.org/10.3390/plants10050908
• South, P. F., Cavanagh, A. P., Lopez-Calcagno, P. E., Raines, C. A., & Ort, D. R. (2018). Optimizing photorespiration for improved crop productivity. J Integr Plant Biol, 60: 1217– 1230.
• Timm, S., & Hagemann, M. (2020). Photorespiration—how is it regulated and how does it regulate overall plant metabolism?, Journal of Experimental Botany, 71(14): 3955–3965. https://doi.org/10.1093/jxb/eraa183