May 20, 2021

Rocket science: Telling the story of the C2 enigma

Written by: Catherine Walsh

Purely by accident, the plan for my PhD research has gained some highly exciting yet potentially controversial hypotheses to test. I am attributing this fresh development to a small extent to the ‘work from home’ policy adopted by Lancaster University during the pandemic but mainly to my supervisor, Dr Marj Lundgren for giving me the opportunity to work with her on a review paper for the journal, ‘Biochemical Society Transactions’.

The journal is the publication of the Biochemical Society and represents all aspects of biochemistry across all disciplines. Dr Lundgren was invited to write a review to discuss current knowledge and global research of C2 photosynthesis with insight into future implications for the mechanism. She invited me to join her on this project as my PhD project investigates the effect of the C2 mechanism to optimise yield, nutrients and flavour in wild rocket crops. I began by reading intensively (from home), although this time mainly supervised by my cat, Marilyn.

To understand what C2 photosynthesis is all about, I read an extensive number of papers, on all kinds of biochemical, metabolic and signalling topics. A luxury afforded by the current pandemic situation. Knowledge of the precise biochemistry of the C2 mechanism is currently limited. It is known to operate in approximately 56 plant species, with rocket being the only commercial crop (Lundgren & Christin 2017). C2 photosynthesis is essentially a carbon concentrating mechanism, that operates a photorespiratory pump across two cell types to salvage previously lost CO2 (Sage et al 2012). The process increases carbon gain under warm, dry conditions without suffering detrimental photorespiratory effects, such as those seen in C3 species (figure 1).

Previous research states that the C2 mechanism sits halfway between C3 and C4 photosynthesis on an evolutionary trajectory. This is also the case when carbon gain is evaluated under hot environments (Vogan & Sage, 2011). Current thinking now suggests that a functional C2 pump is a stable state in some cases without further evolutionary pressure (Lundgren, 2020), such as rocket that has no C4 relatives. Exactly how the C2 species manage to maintain growth under photorespiratory conditions is a complex question that I have been thinking a lot about.

While working on the review paper, I used rocket as a C2 example to get my head around what was going on with the biochemistry. Rocket is well known for its health benefits by being a rich source of glucosinolates, vitamins C, K and flavanols. By ticking off these compounds when they cropped up in the literature, I’ve been able to put together some rudimentary ideas for a hypothetical C2 physiology to suggest how the crop continues to grow in excessive temperatures using highly efficient signalling pathways and integration of carbon, nitrogen and sulphur metabolic pathways. Although, controversially, I am hypothesising that the C2 mechanism is more about generating ammonia in the bundle sheath cell, with CO2 recycling being incidental.

It has been an interesting journey to co-write a review paper. I’ve learnt a lot about  plant biochemistry in general, signalling pathways, the C2 mechanism and how C2 photosynthesis has a role in the C4 evolutionary continuum. I also gained experience of the writing process and how the document progressed with each newly gained insight. I am very grateful to Dr Lundgren for giving me the opportunity to work on the paper and to my cat for supervising the excessive reading program. The whole experience has equipped me with some cool hypotheses that I’m looking forward to putting to the test in the lab over the next few months.


Lundgren MR, Christin PA. 2017. Despite phylogenetic effects, C3-C4 lineages bridge the ecological gap to C4 photosynthesis. Journal of Experimental Botany 68: 241-254.

Sage RF,  Sage TL,  Kocacinar F. 2012. Photorespiration and the evolution of C4 photosynthesis. Annual Review of Plant Biology 63: 19-47.

Vogan PJ, Sage RF. 2011. Water-use efficiency and nitrogen-use efficiency of C3-C4 intermediate species of Flaveria Juss. (Asteraceae). Plant, Cell and Environment 34: 1415-1430.

Lundgren MR. 2020. C2 photosynthesis: a promising route towards crop improvement? New Phytologist 228: 1734-1740.