Reducing respiratory losses of carbon to improve carbon use efficiency and growth in plants

Thesis

Enhancing agricultural productivity remains critical for global food security. While many strategies target improved photosynthetic carbon fixation, reducing respiratory carbon losses has recently emerged as a complementary approach, as suggested by comprehensive mapping of plant metabolic networks.

This thesis investigates a novel strategy to reduce respiratory carbon losses by refixing CO2 produced by nocturnal respiration and making it available again for reassimilation into metabolism before it escapes to the atmosphere.

Using constraint-based metabolic modelling of central metabolism, I examined whether plant metabolic networks possess inherent capacity for nocturnal CO2 refixation under carbon-limited conditions. Multiple approaches to induce carbon limitation in a diel leaf model revealed that CAM cycling emerged as the only plant-native pathway allowing substantial recovery of nocturnal respiratory CO2. Despite additional metabolic costs, CAM cycling enhanced growth under carbon-limited conditions in the absence of water stress.

To explore alternative nocturnal CO2 refixation strategies, I developed a baseline model, which experienced carbon limitation, but was indirectly prevented from using CAM cycling. This model was then used to evaluate the potential of a heterologous CO2-fixing pathway to enhance carbon conversion efficiency and plant growth. Integration of the synthetic crotonyl-CoA/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle into the model revealed that its NADPH costs and the limited capacity of the plant metabolic network to regenerate NADPH at night substantially restricted the cycle’s effectiveness for nocturnal CO2 refixation in plants.

Finally, a reinvestigation of a Marchantia-specific model with experimentally determined parameters indicated that, contrary to previous modelling results, nocturnal CO2 refixation through CAM cycling is likely unnecessary for this plant to achieve observed growth rates.

This research provides insights into the feasibility of nocturnal respiratory CO2 refixation strategies and offers important design considerations for future metabolic engineering approaches.

Authors: Hartinger, C

• DOI: 10.5287/ora-bxjderpmk
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: plant respiration; plant metabolism; Flux Balance Analysis; carbon metabolism; metabolic modelling
• Subjects: Plants--Metabolism