Mathematical modelling of CO₂ delivery and photosynthesis efficiency of the chloroplast.

A transatlantic project.

Motivation. Feeding a growing human population is a major concern world wide; recent food shortages and the ensuing riots putting this on the political agenda. However, scientific advances using old techniques of crop breeding are probably at the limit; these underpinning the so called green agricultural revolution of the last 50 years where significant productivitivity increases were acheived up until late 1990s. The future needs something new, and certainly will take at least a decade to deliver given the complexity of plant development and physiology. One avenue receiving interest from the scientific community is the inherent inefficiency of carbon fixation in most plants, the main enzyme, Rubisco, has a poor specificity for carbon dioxide (CO₂) relative to oxygen (O₂), only the former fixing CO₂ into sugars and thus chemically storing light energy. The latter reaction, in fact requires energy to recycle products and so causes a major loss of efficiency. Successfully overcoming this enzyme inefficiency could potentially increase productivity by 50% and thus make a significant impact in solving global food shortages as human populations grow over the next 20 years.

Schematic of a chloroplast. Carbon dioxide exchanges with the outside by diffusion.

The project. In collaboration with an interdisciplinary transatlantic research team we are working on bioengineering strategies to increase CO₂ delivery to Rubisco. You will work on developing spatial and electrophysiological models of the chloroplast, tailored for the analysis and optimisation of bioengineered CO₂ delivery mechanisms. This includes using bicarbonate as a carbon dioxide store, bicarbonate not being able to diffuse out of the chloroplast. Bicarbonate can be turned into carbon dioxide with the enzyme carbonate anhydrase (CA), but positioning of CA (eg free in the chloroplast lumen, attached to Rubisco) will affect the efficiency of capture by Rubisco. Other ideas include using molecular scaffolds to build a sequential enzyme pathway. Your primary roles will be i) the development of mathematical models of CO₂ in the chloroplast and photosynthetic efficiency, taking into account chloroplast architecture, see Figure ii) the elucidation of optimal designs for CO₂ delivery, iii) perform a cost benefit analysis of each design and estimate the likely maximal efficiency gain. The models will be of two types. Firstly, spatial diffusion models (PDEs) of CO₂ in the chloroplast will be used to investigate the diffusive loss of CO₂ away from the active enzyme and various means to reducing its impact. Secondly, electrophysiological models will be developed to examine the effect of our CO₂ concentrating mechanisms on the chloroplast. For instance, one idea is to use a bicarbonate light driven pump, which will have an secondary affect on the chloroplast ph. Thus, a full electrophysiological (ODE) model incorporating ion channels and a partial metabolic network will be needed. Models for both choroplasts and a cyanobacteria will be developed, the latter being an experimental test bed for the constructs where experimental variates can be determined.

Scientific enquiries can be made to N dot J dot Burroughs at warwick dot ac dot uk.

The team. The modelling is lead by Prof Nigel Burroughs. The rest of the team includes Mike Blatt (Glasgow, PI UK), Julian Hibberd (Cambridge), John Golbeck (Penn State, PI USA), Cheryl Kerfeld (Berkeley) and 4 experimental PDRAs.

The project is funded by BBSRC.