Plants are in fact poor converters of light energy, less than 1/10th of the light energy being converted into biomass, and substantially less is converted into a useful food product.

Source of inefficiency

The inefficiency is due to the crucial enzyme called Rubisco which is; in fact the most abundant protein on the plant has a low preference for utilising carbon dioxide relative to oxygen, while only use of the former fixes carbon dioxide into useful products such as sugars. Some plants have evolved mechanisms to deal with this as oxygen levels have increased, but most crops today, such as rice, remain inefficient because of this poor enzyme.

Rubisco

Rubisco, a critical element in the process of photosynthesis, is short for "ribulose-1,5-biphosphate carboxylase/oxygenase”. It is an enzyme that catalyzes the first major step of carbon fixation in the creation of sucrose and similar molecules. This is the most common chemical reaction by which carbon is fixed into organic molecules. Rubisco is probably the most abundant protein in leaves, and may be the most abundant on Earth. Rubisco is the most rate-limiting enzyme of the Calvin cycle, the first step in photosynthesis. Successfully overcoming this enzyme inefficiency could potentially increase productivity of plants by 50% and thus make a significant impact in solving global food shortages.

Modifying RuBisCO for liquid fuel

Reports on production of liquid fuels directly from the plant indicate that quantity of the carbon dioxide–fixing enzyme RuBisCO can be increased.Researchers from the UCLA Henry Samueli School of Engineering and Applied Science have genetically modified a cyanobacterium to consume carbon dioxide and produce the liquid fuel isobutanol, which holds great potential as a gasoline alternative.Using the cyanobacterium Synechoccus elongatus, researchers first genetically increased the quantity of the carbon dioxide–fixing enzyme RuBisCO. Then they spliced genes from other microorganisms to engineer a strain that intakes carbon dioxide and sunlight and produces isobutyraldehyde gas. The low boiling point and high vapor pressure of the gas allows it to easily be stripped from the system.

MAGIC programme

The Systems Biology Centre in the University of Warwick has just been awarded a research grant from BBSRC together with collaborators in Cambridge, Glasgow and the USA to work on substantially improving the photosynthesis efficiency of plants. University of Warwick researcher Professor Nigel Burroughs and his team is working on this programme.This programme called Multi-level Approaches for Generating Increased CO2, or MAGIC. Under this, the researchers aim to tune up a carbon dioxide capturing mechanism inside the plant using the latest techniques in synthetic biology. This can be thought of as nano bioengineering, the idea being to design a scaffold that funnels carbon dioxide direct to Rubisco. To solve this problem several models will be attempted for the delivery system and identifying which designs will give the most benefit.

Basic idea

The MAGIC team will work on bioengineering strategies to increase CO2 delivery to Rubisco by developing spatial and electrophysiological models of the chloroplast, tailored for the analysis and optimisation of bioengineered CO2 delivery mechanisms. The idea is to use bicarbonate as a carbon dioxide store which can not diffuse out of the chloroplast. Bicarbonate can be turned into carbon dioxide with the enzyme carbonate anhydrase (CA), but positioning of CA 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. The work may involve the development of mathematical models on spatial diffusion and electrophysiology.

Benefits

Researchers say there are obviously significant benefits in undertaking this challenging research. With the world-wide rise in food and energy prices, it is clear that solutions to the competing demands for land must be found to prevent a global crisis.