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Alison Smith
Department of Metabolic Biology
Alison Smith

Manipulation of CO2 concentrations in chloroplasts:

In a collaborative project http://cambridgecapp.wordpress.com/ with Howard Griffiths (University of Cambridge http://www.plantsci.cam.ac.uk/research/howardgriffiths.html), David Fell (Oxford Brookes University) and Martin Jonikas (Stanford University, USA http://dpb.carnegiescience.edu/labs/jonikas-lab), funded by BBSRC and the National Science Foundation (USA) we propose to use the carbon concentrating mechanism (CCM) from green algae to enhance CO2 concentration in C3 chloroplasts, suppressing photorespiration and increasing productivity. The rational for this project is as follows:

In most plants, growth rate is limited by the rate at which carbon dioxide from the atmosphere is taken up and converted to sugars in the process of photosynthesis. The enzyme responsible for the first step in this process, Rubisco, does not work at its potential maximum efficiency at the current levels of carbon dioxide present in the atmosphere. If levels were much higher, photosynthesis would be faster and plants would grow faster. This speeding-up of photosynthesis will happen naturally over the next fifty years or so as atmospheric carbon dioxide levels rise due to human activities. However, there is an immediate requirement for increased crop productivity to provide food for the rising population of the planet. Our project addresses this problem. We are studying a mechanism present in tiny green algae that results in high concentrations of carbon dioxide inside their photosynthesising cells (called a Carbon Concentrating Mechanism, or CCM), enabling Rubisco to work at maximum efficiency. We have recently discovered important new information about this mechanism, and we have invented new and rapid methods to discover algal genes that contribute to it. We have two complementary and parallel aims. First, we will apply our new methods to identify all of the genes required by the algae to achieve high concentrations of carbon dioxide inside the cells, and we will discover exactly how these genes work. Second, we will transfer the most important genes into a plant, and study whether the same CCM can be recreated inside a leaf. If it can, we expect that our experimental plant will have higher rates of photosynthesis and hence a higher rate of growth than normal plants. This work will provide new insights into how plants and algae acquire and use carbon dioxide from the atmosphere, of great importance in predicting and coping with the current rapid changes in the atmosphere and hence in climate. The work will also contribute to strategies to increase global food security, because it will indicate new ways in which crop productivity can be increased.

Our contribution to this project is to introduce known, putative and newly-discovered components of the Chlamydomonas CCM into Arabidopsis plants. Our approach includes: 1) Addition of bicarbonate pumps to the chloroplast envelope inner membrane to maximize bicarbonate accumulation in the chloroplast. 2) Introduction of modified spinach Rubisco small subunit and Chlamydomonas Rubisco small subunit into an Arabidopsis mutant deficient in Rubisco, to investigate whether pyrenoid-like structures can be fomed. In Chlamydomonas, Rubisco proteins are clustered together in a structure called a pyrenoid. This is believed to facilitate elevation of CO2 concentration at the active site of the enzyme.

http://cambridgecapp.files.wordpress.com/2012/07/labelled-chlamy-em.png?w=326&h=246

 

A Chlamydomonas cell, showing the Rubisco-containing pyrenoid inside the single chloroplast