It is reported that scientists at the International Rice Research Institute are pursuing an ambitious project which involves transforming the way in which rice photosynthesises. This will require some serious genetic restructuring.
Most plants use an enzyme called rubisco to convert carbon dioxide into sugars containing three carbon atoms - a process known as C3 photosynthesis. But at temperatures above 25c, rubisco begins to bond with oxygen instead of carbon dioxide, reducing the efficiency of the reaction. As a result certain plants in warm climates have evolved a different mechanism, called C4 photosynthesis, in which other enzymes help to concentrate carbon dioxide around the rubisco and the initial result is a four-carbon sugar. In hot, sunny climes these C4 plants are half as efficient again as their C3 counterparts. They also use less water and nitrogen. The result, in the case of staple crops, is higher yields in tougher conditions.
Turning a C3 plant into a C4 one is tricky since it involves wholesale changes in anatomy.
The IRRI's crop scientists plan to screen the Institute's collection of 6 000 varieties of wild rice to see if any of them display a predisposition for C4 photosynthesis. Other researchers, meanwhile, are trying to isolate the genes responsible for C4 plants' unusual anatomy and biochemistry. A few years ago geneticists managed to get rice to produce some of the enzymes needed for C4 photosynthesis by transplanting the relevant gene from maize.
The task is daunting and will take ten years or more, but the potential is enormous. Success would not only increase yields, but also would reduce the need for water and fertilisers, since C4 plants make more efficient use of both. Other important C3 crops, such as wheat, sweet potatoes and cassava, could also benefit. If it all works, a second and great green revolution beckons.
(See Economist, 9-12-06)