Three into Four Will Go

Most plants use an enzyme called rubisco to convert carbon dioxide (CO₂) into sugars containing three carbon atoms—a process known as C3 photosynthesis. But at temperatures above 25°C, rubisco begins to bond with oxygen instead of CO₂, 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 CO₂ 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: a hectare of rice, a C3 plant, produces a harvest of no more than eight tonnes, whereas maize, a C4 plant, yields as much as 12 tonnes.

Turning a C3 plant into a C4 one, though, is trickier than conferring flood resistance, since it involves wholesale changes in anatomy. C4 plants often absorb CO₂ from the air in one type of cell and then convert it to sugars through photosynthesis in another. C3 plants, by contrast, do both jobs in the same place.

John Sheehy, one of IRRI's crop scientists, plans 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 one of the enzymes needed for C4 photosynthesis by transplanting the relevant gene from maize.

The task, admits Robert Zeigler, IRRI's director, is daunting, and will take ten years or more. But the potential is enormous. Success would not only increase yields, but also 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 green revolution beckons.

Source: The Economist, 2006, December 7

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