Genetic engineers are applying their skills to tropical crops
EVERY hectare of paddy fields in Asia provides enough rice to feed 27 people. Fifty years from now, according to some projections, each hectare will have to cater for 43. Converting more land to paddy is not an option, since suitable plots are already in short supply. In fact, in many of the continent's most fertile river basins, urban sprawl is consuming growing quantities of prime rice-farming land. Moreover, global warming is likely to make farmers' lives increasingly difficult, by causing more frequent droughts in some places and worse flooding in others. （1）Scientists at the International Rice Research Institute （IRRI） doubt it is possible to improve productivity as much as is needed through better farming practices or the adoption of new strains derived from conventional cross-breeding. Instead, they aim to improve rice yields by 50% using modern genetic techniques.
On December 4th the Consultative Group on International Agricultural Research （CGIAR）, a network of research institutes of which IRRI is a member, unveiled a series of schemes intended to protect crop yields against the ill effects of global warming. Many involve genetic engineering—which is generally embraced by farmers in poor countries even if some Western consumers turn their noses up at it. Some, though, only use genetics to identify useful genes. （2）For example, IRRI's scientists have found a gene that allows an Indian rice strain to survive total immersion for several weeks, and have cross-bred it into a strain favoured by farmers in flood-prone Bangladesh. In trials, the new plant produced as much rice as the original under normal conditions, but over twice as much after prolonged flooding. This trait could increase the world's rice harvest dramatically, since flooding damages some 20m hectares （50m acres） of rice each year out of a total crop of 150m hectares.
By far the most ambitious project on CGIAR's list, though, involves transforming the way in which rice photosynthesises. That will require some serious genetic restructuring.
Three into four will go 将C3变成C4就行
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.
大多数植物利用核酮糖-1，5二磷酸羧化酶加氧酶 （rubisco） 将二氧化碳（CO2）转化成含有3个碳原子的糖类，此过程即C3光合作用。但当温度超过25°C时，rubisco就不再与CO2结合，而开始结合氧分子，从而降低了光合效率。因此，处于温暖气候条件下的某些植物就进化产生不同的光合作用，即C4光合作用。在此作用过程中，其它酶类可促进CO2聚集到rubisco周围，反应生成的初始产物是四碳糖类。在炎热、阳光充足的气候下，这些C4植物光合效率比C3植物要高50％。此外，C4植物对水和氮气的需要量也较少。结果是，就主要作物而言，在较为恶劣的环境下产量比较高——一公顷的水稻（C3植物）产量最多为8吨，而一公顷的玉米（C4植物）产量可达12吨。
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.
（3）On the other hand, C4 photosynthesis seems to have evolved more than 50 times, in 19 families of plant. That variety suggests the shift from one form of photosynthesis to the other is not as radical as might appear at first sight. It also gives researchers a number of starting points for the project. Some C4 plants, for example, absorb CO2 and photosynthesise it at either end of special elongated cells, instead of separating the functions out into two different types of cell. Many C3 plants, meanwhile, have several of the genes needed for C4 photosynthesis, but do not use them in the same way. In fact, the distinction between C3 and C4 plants is not always clear-cut. Some species use one method in their leaves and the other in their stems.
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.
（4）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.
1. paddy n. （also `paddy-field） [C] field where rice is grown 稻田. 2 [U] rice that is still growing or in the husk 稻; 稻谷.
2. cater v. 1 （a） [I, Ipr] ~ （for sth/sb） provide food and services, esp at social functions 提供饮食及服务（尤指社交方面）: cater for a party, banquet, etc 为聚会、 宴会等备办食物 * Fifty is a lot of people to cater for! 承办五十人的饮食可够多的! （b） [Tn] （esp US） provide food and services for （a party, banquet, etc） 为（聚会、 宴会等）提供饮食及服务. 2 [Ipr] （a） ~ for sb/sth provide what is needed or desired by sb/sth 由某人[某事物]提供、 迎合: TV must cater for many different tastes. 电视节目必须迎合各种人的爱好. （b） ~ to sth try to satisfy a particular need or demand 满足某种需要或要求: newspapers catering to people's love of scandal 迎合人们爱看丑闻消息的报纸.
3. immersion n. [U] 1 immersing; being immersed 沉浸; 浸沉. 2 baptism by putting the whole body under water 洗礼; 浸礼. immersion heater electric heater fixed inside a hot-water tank in a home 浸入式加热器（家庭用的）.
4. elongated adj. （made） long and thin; stretched out （被拉得）细长的; 伸长的: elongated figures in a painting 画中的细长的人.
5. cassava n. 1 [C] tropical plant with starchy roots 木薯. 2 [U] starch or flour obtained from these roots, used to make tapioca 木薯粉.