Genetic engineers are applying their skills to tropical crops

　　EVERY hectare of paddy[1] fields in Asia provides enough rice to feed 27 people. Fifty years from now, according to some projections, each hectare will have to cater[2] 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.

　　在亚洲，每公顷稻田产出的大米可够养活27个人。据预测，50年后每公顷稻田将必须为43个人供应口粮。由于可耕地本来就不足，因此将更多的土地变成稻田并不可取。事实上，在亚洲大陆许多非常肥沃的江河流域，城市扩张正在吞噬越来越多最适于种植水稻的土地。此外，全球气候变暖由于造成一些地方干旱频发和其它地方水涝加重而使得农民的生活日益艰难。国际水稻研究所（IRRI）的科学家认为，单靠优化耕作方式或采用常规杂交技术培育的新品种不可能进一步增加产量，相反，他们打算利用现代遗传修饰技术使水稻增产50%.

　　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[3] 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.

　　12月4日，国际农业研究磋商组织（CGIAR，一个研究系统，IRRI是其下属的研究所之一）披露了一系列旨在保护粮食产量免受全球气候变暖不利影响的方案，其中多数与基因工程有关（即使有些西方消费者拒绝接受经基因工程改良的粮食，但落后国家的农民还是广泛采用了此项技术），不过有的只是运用遗传学方法确定有益基因。例如，IRRI科学家已经发现了一种基因，它使一种印度水稻在被水完全淹没的环境下仍能存活数周。科学家将其与一种在易发洪涝的孟加拉国深受农民青睐的水稻进行了杂交。试种表明，新品种大米产量正常条件下与原品种产量相当，但在长时间洪涝后则是原品种产量的两倍多。鉴于全球总共1.5亿公顷农作物中每年大约有2千万公顷（5千万英亩）遭受洪涝破坏，这一特性有可能大幅度提高全球水稻收成。

　　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.

　　不过就目前而言，CGIAR开展的研究项目中最具有挑战性的还是改变水稻光合作用的途径，这就必须进行一丝不苟的基因重组。

　　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.

　　不过，将C3植物转化成C4植物由于涉及无数的结构变化，因此比培育抗涝品种要更为复杂。C4植物常常将空气中的CO2吸入到某一类细胞中，然后再在另一类细胞中利用光合作用将其转化为糖类。相比之下，C3植物则是在同一类细胞中完成这两个步骤。

　　（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[4] 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.

　　此外，C4光合作用可见于19科植物中，至今可能经历了50次以上的进化。这么多次的进化表明，从一种形式的光合作用转化为另一形式的光合作用并不怎么“干净利落”。这也为研究人员提供了许多关于该研究的切入点。比如说，有的C4植物对CO2的吸收和光合都是分别在某一类长形的特殊细胞两端进行，而非分散到两类不同的细胞中。同时许多C3植物都含有数种C4光合作用必需的基因，但利用方式均不相同。实际上，C3与C4植物并非总是有着明显的区别。有的植物在其叶部利用一种光合机制，而在其茎部利用的又是另一种光合机制。

　　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.

　　IRRI农作物科学家之一约翰?希伊计划对该研究所收集的6000种野生水稻进行筛选，以弄清哪些品种易于进行C4光合作用。与此同时其他研究人员正致力于分离决定C4植物异常结构和生化特性的基因。几年前，遗传学家通过将玉米中的相关基因转入水稻，从而在水稻中成功合成了C4光合作用必需酶的一种。

　　（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[5], could also benefit. If it all works, a second green revolution beckons.

　　IRRI负责人罗伯特?齐格勒承认此项任务十分艰巨，将需要十年以上的时间才能完成，不过潜力是巨大的。由于C4植物能更为高效地利用两种光合作用，因此研究一旦成功不但会增加水稻产量，而且会减少对水和肥料的需求量。其他重要的C3植物如小麦、甜薯、甜瓜等也会受益。假如一切顺利，就将标志着第二次绿色革命的到来。

　　[NOTES]（OXFORD）

　　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.