基因工程改造速生杨为开发生物质能源打下根基

生物谷

       据physorg网站2006年8月23日报道，一种能够作为石化燃料主要替代品的树木可以在六年内成长到90英尺（约27.5米），并且能够作为田间作物生长在闲置农田中。美国普渡大学的研究员利用基因工具从事于这种树木的研究，使它们能够快速廉价地生产出制造交通燃料替代品所需的物质。

        科学家们被树木细胞壁中一种叫做木纤维的化合物所吸引，它能够提高植物的结构强度但却阻碍纤维素的提取。而含糖的纤维素是制造替代品乙醇燃料所需的物质。

        美国能源部生物及环境研究办公室（DOE）提供140万美元的资金给普渡大学克林特•查布、理查德•米兰和迈克尔•莱德斯基进行为期3年的研究，寻找改变木纤维的方法并检测基因的改变是否会影响制造生物燃料的树木质量。DOE计划的一部分就是将杂交白杨树作为研究基础，到2030年取代美国交通石化燃料年消耗量的30％。

        据2005年统计，生物燃料只取代了美国交通燃料1400亿加仑年消耗量中的40亿――还不到3％。而全国用于生产生物燃料的谷物已占到总数的大约13％。普渡大学农业与能源系的科学专家表示谷物的利用只能作为解决石化燃料替代方案的一部分。

        “在印第安纳州如果想建立单纯以谷物为原料的乙醇生产基地，我们需要进口谷物，”生物化学家查布说。“我们所需要的是适应特定区域的一整套植物生长计划，并且拥有高的产量以服务于生物燃料工业。”

        查布和米兰希望改变杂交白杨树的遗传基因，使木纤维不再阻碍纤维素的提取，以便通过降解得到能够转化为乙醇的可发酵糖。他们说改变的木纤维仍可能用于燃料制造或其它产品的生产。目前植物原料的25％是复杂的木纤维分子，它们现在的形式只能通过燃烧来提供乙醇生产所需的能量而不能转化为燃料替代品。

        改变木纤维的成分或降低其在细胞壁中的含量能够促进酶的进入。而酶的进入能够更有效地促进纤维素向糖的转化。普渡大学森林及自然资源系树木分子生理学家米兰表示，目前从木材和纸中提取木纤维的工艺仍非常落后且污染环境。

        为了改善非石化燃料的制造，查布和米兰利用基因工具改变白杨树，然后研究基因的变化对细胞壁的影响。米兰同时也在试图寻找方法培育无繁殖能力的树以阻止它们将引入的特性传给野生树木。

        当查布和米兰得到满意的结果以后，他们会将木材样本给著名的农业及生物工程教授莱德斯基过目，由他来决定改变的树木是否适应于乙醇的大规模生产。

        他们说，使用杂交白杨树及其亲缘树种作为生物燃料的生产材料对环境、农民和经济的发展都有很多好处。

        “白杨树不需要很多的维护工作，种上之后等待七年就可以收获，”米兰说。“你无需每年喷洒农药；不必走遍各个角落为其压土。每年的落叶降解就能为树木提供源源不断的养料。而且与农田相比，你更可以在多彩多样的杨树林中体验野外生活的乐趣。”

        专家计划像成排的农作物一样大面积种植这种树。这类树木的种植基地将占用DOE及美国农业部所储存的上千英亩未使用土地或闲置地――曾经耕种过的闲置农田，其原因是农民被政府支付费用来闲置这些土地。

        “我们需要一种能在几年内到处生长的生物能作物，”米兰说。“杨树属有大约30个品种生长在从亚热带的佛罗里达到亚高山带的阿拉斯加、北加拿大及欧洲等地区广阔的自然环境中。”

        谷物只能生长在世界上少数几个短生长季的地区。除此之外，在很多地理位置生长的一年期谷物还要满足燃料生产的潜在需求。专家们希望提高每英亩谷物的吨产量以及每吨谷物的乙醇转化量。

        研究人员相信使用目前的杂交白杨树品种每吨木材能够生产70加仑燃料。每年每英亩土地大约有10吨的白杨木材产量，相当于700加仑乙醇。目前谷物每年的英亩产量为4.5吨，折合400加仑乙醇。根据专家的解释，改变木纤维的成分能够将乙醇的年产量提高到每英亩1000加仑，因而在1亿1千万英亩闲置农田上种植这种树木，可以取代美国每年80％的交通石化燃料消耗。

        “我们并不想改变植物的结构完整性，”米兰说。“我们只是想改变木纤维的成分使它更易释放纤维素从而装化成可发酵的糖类并最终变成乙醇。”

英文原文：

　　　　Fast-growing trees could take root as future energy source

The scientists are focused on a compound in cell walls called lignin that contributes to plants' structural strength, but which hinders extraction of cellulose. Cellulose is the sugar-containing component needed to make the alternative fuel ethanol.

The Department of Energy's Office of Biological and Environmental Research is funding a $1.4 million, three-year study by Purdue faculty members Clint Chapple, Richard Meilan and Michael Ladisch to determine ways to alter lignin and test whether the genetic changes affect the quality of plants used to produce biofuels. A hybrid poplar tree is the basis for the research that is part of the DOE's goal to replace 30 percent of the fossil fuel used annually in the United States for transportation with biofuels by 2030.

In 2005 ethanol accounted for only 4 billion gallons of the 140 billion gallons of U.S. transportation fuel used — less than 3 percent. About 13 percent of the nation's corn crop was used for that production. Purdue scientists and experts at the U.S. departments of Agriculture and Energy say corn can only be part of the solution to the problem of replacing fossil fuel.

"If Indiana wants to support only corn-based ethanol production, we would have to import corn," said Chapple, a biochemist. "What we need is a whole set of plants that are well-adapted to particular growing regions and have high levels of productivity for use in biofuel production."

Chapple and Meilan want to genetically modify the hybrid poplar so that lignin will not impede the release of cellulose for degradation into fermentable sugars, which then can be converted to ethanol. The changed lignin also may be useable either in fuel or other products, they said. Currently about 25 percent of the material in plants is the complex molecule lignin, which in its present form could be burned to supply energy for ethanol production, but cannot be transformed into the alternative fuel.

Altering lignin's composition or minimizing the amount present in a cell wall could improve access of enzymes. With easier access, enzymes would be able to more efficiently convert cellulose to sugars. Current treatments used for extracting lignin from woody products for pulp and paper production are harsh and pollute the environment, said Meilan, a Purdue Department of Forestry and Natural Resources molecular tree physiologist.

To advance production of non-fossil fuels, Chapple and Meilan are using genetic tools to modify the poplar and then study how the alterations changed the plants' cell walls. Meilan also is attempting to find ways to produce trees that are reproductively sterile so they are unable to transfer introduced traits to wild trees.

When Chapple and Meilan are satisfied with the results, they will give wood samples to Ladisch, a distinguished professor of agricultural and biological engineering, so he can determine if the changes have created trees suitable for high-yield ethanol production.

Using hybrid poplar and its relatives as the basis for biofuels has a number of advantages for the environment, farmers and the economy, they said.

"Poplar is a low-maintenance crop; plant it and wait seven years to harvest it," Meilan said. "You're not applying pesticides every year; you're not trampling all over the site every year and compacting the soil. You're allowing nutrients to recycle every year when the leaves fall and degrade. In addition, you are more likely to have greater wildlife diversity in poplar plantings than in agricultural fields."

Experts are proposing planting the trees in rows just like any field crop. The basis of these tree plantations will be tens of millions of acres that the DOE and USDA have inventoried as being unused or fallow — previously used farmland that is standing empty because farmers are paid not to grow anything.

"We need a bioenergy crop that can grow many places year-round,” Meilan said. "The genus Populus includes about 30 species that grow across a wide climatic range from the subtropics in Florida to sub-alpine areas in Alaska, northern Canada and Europe."

Corn can be grown only in a few areas of the world and only during a relatively short growing season. Besides needing potential fuel-source crops that can be grown year-round and in many geographical locations, experts also want to increase the per acre tonnage yield of crops and the gallons of ethanol per ton.

Researchers believe that using the hybrid poplar in its present form could produce about 70 gallons of fuel per ton of wood. Approximately 10 tons of poplar could be grown per acre annually, representing 700 gallons of ethanol. Corn currently produces about 4.5 tons per acre per year with a yield of about 400 gallons of ethanol. Changing the lignin composition could increase the annual yield to 1,000 gallons of ethanol per acre, according to experts. Planted on 110 million acres of unused farmland, this could replace 80 percent of the transportation fossil fuel consumed in the United States each year.

"We don't want to compromise the structural integrity of the plant," Meilan said. "We just want to alter the lignin composition to make it easier to liberate the cellulose for conversion to simple sugars that the yeast can gobble up and turn into ethanol."