If you try hard enough, you can make ethanol out of anything that grows. But it has always been more complicated and costly to make ethanol from trees and grass than from edible plants, such as corn.

Now scientists at Dartmouth College have discovered a cheaper way to use a key enzyme for fermenting those tough plants that could make them viable sources of alternative fuel.

A group of researchers at Dartmouth's Thayer School of Engineering in Hanover and Mascoma Corp. in Lebanon genetically engineered a bacterium to make pure ethanol out of woody, or "cellulosic," plant material.

The researchers described how they engineered the bacterium in a paper published online last week in the journal Proceedings of the National Academy of Sciences.

"This is very important work," said Dr. Martin Keller, director of the BioEnergy Science Center at Oakridge National Laboratory.

Normally, a kind of enzyme called cellulase has to be added to help ferment cellulosic plant matter. With the modified bacterium, less cellulase is needed for fermentation.

"That's very important because the cost of cellulase is a key constraint" to producing ethanol from cellulosic plants economically, said Lee Lynd, an environmental engineering professor at Dartmouth and co-founder of the Mascoma Corp.

Cellulase has many commercial uses, and it's an expensive commodity, Lynd said.

But if making ethanol from grass or trees could be done cheaply, Lynd said, they could be a great source of renewable fuel for vehicles. "I think that biofuels are a potential primary energy source for transportation," he said.

As demand for ethanol made from crops like corn has increased, scientists and policymakers have raised concerns about effects on the world's food supply. Many researchers see promise in making ethanol out of inedible plants because they are renewable and don't compete with food crops for farmland.

Lynd said that his genetically modified bacterium could make ethanol with less cellulase, and therefore more cheaply, because the bacterium thrives at high temperatures - up to 140 degrees. Warmer temperatures aid the chemical reactions that break down plant fibers into ethanol.

Keller, whose center works with Mascoma Corp., said that the methods described in Lynd's paper could help researchers genetically engineer bacteria to tweak the fermentation process even more and make not only ethanol, but exotic new fuels.

"It means you can start to manipulate the organism to produce the fuels of our choice," Keller said.