So far, the first generation of biofuels is being made from things like corn, palm oil, and sugar cane. But only a small part of these plants—a part we'd already been using for other things—is actually made into fuel. Being able to make biofuels from the rest of the plant would allow us to get more from existing crops, use the leftover biomass from food production, and allow us to process plants that grow on marginal terrain.

Unfortunately, most of the carbon in a plant is locked up in cellulose, a very tough polymer made from simple sugar molecules. Before we turn a plant into biofuel, we need to figure out how to break down the cellulose. Right now, that process takes harsh conditions and long treatments with enzymes, which significantly adds to the cost. But some bioengineers at a company in Massachusetts have made a plant that carries an enzyme that can help digest itself—but the enzyme remains inactive until the plant is processed.

Hemicellulose is a major component of plant cell walls. Its presence helps protect cellulose from digestion, so digesting it not only liberates sugar for making biofuels, but it also makes the sugar in cellulose easier to access. Digesting away hemicellulose is thus a key early step in biofuel production, and requires either harsh chemicals or expensive enzyme treatments.

The authors reasoned that placing the enzyme itself in the plants they were going to process would reduce the cost and complexity of digesting. But, when they placed the gene for one of those enzymes (a xylanase) into corn, its normal activity kicked in, and it started digesting the plant while it was still alive. The resulting plants were unhealthy, and the kernels of their corn were small and shriveled.

Since biofuel processing normally takes place at high temperatures, they decided to make the xylanase inactive by default, but allow it to be activated on demand. To engineer this version, they turned to a rather unusual form of molecular parasite called an "intein." These are short protein sequences that, when embedded inside a larger protein, splice themselves out, freeing themselves and leaving an intact protein behind. To get an intein that could be controlled, the authors identified one that was active in a species that is fond of high temperatures.

The team took the xylanase gene and inserted the sequence for the intein in the middle of it, which should inactivate the xylanase unless the intein splices itself out. After trying 23 different locations, they found a few where the resulting xylanase was inactive at low temperatures, but got activated as the temperature was raised (thus allowing the intein to remove itself from the resulting enzyme). They then did two rounds of random mutation and selection, screening for a greater difference before and after heat activation. The resulting xylanase had less than 10 percent of its normal activity before heating, but was more than 60 percent active after a two hour heat treatment.

When placed in plants, this didn't cause any of the adverse affects of the normal xylanase. And, more importantly, it made the plants easier to digest. After a two hour treatment at 75°C, yields of sugars from the plant matter were up by over 20 percent. In fact, the actual yield of glucose went up to 90 percent of maximum expected based on the weight of the plant mass. And all of that was obtained from the corn plant after the ears of corn were removed.

There are plenty of ways to potentially improve the intein construct. Additional enzymes can be added, and they can be targeted to tissues like the stalk or leaves, keeping them out of the edible part of the corn. But it's a good start towards bioengineering a plant that can help turn itself into useful biofuels.

Nature Biotechnology, 2012. DOI: 10.1038/nbt.2402 (About DOIs).