Remember the melting witch in The Wizard of Oz?
What if corn stalks, for instance, could be induced to “melt” – that is, to go from tough biomass into a sugary puddle?
If this could be easily accomplished, then would be much more cost-effective to use corn biomass as a source of raw material to make biofuels than is currently the case.
Typically, such plant material is cooked in sulfuric acid to render it suitable for commercial conversion to ethanol.
Thus, despite the fact that cellulosic biomass is by far the most abundant source of material for biofuel production, it is currently not cost effective because of the extensive pretreatment required to make it useful.
But what if digestive enzymes were embedded within the plant biomass itself, to be activated only by special conditions?
“Plant biomass” consists mainly of plant cell wall material, chiefly cellulose (see pie chart right). As cellulose and the other plant cell wall structural components are the plant’s chief line of defense against herbivorous insects and microbes, this is tough stuff.
Some bacteria and fungi, however, have evolved to produce enzymes such as cellulases that can digest plant cell walls. (Even plants have genes that code for cellulase – used in fruit softening and leaf abscission, for example.)
Some especially effective plant biomass-digesting microbes live in the guts of some insects (termites) and animals (cows and even Pandas!). Indeed, cell wall-digesting enzymes obtained from such animals and microbes can be used in the laboratory to breakdown – albeit slowly – plant biomass in vitro.
One of the reasons this process is slow is that the enzymes must digest the material from the outside in. But what if these enzymes could digest plant cell walls from the inside out?
Basically, the idea works like this. Microbial genes coding for potent cell wall-digesting enzymes are incorporated into transgenic corn, for example. These enzymes are designed to be deposited in the plant cell walls. But the key to this plan is to make sure these enzymes remain inactive until “turned-on” by conditions that typically are not experienced naturally, such as temperatures above 140o F.
After corn stalks, for example, are harvested, they then are heated to such temperatures, which activates the digestive enzymes embedded in the biomass itself.
Using cellulosic biomass for biofuel production instead of corn kernels, for example, would help to ease pressure on food prices.
But am I the only one that thinks it’s a bit scary to release transgenes into the wild that code for enzymes designed specifically for the super-efficient self-digestion of plants?
Bottom line: Until lignocellulosic material can be cheaply rendered appropriate for fermentation to ethanol, biofuels will likely remain an impractical solution to America’s current and future energy needs.
1. Jiang, X-r., X-y. Zhou, W-y. Jiang, X-r. Gao, and W-l. Li (2011) “Expressions of thermostable bacterial cellulases in tobacco plant.” Biotechnology Letters, Vol. 33, pp. 1797-1803. (Abstract)
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