There’s two ways I could go about giving myself the ability to digest cellulose (the major structural component of plants).
The first is I could take some cultures of human gut flora (https://en.wikipedia.org/wiki/Gut_flora) and try to grow them on a diet of cellulose. Come back in 11 years or so and some of them are sure to have cracked the problem. Maybe I can borrow some of Richard Lenski’s equipment (https://en.wikipedia.org/…/E._coli_long-term_evolution_expe…). Anyway, you’ll notice that this solution isn’t genetic engineering at all: it’s selective breeding, breeding a microbe (or microbial community) to break down cellulose. If I introduced that community into my gut (by means of some extremely expensive 11-year-old yogurt) I might gain the ability to digest cellulose (assuming that the new bacterial strains replaced my native ones, and that their breakdown of cellulose produced some byproduct that I could make use of). If everything worked perfectly, I’d have that ability forever.
But why be content with boring old Decent With Modification? Why not find something that digests cellulose already and splice its genes into the Lactobacilus or Escherina or whatever that I’ve already got? A good candidate would be Fusarium solani (http://www.pnas.org/content/105/35/12932.full) , a fungus that seems to play some role in the breakdown of lignin (an especially tough cellulose compound that makes up wood) by termites? Termites actually have a whole slew of specialized gut bacteria (some of which have even SMALLER bacteria living inside them: http://www.pnas.org/content/105/35/12932.full) which work together to break down cellulose. It’s not a simple process, and would take a project on par with the moon-landings to port into the human gut, but let’s just assume that this fungus carries the key, and we can just smoosh that key into some yogurt and drink it. Assuming life were that simple (ha!) you’d get the TEMPORARY ability to digest wood, until the modified bacteria were out-competed by your native gut-flora or bred themselves back to normal.
Breaking down lignin is expensive (metabolically costly, in bio-terms) and the bacteria that aren’t burdened with the genes to do it will enjoy a selective advantage. Over time, they’ll come to outnumber the bacteria that can digest lignin, and you’ll be back to where you started. You might stretch out the process by eating lots of sticks and leaves, flooding your system with cellulose so that nothing else is available…and suffer some pretty nasty nutritional deficits.
And why bother? Your doctor and your Monsanto sales rep will be happier if you eat a cup of $50 Yogurmite(R), eat grass for a month along with the rest of your normal diet, then fork out another $50 next month for your next cup. Like the human gut and its microbial ecosystem, capitalism is a complicated system that we don’t want to mess with.