[thechao]

I'll use an analogy with "nanorobots":

When I was doing nanotech 10+ years ago, and people asked me when nanorobots were coming, I'd always ask them why they thought nanorobots would be different than, say, bacteria, or viruses? The constraints "at the bottom" (as Feynman would say) are quite high—and we're not really very close to being able to engineer at that scale. Heck, even making a robot the size of a grain of rice is a pretty daunting task, and a nanorobot would be built on a scale that's 10^12 to 10^18 times smaller (the cubic relationship is a real bitch), depending on what you mean by "nanoscale".

Engineering a "thing" to convert basic chemicals and sunlight into food is a pretty significant task. While I would bet that plants are nowhere near close to optimal, they're already solving the problem. Changing & rethinking an existing system is far easier than simply creating one from whole cloth.

If I had to guess, I'd say that "producing food from manipulating basic elements" using a piece of tech entirely designed by human hands, from scratch, is at least a couple of centuries in the future.