No freaking clue how this will be done, but we need to do it. Some kind of growing medium that needs very little water, no pesticides, can be grown in the barren desert, and could be used as a rotation crop. Cannabis/hemp still needs water, but it's a possibility.
It has to grow like a weed. It can't be a genetically modified plant (unless you consider breeding modification). We can't live on corn alone. Switchgrass sounds nice, I guess.
Of course, a totally synthetic version of "corn" would be nice, something that could be made in a petri dish medium on a grand scale. It's starting to sound sci-fi, but that's because I am no longer a professional biologist.