"It's a stretch to believe that the Earth could be restored to life-sustaining status because of the presence of a single live plant," he said over dinner after the movie.

This is missing the point, I think. This just happens to be the first plant that WALL-E encountered, nothing but an indicator that the world is sufficiently supportive of plant life again. It isn't the spontaneously-appearing engine of rebirth for the entire planetary ecosystem.

Granted, finding it in the closed refrigerator was weird. That didn't make much sense to me, either.