Roosegaarde was inspired for the idea by jellyfish. He proposes if scientists could take the luminescent qualities of jellyfish and fireflies and genetically engineer it with foliage, streets could be lit with a much more aesthetic and environmentally friendly lighting system.
There is a Kickstarter campaign that has raised over half a million dollars to see the idea be put on paper but there may be a disappointment in the works for enthusiasts. The glow from the trees would be 1,000 times less intense than that of a street lamp. Streetlamps release about 10,000 lumens whereas these trees would only exude about 10.
They may not be practical, but they would be pretty.