Currently works are underway to develop a strain of cyanobacteria capable of surviving in Mars ambient conditions. It's a difficult work, but not an impossible one. We have bacteria that can already survive in vacuum, though to make them thrive (and not survive as endospores) they need some more work - they can already thrive at quite a low pressure but not as low as 0.006 bar. They live off water (air moisture would suffice), carbon dioxide (abundant) and sunlight, producing oxygen, and they can survive deep freezing as endospores too, so once the pressure issue is solved they will be capable of growing on Mars. (and even if they aren't, they can be used as oxygen supply for a martian base/colony, kept in 'oxygen-producing solar panels' pressurized to a level survivable for the bacteria.)
Once we get the oxygen production going, and local manufacturing, we can start building underground shelters which could be pressurized to reasonable levels and whatever we want could be grown in them. But without raising the atmospheric pressure not much else could grow on the surface.