It depends on your definition of exploration missions.
Talking about human space flight into low Earth orbit, yes, there have been plenty of plants and successful experiments involving growing plants in space. Some are still going on on-board the ISS. Astronauts also, intentionally or unintentionally, let stuff grow which is actually part of their food supply (fruits etc.) sometimes. However, plants have never been used in a "self-sustaining" way over multiple generations for e.g. generating oxygen. Although, literature on past experiments with plants in space comes to the conclusion that this is possible.
If exploration means out of Earth orbit, there are only very few occasions where plants have been carried. The Soviet circumlunar Zond missions carried, besides lots of different animals, also plants.