Question

For a planet to have breathable air, wouldn’t have to have abundant plant life?

For air to be breathable, it needs to contain oxygen, and oxygen would not likely be present on another planet simply due to circumstance. Given the chemical profile of oxygen, it is much more likely that an atmosphere full of oxygen would be a byproduct of some sort of biochemical processes. Our singular knowledge of Earth life, as you guessed, points to plant life as the likely culprit. However, most scientists believe that plants like trees, shrubs, and grasses may not be as critical in the production of oxygen as we once thought. Instead, many point to plankton and other organisms in the oceans, even plants like kelp (of which there is an enormous amount of biomass) as the primary oxygen producers for our planet. Returning to your question, a planet could perhaps look barren, as in a sci-fi movie, but actually be replete with microbial oxygen-respiring life that could support a breathable air environment. You might want to check out the following books that ask very similar questions to yours. The Natural Selection of the Chemical Elements: The Environment and Life's Chemistry, by R. J. P. Williams and J. R. R. Frausto Da Silva The Search for Life on Other Planets, by Bruce Jakosky Anna Lee Strachan, NASA Astrobiology Institute
May 1, 2002