Plants are found within the natural environment therefore, by all means plants are natural in that sense. Though some of the materials (dna for example) of which they are comprised of, are by all means unique to the environment.
You seem to have an unusual idea of what the term "unique" means.
DNA certainly isn't unique. DNA is abundant. A specific sequence MAY be unique, but then so too would the "plant" which contains it.
However, since we don't address each dandelion as a "unique" kind of plant, it seems a little ridiculous to have such a double standard.
I suppose you could say that DNA/RNA/mDNA was unique _IF_ you had a host of other life forms which didn't utilize any of these sequences. THEN, you could say "Look, life on Earth is 'unique' since all other life is silicon based" or whatever.
But that would assume having discovered other life.