Uhm. You can, in fact, selectively breed any given gene into any creature. You are, of course, aware that plant mutation rates has been accelerated with mutagens for decades in search of desirable changes.

So while it may not be a gene copied directly from a fish, it will be a base-for-base identical copy of the gene found in a fish. Then it can be merged back into the main line of the plant via a recombinant backcross technique.

This is how plant breeding has been done for quite a long time now.