I believe that you can get a patent on a plant if the mutation occurs in cultivation but not if it occurs in the wild. Of course, it also has to have characteristics that make it unique but I think those standards may be a bit loose considering how many patented plants of one species can look so similar.
If you had a plant that spontaneously mutated it would not be the same as their plant and would not be protected by the patent. Your plant would likely be different genetically to begin with unless the parent plant was the same cultivar/clone. Even if the original was the same genetically, the chance that the same exact mutation would occur would be exceedingly unlikely. It might look similar but genetically it would differ.
