To cut a long story short, plants were ineligible for utility patents (in the US at least) because they were considered "products of nature". However, the US Supreme Court decided that because GMOs existed, plants now count as something made by man rather than a product of nature and could be patented - including non-GMO ones produced using traditional breeding techniques that were previously not patentable. It's a strange decision and I'm not sure I follow their logic, but it's the law now.