Constitutionally patents are on very weak legs when applied to plants [IMO], as it says To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries; but plants aren't 'inventions' nor are these modifications 'discoveries' -- add to that the 'perpetual copyright' the Supreme Court basically handed Disney means that these GMO patents may realistically never expire.
A plant variety must be invented and asexually produced by the applicant to be eligible for a patent. Plant patents, like utility patents, expire after 20 years.