Just because something has a basis in something simpler, or is an improvement doesn't mean it's not a new invention; IIRC, there is a little plastic piece that can fit in your pocket that literally replaced a whole roomful of machinery for dialysis.
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.
That's just my point! Plants aren't invented! Soybeans, asexually reproducible, for example, are still soybeans -- even if they have a patented resistance to roundup.