Agreements over genetically modified seeds limiting research?

Andrew Pollack in the NYT has a piece titled Crop Scientists Say Biotechnology Seed Companies Are Thwarting Research which includes the text:

“No truly independent research can be legally conducted on many critical questions,” the scientists wrote in a statement submitted to the Environmental Protection Agency. The E.P.A. is seeking public comments for scientific meetings it will hold next week on biotech crops.

(...)

The problem, the scientists say, is that farmers and other buyers of genetically engineered seeds have to sign an agreement meant to ensure that growers honor company patent rights and environmental regulations. But the agreements also prohibit growing the crops for research purposes.