Cotton is one of the most widely grown farmed plants. It produces copious nutritious seeds. But due to a toxin called "gossypol," people can't eat the seeds – otherwise they'd be enough to feed 500 million people per year. (Correctly processed cottonseed oil can be consumed, though, and cottonseeds can also be fed to "ruminant" farm animals in limited quantities.) The cotton plant produces the toxin apparently to try to defend against insect pests and diseases.
A succession of genetic engineers figured out how to breed a cotton plant with 100 times less gossypol in its seeds (but still producing gossypol in the rest of the plant to fend off insects).
[G. Sunilkumar, L. Campbell, L. Puckhaber, R. Stipanovic, K. Rathore: Engineering cottonseed for use in human nutrition by tissue-specific reduction of toxic gossypol, Proceedings of the National Academy of Sciences 103,48 (2006) 18027-18028.]