Monsanto clones in genes that are resistant to its herbicide, Roundup, which is a simple glycine derivative. These molecules bind tightly to a key enzyme in three amino acid synthesis pathways, and since these molecules bind with such affinity, the enzyme can't dislodge it within a reasonable timeframe/equilibrium. Without the AAs, the plant dies.
Due to patents, Monsanto owns the rights to any plants with the gene--including offspring. Farmers have to continually license the plants to grow them. One unfortunate soul had a neighboring farmer's crops cross pollinate his own, and Monsanto came down hard on him--it was a pretty big incident.
Save for one fact, there is no law which prohibits the owning of animals or planet, or indentured servitude of animals or plants. Thus what Monsanto does is enforceable while this would likely not be.
Edit: I believe it's a form of debt bondage (peonage) that is prohibited by international law too. See: http://en.wikipedia.org/wiki/Debt_bondage