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.