The minimum legth of a codon could be two. If it was only one of the 6 nitrogeneous bases in a codon, we'd only have 6 possible amino acids. If we have, though, a combination of two amino acids out of the 6 nitrogeneous bases, we would have then 36 possible combinations (6 possibilities for the first position × 6 possibilities for the second position) that would allow the existence of the 20 different amino acids. In the human case, for example, with only 4 nitrogeneous bases, a combination of two amino acids would be insufficient (4×4=16) for the 20 amino acids.
One example would be the chemicals they use in the soil to make the plants grow faster and another example would be chemicals they use to kill pest so they wont eat the crops or you could say bioengineering has enhanced the crops
