Disclosed are novel transgenic plant cells that include a heterologous (e.g., human) receptor polypeptide, or fragment thereof. The plant cells can be used to identify molecules (i.e., ligands) that can interact with the receptor polypeptide or fragment. The plant cells can be used to identify ligands that are endogenous to transgenic plant cells, or exogenous ligands that are applied to the plant cells. Such receptor polypeptide ligands can be used to identify novel pharmaceuticals.