The single-celled creatures known as protozoans are primitive, exotic, and sometimes just plain weird, resembling animals, plants, or a combination of both. Researchers now report that one animal-like, parasitic protozoan relies on a biochemical pathway that is strikingly plantlike. The discovery could open up a new method of attacking protozoans that cause diseases such as malaria. Parasitic protozoans are extremely difficult to control because their animal-like biologies are often very similar to those of their hosts. As a result, drugs that target these parasites all too often damage the cells of the patient. Hoping to make headway, a team led by microbiologists Kisaburo Nagamune and L. David Sibley of Washington University School of Medicine in St. Louis, Missouri, took a close look at the protozoan Toxoplasma gondii, a parasite that causes the disease toxoplasmosis.
First, the scientists tried comparing biochemical pathways that they identified in the parasite with those of animals to better understand their function. “When we found few similarities, we thought these animal-like protozoans might not be all that they seemed,” says Sibley. So the team compared the biochemical pathways of Toxoplasma with those of plants. It found that the two had a lot in common. Of particular interest was abscisic acid, a hormone that in plants controls stress responses and dormancy. When the researchers disrupted abscisic acid production using a commonly available herbicide, the parasites inside animal cells in culture remained inactive even after reaching numbers that would normally have led to a violent mass exodus. The reason, the team argues, is that abscisic acid is controlling the shift from dormancy to active growth in protozoans, much as it does in plants. The same herbicide saves mice infected with Toxoplasma, the researchers report tomorrow in Nature.