Freeloading Plants Doing Good?

Our cold winter and late snow seem to have left our local plant life a bit behind schedule. So here's a plant that was still green in March of last year:

Eastern Mistletoe (Phoradendron leucarpum),
Louisville Zoo, Jefferson Co, KY  3/28/2013

This is Eastern Mistletoe (Phoradendron leucarpum) growing on an oak tree at the Louisville Zoo. Mistletoes are parasitic plants, that grow into their host tree's tissues and absorb nutrients from them. Here's the entire tree:

Eastern Mistletoe (Phoradendron leucarpum),
Louisville Zoo, Jefferson Co, KY  3/28/2013

This genus includes over 200 species. There are quite a few other species of mistletoes, but they fall into 5 different families. Apparently parasitism in these guys has evolved independently on several occasions. One of the interesting aspects of parasite evolution is the occurrence of host-race evolution, where a population can split into several, sometimes sympatric races, each specializing on a different host species. Apparently, this happens in Phoradendron, as Glazner, et al. found evidence for it in Desert Mistletoe (P. californicum). (1)

European Mistletoe (Viscum album) has been used as a folk remedy for a number of ailments for centuries, and preparations of it have been marketed as anti-cancer agents. These studies appear to offer a mixed bag of results (2,3,4), although it does appear that it can act through both cytoxic effects (5) and by stimulating the immune system. (6) That cytotoxin, however, should be a concern for anyone rushing down to the corner drugstore -- in at least one case, Mistletoe consumption has been linked to hepatitis. (7)

That's a lot going on for a plant most of us only know as a plastic Christmas ornament!


(1) Glazner, J. T., Devlin, B., & Ellstrand, N. C. (1988). Biochemical and morphological evidence for host race evolution in desert mistletoe, Phoradendron californicum (Viscaceae). Plant systematics and evolution,161(1-2), 13-21.

(2) Joller, P. W., Menrad, J. M., Schwarz, T., Pfüller, U., Parnham, M. J., Weyhenmeyer, R., & Lentzen, H. (1996). Stimulation of cytokine production via a special standardized mistletoe preparation in an in vitro human skin bioassay.Arzneimittel-Forschung, 46(6), 649-653.

(3) Friess, H., Beger, H. G., Kunz, J., Funk, N., Schilling, M., & Büchler, M. W. (1995). Treatment of advanced pancreatic cancer with mistletoe: results of a pilot trial. Anticancer research, 16(2), 915-920.

(4) Stein, G., Henn, W., von Laue, H., & Berg, P. (1998). Modulation of the cellular and humoral immune responses of tumor patients by mistletoe therapy.European journal of medical research, 3(4), 194-202.

(5) Janssen, O., Scheffler, A., & Kabelitz, D. (1993). In vitro effects of mistletoe extracts and mistletoe lectins. Cytotoxicity towards tumor cells due to the induction of programmed cell death (apoptosis). Arzneimittel-Forschung, 43(11), 1221-1227.

(6) Heiny, B. M., & Beuth, J. (1993). Mistletoe extract standardized for the galactoside-specific lectin (ML-1) induces beta-endorphin release and immunopotentiation in breast cancer patients. Anticancer research, 14(3B), 1339-1342.

(7) Harvey, J., & Colin-Jones, D. G. (1981). Mistletoe hepatitis. British medical journal (Clinical research ed.), 282(6259), 186.