Summer is rapidly approaching and I’m so looking forward to a nice long Memorial Day weekend with outdoor cookouts and plenty of time for gardening. Those of us New Englanders that have endured a long, hard winter really appreciate ending our hibernation and spending time outside in the spring and summer. Gardening is one of my favorite activities, and in this region we are strongly advised to wait until Memorial Day to do the majority of our planting. But after hearing that one of my colleagues had just come down with poison ivy, I began to wonder why these plants so often get in the way of enjoying our short season of outdoor life.
Poison ivy, oak and sumac have always been a very annoying part of growing up in New England. They are plants that I never had too many fond thoughts of. Yet, I never really knew much at all about them – other than the itchy, irritating red rash they cause – that is. I decided to do a little digging, reasoning that they must have some redeeming, or at least interesting, biological qualities. After all, it seems that they are only protecting themselves against all of us herbivores. They can’t exactly run away from us, so they have to keep us at bay some how. Their defense mechanism seems quite clever actually.
A quick check in Wikipedia revealed that poison ivy is a member of the Anacardiaceae family of flowering plants. To my surprise cashew and pistachio plants are also members of this same family. Apparently not all members of this plant family are skin irritants at least! The reaction you get from poison ivy is due to contact with urushiol, a very potent oil found in the sap. In fact, only about 1 nanogram is needed to cause a rash (as little as ¼ of an ounce is said to be necessary to cause a rash on every person on earth). The rash, or Toxicodendron dermatitis, is a result of the immune system’s delayed hypersensitivity response – i.e., the reaction may take hours or days to develop. Interestingly, about 20% of the population is not allergic to urushiol. They can wander through poison ivy indefinitely and have no problems (the genetic variations responsible for this trait are certain to be an interesting topic for future work in the genomics and immunology fields). Another surprising fact was that many animals don’t have any type of allergic reaction to urushiol. Deer, goats, horses and cattle are fine with these poisonous plants. In fact, one of the suggested ways to get rid of poison ivy is to get a goat. This seems to be another very interesting genetics of immunity issue – how and why do some animals manage to not only evade these plants, but thrive on them. As more complete genomes are resolved the genes, SNPs, or genetic variations in general, will be uncovered and we should all be enlightened.