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.
Many of us have always believed the “myths” we have grown up with regarding these poisonous plants. “Leaves of three, let them be”. Well it turns out the poison sumac branches can have 7 to 13 leaves. And the rash is NOT contagious. You must have direct contact with urushiol oil. Also, dead plants are really still worrisome. Urushiol can stay active on the surface of a dead plant for up to 5 years. At http://www.poison-ivy.org/html/faq.htm I learned that these plants do possess some redeeming qualities though, with as least some indirect benefits to us. As you may have envisioned – these plants are important to the eco-systems they are found in. Birds and some animals find these plants to be a very good food. They feed on the small berries found on the ivies. These plants also tend to prevent erosion near the ocean and the tangles they form serve as shelter to many species, as well.
Plants not only have to defend themselves against us eating them, but they devote a major amount of their biochemistry to protecting themselves against disease, specifically microbial infection. They have evolved an amazing diversity of chemical substances devoted to this task. If you are a subscriber, this past week’s Science issue has an entire section devoted to plants and their interactions with their microbial environment (Science, Volume 324, 8 May, 2009). The editorial entitled “Next-Generation Communication” highlighted this special section of articles, and discussed why the interaction between plants and microbes is so important. Plants live in soil that is absolutely inundated with bacteria (1010 per gram). Because of this they have evolved some very clever mechanisms to defend themselves against pathogenic microbes. They have also managed to maintain some mutually beneficial symbiotic relationships with some subsets of the colonizing microbes. A better understanding of these mechanisms, and the network of signaling molecules responsible for them, is crucial to maintaining plant health and overall productivity for important crops, such as rice, corn and wheat. Plant disease has a major impact on food production throughout the world. In fact, a recent outbreak of a highly virulent strain of the wheat stem fungus Puccinica graminis tritici (strain Ug99) has resulted in nearly an 80% loss of wheat in some Kenyan fields.
“What’s Bugging Plants?” (on page 741 of the same issue of Science) provides a nice introductory overview to this section and discusses the importance of the series of Science articles. In the following articles you can learn details about how the innate immunity system effectively detects, and then wards off, microbes via pattern recognition receptors (PRRs), and how many bacterial virulence factors are devoted to specifically suppressing plant PRR signaling. There is another article explaining how a series of proteins (resistance, or R, proteins) function as molecular switches to aid in pathogen recognition. And it is certain that solving the three dimensional structure of these proteins would yield some very important information. One article highlights some of the important chemical diversity in plants and focuses on how valuable plant-derived antimicrobial compounds can be. You can also learn lots about the co-evolution of plants and their microbial pathogens, and many of the signaling pathways that are crucial to these defenses in some additional articles in this highly informative and timely Science issue.
Plant Genome Database, or PlantGDB, at http://www.plantgdb.org/ is a great place to go if any of you would like to explore data from the genomes of these and other plants. Here I have searched for genome information on poison ivy (Toxicodendron radican),
You can download or just browse through its nucleotide and protein data here, and link out to other resources. The names of some of the other members of this Anacardiaceae family are: Eastern poison oak (Toxicodendron quercifolium), Western poison oak (Toxicodendron diversilobum), and poison sumac (Toxicodendron vernix) in case you want to plug them in to this great tool too. And if any of you would like to learn more about PlantGDB, OpenHelix has an entire tutorial devoted to teaching you how to use this database!
Here are a couple other nice links to check out too:
But most importantly, don’t let a few pesky plants deter any of you from enjoying outdoor summer fun in New England. After all, it’s only a little urushiol!