A Peculiar Parasite at Berkeley


Parasitic plants are fascinating. I never pass up an opportunity to meet them. On a recent trip to California, my host for the day mentioned that something funny was growing in a patch of ivy on the Berkeley Campus. I had to know what it was. We took a detour from our intended rout and there, growing underneath a pine tree in a dense patch of ivy were these odd purple and brown stalks. This was definitely a parasitic plant.

The plant in question was the ivy broomrape (Orobanche hederae). As both its common and scientific name suggests, it is a parasite on ivy (Hedera spp.). As you can probably guess based on the identity of its host, ivy broomrape is not native to North America. In fact, the population we were looking at is the only known population of this plant you will find in the Americas. How it came to be in that specific location is a bit of a mystery but the proximity to the life sciences building suggests that this introduction might have been intentional. Personally I am quite alright with this introduction as it is parasitizing one of the nastier invasive species on this continent.

The ivy broomrape starts its life as a tiny seed. Upon germination, the tiny embryo sends out a thin thread-like filament that spirals out away from the embryo into the surrounding soils. The filament is looking for the roots of its host. Upon contact with ivy roots, the filament penetrates xylem tissues. The ivy broomrape is now plugged in, receiving all of its water, nutrient, and carbohydrate needs from the ivy. At this point the embryo begins to grow larger, throwing out more and more parasitic roots in the process. These locate more and more ivy roots until the needs of the ivy broomrape are met. Of course, all of this is going on underground.

Only when the ivy broomrape has garnered enough energy to flower will you see this plant. A stalk full of purple tinged, tubular flowers emerges from the ground. At this point its membership in the family Orobanchaceae is readily apparent. Like all members of this family, its parasitic lifestyle is so complete that it is beginning to lose genes for the production of chlorophyll and Rubisco, all things we generally associate with plants. This is why I love parasites so much. Not only are their ecological impacts bewilderingly complex, their evolutionary histories are such a departure from the norm. I will never tire of appreciating such species and I am happy to have met yet another awesome member of this group.

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