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.
Further Reading:
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1925.tb06671.x/pdf
http://cat.inist.fr/?aModele=afficheN&cpsidt=4107447
Newly Discovered Orchid Doesn't Bother With Photosynthesis or Opening Its Flowers
![Photo by Suetsugu Kenji [SOURCE]](https://images.squarespace-cdn.com/content/v1/544591e6e4b0135285aeb5b6/1477880480914-NI1RT36AQBR67J716TOY/image-asset.jpeg)
Photo by Suetsugu Kenji [SOURCE]
A new species of orchid has been discovered on the small Japanese island of Kuroshima. Though not readily recognized as an orchid, it nonetheless resides in the tribe Epidendroideae. Although the flowers of its cousins are often quite showy, this orchid produces small brown blooms that never open. What's more, it has evolved a completely parasitic lifestyle.
The discovery of this species is quite exciting. The flora of Japan has long thought to be well picked over by botanists and ecologists alike. Finding something new is a special event. The discovery was made by Suetsugu Kenji, associate professor at the Kobe University Graduate School of Science. This discovery was made about a year after a previous parasitic plant discovery made on another Japanese island a mere stones throw from Kuroshima.
Coined Gastrodia kuroshimensis, this interesting little parasite flies in the face of what we generally think of when we think of orchids. It is small, drab, and lives out its entire life on the shaded forest floor. Like the rest of its genus, G. kuroshimensis is mycoheterotrophic. It produces no leaves or chlorophyll, living its entire life as a parasite on mycorrhizal fungi underground. This is not necessarily bizarre behavior for orchids (and plants in general). Many different species have adopted this strategy. What was surprising about its discovery is the fact that its flowers never seem to open.
In botany this is called "cleistogamy." It is largely believed that cleistogamy evolved as both an energy saving and survival strategy. Instead of dumping lots of energy into producing large, showy flowers to attract pollinators, that energy can instead be used for seed production and persistence. Additionally, since the flowers never open, cross pollination cannot occur. The resulting offspring share 100% of their genes with the parent plant. Although this can be seen as a disadvantage, it can also be an advantage when conditions are tough. If the parent plant is adapted to the specific conditions in which it grows, giving 100% of its genes to its offspring means that they too will be wonderfully adapted to the conditions they are born into.
As you can probably imagine, pure cleistogamy can be quite risky if conditions rapidly change. In the face of continued human pressures and rapid climate change, cleistogamy as a strategy might not be so good. That is one reason why the discovery of this bizarre little orchid is so interesting. Whereas most species that produce cleistogamous flowers also produce "normal" flowesr that open, this species seems to have given up that ability. Thus, G. kuroshimensis offers researchers a window into how and why this reproductive strategy evolved.
Photo Credit: Suetsugu Kenji
Further Reading: [1]
Rhizanthes lowii
Photo Credit: Ch'ien C. Lee - www.wildborneo.com.my/photo.php?f=cld1500900.jpg
Imagine hiking through the forests of Borneo and coming across this strange object. It's hairy, it's fleshy, and it smells awful. With no vegetative bits lying around, you may jump to the conclusion that this was some sort of fungus. You would be wrong. What you are looking at is the flower of a strange parasitic plant known as Rhizanthes lowii.
Rhizanthes lowii is a holoparasite. It produces no photosynthetic tissues whatsoever. In fact, aside from its bizarre flowers, its doesn't produce anything that would readily characterize it as a plant. In lieu of stems, leaves, and roots, this species lives as a network of mycelium-like cells inside the roots of their vine hosts. Only when it comes time to flower will you ever encounter this species (or any of its relatives for that matter).
The flowers are interesting structures. Their sole function, of course, is to attract their pollinators, which in this case are carrion flies. As one would imagine, the flowers add to their already meaty appearance a smell that has been likened to that of a rotting corpse. Even more peculiar, however, is the fact that these flowers produce their own heat. Using a unique metabolic pathway, the flower temperature can rise as much as 7 degrees above ambient. Even more strange is the fact that the flowers seem to be able to regulate this temperature. Instead of a dramatic spike followed by a gradual decrease in temperature, the flowers of R. lowii are able to maintain this temperature gradient throughout the flowering period.
Photo Credit: Ch'ien C. Lee - www.wildborneo.com.my/photo.php?f=cld1500900.jpg
There could be many reasons for doing this. Heat could enhance the rate of floral development. This is a likely possibility as temperature increases have been recorded during bud development. It could also be used as a way of enticing pollinators, which can use the flower to warm up. This seems unlikely given its tropical habitat. Another possibility is that it helps disperse its odor by volatilizing the smelly compounds. In a similar vein, it may improve the carrion mimicry. Certainly this may play a role, however, flies don't seem to have an issue finding carrion that has cooled to ambient temperature. Finally, it has also been suggested that the heat may improve fertilization rates. This also seems quite likely as thermoregulation has been shown to continue after the flowers have withered away.
Regardless of its true purpose, the combination of lifestyle, appearance, and heat producing properties of this species makes for a bizarrely spectacular floral encounter. To see this plant in the wild would be a truly special event.
Photo Credit: Ch'ien C. Lee - www.wildborneo.com.my/photo.php?f=cld1500900.jpg
Rusty Mustards
Believe it or not, what you are seeing here is the same species of plant. The one on the left is the normal reproductive state of a Boechera (Arabis) mustard while the one on the right is the same species of mustard that has been infected by a rust fungus known as Puccinia monoica.
The interaction of these two species is interesting on so many levels. I spent an entire summer, along with my botanical colleagues, completely stumped as to what this strange orange-colored plant could be only to eventually find out that it was a mustard that has been hijacked! The fungus in question, P. monoica, is part of a large complex of interrelated rust fungi who are quite fond of mustards. The reason for this all boils down to reproduction.
The lifecycle of P. monoica begins when spores land on a young mustard plant and invade the host tissue. As they grow, they gain more and more nutrients from the mustard. Eventually the fungi effectively sterilizes the mustard and causes it to begin forming what are referred to as "pseudoflowers." The pseudoflowers are basically leaves that have been mutated by the fungus to look and smell a lot like other plants blooming in early summer.
The pseudoflowers produce a sticky, nectar-like substance that is very attractive to pollinators. The mimicry even goes as far as to produce yellowish pigments that reflect UV light, making them an even more irrisistable target for passing insects. On each pseudoflower are hundreds of small cups known as spermatogonia. These house the sex cells of the fungus. Visiting insects get covered in these sex cells, which they will then transfers to other infected plants thus achieving sexual reproduction for the fungus.
Still with me?
At this point, the pseudoflowers stop producing color and nectar and instead, the fused sex cells germinate into hyphae that begin to form specialized structures called "aecia." The aceia house the spores that will be responsible for infecting their secondary host plants, which are grasses. Spores blow about on the wind and, with a little luck, a few will land on a blade of grass. The spores germinate and infect the grass. From there, structures called "uredia" are formed that go on to produce even more spores to infect even more grass. Eventually, structures called "telia" are formed on the grass and the cycle finally comes full circle. The telia produce the spores that will go on to infect the original mustard host plants.
Whew! To have stumbled across an evolutionary drama such as this serves as a reminder of just how much in nature goes largely unnoticed every day.
Sandfood
Photo by USFWS Pacific Southwest Region licensed under CC BY 2.0
Photo by Don Davis licensed under CC BY-NC-ND 2.0
Pholisma is yet another amazing genus of parasitic plants. Endemic to the southwestern United States and Mexico, these peculiar members of the borage family tap into the roots of a variety of plant species. They do not photosynthesize and therefore obtain all the nutrients they need from their hosts. Oddly enough, researchers have found that most of their water needs are met by absorbing dew through the stomata on their highly reduced, scale-like leaves. Water is then stored in their highly succulent stems. Throughout their limited range, Pholisma are critically imperiled. Development and agriculture have already eliminated many populations. To add insult to injury, the dunes in which most extant populations are found are owned by the BLM and are open to heavy off-road ATV traffic, which will likely push them to the brink of extinction if nothing is done to limit such recreational use. Unless people speak up about protecting these plants and their habitats, they could disappear for good.
Photo by Vijay Somalinga licensed under CC BY-NC-ND 2.0
Photo by Vahe Martirosyan licensed under CC BY-SA 2.0
Photo by Daniel Jolivet licensed under CC BY 2.0
The Truth About Mistletoe
While perusing a local indie market this weekend I noticed that there was a stand selling mistletoe branches. It was odd to note that, for as ubiquitous of a symbol it is during this time of year, so few people realize what these plants are all about.
A reference to mistletoe could be any number of plants in the order Santalales. It is a large and rather varied grouping but the common thread throughout Santalales is parasitism on some level. Many species grow in the form of a shrub that grows epiphytically and taps into a host tree. While most still undergo photosynthesis, they also obtain nutriment using specialized structures that plug into the vascular tissue of their host. If infestations are intense enough, mistletoe can kill a tree.
However, don't let this fact leave you with any animosity toward mistletoe. Far from being a drain on the ecosystem, evidence is showing that many mistletoe species are keystone organisms where they are native. They produce copious amounts of berries that attract and feed a wide variety of birds and their growth habit makes for great nesting sites as well. Their leaves and shoots are munched on by a multitude of insect life, which then attracts animals higher up the food chain. Because of the preponderance of bird species that frequent mistletoe, other berry producing plants in the area benefit as well. In one study, researchers found that junipers had higher rates of reproduction when growing near mistletoe. Because mistletoe attracts all of those berry eating birds, numbers of seed carriers significantly increase for all other berry producing plants.
The most commonly encountered species of mistletoe is probably Viscum album. This evergreen species ranges from north Africa to southern England all the way to parts of Asia. It is evergreen and seems to really like growing on apple trees (Malus sp.). My favorite thing about this species as well as many other mistletoe is how they manage to reproduce. As mentioned above, they produce plump berries that birds can't resist. The pulp of the berry is quite sticky and thick that when the bird digests what it can and voids the rest, it has to wipe itself on a branch. This causes the mistletoe seed to stick to the branch like glue. When the seed germinates it attaches itself to the tree, living independently at first but later tapping into the trees vascular tissue. Next time you pucker up with someone special under one of these plants, keep that in the back of your mind!
Photo Credit: Martin LaBar
Further Reading:
http://www.kew.org/plants-fungi/Viscum-album.htm
http://www.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.32.081501.114024
http://onlinelibrary.wiley.com/doi/10.2307/4013039/abstract