parasitic plants

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Book Review: In Defense of Plants

Many of us who are plant obsessed didn’t connect with plants right away. It took time. There was a journey we had to go on that would ultimately bring us to the point where plants are now the main thing we think about. After all, plants aren’t the easiest things to relate to. Not immediately anyway. Some of us have to work up to it. Once there, it’s pretty much impossible to go back to our former lives. What was once just a background of green hues is now a rich cast of characters, each with their own name, unique features, and distinct story to tell. Essentially, we went through what Matt Candeias refers to as our ” green revolution.” Candeias – author and host of the long-running blog and podcast, In Defense of Plants – shares his story of learning to love plants and offers a convincing arguement for why you should love them too in his new book, aptly titled, In Defense of Plants.

It’s hard to picture Candeias as anything but a plant lover. If you’ve been following his work, you’ll know he makes it a point to put plants at center stage. It seems that much of the popular content available about plants focuses on the usefulness of plants as they pertain to humans. In many cases it can be easier to find out how to grow a certain plant species than to learn about where it’s from and what it’s like in the wild. Candeias let’s the plants speak for themselves by giving them a voice through his blog, podcast, and now his book. Through the stories he shares we get a peek into the way Candeias sees plants, with the hope being that others might also “be bitten by the botanical bug.”

One of the first plants that captured the attention of Candeias was perennial blue lupine (Lupinus perennis). While assisting with a habitat restoration project at a sand and gravel quarry, Candeias was tasked with improving the establishment of lupine, which is the host plant for the caterpillars of an endangered species of butterfly called Karner blue. The work he did at the quarry and the botanical research that went into it helped Candeias realize that plant’s aren’t at all boring, but are “incredibly interesting organisms worthy of respect and admiration” and that “plants can be both surprisingly relatable and incredibly alien all at once.” His “green revolution” had begun.

The seeds of lupine are dispersed ballistically. As the seed pods dry, tension builds. Then, as Matt Candeias writes in In Defense of Plants, “with an audible pop, the pods eventually explode, catapulting the seeds out into the environment.”

In each chapter of In Defense of Plants we get a peak into the experiences that brought Candeias to where he is now as he discovers the wonder of plants. His personal stories help introduce the main topic of each chapter. Topics include plant sex, plant dispersal, plant defenses, carnivorous plants, and parasitic plants. From countless possible examples, Candeias selects a few of his favorite plant species to help illustrate each topic. Along the way, the reader is presented with various other interesting plant-related facts as Candeias discusses the behaviors of some of the world’s most fascinating plants. In the chapter on dispersal, for example, unlikely agents of seed dispersal (like catfish!) are introduced, as well as phenomena like geocarpy, in which plants are essentially planting themselves.

Carnivorous plants provide an excellent gateway into convincing people who claim to have no interest plants that they actually do. It’s difficult to deny the impressive nature of a meat-eating plant. In the carnivorous plant chapter, Candeias introduces us to the various ways such plants capture and consume their prey, and even wonders if some of these plants should be considered omnivores. After all, certain butterworts digest pollen that falls onto their sticky leaves, and some bladderworts suck in plenty of algae and possibly gain nutrients from the act. If capturing insects inside leaves modified to look like pitchers or on leaves covered in digestive enzyme-producing glands doesn’t impress you, consider the carnivorous actions of corkscrew plants, which drill their leaves into the soil to go after soil-dwelling organisms like protozoans and worms.

Parasitic plants should also excite a reluctant plant lover. These are plants that take all or most of what they need to survive from another plant or host organism. Mistletoes are one of the more familiar parasitic plants, and Candeias describes several, including one that lives almost entirely within the stems of cacti. In fact, “you would never know a cactus had been infected until the mistletoe living within decides to flower,” at which point the flowers push their way out through the sides of the cactus. Dodder is another fairly common, highly specialized, and easy to identify parasitic plant. It basically looks like “a tangled pile of orange spaghetti tossed over the surrounding vegetation.” Orchids, a favorite of Candeias, are known for being mycoheterotrophs, which essentially means they parasitize fungi. Their seeds come unequipped with the energy stores needed to get going, so they borrow resources from mycorrhizal fungi in order to get their start. Years pass before the orchid can offer anything in return.

Datura is a genus of plants that produces toxic compounds like scopolamine and atropine. In his book, In Defense of Plants, Matt Candeias warns, “it would only take a small amount of these chemicals to completely ruin your week and slightly more to put you in a grave.”

After spending more than 200 pages celebrating plants and their amazing abilities and diversity, it’s fitting that Candeias spends the final chapter of his book mourning some of the ways the actions of humans threaten the existence of so many plants. He remarks how unfortunate it is that “plants with their unseeing, unhearing, unfeeling ways of life usually occupy the lowest rung of importance in our society.” Many of us barely notice the loss, yet “plants are the foundation of functioning ecosystems.” Due to that fact, “destroying plant communities causes disastrous ripples that reverberate throughout the entire biosphere of our planet.” Everything suffers when plants are lost. Fortunately, the book doesn’t end on this dark note. Candeias’s overall message is hopeful. When we learn to understand, appreciate, and care about plants, we will want to do everything we can to protect and restore them. With any luck, after reading this book, you too will want to offer your time, energy, and resources in defense of plants.

Listen to Matt talk about his new book on this episode of his podcast.

More Book Reviews on Awkward Botany

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Camel Crickets and the Dust Seeds of Parasitic Plants

A common way for plants to disperse their seeds is to entice animals to eat their seed-bearing fruits – a strategy known as endozoochory. Undigested seeds have the potential to travel long distances in the belly of an animal, and when they are finally deposited, a bit of fertilizer joins them. Discussions surrounding this method of seed dispersal usually have birds and mammals playing the starring roles – vertebrates, in other words. But what about invertebrates like insects? Do they have a role to play in transporting seeds within themselves?

Certain insects are absolutely important in the dispersal of seeds, particularly ants. But ants aren’t known to eat fruits and then poop out seeds. Instead they carry seeds to new locations, and some of these seeds go on to grow into new plants. In certain cases there is an elaisome attached to the seed, which is a nutritious treat that ants are particularly interested in eating. Elaisomes or arils have also been known to attract other insects like wasps and crickets, which may then become agents of seed dispersal. But endozoochory in insects, at first, seems unlikely. How would seeds survive not being crushed by an insect’s mandibles or otherwise destroyed in the digestion process?

camel crickets eating fruits of parasitic plants (via New Phytologist)

While observing parasitic plants in Japan, Kenji Suetsugu wanted to know how their seeds were dispersed. Many parasitic plants rely on wind dispersal, thus their seeds are minuscule, dust-like, and often winged. However, the seeds of the plants Suetsugu was observing, while tiny, were housed in fleshy fruits that don’t split open when ripe (i.e. indehiscent). This isn’t particularly unusual as other species of parasitic plants are known to have similar fruits, and Suetsugu was aware of studies that found rodents to be potential seed disperers for one species, birds to be dispersers of another, and even one instance of beetle endozoochory in a parasitic plant with fleshy, indehiscent fruit. With this in mind, he set out to identify the seed dispersers in his study.

Suetsugu observed three achlorophyllous, holoparisitic plants – Yoania amagiensis, Monotropastrum humile, and Phacellanthus tubiflorus. While their lifestyles are similar, they are not at all closely related and represent three different families (Orchidaceae,  Ericaceae, and Orobanchaceae respectively). All of these plants grow very low to the ground in deep shade below the canopy of trees. Air movement is at a minimum at their level, so seed dispersal by wind is not likely to be very effective. Using remote cameras, Suetsugu captured dozens of hours of footage and found camel crickets and ground beetles to be the main consumers of the fruits, with camel crickets being “the most voracious of the invertebrates.” This lead to the next question – did the feces of the fruit-eating camel crickets and ground beetles contain viable seeds?

Monotropastrum humile via wikimedia commons

After collecting a number of fecal pellets from the insects, Suetsugu determined that the seeds of all three species were “not robust enough to withstand mastication by the mandibles of the ground beetles.” On the other hand, the seeds passed through the camel crickets unscathed. A seed viability test confirmed that they were viable. Camel crickets were dispersing intact seeds of all three parasitic plants via their poop. The minuscule size of the seeds as well as their tough seed coat (compared to wind dispersed seeds of similar species) allowed for safe passage through the digestive system of this common ground insect.

In a later study, Suetsugu observed another mycoheterotrophic orchid, Yoania japonica, and also found camel crickets to be a common consumer of its fleshy, indehiscent fruits. Viable seeds were again found in the insect’s frass and were observed germinating in their natural habitat. Seutsugu noted that all of the fruits in his studies consumed by camel crickets are white or translucent, easily accessible to ground dwelling insects, and give off a fermented scent to which insects like camel crickets are known to be attracted. Camel crickets also spend their time foraging in areas suitable for the growth of these plants. All of this suggests co-evolutionary adaptations that have led to camel cricket-mediated seed dispersal.

Yoania japonica via wikimedia commons

Insect endozoochory may be an uncommon phenomenon, but perhaps it’s not as rare as we once presumed. As mentioned above, an instance of endozoochory by a beetle has been reported, as has one by a species of cockroach. Certainly the most well known example involves the wetas of New Zealand, which are large, flightless insects in the same order as grasshoppers and crickets and sometimes referred to as “invertebrate mice.” New Zealand lacks native ground-dwelling mammals, and wetas appear to have taken on the seed dispersal role that such mammals often play.

Where seeds are small enough and seed coats tough enough, insects have the potential to be agents of seed dispersal via ingestion. Further investigation will reveal additional instances where this is the case. Of course, effective seed dispersal means seeds must ultimately find themselves in locations suitable for germination in numbers that maintain healthy populations, which for the dust seeds of parasitic plants is quite specific since they require a host organism to root into. Thus, effective seed dispersal in these scenarios is also worth a more detailed look.

Further Reading:

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For more stories of seed dispersal check out the first issue of my new zine, Dispersal Stories.

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Tomato vs. Dodder, or When Parasitic Plants Attack

At all points in their lives, plants are faced with a variety of potential attackers. Pathogenic organisms like fungi, bacteria, and viruses threaten to infect them with diseases. Herbivores from all walks of life swoop in to devour them. For this reason, plants have developed numerous mechanisms to defend themselves against threats both organismal and environmental. But what if the attacker is a fellow plant? Plants parasitizing other plants? It sounds egregious, but it’s a real thing. And since it’s been going on for thousands of years, certain plants have developed defenses against even this particular threat.

Species of parasitic plants number in the thousands, spanning more than 20 different plant families. One well known group of parasitic plants is in the genus Cuscuta, commonly known as dodder. There are about 200 species of dodder located throughout the world, with the largest concentrations found in tropical and subtropical areas. Dodders generally have thread-like, yellow to orange, leafless stems. They are almost entirely non-photosynthetic and rely on their host plants for water and nutrients. Their tiny seeds can lie dormant in the soil for a decade or more. After germination, dodders have only a few days to find host plants to wrap themselves around, after which their rudimentary roots wither up. Once they find suitable plants, dodders form adventitious roots with haustoria that grow into the stems of their host plants and facilitate uptake of water and nutrients from their vascular tissues.

A mass of dodder (Cuscuta sp.) - photo credit: wikimedia commons

A mass of dodder (Cuscuta sp.) – photo credit: wikimedia commons

Some plants are able to fend off dodder. One such instance is the cultivated tomato (Solanum lycopersicum) and its resistance to the dodder species, Cuscuta reflexa. Researchers in Germany were able to determine one of the mechanisms tomato plants use to deter dodder; their findings were published in a July 2016 issue of Science. The researchers hypothesized that S. lycopersicum was employing a similar tactic to that of a microbial invasion. That is, an immune response is triggered when a specialized protein known as a pattern recognition receptor (PRP) reacts with a molecule produced by the invader known as a microbe-associated molecular pattern (MAMP). A series of experiments led the researchers to determine that this was, in fact, the case.

The MAMP was given the name Cuscuta factor and was found “present in all parts of C. reflexa, including shoot tips, stems, haustoria, and, at lower levels, in flowers.” The PRP in the tomato plant, which was given the name Cuscuta receptor 1 (or CuRe 1), reacts with the Cuscuta factor, triggering a response that prohibits C. reflexa access to its vascular tissues. Starved for nutrients, the dodder perishes. When the gene that codes for CuRe 1 was inserted into the DNA of Solanum pennellii (a wild relative of the cultivated tomato) and Nicotiana benthamiana (a relative of tobacco and a species in the same family as tomato), these plants “exhibited increased resistance to C. reflexa infestation.” Because these transgenic lines did not exhibit full resitance to the dodder attack, the researchers concluded that “immunity against C. reflexa in tomato may be a process with layers additional to CuRe 1.”

photo credit: wikimedia commons

photo credit: wikimedia commons

A slew of crop plants are vulnerable to dodder and other parasitic plants, so determining the mechanisms behind resistance to parasitic plant attacks is important, especially since such infestations are so difficult to control, have the potential to cause great economic damage, and are also a means by which pathogens are spread. It is possible that equivalents to CuRe 1 exist in other plants that exhibit resistance to parasitic plants, along with other yet to be discovered mechanisms involved in such resistance, so further studies are necessary. Discoveries like this not only help us make improvements to the plants we depend on for food, but also give us a greater understanding about plant physiology, evolutionary ecology, and the remarkable ways that plants associate with one another.

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