Screening for Invasive Plants at Botanical Gardens and Arboreta

As discussed in last week’s post, many of the invasive species that we find in our natural areas were first introduced to North America via the horticulture trade. As awareness of this phenomenon grows, steps are being taken by the horticulture industry to address this issue. The concluding remarks by Sarah Reichard and Peter White in their 2001 article in BioScience describe some recommended actions. One of them involves the leadership role that botanical gardens can play by both stopping the introduction and spread of invasive species and by presenting or promoting public education programs.

Reichard and White offer North Carolina Botanical Garden as an example, citing their “Chapel Hill Challenge,” which urges botanical gardens to “do no harm to plant diversity and natural areas.” Reichard and White also encourage botanical gardens and nurseries to adopt a code of conservation ethics addressing invasive species and other conservation issues. Codes of conduct for invasive species have since been developed for the botanical garden community and are endorsed by the American Public Gardens Association.

 

Botanical gardens that adopt this code have a number of responsibilities, one of which is to “establish an invasive plant assessment procedure,” preferably one that predicts the risks of plant species that are new to the gardens. In other words, botanical gardens are encouraged to screen the plants that are currently in their collections, as well as plants that are being added, to determine whether these plants currently exhibit invasive behavior or have the potential to become invasive. Many botanical gardens now have such programs in place, and while they may not be able to predict all invasions, they are a step in the right direction.

In an article published in Weed Technology (2004), staff members at Chicago Botanic Garden (CBG) describe the process they went through to determine a screening process that would work for them. CBG has an active plant exploration program, collecting plants in Asia, Europe, and other parts of North America. Apart from adding plants to their collection, one of the goals of this program is to find plants with horticulture potential and, through their Ornamental Plant Development department, prepare these plants to be introduced to the nursery industry in the Chicago region. As their concern about invasive species has grown, CBG (guided by a robust Invasive Plant Policy) has expanded and strengthened its screening process.

In order to do this, CBG first evaluated three common weed risk assessment models. The models were modified slightly in order to adapt them to the Chicago region. Forty exotic species (20 known invasives and 20 known non-invasives) were selected for testing. Each invasive was matched with a noninvasive from the same genus, family, or growth form in order to “minimize ‘noise’ associated with phylogenetic differences.” The selected species also included an even distribution of forbs, vines, shrubs, and trees.

Weed risk assessment models are used to quickly determine the potential of a plant species to become invasive by asking a series of questions about the plant’s attributes and life history traits, as well as its native climate and geography. A plant species can be accepted, rejected, or require further evaluation depending on how the questions are answered. For example, if a plant is known to be invasive elsewhere and/or if it displays traits commonly found in other invasive species, it receives a high score and is either rejected or evaluated further. Such models offer a quick and affordable way to weed out incoming invasives; however, they are not likely to spot every potential invasive species, and they may also lead to the rejection of species that ultimately would not have become invasive.

After testing the three models, CBG settled on the IOWA-modified Reichard and Hamilton model “because it was extensively tested in a climatic zone reasonably analogous to … Illinois,” and because it is easy to use and limits the possibility of a plant being falsely accepted or rejected. The selected model was then tested on 208 plants that were collected in the Republic of Georgia. Because few details were known about some of the plants, many of the questions posed by the model could not be answered. This lead CBG to modify their model to allow for such plants to be grown out in quarantined garden plots. This way pertinent information can be gathered, such as “duration to maturity; self-compatibility; fruit type and potential methods of seed or fruit dispersal; seed production, viability, and longevity in the field; and vegetative spread.” CBG believes that evaluations such as this will help them modify their model over time and give them more confidence in their screening efforts.

More about botanical gardens and invasive species: Botanic Gardens Conservation International – Invasive Alien Species

More about weed risk assessment models: Weed Risk Assessment – A way forward or a waste of time? by Philip E. Hulme

Horticulture’s Role in the Spread of Invasive Plants

I live in the city of Boise – a bustling metropolis by Idaho’s standards. It is located in the high desert of the Intermountain Northwest in a region called the sagebrush steppe. Our summers are hot and dry, and our native flora reflects this.

When I leave my apartment I am greeted by a flowering quince (Chaenomeles sp.). At this time of year it is in full bloom and looking amazing. It originated in East Asia. To my left I see a tree of heaven (Ailanthus altissima), a common urban tree that came to America from China via Europe. To my right there is a row of Norway maples (Acer platanoides), another popular urban tree. As its common name suggests, it is a European species that is distributed across large portions of eastern and central Europe. None of these plants are native to the sagebrush steppe, nor would they survive the harsh conditions without supplemental irrigation. All are horticultural introductions.

Tree of Heaven (Ailanthus altissima) – photo credit: wikimedia commons

But there is another thing that at least two of these species have in common. Tree of heaven and Norway maple are considered invasive species in North America due to their propensity to spread into natural areas and disrupt native ecosystems. They also have a reputation of being pesky urban weeds.

My experience isn’t unique. Yards across North America are planted largely with species that are not native to this continent, and while most species stay where we plant them, a significant portion of them have leaped out of our tidy landscapes and disseminated themselves across natural areas, earning them the title invasive species.

In a paper published in BioScience (2001), Sarah Hayden Reichard and Peter White discuss the role that horticulture has played in introducing invasive species to the United States. Humans have a long history of moving plants from one part of the world to another for food, fuel, and fiber. However, collecting plants from around the world and organizing them into gardens for aesthetic purposes is, by comparison, a more recent thing. Species used for ornamental horticulture are what Reichard and White are concerned about.

As an introduction, Reichard and White offer a quick history of the beginnings of ornamental horticulture in the United States. This period is summed up well in an article by Richard Mack and Mark Lonsdale in the same issue of BioScience:

As colonists became more secure in their new environments, they began to import ornamental species from their homelands and elsewhere, in simultaneous quests for both familiar and unfamiliar plants. These plant importations sprang from deep-seated or primal aspects of human behavior shared by people in former colonies and homelands alike. … Many needed to be reassured with familiar plants from home, and they also had seemingly antithetical desires to experience novel, exotic ornamental plants.

Today, plant explorations continue throughout the world, often with the goal of introducing new plant species to the horticulture trade, and avid gardeners remain eager to find something new and interesting to add to their yards. There is nothing inherently wrong with this. Nor is there anything inherently wrong with filling our yards with exotic plants. The trouble comes when these plants escape cultivation and cause problems in neighboring ecosystems. Bringing awareness to this darker side of ornamental horticulture is what Reichard and White endeavor to do.

“Thomas Jefferson, an avid horticulturist, also introduced several species. He may have been the first person to introduce Cytisus scoparius (Scotch broom) as an ornamental species; that plant is now an invasive species in many parts of North America.” — Reichard and White (2001) [photo credit: www.eol.org]

Major players involved in the global movement of horticultural specimens include botanical gardens and arboreta, nurseries, garden clubs and horticultural societies, and the seed trade industry. The motives for transporting species vary among the groups, as do their roles in addressing the invasive species issue. Many botanical gardens have extensive plant exploration programs, which today are often more conservation focused than they were in the past; however, some of the species acquired during these explorations are released to the public, often without certainty that they won’t spread.

Even though most nurseries don’t have active plant exploration programs, they may acquire plants from nurseries or other institutions that do. For business reasons, plants may be sold before they have been properly screened for invasive-ness. Some retail nurseries make an effort to not sell plants that are known invasives in their regions. However, there are plenty of mail order nurseries that may not be aware of or may simply ignore the fact that they are shipping plants to regions where they are invasive. Seed exchanges between garden clubs and botanical societies, as well as the seed trade industry, are also responsible for shipping species to areas where they are currently or may become invasive.

“Uninformed people sometimes dump their aquarium water and plants into local water sources, and many of the aquarium plants survive and multiply. Hydrilla verticillata, a very aggressive aquatic weed in the South, was probably introduced to provide a domestic source of this plant for the aquarium trade.” — Reichard and White (2001) [photo credit: wikimedia commons]

Plant exploration will continue, and many new plants will be introduced to the public through the horticulture trade. Rules and regulations help restrict some plant movement, but in a capitalist society such restrictions will ultimately be, as Reichard and White write, “a compromise between ideal invasive plant exclusion and trade facilitation.” Plants can be screened for invasivibility, but it is difficult to know if, when, and where a species may become invasive. Furthermore, given enough time, a species that appeared to stay put can suddenly start to spread (or could have been spreading all along unnoticed).

Reichard and White acknowledge that “the burden of finding a solution to the problems posed by invasive plants does not necessarily fall on the shoulders of [the horticulture] industry.” Various groups from broad disciplines will have to come to together to work towards a solution. Reichard and White offer some suggestions for working together. For example, invasive species biologists can share their research with the horticulture industry which can, in turn, communicate this information to the public through garden writers and speakers. Botanical gardens can take a leadership role by vowing to “first do no harm to plant diversity and natural areas” and by providing public education about the issue.

Efforts can be made to ban the sale of problematic plants and to encourage proper screening of new introductions, but public demand for certain plants may remain. So, “better communication from ecologists to the public about which species are causing problems will discourage people from buying them.” Involving the public in eradication efforts can also help raise awareness, as people can see first hand that plants in their yards have invaded the wild.

Poisonous Plants: Yews

Wildfires last summer followed by a particularly harsh winter has driven herds of elk, deer, antelope, and other ungulates closer to urban and suburban areas in southern Idaho. This has resulted in several of the animals making a meal out of a particularly poisonous plant and then promptly dying. The plant is a yew, an ornamental shrub or tree that is commonly used in residential and commercial landscapes. Seven elk died after eating Japanese yew in the Boise Foothills. Fifty pronghorn antelope died after eating the same plant species in the small city of Payette. Eight more elk were found dead in North Fork and Challis, poisoned by yew; eight others were found dead outside of Idaho Falls having suffered a similar fate. And this is just a sampling. Needless to say, such tragedies have spawned a greater awareness of this and other deadly poisonous plants – plants that were purposely planted in our yards, thought benign, but lying in wait to kill.

Japanese yew (Taxus cuspidata) - photo credit: wikimedia commons

Japanese yew (Taxus cuspidata) – photo credit: wikimedia commons

Yews, plants in the genus Taxus, are in the family Taxaceae, a coniferous family that consists of around 5-7 genera and up to 30 species (sources vary). Taxus is one of the largest genera in the family with between 9 and 11 species. The genus occurs across three continents, with at least four species naturally occurring in North America (T. canadensis, T. brevifolia, T. globosa, and T. floridana). The species most commonly grown as ornamentals include Japanese yew (T. cuspidata), English yew (T. baccata), and a hybrid of the two (T. x media).

Generally speaking, yews are evergreen shrubs or trees with inch long, dark green needles that come to a sharp point. Branches are alternately arranged and the bark is scaly and reddish-brown. As trees they can reach heights of more than 60 feet, but in a garden setting the plants are usually hedged into more managable-sized shrubs. Taxus species are dioecious, which means that individuals are either male or female. The females produce fleshy, round, cup-shaped fruits that are pink, red, or green. This structure is called an aril and is produced by the swelling of the stem around a single seed. All parts of the plant are poisonous, with only one exception – the aril. This is problematic because the bright-colored aril can appear quite appetizing. And it is edible; however, when the seed is consumed along with it, the plant’s poison makes its way into the body.

The fruits of yew (Taxus sp.)

The fruits of yew (Taxus sp.)

Yew poisoning is unfun. Death can occur in a matter of a few hours, depending on the parts of the plant and amount consumed. The North American Guide to Common Poisonous Plants and Mushrooms lists these symptoms: “nausea, dry throat, severe vomiting, diarrhea, rash, pallor, drowsiness, abdominal pain, dizziness, trembling, stiffness, fever, and sometimes allergy symptoms.” Symptoms of severe poisoning include, “acute abdominal pain, irregular heartbeat, dilated pupils, collapse, coma, and convulsions, followed by a slow pulse and weak breathing.” The cause of death is respiratory and heart failure.

Yews contain a number of toxic compounds, including volatile oils and a cyanogenic glycoside. The compound responsible for yew’s high toxicity is taxine, a potent cardiotoxin and, as it turns out, an effective drug against certain types of cancer. Very small doses of this poison can be deadly. One or two yew seeds can kill a small child, and a handful or two of the needles can kill an animal, depending on its size. Even dried branches and leaves remain toxic, so wreaths made with yew should be disposed of in a landfill rather than tossed into a yard or field where domestic animals and livestock can find them. Yew consumption should be promptly addressed by visiting an emergency room or calling the Poison Control Center.

Yew’s deadly reputation is not something to take lightly. They are a popular ornamental because of their attractive fruits and evergreen foliage, their tolerance of shade, and their low maintenance requirements, but homeowners with children, pets, or proximity to horses, cows, or wild animals should consider removing them. If a decision is made to keep them, the shrubs can be wrapped in burlap during the winter to prevent hungry animals from coming in for a bite, particularly on properties that are adjacent to natural areas.

For more information about yew identification and removal, check out this article in the Idaho Statesman. Also, consider this wise counsel by Amy Stewart from her book, Wicked Plants:

Do not experiment with unfamiliar plants or take a plant’s power lightly. Wear gloves in the garden; think twice before swallowing a berry on a trail or throwing a root into the stew pot. If you have small children, teach them not to put plants in their mouths. If you have pets, remove the temptation of poisonous plants from their environment. The nursery industry is woefully lax about identifying poisonous plants; let your garden center know that you’d like to see sensible, accurate labeling of plants that could harm you. Use reliable sources to identify poisonous, medicinal, and edible plants.

More Poisonous Plant Posts on Awkward Botany:

Eating the Invasives

Happy National Invasive Species Awareness Week! It’s a fine time to get educated about invasive species, and perhaps even play a role in mitigating them. Opportunities for getting involved are myriad and include volunteering with local conservation groups, replacing invasive plants in your yard with non-invasive alternatives, and being mindful when you visit natural areas not to bring along weed seeds and other pests and diseases. Another strategy in the battle against invasive species is to eat them, which is precisely what I plan on doing. If you are interested in doing the same, this revised post (originally published in November 2013) will help get you started.

Invasivore: One Who Consumes Invasive Species

Invasive species are a major ecological concern, and considerable effort is spent controlling them. The ultimate goal  – albeit a lofty one in many cases – is to eradicate them and to prevent future outbreaks. The term “invasive species” describes plants, animals, and microorganisms that have been intentionally or unintentionally introduced into an environment outside of their native range. They are “invasive” because they have established themselves and are causing adverse effects in their non-native habitats. Some introduced species cause no discernible adverse effects and so are not considered invasive. Species that are native to a specific habitat and exhibit adverse effects following a disturbance can also be considered invasive. (White-tailed deer are an example of this in areas where human activity and development have reduced or eliminated their natural predators resulting in considerably larger deer populations than would otherwise be expected.) Defining and describing invasive species is a challenging task, and so it will continue to be a topic of debate among ecologists and conservation biologists for the foreseeable future.

The adverse effects of invasive species are also not always straightforward. Typical examples include outcompeting native flora and fauna, disrupting nutrient cycles, shifting the functions of ecosystems, altering fire regimes, and causing genetic pollution. Countless hours of research and observation are required in order to determine the real effects of invaders. The cases are too numerous and the details are too extensive to explore in this post; however, I’m sure I will cover this topic more thoroughly in the future.

There are many approaches to eradicating invasive species, but one fairly unconventional method is to simply eat them. Why not, right? Historically, the voracious appetite of humans has helped drive several species to extinction, so why not employ our stomachs in the removal of introduced species from their non-native habitats? The folks at Invasivore are suggesting just that. By encouraging people to consume invasive species, they are also promoting awareness about them – an awareness they hope “will lead to decreasing the impacts of invasive species by preventing introductions, reducing spread, and encouraging informed management policies.”

“If you can’t beat ’em, eat ’em!” And so they provide recipes in order to encourage people to harvest, prepare, and consume the invasive species in their areas. Some of the invasive plant species they recommend eating are Autumn Olive (Autumn Olive Jam), garlic mustard (Garlic Mustard Ice Cream), Japanese honeysuckle (Honeysuckle Simple Syrup), purslane (Purslane Relish), and Canada goldenrod (Strawberry-Goldenrod Pesto). And that’s just a sampling. One might ask if we are encouraged to eat invasive species and ultimately find them palatable, won’t our demand result in the increased production of these species? The Invasivores have considered this, and that is why their ultimate goal is raising awareness about the deleterious effects of invasive species. In the end, we should expect to see our native habitats restored. Our craving for Burdock Chips on the other hand will have to be satisfied by some other means.

lonicera japonica

Japanese honeysuckle (Lonicera japonica) – photo credit: wikimedia commons

More about eating invasive species:

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What are you doing to celebrate National Invasive Species Awareness Week? Let us know in the comment section below.

Ethnobotany: The Henna Tree

A hair dye used in pre-dynastic Egypt is still used today. This enduring plant-based dye has found its way into a great number of cultures going back as long as 6000 years. Its popularity is thanks in part to the broad distribution of the plant itself, but is largely a result of the diverse religious traditions that have incorporated the dye into their rituals. The plant’s use in such traditions continues, while its current popularity extends well beyond that.

Lawsonia inermis is the only species in its genus. It is a member of the plant family, Lythraceae, a family that includes crepe myrtles (Lagerstroemia), cigar plants (Cuphea), and pomegranates (Punica). L. inermis has many common names, including mignonette tree and Egyptian privet. It is most commonly known as, henna, a term that refers to the plant itself, the dye derived from the plant, and the body art made using the dye.

Henna is a shrub or small tree that reaches a height of about 6 meters. The leaves are smooth, elliptically-shaped, and oppositely-arranged on branches that are spine-tipped. Inflorescences are many-branched with numerous small, fragrant flowers. The most prominent features of the flowers are four sepals forming a bowl shape and several white to red stamens reaching towards the sky. The fruits are small, round, brown capsules full of tiny seeds. Henna thrives in dry environments with poor soil; however, it does not tolerate frost. It occurs in tropical and subtropical regions of Africa, western and southern Asia, and northern Australasia. Cultivation by humans has broadened its distribution well beyond its original boundaries.

Lawsonia inermis - photo credit: wikimedia commons

Lawsonia inermis – photo credit: wikimedia commons

Henna has been used to dye the skin, nails, and hair of women and men in many cultures and religions across its area of natural distribution and beyond. Its use has been especially common among women as part of fertility and marriage celebrations. The plant’s dye may have been first discovered around the mouths of browsing livestock – the persistent red-orange color having the appearance of blood. Henna plants are drought-deciduous, but they burst back to life when rain returns, producing abundant new branches, leaves, and flowers. This period of growth coincides with celebrations of marriage and fertility and may explain why it found common use in such traditions.

Dyes are made by crushing dried leaves into a fine powder and then mixing it into a paste using water, lemon juice, tea, or other liquids. A soap or shampoo is produced when henna is mixed with plant extracts containing saponin, and the addition of certain essential oils can enhance the performance of the dye. The compound in the leaves that produces the red-orange dye is called lawsone and is found in varying concentrations depending on the conditions in which the plant was grown. High heat and low soil moisture is said to produce the highest levels of lawsone. More than just a dye, lawsone also has antifungal properties and strongly absorbs UV light, thus its application is beyond cosmetic as it has proven useful against fungal diseases like athlete’s foot and as a sunscreen. And that’s just the beginning.

Henna applied to hair - photo credit: wikimedia commons

Henna applied to hair – photo credit: wikimedia commons

A study published in the Journal of Ethnopharmacology by Semwal, et al. reveals that nearly a hundred phytoconstituents (or “biologically active compounds”) have been isolated from all parts of the henna plant. Henna has long been used medicinally to treat a wide range of ailments, and while it may not be an effective treatment for all that it has been historically used for, it has been found effective for certain things and has great potential for further use.

In the paper, the authors review dozens of studies exploring the many “biological activities” that henna is claimed to have, which include “antifungal, antibacterial, virucidal, antiparasitic, anti-inflammatory, analgesic, and anticancer properties,” etc. Research into these properties is limited and has been “complicated and hampered” by the widespread practice of adding other ingredients (some of them harmful) to henna products. In order for henna’s “therapeutic potential” to be properly explored, the authors advise identifying and standardizing the plant’s active components.

Henna continues to be used in cultures across the world and is particularly prominent in Hinduism and Muslim practices. It is most commonly used to dye hair and create temporary body art (also known as mehndi). Henna art is often applied to the hands and feet, where the skin is thick and absorbs more of the lawsone. It is applied as a paste and either squeezed through a plastic cone or syringe or painted on with a stick or brush. The longer the paste is left on, the darker the stain will be. After a week or so the henna begins to fade as old skin cells slough off.

photo credit: wikimedia commons

photo credit: wikimedia commons

In ancient cultures, henna was thought to ward off the Evil Eye as well as bring good luck and blessings, a trait known as baraka. This belief is part of the reason why henna was incorporated into marriage ceremonies and other religious rituals. Because of henna’s antifungal, analgesic, and anti-inflammatory properties, etc., real benefits are seen when henna is applied to various parts of the body. Semwal, et al. argue that a scientific understanding was not necessary for “recognition of benefit.” Today however, “scientific investigation and quantification of henna’s ‘baraka’ should expand and optimize these traditional qualitative understandings.”

Because of henna’s widespread use and long history, it is not feasible to fit henna’s entire story into a single blog post. Henna is worth exploring on your own. Here are a few more interesting tidbits for now. In Semwal, et al.’s summary there is a mention of henna twigs being “rubbed over the teeth for effective dental self-care” – something to keep in mind in case you find yourself without a toothbrush, and a henna plant happens to be nearby. A paper published in a 1993 issue of Thaiszia – Journal of Botany discusses the historical use of henna in the Balkans. Slavs in the area reportedly treated typhoid fever using a mixture of henna and “the juice of twenty heads of garlic” heated in water. Finally, henna has been used to dye many things, including leather, cloth, and animal fur. Persians have long used henna to dye the manes, tales, and hooves of their horses and other animals. A practice that continues today.

More Ethnobotany Posts:

What Is a Plant, and Why Should I Care? part four

What Is a Plant?

Part one and two of this series have hopefully answered that.

Why should you care?

Part three offered a pretty convincing answer: “if it wasn’t for [plants], there wouldn’t be much life on this planet to speak of.”

Plants are at the bottom of the food chain and are a principle component of most habitats. They play major roles in nutrient cycling, soil formation, the water cycle, air and water quality, and climate and weather patterns. The examples used in part three of this series to explain the diverse ways that plants provide habitat and food for other organisms apply to humans as well. However, humans have found numerous other uses for plants that are mostly unique to our species – some of which will be discussed here.

But first, some additional thoughts on photosynthesis. Plants photosynthesize thanks to the work accomplished by very early photoautotrophic bacteria that were confined to aquatic environments. These bacteria developed the metabolic processes and cellular components that were later co-opted (via symbiogensis) by early plants. Plants later colonized land, bringing with them the phenomena of photosynthesis and transforming life on earth as we know it. Single-celled organisms started this whole thing, and they continue to rule. That’s just something to keep in mind, since our focus tends to be on large, multi-cellular beings, overlooking all the tiny, less visible beings at work all around us making life possible.

Current representation of the tree of life. Microorganisms clearly dominate. (image credit: nature microbiology)

Current representation of the tree of life. Microorganisms clearly dominate. (image credit: nature microbiology)

Food is likely the first thing that comes to mind when considering what use plants are to humans. The domestication of plants and the development of agriculture are easily among the most important events in human history. Agricultural innovations continue today and are necessary in order to both feed a growing population and reduce our environmental impact. This is why efforts to discover and conserve crop wild relatives are so essential.

Plants don’t just feed us though. They house us, clothe us, medicate us, transport us, supply us, teach us, inspire us, and entertain us. Enumerating the untold ways that plants factor in to our daily lives is a monumental task. Rather than tackling that task here, I’ll suggest a few starting points: this Wikipedia page, this BGCI article, this Encylopedia of Life article, and this book by Anna Lewington. Learning about the countless uses humans have found for plants over millennia should inspire admiration for these green organisms. If that admiration leads to conservation, all the better. After all, if the plants go, so do we.

Humans have a long tradition of using plants as medicine. Despite all that we have discovered regarding the medicinal properties of plants, there remains much to be discovered. This one of the many reasons why plant conservation is so important. (photo credit: wikimedia commons)

Humans have a long tradition of using plants as medicine. Despite all that we have discovered regarding the medicinal properties of plants, there remains much to be discovered. This is one of the many reasons why plant conservation is imperative. (photo credit: wikimedia commons)

Gaining an appreciation for the things that plants do for us is increasingly important as our species becomes more urban. Our dense populations tend to push plants and other organisms out, yet we still rely on their “services” for survival. Many of the functions that plants serve out in the wild can be beneficial when incorporated into urban environments. Plants improve air quality, reduce noise pollution, mitigate urban heat islands, help manage storm water runoff, create habitat for urban wildlife, act as a windbreak, reduce soil erosion, and help save energy spent on cooling and heating. Taking advantage of these “ecosystem services” can help our cities become more liveable and sustainable. As the environmental, social, and economic benefits of “urban greening” are better understood, groups like San Francisco’s Friends of the Urban Forest are convening to help cities across the world go green.

The importance of plants as food, medicine, fuel, fiber, housing, habitat, and other resources is clear. Less obvious is the importance of plants in our psychological well being. Numerous studies have demonstrated that simply having plants nearby can offer benefits to one’s mental and physical health. Yet, urbanization and advancements in technology have resulted in humans spending more and more time indoors and living largely sedentary lives. Because of this shift, author Richard Louv and others warn about nature deficit disorder, a term not recognized as an actual condition by the medical community but meant to describe our disconnect with the natural world. A recent article in BBC News adds “nature knowledge deficit” to these warnings – collectively our knowledge about nature is slipping away because we don’t spend enough time in it.

The mounting evidence for the benefits of having nature nearby should be enough for us to want to protect it. However, recognizing that we are a part of that nature rather than apart from it should also be emphasized. The process that plants went through over hundreds of millions of years to move from water to land and then to become what they are today is parallel with the process that we went through. At no point in time did we become separate from this process. We are as natural as the plants. We may need them a bit more than they need us, but we are all part of a bigger picture. Perhaps coming to grips with this reality can help us develop greater compassion for ourselves as well as for the living world around us.

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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