Fungus, parasites, and witches, oh my!

Figure 1: Festuca grass. Sometimes livestock that ingest Festuca or certain other grasses can cause them to have symptoms akin to extreme drunkenness: stumbling, disorientation, and paralysis.

Imagine a an old wild west town having its bank robbed. Two outlaws are running out of the bank, firing pistols into the morning sky and jumping on their horses while townsfolk are running to get the sheriff. A cross country pursuit begins: horses racing across the grassy prairie as the outlaws try to reach their hideout. At a river crossing one outlaw ties up his horse while helping the other cross. Later, his horse is stumbling and can't walk straight, finally, it lays down, hind legs paralyzed! The outlaws continue on one horse, but with two riders it moves more slowly and they are caught by the sheriff.

Ingesting festuca and other grasses can cause toxicosis - Unknown to the outlaws in the story above, while they crossed the river the waiting horse munched on a species of grass in the Festuca genus (Fig. 1). These grasses are well-known for causing toxicosis in grazing animals, leading to shaking or trembling, partial paralysis, and in some cases death. Today: the chemistry of these toxins.

Figure 2: Fungus produces ergot alkaloids. Plant scientists and mycologists have discovered that the drunkenness effects are due to toxins produced produced not by the plant, but by a fungus that lives inside some plants. Left: isolated fungus growing in a petri dish (top), microscope images of isolated endophytic fungus (bottom). Photo from Cabral et al. [1] Right: the basic chemical structure of ergot alkaloids.

Fungal endophytes produce toxins - Researchers tried to grow some toxicosis-causing plants in sterile, laboratory conditions to study their properties, but were surprised when, in addition to a plant growing, a large amount of fungus also grew along with the plant! They were able to isolate the fungus (Fig. 2, left), and after some tests, it became clear that it is not actually the plants that produce the toxins, but the fungus that lives inside the plant. In exchange for poisoning any attacking herbivore, the plant allows the fungus to live inside its walls, making the fungus an endophyte (endo=inside, phyte=grower). The fungus obliges by producing ergot toxins (Fig. 2, right), complicated molecules that belong to the class of chemicals called alkaloids (alkaloids contain at least one nitrogen atom, for example, caffeine and nicotine).

Colonies of fungal endophytes are transferred to plant offspring - In a another amazing twist, scientists found that a small amount of the fungus is stored inside the plant's seeds so that when new plants start growing, they already have a fungus colony inside them, ready to defend the plant as soon as the seed germinates. Overall, plants that host the fungus have an advantage in grazing fields. This advantage is so substantial that parasitic plants growing on roots of Festuca have evolved to steal some of the ergot toxins to protect themselves from herbivores as well.

Figure 3: Rye with visible endophyte fungus. Some crops can also host fungal endophytes, which can cause terrible symptoms in people who ingest them.

Ergot alkaloids can also affect humans - In addition to affecting grazing animals, the ergot alkaloids produced by fungal endophytes can also affect humans. If a human has a long-term diet that consists mainly of a grain containing a toxin-producing endophyte, alarming symptoms such as convulsions, itching, psychosis, and peeling skin can appear. Some scientists and historians have argued that ergot alkaloids were one cause of the Salem witch trials! The symptoms caused by ergot poisoning match those reported at the trials, and the townspeople ate lots of rye grown in a climatic region that would support the fungus (Fig. 3). Still, this idea is somewhat debated because several historians think Salem residents would have known of and recognized ergot poisoning.

Fungal endophytes in biotechnology - Whether or not fungal toxins played a role in the Salem witch trials, they will almost certainly play a role in the future of agriculture. Scientists are working on better understanding the nature of the relationship between the fungus and its host. So far they have found that, for example, fungal endophytes assist the plant in acquiring nitrogen from the soil [2], and that they help plants tolerate very hot environments [3]. These results suggest that we could increase the heat tolerance of our crops and decrease their need for fertilizer if we could introduce them to a fungal endophyte that the worked together with the plant in nitrogen acquisition and heat tolerance, but didn't produce toxins. This would decrease the environmental impacts of our agricultural system (fewer fertilizer applications) and make it more secure (less susceptible to extreme heat). Go crop science!

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[1] Cabral, Daniel, et al. "Evidence supporting the occurrence of a new species of endophyte in some South American grasses." Mycologia (1999): 315-325.
[2] Behie, S. W., P. M. Zelisko, and M. J. Bidochka. "Endophytic insect-parasitic fungi translocate nitrogen directly from insects to plants." Science 336.6088 (2012): 1576-1577.
[3] Redman, Regina S., et al. "Thermotolerance generated by plant/fungal symbiosis." Science 298.5598 (2002): 1581-1581.

Plant pigments of all colors

I love seeing all the plants getting green and filled out in the spring. Suddenly the landscape goes from bare and brown to many shades of green. This got me thinking: I know that leaves are green because of their photosynthetic pigments, but what about red and brown algae in ponds? If they're photosynthetic too, why aren't they green? So, today, a look at plant light-harvesting pigments - of all colors! - and what they're good for in a plant's life and human use.

Figure 1: Light that can penetrate Earth's atmosphere. Visible light can make it through the atmosphere (so it moves further down the y-axis in the diagram) while UV and infrared light is blocked (does not move far down the y-axis).

Visible light makes it through the atmosphere to create color - The sun emits many forms of energy, including UV, visible, and infrared light. Earth's atmosphere blocks much of the UV and infrared light, leaving visible light to reach the surface (Figure 1). Remember ROY G BIV? These are the components (red, orange, yellow, green, blue indigo-formerly-and violet) of visible light. The way surfaces absorb and reflect these different types of light creates their color.

Leaves harvest light with pigments - Plants are masters at capturing visible light and transforming it into sugar - a source of chemical energy that feeds all the animals on the planet, including humans. But leafy plants really only absorb the red/orange/yellow and blue/purple light for sugar production - the green light is reflected away, which makes most leaves appear green. To harvest light, plants use specialized chemicals called pigments. Each pigment can only capture a certain color of light and transfer the light's energy into the plant's sugar-making processes. This means that the plant has to make a few different types of pigments if it wants to harvest energy from as many colors of light as possible. Most land plants make chlorophyll A, which collects purple and orange/red light, and chlorophyll B, which harvests blue and yellow light.

Figure 2: Plant pigments that harvest green light. Fucoxanthin and phycobilins are two examples of light harvesting chemicals that water plants make to harvest the green light that they have access to.

Not all plants use chlorophyll - Chlorophyll makes most land plants green - but what about water plants? Could water, like the atmosphere, also filter out some light, leaving even less for water plants to feed on? Yes! Water absorbs lots of red/orange/yellow and purple/blue light, allowing mainly green light through (Figure 2). Since the chlorophyll that land plants use isn't very good for harvesting this green light, water plants make light-collecting pigments called fucoxanthin or phycobilins instead. These specialize in collecting green light, leaving the small amount of red and blue/purple light that does make it through the water to be reflected away from the water plant, resulting in its brown/red appearance. Nifty!

Fucoxanthin is bioactive! - As with many plant chemicals, fucoxanthin (the brown algae pigment) has beneficial effects on the human body. There is some evidence that this chemical promotes fat burning in fatty tissues, that it is an antioxidant, and a cancer-killing compound. This has led to its sale as a dietary supplement.

The next time you are admiring plants in the park, or swimming with aquatic plants, perhaps you will think about the wonderful variety of plant pigments and how they create the vibrant colors we see.

Plant leaves - hijacked as insect and bacterial nurseries

Figure 1: Large, red growths on tree leaves. What are they?

Several months ago we were hiking in Washington state and saw some bizarre red growths on the leaves of many of the trees in the area (Fig. 1). We hadn't seen anything like these gross bulbous growths before. What are they? Cancer? Blisters? Aliens?!? Let's explore the answer to this question.

Burls and cankers are two types of plant outgrowths - Burls, bark-covered growths that look like warts, often form around wounds or insect infestations. Burls most frequently occur underground on the roots of the plant, but also occur above ground (Figure 2A). In these structures, the cell pattern that makes up the grain of the wood is chaotic - forming whirls and other complex designs that are very beautiful (Figure 2B). For this reason, wood from burls is highly prized, and has unfortunately led to burl poaching in some U.S. national parks [1]. Cankers, a broad class of plant diseases caused by microorganisms and viruses, can also cause visual physical deformities on the plant, which can vary from discolored bark to bulbous growths. Canker-causers are usually species-specific, meaning that each disease spreader affects only one particular species. Cankers vary greatly in the amount of harm they cause to the plant: some are not very harmful, and some are deadly.

Galls are outgrowths induced by parasites - Galls, unlike the large masses of disorganized cells characteristic of the burls and cankers, are highly organized abnormal growths that are induced by a parasite, often a fungus, bacteria, insect, or mite. These parasites inject specialized chemicals into regions of the plant that are undergoing rapid growth. The rapidly growing cells then quickly develop into structures that shelter and feed the parasite!

Figure 2: Burl outgrowths. One type of outgrowth plants can develop is called a burl. These form around wounds and infestations and have a wart-like appearance (A). Inside, burls have beautiful grain patterns that make them prized for woodworking (B).

Gall-inducers can modify plant defense chemistry - In some cases, the gall-inducing organisms are able to modify the plant in amazing ways. As an example, let's look at the chestnut oak, Quercus prinus, and the gall wasp, Andricus petiolicolus. When the wasp lays its eggs on the oak leaves, leaf-altering substances from the wasp are injected into the leaf, causing the leaf to grow a protective casing (a gall) around the eggs, which protects them from the elements as they develop into larvae and begin to consume the leaf for sustenance. These galls look similar to those we observed in Washington (Figure 1). Usually when something eats the leaves of the oak tree, the tree quickly fills its leaves with polyphenols - distasteful and toxic chemicals designed to repel the predator. Amazingly, when the wasp reprograms the leaf to form the gall around its young, it also modifies the polyphenol defense program. Instead of being present throughout the leaf, the polyphenols leave the area of the leaf inside the gall, leaving behind a tender, nutritious tissue for the larvae to eat [2]. The polyphenols instead congregate in the shell of the gall, protecting the larvae from insects that may otherwise eat the leaf, gall and all. Devious!

Agrobacterium modifies plants by injecting DNA - Another amazing gall-inducing organism is Agrobacterium. This bacteria senses chemicals that are unique to plants and moves towards them. When the bacteria get on the surface of the plant, usually underground, they synthesize small fibers, anchor themselves to the plant, and form a small colony. Then the bacterial cells synthesize specialized tunnels between themselves and the adjacent plant cells. Through these tunnels the bacteria pump small pieces of their DNA into the plant cells. The injected DNA fragments, once inside the plant, make the plant synthesize a large gall to protect the bacterial colonies. The bacterial DNA also forces the plant to combine some of its most crucial metabolic resources, amino acids and keto acids, to form new molecules called opines, a special food that only the Agrobacterium can eat [3].

Figure 3: Galls induced by Agrobacterium. Agrobacterium infect the roots of many different plants and inject bacterial DNA into the root cells. This DNA causes the plant to produce these large shelters (galls) for the bacteria (A). The plant is also forced to use its own keto acids and amino acids to produce special food for the bacteria called opine (B).

More than just Agrobacteria infect others with their DNA - DNA is a tightly controlled and protected component of living cells - it contains instructions that influence each and every process that makes the cell work. For this reason scientists were astonished to find that Agrobactiera play fast and loose with this important biomolecule: not only do they infect others with fragments of their DNA, but Agrobacteria can also transfer DNA directly to one another and take in pieces of DNA lying around in the soil [4]. The ability of these bacteria to transfer and uptake DNA has made them a major subject of scientific investigation, the results of which actually suggest that there are many species of bacteria that inject small pieces of DNA into other organisms - apparently this process is more common than our intuition tells us!

Nature's genetic engineers - Due to the shocking effects that gall-inducing insects have on the internal chemistry of plants, they have been studied in substantial detail, resulting in a remarkable discovery: bacteria that transfer DNA between organisms, and the use of such transfers to manipulate the internal chemistry of plants. I'm not sure about you, but the next time I see little bumps on the leaves of oaks near my house, I won't be so quick to move on to my next thought... inside are nature's own genetic engineers, hijacking plant leaves to create nurseries for their young.

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[1] National Park Service: https://www.nps.gov/redw/learn/news/arrest-made-in-burl-poaching-case.htm
[2] Allison, Steven D., and Jack C. Schultz. "Biochemical responses of chestnut oak to a galling cynipid." Journal of chemical ecology 31.1 (2005): 151-166.
[3] Zupan, John, et al. "The transfer of DNA from Agrobacterium tumefaciens into plants: a feast of fundamental insights." The Plant Journal 23.1 (2000): 11-28.
[4] Demanèche, Sandrine, et al. "Natural transformation of Pseudomonas fluorescens and Agrobacterium tumefaciens in soil." Applied and environmental microbiology 67.6 (2001): 2617-2621.

Why are roses red and violets blue?

What do flower colors mean in plant language? - Around this time each year, thousands of people will ask Google the meaning of the different rose colors. Red is romance, yellow is friendship, lavender enchantment, and white purity... among many others. But in my opinion, the real question is: what is the meaning of these colors in their native tongue?! What do they mean in plant language? Today, Plants are Chemists explores the answers to these questions.

Figure 1: Three major classes of plant flower pigment molecules. Different plants synthesize different molecules to color their petals. The three major classes are carotenoids, which generally make yellow, orange, and red colors, flavonoids, which can make yellow and red but also purple and bluish colors, and finally betalains, which usually make purple colors. For the most part, plants accumulate these molecules in their flower petals to attract pollinators, or in their fruits to attract mammals or birds.

Flowers are colored because of pigment molecules they create - Just like human skin, the color of flower "skin" comes from pigment molecules present in the epidermis. While you have probably heard of the light-harvesting plant pigment that makes leaves green (chlorophyll), flower color pigments are different, belonging to groups called carotenoids, flavonoids, and betalains. Members of each of these families can give rise to a great diversity of colors (Figure 1).

Plants use flower pigments to attract pollinators - Plants cannot get up and walk around to find mates like mammals, nor are all plants adept at spreading their seeds far and wide on their own. Many plants, including melons, tomatoes, berries, and peppers, rely on insects, animals, or birds to spread their pollen or seeds. To attract these bugs and beasts, plants develop brightly colored flowers and berries. These large, colorful displays proclaim "Here is your nectar reward! Right this way to a nectar reward in exchange for pollination!" or, "Here are the berries! Eat them and disperse my seeds!"

Figure 2: The color-changing flowers of Weigela coraeensis. New flowers of the common garden shrubbery W. coraeensis are white and full of nectar, but after pollination they change their color to pink. Ecologists and botanists have observed that W. coraeensis achieves very high rates of pollination (almost 100% of its flowers are pollinated), while the flowers of its sister species that do not change color only have a 25% pollination rate, perhaps because pollinators less efficiently find unpollinated flowers on the sister species. This has led to the idea that W. coraeensis is deliberately changing flower color to increase pollination efficiency.

Some plants use flower color to speak to insects - Some plant species give flower color additional meaning. For example, prior to pollination, the flowers of the common garden shrubbery Weigela coraeensis are white, but after pollination their color changes to red/purple, and the flowers stop producing nectar [1]. Furthermore, when compared to its sister species Weigela hortensis that does not change flower color, W. coraeensis flowers achieve near 95% pollination, while W. hortensis only around 25% [2]. These observations suggest that W. hortensis uses flower color change as a cue to insects that red/purple flowers do not contain nectar (and that they have already been pollinated) and that the insect should visit the white flowers instead - leading to fewer repeat visits to a single flower and thus leading to a higher percentage of the flowers being pollinated! Cool! It is not clear if white to red/purple color changes are common color cues used by many plant species to alert insects. Seeing as there are some species whose flowers are always red or purple, it is certainly not a universal characteristic among plants. As research in this area continues, it will be interesting to see if other color change patters are discovered that have other meanings or if there are other types of specific cues flower colors give to insects, birds, or animals.

Flower color and humans - If you see flowers today, pause and think about how successful flowering plants have been not only in attracting and signalling insects and birds with their bright colors, but also in attracting human attention. Because of their color chemistry, roses have become a highly successful species via their relationship with humans. They get whole gardens dedicated to them, and their color diversity plays a role in our language. Happy Valentine's Day!

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[1] Suzuki, Miki F., and Kazuharu Ohashi. "How does a floral colour‐changing species differ from its non‐colour‐changing congener?–a comparison of trait combinations and their effects on pollination." Functional ecology 28.3 (2014): 549-560.
[2] Ruxton, Graeme D., and H. Martin Schaefer. "Floral colour change as a potential signal to pollinators." Current Opinion in Plant Biology 32 (2016): 96-100.

Preserving endangered trees, one phytochemical at a time.

Valuable phytochemicals give rise to plant poaching - Phytochemicals, chemicals produced by plants, have important roles in our economy, human health, and large scale ecosystems. Previous posts on this blog contain numerous interesting and hopefully entertaining examples. Over 30,000 species are used by humans for all sorts of things from fuels, to textiles, to food [1]. Lots of these plants have high monetary value, especially those that produce hard-to-find medicines or particularly complex molecules. Since they are worth quite a bit, such species are harvested from the wild, sometimes on large scales. Harvesting phytochemicals from wild plants that are extremely widespread or fast growing is often not a problem. However, harvest is a problem for species that are slow growing, or only grow in certain regions. Tree species are especially susceptible since they are slow growing and are often harvested faster than they can regenerate. As world population increases, how can we reconcile our increasing need for valuable phytochemicals with dwindling forest sizes?

Figure 1: Sandalwood scent molecules. Sandalwood, a popular incense and essential oil, comes from an endangered tree. Four chemical compounds, shown in white, comprise the sandalwood aroma. Genes encoding the molecular machinery that create these molecules were identified in Santalum album and have been transferred to yeast. Now it is possible to obtain these scent molecules without harming tree populations.

Valuable plant-derived scent molecules can be produced industrially after plant genome analysis - Fortunately, plant scientists and engineers are working together to solve this problem. For example, many native populations of the sandalwood tree, a popular source of scent molecules, are becoming endangered in certain areas due to over-harvesting [2]. In response, researchers at the University of British Columbia performed detailed analyses of sandalwood extract and found a family of molecules called santalols (lol) that are responsible for the sandalwood fragrance (Figure 1) [3]. Next, they performed detailed genetic analyses of several sandalwood trees and identified genes that are capable of producing the santalol molecule. Finally, they transferred these genes into a yeast culture, and after growing the yeast for several days, were able to extract the santalol molecules from the yeast. This research has laid the groundwork for a system in which the sandalwood scent could be produced from yeast cultures instead of harvesting it from endangered sandalwood trees, leaving the trees to thrive and contribute to their natural ecosystems.

Plant genome analysis enables acquisition of plant-derived medicines from crop species instead of endangered, wild species - Phytochemicals from slow-growing plants are also important medicines and therapeutics. The mayapple, also called the American mandrake(!), produces a compound called podophyllotoxin. By harvesting the mayapple, podophyllotoxin can be extracted and converted into another chemical called etoposide that kills dozens of types of malignant cancers (Figure 2). For this reason, the mayapple is harvested extensively and is now endangered in the eastern half of North America [4]. Chemical engineers at Stanford University carefully analyzed the mayapple's genome and found the six genes that create podophyllotoxin. They transferred these genes into tobacco plants, which are easy to grow, and these plants can now produce podophyllotoxin. In the future it will be possible to obtain podophyllotoxin by harvesting these modified tobacco plants instead of wild mayapple. Tobacco-derived podophyllotoxin can then be used to produce the cancer drug etoposide, likely lending an ironic twist to the history of the tobacco industry and cancer in humans.

Figure 2: Obtaining anti-cancer-etoposide from the mayapple. The mayapple can be harvested and extracted to obtain podophyllotoxin. While this compound has some medicinal properties, it can be converted into a potent chemotherapeutic called etoposide via chemical synthesis (that is, using chemical reactions in a chemistry lab). After genome analysis and molecular biology experiments, scientists have found how to create podophyllotoxin in tobacco plants, instead of harvesting mayapples from the wild.

Plant science for the environment! For biodiversity! - These two examples highlight the potential of genetic techniques to lessen our impact on our environment while increasing access to economically important and health-related chemical compounds. These examples also draw attention to the importance of publically-funded plant research programs; these programs stimulate industry, create new markets, and advance biotechnology. With more than 2,000 new species being discovered each year [1], who knows what plant scientists will discover next!

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[1] Anderson, Seona, et al. "State of the world's plants - 2016." (2016).
[2] Arun Kumar, A.N., Joshi, G. and Mohan Ram, H.Y. (2012) Sandalwood: history, uses, present status and the future. Curr. Sci. 103, 1408–1416.
[3] Celedon, Jose M., et al. "Heartwood‐specific transcriptome and metabolite signatures of tropical sandalwood (Santalum album) reveal the final step of (Z)‐santalol fragrance biosynthesis." The Plant Journal (2016).
[4] USDA Natural Resources Conservation Service https://plants.usda.gov/core/profile?symbol=POPE
[5] Warren Lau, and Elizabeth S. Sattely. "Six enzymes from mayapple that complete the biosynthetic pathway to the etoposide aglycone." Science 349.6253 (2015): 1224-1228.

Why do Christmas trees smell good?

Why do Christmas trees smell good? - Many of us have fond memories of the wonderful aroma that infuses the air when the christmas tree is brought into the house. But my thoughts about this perfume seldom extend beyond "wow, this tree smells good". What, specifically, is the characteristic scent? Why does the tree even bother making it in the first place? Do other plants make the same smells? Where can I buy a small vial of this concentrated essence so that I can invoke christmas spirit all year round!? And how is that concentrate even made? Today, Plants Are Chemists answers these questions.

Figure 1: Conifer resin. Conifer species like pines, firs, and spruces make resins to seal any wounds they sustain. These resins contain terpenes that kill organisms invading the wound and attract other organisms that might kill the invaders.

Christmas trees are conifers - Christmas trees are usually pine, spruce, or fir trees. These are all conifers, a division of land plants that bear cones and have internal water conduction systems. Though there are relatively few species of conifers, they cover vast areas in great numbers; they are earth's second largest above-ground carbon sink, after tropical forests [1]. They also provide about 45% of the world's lumber.

Conifers produce resins to protect themselves - Conifers are well known for the resins they produce (Figure 1). Maybe you've had a (frustrating?) experience with resin from one of these trees - it can be very difficult to get off of your hands. This resin is the tree's means of sealing up any wounds it sustains. It is composed mainly of a special class of plant chemicals called terpenes. Broadly speaking, terpene molecules fall into two classes: those that are small and volatile (they evaporate quickly and spread through the air), and those that are large and non-volatile. Often, the larger terpenes are toxic to insects, bacteria, or fungi that might be invading the wound. In contrast, the airborne terpenes can signal nearby organisms that might eat the invaders. Clearly, terpenes are another testament to the awesome power of the plant kingdom's skills in applied chemistry.

Resins mainly contain terpenes - The identity of the terpenes that make up conifer resins have been studied quite a bit. Common resin terpenes include limonene, terpinolene, alpha- and beta-pinene, delta-3 carene, and sabinene, just to name a few [2]. Terpenes molecules typically contain carbon atoms linked into many complicated rings (Figure 2, left). These rings are put together in a very ingenious way. First, the plant produces a carbon chain that has no rings but has one very reactive carbon that is initially covered up with phosphate to prevent the carbon from reacting (Figure 2, middle). This chain is sort of like one of those snap bracelets that you may have played with as a kid (and maybe still play with today!?) when in its flattened form. When the plant wants to make a terpene, it takes one of these chains, puts it in strong, specialized box (an enzyme) and removes the phosphate, unleashing the reactive carbon. The energy from the reactive carbon bounces all around the chain, causing immensely complex carbon rings to form (sort of like snapping the snap bracelet) (Figure 2, right). Then the terpene is released from the box. Depending on the type of rings and the kind of terpene the plant wants to make, it can use boxes (enzymes) with different characteristics.

Figure 2: Terpene synthesis. Plants put a carbon chain with an active (but restrained carbon) into a box (enzyme), then let the reactive carbon free, causing the energy from the reactive carbon to bend the chain into rings and form a terpene (shown above the dotted line). This is analogous to a snap bracelet (shown below the dotted line): in its restrained, linear form, the energy can be released causing the bracelet to bend into rings.

Figure 3: The Christmas tree smell. The molecules shown here, alpha-pinene, beta-pinene, and bornyl acetate, are the compounds that, together, are the Christmas tree smell.

The christmas tree smell is three terpenes that can be isolated with steam distillation - Using this technique conifer trees make the major aroma molecules that are associated with the classic christmas tree smell: alpha- and beta-pinene and bornyl acetate (Figure 3) [3]. For this reason these molecules, or more accurately their aroma, are very popular. These molecules are quite volatile - they evaporate at relatively low temperatures. This characteristic makes them easy to isolate - the plant is boiled in water, releasing the terpenes into the steam, then the steam is collected and concentrated into an essential oil. A quick internet search will turn up dozens of outlets where one can buy pine or douglas fir essential oils, as well as many other "Christmas blend" essential oils also containing orange, cinnamon, and other spices. Whether from a concentrated oil or from a real christmas tree, if you experience that Christmas tree smell this year, hopefully you'll be reminded a little bit about terpene chemistry and the amazing defense mechanisms of conifer trees. Happy holidays.

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[1] https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter9.pdf
[2] Clark, Erin L., et al. "Comparison of lodgepole and jack pine resin chemistry: implications for range expansion by the mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae)." PeerJ 2 (2014): e240.
[3] (2011), Festive fragrances. Chemistry & Industry, 75: 17–20. doi:10.1002/cind.7524_8.x

Why do chilis bother making themselves spicy anyway?

Why are chilis spicy? - Spicy food holds a special place in many hearts - some people love it and some hate it. We know that chili peppers are used to make spicy food, but why are chilis spicy in the first place? What's in it for the chili plant? Shouldn't those plants be directing all their energy into surviving or reproducing or something?

Mammals threaten chili pepper seeds - Imagine you're a chili plant: you've just spent all year packaging up your children in stored energy and nutrients (seeds). You've even wrapped your kids up in a tasty, shiny red treat (the meaty part of the pepper), to try and entice a bird to eat your pepper, carry it off far away, and poop your children out into a great new environment to start their lives (I know - plant parents have weird hopes for their children). Despite all this, some mammal suddenly comes by and gobbles up your children, using its molar teeth to grind them into a lifeless pulp, and poops them out less than a mile away to rot. What a nightmare!

Spicy capsaicin protects chili seeds from mammals - Fortunately, as a member of the Capsicum plant family, you have a secret weapon against the murderous mammals - you are a master of organic chemistry! You find (after millions of years of experimentation) that by combining a simple fatty acid and a compound similar to vanilla you can create a remarkable molecule that humans call capsaicin (Fig. 1). By coating your children in this compound you successfully make them extremely unpallatable to marauding mammals. Birds, who are unaffected by capsaicin, are then free to eat your peppers [1]. Since birds don't have the same grinding teeth mammals do, your children pass through the bird intact, and are dispersed widely over the globe to colonize new areas.

Figure 1: Capsicum plants, seeds, and capsaicin. Species in the pepper plant family (example in top left), product fruit with seeds (top right), that contain spicy capsaicin (bottom) to protect the seeds against mammals.

Humans (bizarre mammals) cultivate the chili's capsaicin spice - To humans, capsaicin has a spicy taste that many enjoy. Capsicum species were originally domesticated in Central America over 6000 years ago. After european explorers ventured to the Americas, these plants and their peppers were brought all over the world as part of the Columbian Exchange. Now, cultures all over the globe use spicy peppers in their cooking. Since then, humans have been breeding pepper plants that make incredibly hot peppers, and have developed the Scoville heat unit as a scale to describe how hot different peppers are.

Medicinal properties of capsaicin - Some claim to experience euphoria after eating capsaicin [4], supposedly related to a release of endorphins [5], though I suspect that this is folklore and varies greatly between different people. More widely experienced, and more recent, is the use of capsaicin as a pain-reliever. It is currently approved for topical application in the treatment of backache, arthritis, and sprains.

Chilis are used as a deterrent in agriculture - In large doses, capsaicin creates a burning sensation wherever it touches mammals; it is the primary active ingredient in pepper spray and bear mace. In some parts of Africa and Asia, farmers' crops are in danger of being eaten by elephants, which can step over high fences that keep out other herbivores. Elephants have large and sensitive noses, so farmers can keep them away by planting chili pepper plants in a defensive ring around other crops, and the chilis the barrier plants produce can be sold at market.

Chilis can be used as deterrents in home gardens - Chili seeds can also be added to bird seed to deter squirrels and rodents [2], and applied around flowers and other ornamentals to deter deer and rabbits [3]. Some gardeners report no deterring effects when using certain varieties of paprika (100% ground chilis) or chili powder (ground chilis + cumin and other spices). This is probably because paprika is often made with pimento peppers - chilis that are 4x less spicy than jalapenos, and barely register on the Scoville heat scale. A gardener looking to deter pests should search out a paprika with a punch!

Capsaicin is one of many valuable chemicals that plants have developed over millions of years- Capsicum species and the unique chemical called capsaicin they produce have applications in cooking, medicine, agriculture, and home gardening; all of which make the pepper plant a valuable economic commodity. Indeed, over 10 million acres of chili peppers are cultivated world wide. The plant kingdom is full of species, both known and undiscovered, that synthesize chemical compounds of enormous importance to human life. So, it is important that plant scientists work towards discovering and understanding these natural chemicals to improve both our quality of life and our agricultural practices.

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[1] Tewksbury, J. J.; Nabhan, G. P. (2001). "Seed dispersal. Directed deterrence by capsaicin in chilies". Nature. 412 (6845): 403–404.
[2] Jensen, P. G.; Curtis, P. D.; Dunn, J. A.; Austic, R. E.; Richmond, M. E. (2003). "Field evaluation of capsaicin as a rodent aversion agent for poultry feed". Pest Management Science. 59 (9): 1007–1015.
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