A plant's sugar high

I love breakfast food. I especially love those Ma and Pa breakfast diners where one can get two giant pancakes, hashbrowns, and eggs for about $5. In my mind's eye, I sit down in one of these places, the server brings over the coffee, and there's this little container on the table that has small, pastel colored packets of sweeteners, and maybe a brown packet with raw cane sugar. If I want to treat myself, I tear open one of the brown packets and empty the small crystals into my morning cup. Where do these crystals come from, and why are they brown? Today: sweeteners from plants (volume 1)!

Figure 1: Sugar cane plants. Here, the beautiful colors on the canes are from the fields having been burned to remove excess foliage. Though this reduces the amount of manual labor required to harvest the plants, it releases substantial amounts of carbon dioxide into the atmosphere.

Cane sugar comes from Saccharum officinarum (Figure 1), a plant with Southeast Asian origins, and was the earliest sweetener used on a grand scale. This grass species is now cultivated extensively in its home territory and in South and Central America, having been brought there as part of the Columbian exchange, and accounts for around 70% of the world's sugar production [1], most of the rest coming from sugar beets.

Sugar cane is relatively unique in how it stores sugar - Virtually all plants operate by photosynthesizing to harness the energy of the sun to convert CO2 gas into the solid chemical glucose. Plants then stash this high-energy glucose away in a storage unit for use during the night, a later season, or to give their children a better chance in the dangerous wild. For plants like the potato, their storage unit is a big fat tuber (which is actually a modified stem!). They link many glucose units together into long polymers or chains called starch - these are easy to pack together for storage (Figure 2). Many plants hoard starch in their roots, fruits, or grains. However, unlike most plants, sugar cane stores each of its glucose units linked to just a single partner to make sucrose (these two together are also called a disaccharide), and it stores the sucrose in its stalk. It is not entirely clear why Saccharum does this, but it could be because it allows the plant to continue rapid growth immediately after photosynthesis stops, while its competitors need time to get glucose out of starch storage, giving Saccharum an advantage in the short-term.

Figure 2: Plant sugar storage. Most plants store sugar as a large polymer, many glucose units linked together (top). Sugar cane, together with sugar beets and sweet sorghum, is an exception in that it stores sugar as sucrose, a disaccharide (bottom).

Cane sugar is purified by crystallization - By forcing sugar cane stalks through a press, a watery liquid containing the disaccharides can be obtained. This liquid is then heated to kill enzymes that might break down the disaccharides (and destroy the sweet taste!), and finally boiled to get rid of some of the water and concentrate the sugars into a juice. Ever notice how salt or sugar dissolve most fastest in boiling water? This is because the water molecules are vibrating really quickly and can easily smash apart the salt or sugar crystals. However, when they are vibrating so quickly, hot water molecules can bite off more than they can chew: they dissolve more sugar than they would be able to if they were cold. So, when sugar cane juice becomes very concentrated while boiling, lots of sucrose is dissolved in a relatively small amount of water. When the juice cools, the water can no longer dissolve all the disaccharides it could when it was hot, and the sucrose molecules coordinate and form themselves into crystals that can be collected in a filter. Though purification by crystallization is an elegant chemical process, in practice, at least historically, it was difficult, dangerous, and carried out by slaves at a high cost to human life.

Figure 3: Cane sugar crystallization. Sucrose crystals can be obtained from the liquid of pressed sugar cane by heating and boiling it to concentrate it to a point where the ratio of sucrose to water is very high, then allowed to cool so the sugar crystallizes and can be isolated by filtration.

Other plants also accumulate disaccharides - Sugar beets and sweet sorghum also defy the starch trend and belong to the disaccharide club. Instead of storing their disaccharides in their stalks though, a sugar beet's cache is, of course, in its roots. For processing, these roots are chopped into small pieces and extracted with water, which can then be concentrated and the sucrose obtained by crystallization in a process roughly similar to that used for cane sugar. Of course, after filtering crystals from either cane or beet processing, the liquid is left over (Figure 3, far right, blue liquid). This liquid can be subjected to another round of recrystallization to obtain more crystals, and after that the remaining liquid itself can be concentrated into the brown molasses sold in grocery stores. This is most often done during sugar cane processing, as the molasses obtained from beet processing is not as palatable. Sugar crystals that contain trace amounts of molasses have a brown color.

Some plants are sweet without creating lots of sucrose - There are several other plant-based sweeteners that are not related to sucrose that are beginning to be used widely in U.S. consumer products. These have the advantage of not having the negative health effects associated with sucrose. However, discussion of these phytochemicals will have to wait until next time. See you then!

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[1] Lakshmanan O, Geijskes RJ, Aitken KS, Grof CPL, Bonnett GD, Smith GR. 2005. Sugarcane biotechnology: the challenge and opportunities. In Vitro Cellular and Developmental Biology-Plant 41, 345–363.
[2] A. J. McCormick, D. A. Watt, and M. D. Cramer. 2009. Supply and demand: sink regulation of sugar accumulation in sugarcane. Journal of Experimental Botany, Vol. 60, No. 2, pp. 357–364

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