O Tannenbaum

The decoration of Christmas trees is a tradition started in Germany during the Renaissance. However, its roots go far back in time; evergreen plants had a special meaning in many cultures long before the advent of Christianity. Nowadays, decorating a tree with garlands, baubles, tinsel, candy canes, blinking lights (or candles), etc. is a worldwide spread tradition, with many local variation of the theme.
Although the popular Christmas carol,  “O Tannenbaum”, refers to a fir (Tanne in German), several different conifers are actually used, like species of fir, spruce, pine, etc. (1), often depending on the geographical area. The most common in Europe, for example, is Norway Spruce (Picea abies).
Unfortunately, these conifer species face several problems: climate change-related issues, but also several biotic disturbances (often exacerbated by the abiotic stress), like the fungi-induced canker and needle cast diseases, herbivore feeding, and attacks by boring insects (2). On the other hand, the plants have evolved several physical and chemical-based defence mechanisms (3,4).

Conifer trees and resin

While hiking through a conifer forest or just passing by a spruce or a pine tree, you might have noticed the emission of a light yellow or amber-coloured and nicely scented material from trunks or branches. This material is resin (or oleoresin), a mixture of monoterpenes (C10), and diterpenes (C20), and to a lesser extent sequiterpenes (C15), produced by many conifer plant species.1 Although the resin composition varies considerably across different species and between populations and individuals of the same species, depending on several factors (season, geographical area, altitude, etc.), examples of monoterpenes usually present are α- and β-pinene, limonene, myrcene, sabinene, etc., some of the sesquiterpenes are caryophyllene, longifolene, humulene, while for the diterpenes, the most abundant compounds are diterpenoid acids, especially abietic-type and pimaric-type acids (3).

Why is resin produced? Well, it seems to be involved in many defence-related biological processes. After all, many of the main components have demonstrated biological activities. However, the resin also represents a first line of defence, acting as a physical barrier by sealing wounds. When a mechanical damage occurs, the resin is emitted on the surface and upon exposure to the atmosphere, the C10/C15 volatile compounds (which might serve as a solvent for the resin acids) evaporate, while the C20 resin acids oxidatively polymerize to form a hardened barrier that seals the wound and often traps and expels potential invaders (a process called “pitching out”) (5).  

Depending on the species, resin production may be constitutive or inducible. In the first case, it is made independently from any external trigger and stored in specialized secretory structures, like the so-called resin ducts, vesicles, blisters, glands, or cells (3). In the second, it is produced in response to stress (3). Norway spruce, for example, has constitutive resin ducts, but it has been shown that upon exposure to stress it also produces de novo (traumatic) resin ducts.

Resin in chemical ecology

Among the above-mentioned pests, bark boring beetles (so called cause they make holes in the bark and then in other organs of the tree) are the most destructive agents (6). Although most bark beetles live and feed on dead trees, some species need living trees, which then usually do not survive the colonization. The insects are guided by visual, tactile, and olfactory cues and select a host tree, usually one that is already physically or physiologically damaged.
The volatile monoterpenes, despite being toxic to the beetles, provide a species-specific signature and, due to their nature, they work very well as chemical olfactory cues to locate hosts, given that they are even emitted in higher amounts when the tree is damaged (6). Once the host has been located, the pioneer beetles signal for mass attack through aggregation pheromones (i.e., chemicals that induce group formation by bringing many individuals together). Oleoresin monoterpenes are also involved in this process. Many bark beetle species synthesize aggregation pheromones from the volatile components of the host oleoresin (6). It has been proposed that this phenomenon might have arisen as a detoxification mechanism and resulted afterwards in the use of the detoxification products as pheromone signals.
For example, the beetle Ips typographus produces (+)-cis-verbenol and (+)-trans-verbenol through oxidation of Norway spruce derived (-)-α-pinene and (+)-α-pinene, respectively. It uses these compounds to signal for mass attack. It is interesting that only cis-verbenol acts as an aggregation pheromone for Ips typographus. Therefore, the presence of the two isomers of pinene “dilutes” the efficacy of the message (5).
When the population gets too crowded, the beetles signal to co-specific that it is time to search for a new host: the verbenol is further oxidized to verbenone, which acts as a dispersion pheromone (5).
It must be said at this point that there are specificities for each tree species and for the different insects and that the interactions are even more complex than here described. Different monoterpenes and mechanisms are involved, but all are equally or even more fascinating. In some cases, for example, co-existing beetle species use “complementary” messages. Furthermore, some of the monoterpenes, are also attractive to parasitoids and predators of bark beetles.
After the mass attack the beetles mate and lay eggs in tunnels excavated in the tree. The larvae hatch, feed in the tunnels and develop there, causing big problems to the trees, since they destroy important anatomical structures (6). This usually results in the death of the host, from which then young adults emerge and the cycle starts over. The damage is also exacerbated by the fact that these beetles bring with them pathogenic fungi, which “help” them in destroying the tree (6).

The relationships between conifers, their defences and co-evolved pests that became able to overcome them is very interesting. Under normal conditions, most of the resin-producing conifer trees are very well protected, thanks to many chemical (not only resin, but also flavonoids and phenolics in general) and physical barriers (4). Only already damaged and weakened trees are infested. However, recently, we have witnessed more and more infestation out brakes and anthropogenic impact is very likely one of the determining factors (3).

Mankind use of conifer resins

Going back to the resin, we know that mankind often uses the amazing chemicals from plants for different aims, and this is also true for the single components of the resins. The pure compounds are usually obtained from less complex matrixes. However, resin itself (or fractions of it) is also widely used. It has for example been used for thousands of years by ship builders to waterproof ropes and tarps and to seal the seams of wooden ships. It was also used to waterproof and seal many other objects. Furthermore, it is very persistent and the analysis of the resin components of wood can help in the identification of the tree source (7). These are some of the reasons why resin is very important also from the archaeological and art history point of view. How not to mention here also the fossilized resin, amber, which not only is extensively used in art and jewellery, but has also preserved insects and other small organisms that were imbedded before it hardened (8).
It is possible to separate the volatile part and the acid part of the resin, obtaining turpentine and rosin, respectively. Turpentine is used as a solvent for paints and in the chemical industry as a base to produce solvents, cleaners, fragrances, food flavourings, pharmaceuticals, insecticides, etc. (9). Rosin was initially discarded as a waste product and now is used as an ingredient of paints, varnishes, plastics, lubricants, glues, asphalt, rubber, insecticides, germicides, cosmetics, chewing gums, and numerous pharmaceutical products, and widely used in the paper industry (9).
Finally, there is a record of medicinal applications of resin for wound healing (10).

Note
1. Terpenes are compounds built in a modular way starting from a C5 unit (well, actually there are two units that react with each other in the first step of the biosynthesis: dimethylallyl diphosphate and isopentenyl diphosphate) through the methylerythritol phosphate (MEP) and mevalonic acid (MEV) pathways. These units are linked together and then the so synthesized chains undergo processes of cyclization, rearrangements, oxidation, etc. thanks to specific enzymes and to reactions that involve carbocations intermediates. The diversity of compounds obtained in such a way is amazing. These compounds include several plant metabolites besides the terpenoids that constitute oleoresin.

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