Monoterpene Defensive Compounds

Stingless bees lay chemical trails with mandibular gland constituents which have been identified as normal aliphatic alcohols or monoterpene aldehydes. Trigona spinipes generates a trail with a mixture of 2-heptanol, 2-undecanol, and 2-tridecanol, and it has been possible to successfully lay artificial trails with these alcohols (136). Trail following in workers of Trigona subterranea is released by citral (130), the stereoisomers of which are also utilized as alarm pheromones and defensive compounds. Such pheromonal parsimony appears to be especially typical of eusocial bees and ants. 

Defensive Compounds. Larvae of the weevil Oxyops vitiosa produce a shiny orange secretion that covers their integument and probably acts as deterrent against ants [420]. The composition of the secretion resembles the terpenoid pattern of the host foliage (Melaleuca quinquenervia) from where it is sequestered (concentration about twice that of the host foliage). It contains the sesquiterpene (+)-viridoflorol 230 (Scheme 25), the monoterpene hydrocarbons a-pinene 45, P-pinene 46, limonene 171, a-terpinene 231, and y-terpinene 232 as well as the oxygenated monoterpenesl,8-cineole 58, a-terpineol 233, and terpinen-4-ol 234. 

Evidence accumulated for and against the paradigm that bark beetle pheromone biosynthesis involved direct modification of host precursor monoterpenes. For 1. pini, the issue was laid to rest with the demonstration that male tissues incorporate radio-labeled acetate into ipsdienol in a manner consistent with pheromone production. Similar experiments proved the de novo biosynthesis of frontalin, an important isoprenoid-derived semiochemical produced by male Dendroctonus jeffreyi It is probable that other Coleoptera can also synthesize monoterpenes, either as pheromone components " or defensive compounds. Despite the capacity for de novo biosynthesis, plant precursor modification is likely an important source of pheromone components for some species. In these cases, plant chemicals could enter the pheromone biosynthetic pathway at later steps. 

The terpenoids, which are composed of the five-carbon isoprenoids, constitute the largest family of natural products with over 22,000 individual compounds in this class having been described. The terpenoids (hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, polyterpenes, and the like) play diverse functional roles in plants as hormones, photosynthetic pigments, electron carriers, mediators of polysaccharide assembly, structural components of membranes, and defense compounds. Many compounds used by man including resins, latex, waxes, and oils contain plant terpenoids. 

A number of monoterpenes are used as defensive compounds by the walking stick insect Anisomorpha bupresti-odes and the ant Acanthomyops claviger. The compounds are apparently synthesized within the insect. Other allomonal compounds (57-59) produced by ants are probably terpene derived. a-Pinene (14) is used by some termites for defense. 

Mono- and sesquiterpenes are the main constituents of the essential oils, while the other terpenes are constituents of balsams, resins, waxes, and rubber. Oleoresin is a roughly equal mixture of turpentine (85 % Cio-monoterpenes and 15 % C15- sesquiterpenes) and rosin (C2o-diterpene) that acts in many conifer species as a toxic material to invading insects and their pathogenic fungi [12]. A number of angiosperm species have inducible terpenoid defensive compounds (phytoalexins) [13]. These include both sesquiterpenoid and diterpenoid t)pes. Isoprenoid units are also found within the framework of other natural molecules. Thus, indole alkaloids, several quinones (vitamin K), alcohols (vitamin E, vitamin A formed from p-carotene), phenols, and isoprenoid alcohols (also known as terpenols or polyprenols) also contain terpenoid fragments. The biogenesis. 

In 1949 Pavan isolated the substance he called iridomyrmecin from the Argentine ant Iridomyrmex humilis (now Linepithema humile). Iridomyrmecin, dolichodial and other monoterpenes with a methylcyclopen-tanoid structure are called iridoids (Figure 6.12). They act as defensive compounds in ants, stick insects, rove beetles and leaf beetle larvae. Some of the group (e.g. the isomer of nepetalactone shown in Figure 6.12) are also sex pheromones in aphids. Reaction of iridomyrmecin with... 

Cephalic alarm pheromones are secreted by soldiers of termites in the genera Drepanotermes and Amitermes (29). These compounds have been identified as limonene and terpinolene, two of several monoterpene hydrocarbons forming part of the defensive battery of these insects. 

Conifer resin, which is a mixture of monoterpenes and diterpenes is an important protective compound against bark beetles and other conifer herbivores. The volatile monoterpenes emanating from a specific tree is often the cue for bark beetles to find a tree where the tree defenses could be compromised from abiotic or biotic stresses. The synchronized mass attack is the strategy bark beetles use to reduce the effects of resin-based defenses in conifers. Aggregation hormones released by bark beetles are oxidized monoterpenoids such as ipsdienol, ipsenol and verbenol. It is believed that these compounds can be oxidation products of host plant monoterpenes such as myrcene. Recently it has been observed that most of the monoterpenoid aggregation pheromone components are biosynthesized de novo in bark beetles [7]. 

Plant feeding insects are able to store host plant monoterpenes and use the compounds in their own defense. Pine sawfly larvae store mono- and diterpenes of host plant resin for defensive purposes (see diterpenes). Larvae of Chrysomelid leaf beetles feeding on Salix and Populus are able to store and sequester precursors of iridoids (cyclopentane monoterpenoids) derived from host plant. In addition to this sequestration iridoids may even have a dual origin as larvae of Plagiodera versicolora and Phratora laticollis have the potential to de novo produce iridoids from the precursors in their food [8]. 

Monoterpenes originate from one molecule DMAPP and one molecule IPP that are joined in most cases head-to-tail, yielding all- ra/25 geranyldiphosphate (GPP) (Fig. 28). GPP can be folded into mono-, bi- and tricyclic structures and may undergo other modifications to yield more than 1,000 different monoterpenes. Monoterpenes are lipophilic volatile compounds that occur in defensive resins of conifers, essential oils, and floral scents and contribute to the characteristic flavor or aroma of many plants. 

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