SESQUITERPENES

Sesquiterpenoids are defined as the group of 15 carbon compounds derived by the assembly of 3 isoprenoid units and they are found mainly in higher plants but also in invertebrates. Sesquiterpenes, with monoterpenes, are an important constituent of essential oils in plants. They are the most diverse group of isoprenoids. In plants, they function as pheromones and juvenile hormones.
Sesquiterpene structures present several acyclic, mono-, bi-, tri-, and tetracyclic systems. Some of natural sesquiterpenoids are shown below.

Acyclic compounds
The acyclic representative are also called farnesans, term derived from the basic structure, farnesol. Farnesol and nerolidol are very common and are isolated from essential oils of various sources. 

Farnesol is widely distributed in many essential oils such as citronella, neroli, cyclamen, lemon grass, tuberose, rose, musk, and balsam. It is used in perfumery to emphasize the odors of perfumes. Moreover, it is a natural pesticide for mites and is also a pheromone for several insects and mammals, including elephants (teritorial marking, individual recognition, mate attraction). Farnesol is frequently esterified with one fatty acid having 8 to 12 carbon atoms.
Nerolidol is present in neroli, ginger, jasmine, lavender, tea tree and lemon grass. The aroma of nerolidol is woody and reminiscent of fresh bark. It is used as a flavoring agent and in perfumery.

Among the acyclic species, two compounds are well known for their importance in invertebrate endocrinology. One, methyl farnesoate is now considered as the crustacean juvenile hormone (Homola E et al., Comp Biochem Physiol 1997, 117B, 347) and is synthesized by the mandibular organs and is present in the hemolymph. Its structure is nearly identical to the other one, the insect juvenile hormone III which regulates metamorphosis and reproduction. Both are synthesized from farnesoic acid in the corpora allata.

Cyclic compounds
Abscisic acid plays a key role in plants in the regulation of stomatal closure by regulating ion channel activities and water exchanges across the plasma membrane of guard cells. It has also a variety of roles in plant development, bud and seed dormancy, germination, cell division and movement. It induces also storage protein synthesis in seeds  and may be involved in defense against insect attack. Abscisic acid is biosynthesized via carotenoids (zeaxanthin, neoxanthin, violaxanthin) in roots and mature leaves. Its direct precursor is xanthoxin which is a natural inhibitor of plant growth. Abscissic acid is ubiquitous in lower and higher plants, it is present also in algae. Only the C2-cis, C4-trans isomer is biologically active. It was shown that diacylglycerol pyrophosphate plays a role as phospholipid second messenger in abscisic signaling (Zalejski  C et al., Plant J 2005, 42,145). A review of the signaling network may be found in the "Plant hormones" textbook (Litwack G Ed, Elsevier, 2005).
Cadalene has the cadinane skeleton and is present in essential oils and in many plants. It is used as a biomarker in paleobotanic studies. In connection with retene, it enables the estimation of the importance of Pinaceae in ancient forests.

Cadalene