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.
Farnesene, an analogue of farnesol,  is known to act as an alarm pheromone in aphids. Released during predator attack, it causes aphids to stop feeding, disperse, and give birth to winged (rather than wingless) forms, which leave their host plants.
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. 

Cyclic ADP-ribose (cADPR) has been shown to mediate signaling of abscisic acid in the drought-stress response leading to activation of gene transcription and to stomatal closure (Leckie C P et al., Proc Natl Acad Sci U S A 1998, 95, 15837). 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 a paper by Giraudat J (Curr Opin Cell Biol 1995, 7, 232)and in the "Plant hormones" textbook (Litwack G Ed, Elsevier, 2005) and . 
Abscisic acid 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. Abscisic acid is ubiquitous in lower and higher plants, it is present also in algae. Only the C2-cis, C4-trans isomer is biologically active. A mechanism of abscisic signaling in connection with cyclic ADP-ribose and calcium movement has been demonstrated to mediate temperature signaling in sponges (Zocchi E et al., Proc Natl Acad Sci USA 2001, 98, 14859) as well as tissue regeneration in Cnidaria (Puce S et al., J Biol Chem 2004, 279, 39783).
Abscisic acid is not restricted to the plant kingdom and primitive invertebrates since it has been shown to be present in the central nervous system of pigs and rats (Le Page-Degivry MT et al., Proc Natl Acad Sci USA 1986, 83, 1155). Evidence was provided that it is also involved in the stimulation of human granulocytes with cyclic ADP-ribose as second messenger (Bruzzone S et al., Proc Natl Acad Sci USA 2007, 104, 5759). This lipid may be considered as a new pro-inflammatory cytokine in humans. Abscisic acid was shown to be a endogenous stimulator of insulin release from human pancreatic b cells with cyclic ADP-ribose as second messenger (Bruzzone S et al., J Biol Chem 2008, 283, 32188). This observation suggests that this lipid phytohormone may be involved in the physiology of insulin release, mainly in its dysregulation under conditions of inflammation.

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 (1-methyl-7-isopropyl phenanthrene), it enables the estimation of the importance of Pinaceae in ancient forests.

Some important sesquiterpenes

Gossypol, a sesquiterpene dimer found in cotton that is formed from two cadinane units. All the cotton plant contains gossypol. That terpene occurs as a mixture of two enantiomers but each has different biological activities. For nonruminant animals (rodents, chickens, humans), (–)-gossypol is significantly more toxic than the (+) enantiomer. It has anti-cancer properties and inhibits male fertility in humans. In contrast, cotton plants containing high levels of (+)-gossypol are resistant to insect damage. These terpenes must be removed from the plant parts and oil before use as animal foods. 

Bicyclic sesquiterpenes with a driman unit are widespread in plants, fungi and certain marine organisms (Jansen BJ et al., Nat Prod Rep 2004, 21, 449).

Driman squeleton

 They have generally potent antibacterial and antifungal activities, and they are toxic to several invertebrates and also in fish. In addition, they deter feeding by insects on plants and by fish on sponges.