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
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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.