predators [65]. Depending on the specific insect species and plant-insect interactions,
different volatiles are released by plants. VOCs mainly include terpenes, terpenoids,
green leaf volatiles (GLV), methyl salicylate, and others.
Terpenes are the most diverse group of plant-volatile compounds. These are
modified unsaturated compounds made up of isoprene units. More than 40,000
terpenes are known to date [66]. Among terpenes, isoprene and monoprenes are
dominantly occurring plant volatiles. The levels of biosynthesis of terpenes are
dependent on various factors like tissue, developmental stage, and phenological
status. Studies conducted in Malus domestica and Prunus avium show high levels
of terpenes in reproductive parts of the flower with the highest levels found during
and after flowering [18]. In addition to reproductive parts, they are also extensively
synthesized in vegetative parts like leaves. However, young leaves have higher
terpene levels compared to the old ones [67].
Terpenoids are synthesized in the epidermal cells of leaves and roots. 1,8-cineole,
a monoterpene volatile, and sesquiterpene (E)-β-caryophyllene are examples of
terpenoids produced in roots [68]. They are also found in special secretory structures
like resin ducts or lactifers in conifers, glandular trichomes seen in Artemisia annua,
and Ocimum basilicum. The concentration of terpenoids also differs within the same
tissue depending upon localization [69]. In case of ponderosa pine needles, levels of
monoterpene cyclase in the base of the needle were highest which caused the
Arctiinae larvae to feed on upper part of the needle. It has been observed that
terpenoid levels depend on various physiological factors [70]. Several terpenoids
are found to be neurotoxic to insects, by inhibiting acetylcholine esterase, causing
anoxia and further death.
Besides terpenoids, GLVs (fatty acid derivatives) also play an acute role in host
perception and localization of herbivore to its natural enemies. These include C6
aldehydes, alcohols, and esters. They were first identified in braconid Microplitis
croceipes and Ichneumon netelia. The Liriomyza huidobrensis larvae elicit the
release of GLVs in a number of host and non-host plants. (Z)-3-hexanol is one
such GLV that attracts Opius dissitus, a parasite of L. huidobrensis [71]. Many other
GLVs serve as an attractant to an aphid parasitoid, Aphidius ervi [72]. Thus, GLVs
resist insect indirectly by attracting the parasitoid of the infesting herbivore.
Along with the indirect defense mechanism, GLVs might serve as DAMPs as they
cause expression of plant defense genes [73]. They also play an essential role in host
location by natural enemies of the herbivore and guide them to the damaged site. The
GLVs found in Solanum tuberosum cause excitation of olfactory sensilla of Colorado beetle, Leptinotarsa decemlineata [74]. In another study, the female braconid
parasitoids, M. croceipes, were attracted towards the GLVs – Z-3-hexen-l-ol and Z3-hexenyl acetate released by caterpillar feeding [75]. Thus, GLVs guide the herbivore parasites to the damage site.
Other VOCs found are methyl-salicylate, 6-methyl-5-hepten-2-one, indole, and
nitrogenous compounds [76, 77]. Nitrogenous compounds like aldoximes, nitriles,
methyl butyronitriles, and benzyl cyanides are synthesized in minor quantities in
plants. However, these have shown to be essential for attracting caterpillar parasitoids – Cotesia glomerata and Cotesia rubecula [78].
30
S. S. Zunjarrao et al.
different volatiles are released by plants. VOCs mainly include terpenes, terpenoids,
green leaf volatiles (GLV), methyl salicylate, and others.
Terpenes are the most diverse group of plant-volatile compounds. These are
modified unsaturated compounds made up of isoprene units. More than 40,000
terpenes are known to date [66]. Among terpenes, isoprene and monoprenes are
dominantly occurring plant volatiles. The levels of biosynthesis of terpenes are
dependent on various factors like tissue, developmental stage, and phenological
status. Studies conducted in Malus domestica and Prunus avium show high levels
of terpenes in reproductive parts of the flower with the highest levels found during
and after flowering [18]. In addition to reproductive parts, they are also extensively
synthesized in vegetative parts like leaves. However, young leaves have higher
terpene levels compared to the old ones [67].
Terpenoids are synthesized in the epidermal cells of leaves and roots. 1,8-cineole,
a monoterpene volatile, and sesquiterpene (E)-β-caryophyllene are examples of
terpenoids produced in roots [68]. They are also found in special secretory structures
like resin ducts or lactifers in conifers, glandular trichomes seen in Artemisia annua,
and Ocimum basilicum. The concentration of terpenoids also differs within the same
tissue depending upon localization [69]. In case of ponderosa pine needles, levels of
monoterpene cyclase in the base of the needle were highest which caused the
Arctiinae larvae to feed on upper part of the needle. It has been observed that
terpenoid levels depend on various physiological factors [70]. Several terpenoids
are found to be neurotoxic to insects, by inhibiting acetylcholine esterase, causing
anoxia and further death.
Besides terpenoids, GLVs (fatty acid derivatives) also play an acute role in host
perception and localization of herbivore to its natural enemies. These include C6
aldehydes, alcohols, and esters. They were first identified in braconid Microplitis
croceipes and Ichneumon netelia. The Liriomyza huidobrensis larvae elicit the
release of GLVs in a number of host and non-host plants. (Z)-3-hexanol is one
such GLV that attracts Opius dissitus, a parasite of L. huidobrensis [71]. Many other
GLVs serve as an attractant to an aphid parasitoid, Aphidius ervi [72]. Thus, GLVs
resist insect indirectly by attracting the parasitoid of the infesting herbivore.
Along with the indirect defense mechanism, GLVs might serve as DAMPs as they
cause expression of plant defense genes [73]. They also play an essential role in host
location by natural enemies of the herbivore and guide them to the damaged site. The
GLVs found in Solanum tuberosum cause excitation of olfactory sensilla of Colorado beetle, Leptinotarsa decemlineata [74]. In another study, the female braconid
parasitoids, M. croceipes, were attracted towards the GLVs – Z-3-hexen-l-ol and Z3-hexenyl acetate released by caterpillar feeding [75]. Thus, GLVs guide the herbivore parasites to the damage site.
Other VOCs found are methyl-salicylate, 6-methyl-5-hepten-2-one, indole, and
nitrogenous compounds [76, 77]. Nitrogenous compounds like aldoximes, nitriles,
methyl butyronitriles, and benzyl cyanides are synthesized in minor quantities in
plants. However, these have shown to be essential for attracting caterpillar parasitoids – Cotesia glomerata and Cotesia rubecula [78].
30
S. S. Zunjarrao et al.