plant immunity at an extremely low concentration of 1 fmol [42]. Expression of plant
defense genes like pathogen-associated molecular patterns (PAMPs) in Arabidopsis
thaliana was triggered during P. brassicae oviposition [43]. In case of Diprion pini,
bruchins caused an outburst of plant terpenoids and reduction in ethylene in Pinus
sylvestris [44].
Along with esters, benzyl cyanide may also be discharged during oviposition.
Benzyl cyanide is obtained by female flies from males that acts as antiaphrodisiac
and prevents further female mating [45]. It is secreted by the female accessory gland
and induces plant defense. In P. brassicae, it is known to elevate the ROS levels in
Arabidopsis. Benzyl cyanide also causes leaf surface aberrations, callose formation,
and restricts the entry of Trichogramma brassicae newborns [46, 43].
Thus, there are several chemical strategies that plant adapts to perceive insect
presence and activity. Further on insect detection, plant activates a spectrum of
chemical defense mechanisms to maintain its survival and fitness.
3
Evolution of Chemical Defense in Plant against Herbivore
As biotic stress, herbivores impose a great threat to the plants. To counterattack
herbivores, plants produce a pool of toxic, deterrent, and volatile compounds. Plant
chemical defense system against herbivores can be direct or indirect. Also, it could
be constitutive (phytoanticipins; already present in the tissue) or inducible (phytoalexins; synthesized and released only after the attack of herbivore) (Fig. 2).
Till date, it has been observed that compounds called secondary metabolites
released by plants in response to stress have a pivotal role in herbivore defense
[47]. They mostly possess a deterrent and toxic activity against the insects. But the
role and mode of action of most of the secondary metabolites in insect resistance are
enigmatic. Many studies have been carried out to uncover the ancestral metabolite’s
prime function and their evolution [48]. In recent past, with advancement in analytical methods and sophisticated tools, researchers have elucidated the role of some
specific metabolites against the insect.
Depending on the chemical structure, secondary metabolites can be classified into
three groups: phenolic- (lignin, tannins, flavonoids, coumarins, ravonaids, and
phenolic acids), nitrogen and sulfur-containing compounds (glucosinolates and
terpenoids), and nitrogen-containing compounds (alkaloids). They display distinction in terms of their occurrence and abundance throughout plant tissues. This
variability in distribution and concentration of phytochemicals could be attributed
to their tissue specificity, developmental stage specificity, or stress response. Upon
insect feeding, secondary metabolite level upsurge in localized tissue, followed by
their increment in systemic tissues. The disparity in feeding behavior could instigate
production of specific secondary metabolites. A clear difference is observed in the
plant defense response against chewing and sap-sucking insects. Chewing insects
(Orthoptera, Coleoptera) elicit a strong plant response similar to that of wounding,
whereas mild plant defense response is triggered by sap-sucking insects [7]. For
example, Spodoptera littoralis (chewing insect) feeding increased JA levels and a
26
S. S. Zunjarrao et al.
defense genes like pathogen-associated molecular patterns (PAMPs) in Arabidopsis
thaliana was triggered during P. brassicae oviposition [43]. In case of Diprion pini,
bruchins caused an outburst of plant terpenoids and reduction in ethylene in Pinus
sylvestris [44].
Along with esters, benzyl cyanide may also be discharged during oviposition.
Benzyl cyanide is obtained by female flies from males that acts as antiaphrodisiac
and prevents further female mating [45]. It is secreted by the female accessory gland
and induces plant defense. In P. brassicae, it is known to elevate the ROS levels in
Arabidopsis. Benzyl cyanide also causes leaf surface aberrations, callose formation,
and restricts the entry of Trichogramma brassicae newborns [46, 43].
Thus, there are several chemical strategies that plant adapts to perceive insect
presence and activity. Further on insect detection, plant activates a spectrum of
chemical defense mechanisms to maintain its survival and fitness.
3
Evolution of Chemical Defense in Plant against Herbivore
As biotic stress, herbivores impose a great threat to the plants. To counterattack
herbivores, plants produce a pool of toxic, deterrent, and volatile compounds. Plant
chemical defense system against herbivores can be direct or indirect. Also, it could
be constitutive (phytoanticipins; already present in the tissue) or inducible (phytoalexins; synthesized and released only after the attack of herbivore) (Fig. 2).
Till date, it has been observed that compounds called secondary metabolites
released by plants in response to stress have a pivotal role in herbivore defense
[47]. They mostly possess a deterrent and toxic activity against the insects. But the
role and mode of action of most of the secondary metabolites in insect resistance are
enigmatic. Many studies have been carried out to uncover the ancestral metabolite’s
prime function and their evolution [48]. In recent past, with advancement in analytical methods and sophisticated tools, researchers have elucidated the role of some
specific metabolites against the insect.
Depending on the chemical structure, secondary metabolites can be classified into
three groups: phenolic- (lignin, tannins, flavonoids, coumarins, ravonaids, and
phenolic acids), nitrogen and sulfur-containing compounds (glucosinolates and
terpenoids), and nitrogen-containing compounds (alkaloids). They display distinction in terms of their occurrence and abundance throughout plant tissues. This
variability in distribution and concentration of phytochemicals could be attributed
to their tissue specificity, developmental stage specificity, or stress response. Upon
insect feeding, secondary metabolite level upsurge in localized tissue, followed by
their increment in systemic tissues. The disparity in feeding behavior could instigate
production of specific secondary metabolites. A clear difference is observed in the
plant defense response against chewing and sap-sucking insects. Chewing insects
(Orthoptera, Coleoptera) elicit a strong plant response similar to that of wounding,
whereas mild plant defense response is triggered by sap-sucking insects [7]. For
example, Spodoptera littoralis (chewing insect) feeding increased JA levels and a
26
S. S. Zunjarrao et al.