[83]. This suggested that insects are capable of detecting previously elicited response
and thus reduce contact with induced plants. M. persicae, an aphid, avoids ingesting
the toxic nicotine present in the xylem by feeding on the phloem tissue [84]. While
feeding on Solanum sp., larvae of Mechanitis isthmia spin a silk fabric over spines,
allowing them to move and feed without being affected by the defensive trichomes
[85]. Another way in which insects refrain from plant defenses is by leaf vein
severing or cutting trenches across leaves before feeding so as to depressurize the
secretary canals and get rid of toxins at the site before feeding [86]. For example,
chrysomelid beetles of the Blepharida genus, that feed on certain Bursera species,
puncture leaf veins to stop the flow of terpene-containing resins stored in leaf canals
[87]. Therefore, avoidance in insects offers the first line of defense against plant
allelochemicals. In spite of this, the insects might sometimes ingest the toxins.
Hence, they have adapted other resistance mechanisms like detoxification, which
ensures the conversion and elimination of the ingested secondary metabolites.
4.2
Detoxification
Detoxification is an enzyme-mediated conversion of toxic compounds into non-toxic
or less toxic forms. Detoxification generally occurs in distinct phases. Phase I
involves the hydrolysis or oxidation of secondary metabolites and Phase II conjugates Phase I products with endogenous compounds. The predominant enzymes
used for detoxification are cytochrome P450 monooxygenases (P450s), esterases,
UDP glucosyl-transferases (UGTs), glutathione S-transferases (GSTs), and ABC
transporters [58, 88].
4.2.1 Cytochrome P450
P450s are Phase I detoxifying enzymes. In insects, P450s play an important role in
the biosynthesis of hormones, fat metabolism, and insecticide resistance [89]. P450s
are extensively found in microsomal membranes and possess different electrontransfer partners. Despite having a common catalytic chemistry, these enzymes
exhibit different metabolic capabilities. P450s get their common name since they
bind to carbon monoxide (CO) in their reduced state and form a P450:CO complex.
The microsomal P450s are heme-dependent, mixed-function oxidases, and use
NADPH and/or NADH for reduction [90]. P450s are extremely versatile and thus
play a central position in the evolution of interspecies defense strategies. This is
because of their biochemical flexibility of multiple substrate recognition.
There are many instances of different classes of P450s assisting detoxification. A
cytochrome P450 was detected in Papilio species. In this case, exposure to
furanocoumarins caused expression of CYP450 from the CYP6B class [89]. An
engaging distinction of this enzyme class has been observed in specialist and
generalist insects. Molecular modeling studies indicated that CYP6B enzyme from
H. zea (generalist) had a flexible catalytic pocket and an additional substrate access
channel compared to Papilio polyxenes (specialist) [91]. These results indicated that
generalist P450s can accept structurally diverse compounds as compared to
32
S. S. Zunjarrao et al.
and thus reduce contact with induced plants. M. persicae, an aphid, avoids ingesting
the toxic nicotine present in the xylem by feeding on the phloem tissue [84]. While
feeding on Solanum sp., larvae of Mechanitis isthmia spin a silk fabric over spines,
allowing them to move and feed without being affected by the defensive trichomes
[85]. Another way in which insects refrain from plant defenses is by leaf vein
severing or cutting trenches across leaves before feeding so as to depressurize the
secretary canals and get rid of toxins at the site before feeding [86]. For example,
chrysomelid beetles of the Blepharida genus, that feed on certain Bursera species,
puncture leaf veins to stop the flow of terpene-containing resins stored in leaf canals
[87]. Therefore, avoidance in insects offers the first line of defense against plant
allelochemicals. In spite of this, the insects might sometimes ingest the toxins.
Hence, they have adapted other resistance mechanisms like detoxification, which
ensures the conversion and elimination of the ingested secondary metabolites.
4.2
Detoxification
Detoxification is an enzyme-mediated conversion of toxic compounds into non-toxic
or less toxic forms. Detoxification generally occurs in distinct phases. Phase I
involves the hydrolysis or oxidation of secondary metabolites and Phase II conjugates Phase I products with endogenous compounds. The predominant enzymes
used for detoxification are cytochrome P450 monooxygenases (P450s), esterases,
UDP glucosyl-transferases (UGTs), glutathione S-transferases (GSTs), and ABC
transporters [58, 88].
4.2.1 Cytochrome P450
P450s are Phase I detoxifying enzymes. In insects, P450s play an important role in
the biosynthesis of hormones, fat metabolism, and insecticide resistance [89]. P450s
are extensively found in microsomal membranes and possess different electrontransfer partners. Despite having a common catalytic chemistry, these enzymes
exhibit different metabolic capabilities. P450s get their common name since they
bind to carbon monoxide (CO) in their reduced state and form a P450:CO complex.
The microsomal P450s are heme-dependent, mixed-function oxidases, and use
NADPH and/or NADH for reduction [90]. P450s are extremely versatile and thus
play a central position in the evolution of interspecies defense strategies. This is
because of their biochemical flexibility of multiple substrate recognition.
There are many instances of different classes of P450s assisting detoxification. A
cytochrome P450 was detected in Papilio species. In this case, exposure to
furanocoumarins caused expression of CYP450 from the CYP6B class [89]. An
engaging distinction of this enzyme class has been observed in specialist and
generalist insects. Molecular modeling studies indicated that CYP6B enzyme from
H. zea (generalist) had a flexible catalytic pocket and an additional substrate access
channel compared to Papilio polyxenes (specialist) [91]. These results indicated that
generalist P450s can accept structurally diverse compounds as compared to
32
S. S. Zunjarrao et al.