specialist defense proteins, which draws generalists to a wide host range [88, 90, 92].
Similarly, the CYP321 class of enzymes can accommodate a diversity of structural
classes of phytochemicals and are present in generalists, whereas absent in specialist
(Bombyx mori) [93]. Usually, insects show resistance on coming in contact with the
plant secondary metabolites. However, the generalist insect (H. zea) has shown to
activate the expression of four CYP450s in response to plant phytohormones – JA
and SA, that further detoxify furanocoumarins and other plant toxins [94]. Thus, the
insect protects itself from toxins by activating the resistance either prior to or
concomitantly with the synthesis of allelochemicals [94].
P450 is one of the major players in the plant-insect arms race. Thus, the evolution
of plant allelochemicals has effected an increase in P450 genes. Phylogenetic studies
on lepidopteran P450s suggest active duplication of gene loci and increase in their
copy number. This can be attributed to the fine-tuning of substrate specificity
conferring them resistance to P450 inhibitors of plant origin [95, 96].
4.2.2 Esterase
Esterases are also Phase I detoxification enzymes which catalyze the hydrolysis of
carboxylic acid esters. They are essential for development, neurogenesis, pheromone
degradation, hydrolysis of acetylcholine, and juvenile hormones. Esterases introduce
hydrophilic groups into apolar molecules and enhance their water solubility.
Carboxylesterases are important multifamily enzymes of the esterase. The mechanism of action of insect esterases for detoxifying plant secondary metabolites has not
been extensively studied. However, several studies conducted have shown that there
is a high degree of overexpression of esterases in response to plant allelochemicals.
Detoxification due to increased activity of esterases in response to Nicotiana
tabacum was reported in M. persicae [97].
Esterases are mainly involved in phenolic glycosides. In the Gypsy moth,
Lymantria dispar, survival rate while feeding on phenolic glycosides has been
positively correlated with esterase activity [98]. Similarly, in Papilio canadensis
and Papilio glaucus, carboxylesterases are induced in response to phenolic glycosides in their salicaceous host plants [99]. Reports suggest that carboxylesterases
also perform detoxification against the plant glycoside rutin in Spodoptera litura.
The enzymes can also detoxify an indole alkaloid: gramine, quercetin, and 2tridaconone [100, 101].
4.2.3 Glutathione-S-Transferases (GSTs)
Glutathione-S-transferases (GSTs) are found in all aerobic organisms and are responsible for detoxification of endogenous and xenobiotic compounds, intracellular
transport, hormone synthesis, and protection against oxidative stress [102, 103].
These enzymes are primarily Phase II enzymes, which metabolize secondary products generated in Phase I by P450s and esterases. Sometimes, they also show Phase I
detoxification by directly binding and sequestering the toxins [104]. It is interesting
to know the mode of action of GSTs. GSTs exhibit binding sites for glutathione
(GSH) and other toxic compounds [105]. In the reaction, the active site residue of the
GST interacts with GSH sulfhydryl group (-SH) to generate the catalytically active2 Plant-Insect Interaction: The Saga of Molecular Coevolution
33
Similarly, the CYP321 class of enzymes can accommodate a diversity of structural
classes of phytochemicals and are present in generalists, whereas absent in specialist
(Bombyx mori) [93]. Usually, insects show resistance on coming in contact with the
plant secondary metabolites. However, the generalist insect (H. zea) has shown to
activate the expression of four CYP450s in response to plant phytohormones – JA
and SA, that further detoxify furanocoumarins and other plant toxins [94]. Thus, the
insect protects itself from toxins by activating the resistance either prior to or
concomitantly with the synthesis of allelochemicals [94].
P450 is one of the major players in the plant-insect arms race. Thus, the evolution
of plant allelochemicals has effected an increase in P450 genes. Phylogenetic studies
on lepidopteran P450s suggest active duplication of gene loci and increase in their
copy number. This can be attributed to the fine-tuning of substrate specificity
conferring them resistance to P450 inhibitors of plant origin [95, 96].
4.2.2 Esterase
Esterases are also Phase I detoxification enzymes which catalyze the hydrolysis of
carboxylic acid esters. They are essential for development, neurogenesis, pheromone
degradation, hydrolysis of acetylcholine, and juvenile hormones. Esterases introduce
hydrophilic groups into apolar molecules and enhance their water solubility.
Carboxylesterases are important multifamily enzymes of the esterase. The mechanism of action of insect esterases for detoxifying plant secondary metabolites has not
been extensively studied. However, several studies conducted have shown that there
is a high degree of overexpression of esterases in response to plant allelochemicals.
Detoxification due to increased activity of esterases in response to Nicotiana
tabacum was reported in M. persicae [97].
Esterases are mainly involved in phenolic glycosides. In the Gypsy moth,
Lymantria dispar, survival rate while feeding on phenolic glycosides has been
positively correlated with esterase activity [98]. Similarly, in Papilio canadensis
and Papilio glaucus, carboxylesterases are induced in response to phenolic glycosides in their salicaceous host plants [99]. Reports suggest that carboxylesterases
also perform detoxification against the plant glycoside rutin in Spodoptera litura.
The enzymes can also detoxify an indole alkaloid: gramine, quercetin, and 2tridaconone [100, 101].
4.2.3 Glutathione-S-Transferases (GSTs)
Glutathione-S-transferases (GSTs) are found in all aerobic organisms and are responsible for detoxification of endogenous and xenobiotic compounds, intracellular
transport, hormone synthesis, and protection against oxidative stress [102, 103].
These enzymes are primarily Phase II enzymes, which metabolize secondary products generated in Phase I by P450s and esterases. Sometimes, they also show Phase I
detoxification by directly binding and sequestering the toxins [104]. It is interesting
to know the mode of action of GSTs. GSTs exhibit binding sites for glutathione
(GSH) and other toxic compounds [105]. In the reaction, the active site residue of the
GST interacts with GSH sulfhydryl group (-SH) to generate the catalytically active2 Plant-Insect Interaction: The Saga of Molecular Coevolution
33