excreted as waste, lost during molting or sequestered for the insect’s defense [114].
Sequestration aids the uptake, transfer, and concentration of phytochemicals, which
may or may not be modified and are stored in tissues or hemolymph. It is a
biochemically sophisticated process and has been observed to be independently
evolved as plant toxins are chemically diverse. The sequestered secondary metabolites are used for defense against predators, as pigments for adult coloration, as
pheromones or for protection against UV radiation and photoactivated phytotoxins
like furanocoumarins [115, 116].
The key enzymes involved in sequestration are ABC transporters, as they promote the uptake of toxins and their sequestration. In a research held on poplar beetle
larvae (Chrysomela populi), defensive glands showed significant expression of
salicin-transporting ABCC protein CpABC35 compared to other tissues [117].
This pointed out the role of ABCs in accumulating salicin in storage compartments
of the gland cells to exocytose into the glandular reservoir. In another observation, it
was noted that C. populi and Phratora vitellinae are evolutionarily adapted to
transport and sequester host plant glycosides. These glycosides are further delivered
to dorsal glandular reservoirs, which are signaled to release defensive secretions on
any disturbance or attack [118]. Larvae of a chrysomelid beetle sequester a phenol
glucoside, salicin, and hydrolyze it to salicylic acid, which is used as a defensive
secretion. The hydrolysis of salicin liberates glucose in the insect body implying that
sequestration is not an energy intensive process [119]. A large number of studies
have been carried out to understand the sequestration mechanism of insects against
an array of plant defense compounds. One such interesting study is that a polyphagous lepidopteran moth, Estigmene acrea, which feeds on Asteracea. It sequesters
pyrrolizidine alkaloids while detoxifying them by N-oxidation [120].
Other plant secondary metabolites like cyanogenic glycosides are also processed
using this resistance strategy. The cyanogenic glycosides are either metabolized by
β-cyanoalanine synthase, which converts the cyanide moiety to asparagine or they
are sequestered in insect organs [121].
4.4
Mutation of the Target Site of Plant Secondary Metabolites
Most of the plant secondary metabolites act on insects by binding to a specific
receptor. Therefore, insects have developed mutations of these target sites to prevent
the binding. The most well-documented example of this type of adaptation strategy
is Na
+
/K
+ ATPase mutations. Cardenolides, a toxic secondary metabolite found in
Apocynaceae plant, inhibit Na
+
/K
+ ATPase and disrupt the sodium pump [122].
Thus, insects have employed mutations in this receptor to render resistance against
the cardenolides. Till date, the widely observed mutation in insect species is N122H
in the α-subunit of Na
+ /K
+ ATPase and has evolved in very few cardenolide
sequestration insects – monarch butterfly (Danaus species), milkweed bug
(Oncopeltus fasciatus), fruit fly (D. melanogaster), and beetles (Chrysochus auratus
and Chrysochus cobaltinus) [123, 124]. However, another substitution L111 V has
also been found in the Danai species – D. chrysippu, D. genutia, and the Trimala
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
35
Sequestration aids the uptake, transfer, and concentration of phytochemicals, which
may or may not be modified and are stored in tissues or hemolymph. It is a
biochemically sophisticated process and has been observed to be independently
evolved as plant toxins are chemically diverse. The sequestered secondary metabolites are used for defense against predators, as pigments for adult coloration, as
pheromones or for protection against UV radiation and photoactivated phytotoxins
like furanocoumarins [115, 116].
The key enzymes involved in sequestration are ABC transporters, as they promote the uptake of toxins and their sequestration. In a research held on poplar beetle
larvae (Chrysomela populi), defensive glands showed significant expression of
salicin-transporting ABCC protein CpABC35 compared to other tissues [117].
This pointed out the role of ABCs in accumulating salicin in storage compartments
of the gland cells to exocytose into the glandular reservoir. In another observation, it
was noted that C. populi and Phratora vitellinae are evolutionarily adapted to
transport and sequester host plant glycosides. These glycosides are further delivered
to dorsal glandular reservoirs, which are signaled to release defensive secretions on
any disturbance or attack [118]. Larvae of a chrysomelid beetle sequester a phenol
glucoside, salicin, and hydrolyze it to salicylic acid, which is used as a defensive
secretion. The hydrolysis of salicin liberates glucose in the insect body implying that
sequestration is not an energy intensive process [119]. A large number of studies
have been carried out to understand the sequestration mechanism of insects against
an array of plant defense compounds. One such interesting study is that a polyphagous lepidopteran moth, Estigmene acrea, which feeds on Asteracea. It sequesters
pyrrolizidine alkaloids while detoxifying them by N-oxidation [120].
Other plant secondary metabolites like cyanogenic glycosides are also processed
using this resistance strategy. The cyanogenic glycosides are either metabolized by
β-cyanoalanine synthase, which converts the cyanide moiety to asparagine or they
are sequestered in insect organs [121].
4.4
Mutation of the Target Site of Plant Secondary Metabolites
Most of the plant secondary metabolites act on insects by binding to a specific
receptor. Therefore, insects have developed mutations of these target sites to prevent
the binding. The most well-documented example of this type of adaptation strategy
is Na
+
/K
+ ATPase mutations. Cardenolides, a toxic secondary metabolite found in
Apocynaceae plant, inhibit Na
+
/K
+ ATPase and disrupt the sodium pump [122].
Thus, insects have employed mutations in this receptor to render resistance against
the cardenolides. Till date, the widely observed mutation in insect species is N122H
in the α-subunit of Na
+ /K
+ ATPase and has evolved in very few cardenolide
sequestration insects – monarch butterfly (Danaus species), milkweed bug
(Oncopeltus fasciatus), fruit fly (D. melanogaster), and beetles (Chrysochus auratus
and Chrysochus cobaltinus) [123, 124]. However, another substitution L111 V has
also been found in the Danai species – D. chrysippu, D. genutia, and the Trimala
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
35