In such a way, plants defense system subject insects to multitudinous phytochemicals, with each chemical having a different mode of action. As a result of decades of
association of plants and insects, insects have developed numerous resistance mechanisms of circumventing plant defense systems and surviving in this arms race.
4
Insect Resistance to Plant Defense
The various modes of plant defenses have been discussed by us so far. We will now
look into the counter strategies employed by insects in response to them. Insects
have devised methods to protect themselves from the hazards of the toxic chemicals
released by plants using enzymatic detoxification, followed by excretion or sequestration, physiological tolerance or behavioral avoidance. They have evolved novel
mechanisms of detoxification through gene recruitment, neofunctionalization, and
horizontal gene transfer. The various molecular mechanisms of insect resistance
have been discussed in the following section.
4.1
Avoidance
Chemosensation plays a crucial role in insect avoidance of secondary metabolites.
Insects possess the ability to avoid ingestion of toxins by detecting them visually,
through olfaction or by contact. Chemosensation in insects is facilitated by the
transmembrane proteins – gustatory receptors (GRs) and olfactory receptors (ORs)
present in gustatory receptor neurons (GRNs) and olfactory receptor neurons
(ORNs), respectively. These neurons are present in hair-like projections called
“sensilla,” which are distributed throughout the insect body surface. GRs are
involved in metabolite detection, whereas ORs detect volatile compounds. GRs are
further classified as sweet, bitter, umami, salt, and carbon dioxide based on the type
of ligand binding. The deterrent secondary metabolites bind to the bitter receptors
and activate the downstream cascade. This aversive mechanism is genetically
determined or learned. Studies have shown that certain females avoid oviposition
on unsuitable plants, due to genetic cues [79].
Phenological shifts are also seen in certain insects to refrain from feeding on toxic
compounds. This means, that insects restrict themselves to toxin-free plant organs or
they feed on the plant at a stage when the toxin is not produced or is present at low
levels [80]. Insects are also aversive to bitter compounds and may sometimes avoid
them even if the compounds are non-toxic. For example, M. sexta on encountering a
non-toxic phenolic compound (salicin) and a toxic alkaloid (caffeine) activate the
bitter-signaling pathway [81]. Similarly, grasshoppers and weevils avoid bittertasting cyanogenic glucosides even when they are present in non-toxic concentrations [82]. Thus, bitterness as a signal restricts the range of host plants and increases
the cost of avoidance since it is not always triggered by a toxic compound.
In another intriguing study conducted by Perkins et al. (2013), it was observed
that H. armigera larvae avoid elicited and closely connected leaves of Arabidopsis
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
31
association of plants and insects, insects have developed numerous resistance mechanisms of circumventing plant defense systems and surviving in this arms race.
4
Insect Resistance to Plant Defense
The various modes of plant defenses have been discussed by us so far. We will now
look into the counter strategies employed by insects in response to them. Insects
have devised methods to protect themselves from the hazards of the toxic chemicals
released by plants using enzymatic detoxification, followed by excretion or sequestration, physiological tolerance or behavioral avoidance. They have evolved novel
mechanisms of detoxification through gene recruitment, neofunctionalization, and
horizontal gene transfer. The various molecular mechanisms of insect resistance
have been discussed in the following section.
4.1
Avoidance
Chemosensation plays a crucial role in insect avoidance of secondary metabolites.
Insects possess the ability to avoid ingestion of toxins by detecting them visually,
through olfaction or by contact. Chemosensation in insects is facilitated by the
transmembrane proteins – gustatory receptors (GRs) and olfactory receptors (ORs)
present in gustatory receptor neurons (GRNs) and olfactory receptor neurons
(ORNs), respectively. These neurons are present in hair-like projections called
“sensilla,” which are distributed throughout the insect body surface. GRs are
involved in metabolite detection, whereas ORs detect volatile compounds. GRs are
further classified as sweet, bitter, umami, salt, and carbon dioxide based on the type
of ligand binding. The deterrent secondary metabolites bind to the bitter receptors
and activate the downstream cascade. This aversive mechanism is genetically
determined or learned. Studies have shown that certain females avoid oviposition
on unsuitable plants, due to genetic cues [79].
Phenological shifts are also seen in certain insects to refrain from feeding on toxic
compounds. This means, that insects restrict themselves to toxin-free plant organs or
they feed on the plant at a stage when the toxin is not produced or is present at low
levels [80]. Insects are also aversive to bitter compounds and may sometimes avoid
them even if the compounds are non-toxic. For example, M. sexta on encountering a
non-toxic phenolic compound (salicin) and a toxic alkaloid (caffeine) activate the
bitter-signaling pathway [81]. Similarly, grasshoppers and weevils avoid bittertasting cyanogenic glucosides even when they are present in non-toxic concentrations [82]. Thus, bitterness as a signal restricts the range of host plants and increases
the cost of avoidance since it is not always triggered by a toxic compound.
In another intriguing study conducted by Perkins et al. (2013), it was observed
that H. armigera larvae avoid elicited and closely connected leaves of Arabidopsis
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
31