peptides. These salivary peptides from insects are further detected by plants via
pattern recognition receptors (PRR). These receptors present on the plasma membrane of plant cells possess an extracellular domain that specifically binds to the
herbivore or microbe-associated molecular patterns (HAMPs or MAMPs) [34].
These endogenous peptides (derived from plants) are classified into several
families such as systemins, Peps (plant elicitor peptides), HypSys (Hydroxyproline-rich systemin), Inceptin, and Subpep [35, 36]. Peptide elicitors were first
discovered in fungus Trichoderma viride and were later reported in various insect
species [37]. Here, we have discussed about inceptin, as it is the only peptide type
that occurs in insects, acting as herbivore-associated molecular patterns (HAMPS).
The other four families of peptides are present in plant and get activated on
wounding or herbivore attack. Thus, these are recognized as damage-associated
molecular patterns (DAMPS) [34].
Inceptin, an active 13 residue protein, is a proteolytic fragment of ATP synthase γ
subunit derived from plant chloroplast and is known to trigger the production of
ethylene, JA, and SA [38]. In case of Spodoptera frugiperda, inceptin was reported
to be involved in indirect insect recognition by inducing the release of ethylene in
Vigna unguiculata [39]. Inceptins bind to the PRRs and commence the downstream
defense signaling cascade in plants. As inceptin is specific to chloroplast ATP
synthase, stem, root, and pod borer insects might not be perceived by plants through
inceptins [34].
2.1.4 Fatty Acids
Fatty acid elicitors dominantly present in insect oral secretions are sulfoxy fatty acids
[7, 40]. This class of elicitors typically contains saturated and monounsaturated
sulfated α-hydroxy fatty acids. As these fatty acids are commonly found in Caelifera
(Orthoptera), they are termed as “caeliferins.” They were first determined in S.
americana and occur in insect regurgitate [41]. In most of the cases, they activate
ethylene and JA pathways of plant defense. It is demonstrated that synthetic fatty
acids induce the release of ethylene and JA in Arabidopsis [10]. Interestingly, these
are present in oviposition fluid as well. However, the molecular mechanism of fatty
acid elicitation needs to be studied.
2.2
Elicitors from Insect Oviposition Fluid
Along with food and shelter, plants also serve as a site for insect’s oviposition.
Insects secrete an array of fluids from the ovary and posterior parts of the body
during oviposition. These secretions either coat the newly laid eggs or are present at
the plant-egg interface and trigger plant defense in both circumstances. Chemically,
these elicitors largely encompass esters.
Esters are long-chained α,ω-diols (C 22 to C 24 ), which are mono- or diesterified by
3-hydroxypropanoic acid. These have been distinguished in bruchid beetles and are
also known as “bruchins” [42]. As a result of these bruchins, plants produce a tumorlike structures at oviposition site to obstruct entry of newborn larvae and also induce
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
25
pattern recognition receptors (PRR). These receptors present on the plasma membrane of plant cells possess an extracellular domain that specifically binds to the
herbivore or microbe-associated molecular patterns (HAMPs or MAMPs) [34].
These endogenous peptides (derived from plants) are classified into several
families such as systemins, Peps (plant elicitor peptides), HypSys (Hydroxyproline-rich systemin), Inceptin, and Subpep [35, 36]. Peptide elicitors were first
discovered in fungus Trichoderma viride and were later reported in various insect
species [37]. Here, we have discussed about inceptin, as it is the only peptide type
that occurs in insects, acting as herbivore-associated molecular patterns (HAMPS).
The other four families of peptides are present in plant and get activated on
wounding or herbivore attack. Thus, these are recognized as damage-associated
molecular patterns (DAMPS) [34].
Inceptin, an active 13 residue protein, is a proteolytic fragment of ATP synthase γ
subunit derived from plant chloroplast and is known to trigger the production of
ethylene, JA, and SA [38]. In case of Spodoptera frugiperda, inceptin was reported
to be involved in indirect insect recognition by inducing the release of ethylene in
Vigna unguiculata [39]. Inceptins bind to the PRRs and commence the downstream
defense signaling cascade in plants. As inceptin is specific to chloroplast ATP
synthase, stem, root, and pod borer insects might not be perceived by plants through
inceptins [34].
2.1.4 Fatty Acids
Fatty acid elicitors dominantly present in insect oral secretions are sulfoxy fatty acids
[7, 40]. This class of elicitors typically contains saturated and monounsaturated
sulfated α-hydroxy fatty acids. As these fatty acids are commonly found in Caelifera
(Orthoptera), they are termed as “caeliferins.” They were first determined in S.
americana and occur in insect regurgitate [41]. In most of the cases, they activate
ethylene and JA pathways of plant defense. It is demonstrated that synthetic fatty
acids induce the release of ethylene and JA in Arabidopsis [10]. Interestingly, these
are present in oviposition fluid as well. However, the molecular mechanism of fatty
acid elicitation needs to be studied.
2.2
Elicitors from Insect Oviposition Fluid
Along with food and shelter, plants also serve as a site for insect’s oviposition.
Insects secrete an array of fluids from the ovary and posterior parts of the body
during oviposition. These secretions either coat the newly laid eggs or are present at
the plant-egg interface and trigger plant defense in both circumstances. Chemically,
these elicitors largely encompass esters.
Esters are long-chained α,ω-diols (C 22 to C 24 ), which are mono- or diesterified by
3-hydroxypropanoic acid. These have been distinguished in bruchid beetles and are
also known as “bruchins” [42]. As a result of these bruchins, plants produce a tumorlike structures at oviposition site to obstruct entry of newborn larvae and also induce
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
25