oxidation of glucose to form gluconic acid and H 2 O 2. An increase in endogenous
levels of H 2 O 2 has shown to levitate ethylene production in tobacco [18]. Thus, it has
been postulated that GOX triggers plant defense by increasing H 2 O 2 levels. Apart
from various Lepidopteran species, GOX has also been determined in Apis mellifera,
Myzus persicae, and Schistocerca americana [19–22]. Another enzyme discovered
in Schistocerca gregaria is lipase, which is found to trigger high levels of
cyclopentenone 12-oxophytodienoic acid (OPDA) [23]. OPDA is a substrate of JA
and has also shown a direct effect on plant defense activation [24, 25]. Other
enzymes like alkaline phosphatase in Bemisia tabaci, and digestive enzymes in
aphids have also been discovered as elicitors in insect saliva [26]. However, their
role in elicitation of plant defense system is yet unexplained.
2.1.2 Fatty Acid Amino Acid Conjugates
Fatty acid amino acid conjugates (FACs) is a broad class of insect elicitors and
accord to a majority of insect oral secretions. It comprises of a fatty acid moiety
(mostly linoleic or linolenic acid or their hydroxylated and phosphorylated forms)
linked to an amino acid [27]. Predominantly in FACs, the fatty acid component is of
plant origin, whereas the amino acid is derived from insect gut [12]. An exception to
this is volicitin, wherein both the fatty acid and amino acid (glutamine) are plantderived.
Volicitin (N-(17-hydroxylinolenoyl)-L-glutamine) was the first determined insect
elicitor in Spodoptera exigua and caused the release of terpenoids [27]. Insects
acquire linolenic acid from plants, which is then hydroxylated to glutamine for the
production of volicitin [28]. Additionally, other volicitin conjugates, N-(17hydroxylinoleoyl)-L-glutamine, N-linolenoyl-L-glutamine and N-linoleoyl-L-glutamine were also found in S. exigua oral secretions [29]. N-linolenoyl-L-glutamine
and/or N-linoleoyl-L-glutamine were the prime FACs found in Manduca sexta,
Teleogryllus taiwanemma, Teleogryllus emma, Drosophila melanogaster, and primitive Lepidopteran species, Helcystogramma triannulella [8, 30, 31]. Thus, it was
proposed that they might be the ancestral compounds [31]. Recently, insect Nlinolenoyl-L-glutamine induced an outburst of Reactive Oxygen Species (ROS) in
Arabidopsis [32]. L-glutamine is prevalent in FACs and perhaps plays a crucial role
in protein binding and activating plant volatile release. Moreover, a study showed
that among the L-linolenic acid conjugates, only L-glutamine exhibited elicitation in
Zea mays [33].
Volicitin showed strong binding to the plasma membrane fractions of Z. mays,
suggesting binding of volicitin to plant membrane protein. This further activates
MAPK pathway and plant defensive response [24]. Also, FACs are involved in
nitrogen metabolism of insects. Any variation in FACs metabolism can lead to
various adverse effects in insects. Therefore, insects preferably might not modify
FACs.
2.1.3 Peptides
Peptides indirectly allow plants to perceive the feeding damage caused by insects.
Due to the action of insect salivary proteases, plant proteins get digested into small
24
S. S. Zunjarrao et al.
levels of H 2 O 2 has shown to levitate ethylene production in tobacco [18]. Thus, it has
been postulated that GOX triggers plant defense by increasing H 2 O 2 levels. Apart
from various Lepidopteran species, GOX has also been determined in Apis mellifera,
Myzus persicae, and Schistocerca americana [19–22]. Another enzyme discovered
in Schistocerca gregaria is lipase, which is found to trigger high levels of
cyclopentenone 12-oxophytodienoic acid (OPDA) [23]. OPDA is a substrate of JA
and has also shown a direct effect on plant defense activation [24, 25]. Other
enzymes like alkaline phosphatase in Bemisia tabaci, and digestive enzymes in
aphids have also been discovered as elicitors in insect saliva [26]. However, their
role in elicitation of plant defense system is yet unexplained.
2.1.2 Fatty Acid Amino Acid Conjugates
Fatty acid amino acid conjugates (FACs) is a broad class of insect elicitors and
accord to a majority of insect oral secretions. It comprises of a fatty acid moiety
(mostly linoleic or linolenic acid or their hydroxylated and phosphorylated forms)
linked to an amino acid [27]. Predominantly in FACs, the fatty acid component is of
plant origin, whereas the amino acid is derived from insect gut [12]. An exception to
this is volicitin, wherein both the fatty acid and amino acid (glutamine) are plantderived.
Volicitin (N-(17-hydroxylinolenoyl)-L-glutamine) was the first determined insect
elicitor in Spodoptera exigua and caused the release of terpenoids [27]. Insects
acquire linolenic acid from plants, which is then hydroxylated to glutamine for the
production of volicitin [28]. Additionally, other volicitin conjugates, N-(17hydroxylinoleoyl)-L-glutamine, N-linolenoyl-L-glutamine and N-linoleoyl-L-glutamine were also found in S. exigua oral secretions [29]. N-linolenoyl-L-glutamine
and/or N-linoleoyl-L-glutamine were the prime FACs found in Manduca sexta,
Teleogryllus taiwanemma, Teleogryllus emma, Drosophila melanogaster, and primitive Lepidopteran species, Helcystogramma triannulella [8, 30, 31]. Thus, it was
proposed that they might be the ancestral compounds [31]. Recently, insect Nlinolenoyl-L-glutamine induced an outburst of Reactive Oxygen Species (ROS) in
Arabidopsis [32]. L-glutamine is prevalent in FACs and perhaps plays a crucial role
in protein binding and activating plant volatile release. Moreover, a study showed
that among the L-linolenic acid conjugates, only L-glutamine exhibited elicitation in
Zea mays [33].
Volicitin showed strong binding to the plasma membrane fractions of Z. mays,
suggesting binding of volicitin to plant membrane protein. This further activates
MAPK pathway and plant defensive response [24]. Also, FACs are involved in
nitrogen metabolism of insects. Any variation in FACs metabolism can lead to
various adverse effects in insects. Therefore, insects preferably might not modify
FACs.
2.1.3 Peptides
Peptides indirectly allow plants to perceive the feeding damage caused by insects.
Due to the action of insect salivary proteases, plant proteins get digested into small
24
S. S. Zunjarrao et al.