3.2
Nitrogen-Containing Compounds
After flavonoids, the next largest group of secondary metabolites is alkaloids. These
are nitrogen-containing chemicals. Till date, 10,000 different derivatives of alkaloids
have been revealed from different plant species [47]. Alkaloids have been classified
into three major groups: true-alkaloids (nicotine and atropine), pseudo-alkaloids
(caffeine and solanidine), and proto-alkaloids (mescaline). Alkaloids are found in
gymnosperms, angiosperms, and in other primitive plant genera like Lycopodium
[57]. Secondary metabolites, along with terpenes and phenolics, are also present in
trichomes of the plants. Trichomes appeared along with the true leaves in late
Paleozoic era with the appearance of alkaloids [48]. The insect toxicity arises as a
result of interference of these compounds in biological activities like neuronal signal
transduction, DNA replication, protein synthesis, and enzyme activity [5, 58, 59].
3.3
Nitrogen and Sulfur-Containing Compounds
Predominant class in this category of compounds is glucosinolates, which are
characterized by the presence of nitrogen and sulfur in their basic scaffold. In plants,
they are sequestered in compartments to protect them from the action of myrosinase
enzyme [4]. On insect attack or wounding, these compounds come in contact of
myrosinase and are converted to glucon isothiocyanates, which are found to be toxic
to insects [15]. Glucosinolates are found only in the order Capparales and genus
Drypetes of the Euphorbiaceae [4]. Brassica napus is studied extensively for the
herbivore-resistance activity of glucosinolates [60, 61]. Due to the similarity in
chemical structure and synthesis process, glucosinolates are hypothesized to have
been evolved from primitive cyanogenic glucosides [62]. Aldoxime-metabolizing
enzyme, belonging to the cytochrome (CPY) family, is involved in the synthesis of
both cyanogenic glucosinolates and glucosinolates. It is reported that cyanogenic
glucosinolates are present ubiquitously in ferns, gymnosperms, and angiosperms
[63]. The evolutionary theory suggests that the mutation in CPY family enzyme of
cyanogenic glucosinolates has caused the formation of a toxic compound –
glucosinolate [15, 62, 64]. Glucosinate levels are found highest in young leaves
and reproductive parts – seeds and siliques.
3.4
Plant-Volatile Compounds
Volatile compounds (VOCs) are phytoalexins that are released by plants on herbivore
attack, also called as herbivore-inducible plant volatiles (HIPVs) [40]. Feeding of the
herbivore induces downstream cascade in plant cells, releasing specific VOCs. As
discussed previously, insect elicitors play an essential role in the release of these VOCs.
Though VOCs cause indirect defense, many of these like indoles, (E)-β-caryophyllene
are found to be toxic to the herbivores (direct defense). This suggests that the primary
function of VOCs might have been toxicity, which evolved further to attract herbivore
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
29
Nitrogen-Containing Compounds
After flavonoids, the next largest group of secondary metabolites is alkaloids. These
are nitrogen-containing chemicals. Till date, 10,000 different derivatives of alkaloids
have been revealed from different plant species [47]. Alkaloids have been classified
into three major groups: true-alkaloids (nicotine and atropine), pseudo-alkaloids
(caffeine and solanidine), and proto-alkaloids (mescaline). Alkaloids are found in
gymnosperms, angiosperms, and in other primitive plant genera like Lycopodium
[57]. Secondary metabolites, along with terpenes and phenolics, are also present in
trichomes of the plants. Trichomes appeared along with the true leaves in late
Paleozoic era with the appearance of alkaloids [48]. The insect toxicity arises as a
result of interference of these compounds in biological activities like neuronal signal
transduction, DNA replication, protein synthesis, and enzyme activity [5, 58, 59].
3.3
Nitrogen and Sulfur-Containing Compounds
Predominant class in this category of compounds is glucosinolates, which are
characterized by the presence of nitrogen and sulfur in their basic scaffold. In plants,
they are sequestered in compartments to protect them from the action of myrosinase
enzyme [4]. On insect attack or wounding, these compounds come in contact of
myrosinase and are converted to glucon isothiocyanates, which are found to be toxic
to insects [15]. Glucosinolates are found only in the order Capparales and genus
Drypetes of the Euphorbiaceae [4]. Brassica napus is studied extensively for the
herbivore-resistance activity of glucosinolates [60, 61]. Due to the similarity in
chemical structure and synthesis process, glucosinolates are hypothesized to have
been evolved from primitive cyanogenic glucosides [62]. Aldoxime-metabolizing
enzyme, belonging to the cytochrome (CPY) family, is involved in the synthesis of
both cyanogenic glucosinolates and glucosinolates. It is reported that cyanogenic
glucosinolates are present ubiquitously in ferns, gymnosperms, and angiosperms
[63]. The evolutionary theory suggests that the mutation in CPY family enzyme of
cyanogenic glucosinolates has caused the formation of a toxic compound –
glucosinolate [15, 62, 64]. Glucosinate levels are found highest in young leaves
and reproductive parts – seeds and siliques.
3.4
Plant-Volatile Compounds
Volatile compounds (VOCs) are phytoalexins that are released by plants on herbivore
attack, also called as herbivore-inducible plant volatiles (HIPVs) [40]. Feeding of the
herbivore induces downstream cascade in plant cells, releasing specific VOCs. As
discussed previously, insect elicitors play an essential role in the release of these VOCs.
Though VOCs cause indirect defense, many of these like indoles, (E)-β-caryophyllene
are found to be toxic to the herbivores (direct defense). This suggests that the primary
function of VOCs might have been toxicity, which evolved further to attract herbivore
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
29