Adverse environmental conditions, biotic or abiotic generates stress on plants;
consequently modification in physiological signaling and metabolism results in the
production of defense-related metabolites. Biosynthesis, transport, and concentration of metabolites are affected by stress on plants [67]. Abiotic stresses such as
drought, salinity, cold, toxic metals/metalloids, ozone, and UV-B radiation are
important factors affecting plant growth, development, and reproduction. Plant
evolved various strategies to combat these adverse and changing environmental
conditions. Several genes, including those for protein kinases and transcription
factors, have been identified which are involved in abiotic response of plants
[68]. According to John Thompson [69], locations where different abiotic and biotic
conditions promote evolution are called “co-evolutionary hotspots,” and locations
where conditions do not promote co-evolution are called as cold spots. Both plantplant and plant-environment interactions generate stress on plants. Plants have to
evolve and develop mechanism to cope with this situation or perish with time.
5
Role in Plant-Insect Interaction
Plants defend themselves against insects and microorganisms by physical means
(e.g., thick cuticle, spines) or physiological actions (the presence of toxic chemicals
or antifeedants). However, they cannot protect themselves from large herbivores
(mammals and others). Plant-herbivore contact is one of the largest interactions
involving from microorganisms to vertebrates. All groups of animals (~25–30%)
and phytophagous insects (26%) feed on green plants [70, 71]. Plants have evolved
defense mechanism against insect herbivores over a period of 400 million years [72].
Herbivory is an important selection pressure on plants resulting in the development of new chemicals and physical traits for defense. Simultaneously, herbivores
develop new detoxifying mechanisms to tolerate or degrade the toxic chemicals of
plant defense, e.g., Argemone species are equipped with prickles as well as alkaloidcontaining resin [7]. There is intra- and interspecific variation in defense chemicals
in this plant and its organs. It is not possible to identify modification against a
weapon, but process is continuous.
Living organisms, as small as bacteria, protozoa, and fungi to as large as
vertebrates, can detect and select useful odor among thousands of odors available
in the surrounding. Olfactory receptors are responsible for this property of living
things [73]. About 2500 plant species are known to contain cyanogenic glucosides.
When plant tissues are damaged by herbivores, β-glycosidase and α-hydroxynitrile
lyase enzymes degrade cyanogenic glucosides to toxic hydrogen cyanide. Hydrogen
cyanide is a potent mitochondrial respiratory chain inhibitor and thus provides
defense against herbivores. Findings with transcriptome analysis of plant showed
that the gene for cyanogenic glucoside degradation might have horizontally transferred from bacteria [74]. Herbivore insects maintain a close relationship with their
host plants because hosts provide food, mating site, oviposition site, and habitat for
whole or part of their life cycle. This requires quick adaptation and possible changes
to cope with variation of the host plants. This selection pressure can be noticed in
survival of insects with new varieties showing phenotypic and physiological changes
12
K. G. Ramawat and S. Goyal
consequently modification in physiological signaling and metabolism results in the
production of defense-related metabolites. Biosynthesis, transport, and concentration of metabolites are affected by stress on plants [67]. Abiotic stresses such as
drought, salinity, cold, toxic metals/metalloids, ozone, and UV-B radiation are
important factors affecting plant growth, development, and reproduction. Plant
evolved various strategies to combat these adverse and changing environmental
conditions. Several genes, including those for protein kinases and transcription
factors, have been identified which are involved in abiotic response of plants
[68]. According to John Thompson [69], locations where different abiotic and biotic
conditions promote evolution are called “co-evolutionary hotspots,” and locations
where conditions do not promote co-evolution are called as cold spots. Both plantplant and plant-environment interactions generate stress on plants. Plants have to
evolve and develop mechanism to cope with this situation or perish with time.
5
Role in Plant-Insect Interaction
Plants defend themselves against insects and microorganisms by physical means
(e.g., thick cuticle, spines) or physiological actions (the presence of toxic chemicals
or antifeedants). However, they cannot protect themselves from large herbivores
(mammals and others). Plant-herbivore contact is one of the largest interactions
involving from microorganisms to vertebrates. All groups of animals (~25–30%)
and phytophagous insects (26%) feed on green plants [70, 71]. Plants have evolved
defense mechanism against insect herbivores over a period of 400 million years [72].
Herbivory is an important selection pressure on plants resulting in the development of new chemicals and physical traits for defense. Simultaneously, herbivores
develop new detoxifying mechanisms to tolerate or degrade the toxic chemicals of
plant defense, e.g., Argemone species are equipped with prickles as well as alkaloidcontaining resin [7]. There is intra- and interspecific variation in defense chemicals
in this plant and its organs. It is not possible to identify modification against a
weapon, but process is continuous.
Living organisms, as small as bacteria, protozoa, and fungi to as large as
vertebrates, can detect and select useful odor among thousands of odors available
in the surrounding. Olfactory receptors are responsible for this property of living
things [73]. About 2500 plant species are known to contain cyanogenic glucosides.
When plant tissues are damaged by herbivores, β-glycosidase and α-hydroxynitrile
lyase enzymes degrade cyanogenic glucosides to toxic hydrogen cyanide. Hydrogen
cyanide is a potent mitochondrial respiratory chain inhibitor and thus provides
defense against herbivores. Findings with transcriptome analysis of plant showed
that the gene for cyanogenic glucoside degradation might have horizontally transferred from bacteria [74]. Herbivore insects maintain a close relationship with their
host plants because hosts provide food, mating site, oviposition site, and habitat for
whole or part of their life cycle. This requires quick adaptation and possible changes
to cope with variation of the host plants. This selection pressure can be noticed in
survival of insects with new varieties showing phenotypic and physiological changes
12
K. G. Ramawat and S. Goyal