including stomatal movement, seed germination [8], and floral transition besides
having a significant role as anti-stress compound against a plethora of abiotic
and biotic stresses such as drought, salinity, temperature extremes, UV-B, and
heavy metal toxicity. Several reviews delineated the functions of NO in most of
environmental abiotic stresses [2]. Besides, pharmacological studies using various
NO donors and scavengers also demonstrated the pivotal role of NO in increasing
plant tolerance to abiotic cues [2, 9]. Till date, it has been reported that exogenous
application of NO donors could enhance stress tolerance in many species of plants
including reed, sunflower, wheat, rice, bitter orange, tobacco, and Arabidopsis. NO
is critical for stress tolerance by modulating osmolytes accumulation and metabolite
reprogramming [10].
Besides, number of researchers reviewed the crucial role of NO in moderating
various plant hormone-mediated development and stress responses [11]. The protective effect of NO in most reports has been attributed to its antioxidant role due
to its ability to activate antioxidant enzymes [2]. There are several studies that
support NO inducing stimulation of major antioxidant enzymes such as ascorbate
peroxidase (APX), catalase (CAT), and superoxide dismutase (SOD) during stress
conditions [12, 13]. Despite the emerging knowledge about NO-mediated plant
stress responses including decreasing reactive oxygen species (ROS) levels and
protecting membranes from oxidative damage, osmolyte accumulation, and regulation of various hormone-mediated signaling events, its functional status has been
far from clarity. Nonetheless, the diffusible nature, short life, and complex chemistry
in living systems of NO pose a great challenge to NO researchers [13].
1.1.2 Synthesis and Signaling of NO
In mammalian systems, NO is synthesized through well-characterized forms of nitric
oxide synthase (NOS), iNOS, nNOS, and eNOS. Although the pace of investigations
was bit slower in plants, initially two plant enzymes, nitric oxide synthase (NOS) and
nitrate reductase (NR) [10], have been attributed for plant NO biosynthesis. Plants
can also produce NO as a by-product of metabolic pathways including nitrogen
fixation and respiration.
Later four major sources for NO generation have been deciphered in plants:
nitrate reductase (NR) pathway, NOS pathway [14], and other enzymatic and
nonenzymatic pathways [15]. Oxidative NO synthesis from L-arginine through
NOS activity has been reported across the kingdoms including prokaryotes, unicellular eukaryotes, invertebrates, and mammals. However, the identification of
NOS sequences from higher plants having high homology to already known NOS
encoding genes [16] in other taxa is still awaiting. During the past decade, the first
plant NOS-like gene (AtNOS1) in higher plants is identified in Arabidopsis having
homology to a snail NOS. Additionally, the chlorotic symptoms of Atnos1 seedlings
disappeared by exogenous NO. Furthermore, the overexpression of AtNOS1
enhanced NO synthesis in Escherichia coli while proved to possess NOS activity
by converting L-arginine to L-citrulline using commercial kits. However, its orthologs from maize and rice failed to show NOS activity indicating its function more as
a regulator of NO rather than the actual gene coding for synthesis. Although few
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
597
having a significant role as anti-stress compound against a plethora of abiotic
and biotic stresses such as drought, salinity, temperature extremes, UV-B, and
heavy metal toxicity. Several reviews delineated the functions of NO in most of
environmental abiotic stresses [2]. Besides, pharmacological studies using various
NO donors and scavengers also demonstrated the pivotal role of NO in increasing
plant tolerance to abiotic cues [2, 9]. Till date, it has been reported that exogenous
application of NO donors could enhance stress tolerance in many species of plants
including reed, sunflower, wheat, rice, bitter orange, tobacco, and Arabidopsis. NO
is critical for stress tolerance by modulating osmolytes accumulation and metabolite
reprogramming [10].
Besides, number of researchers reviewed the crucial role of NO in moderating
various plant hormone-mediated development and stress responses [11]. The protective effect of NO in most reports has been attributed to its antioxidant role due
to its ability to activate antioxidant enzymes [2]. There are several studies that
support NO inducing stimulation of major antioxidant enzymes such as ascorbate
peroxidase (APX), catalase (CAT), and superoxide dismutase (SOD) during stress
conditions [12, 13]. Despite the emerging knowledge about NO-mediated plant
stress responses including decreasing reactive oxygen species (ROS) levels and
protecting membranes from oxidative damage, osmolyte accumulation, and regulation of various hormone-mediated signaling events, its functional status has been
far from clarity. Nonetheless, the diffusible nature, short life, and complex chemistry
in living systems of NO pose a great challenge to NO researchers [13].
1.1.2 Synthesis and Signaling of NO
In mammalian systems, NO is synthesized through well-characterized forms of nitric
oxide synthase (NOS), iNOS, nNOS, and eNOS. Although the pace of investigations
was bit slower in plants, initially two plant enzymes, nitric oxide synthase (NOS) and
nitrate reductase (NR) [10], have been attributed for plant NO biosynthesis. Plants
can also produce NO as a by-product of metabolic pathways including nitrogen
fixation and respiration.
Later four major sources for NO generation have been deciphered in plants:
nitrate reductase (NR) pathway, NOS pathway [14], and other enzymatic and
nonenzymatic pathways [15]. Oxidative NO synthesis from L-arginine through
NOS activity has been reported across the kingdoms including prokaryotes, unicellular eukaryotes, invertebrates, and mammals. However, the identification of
NOS sequences from higher plants having high homology to already known NOS
encoding genes [16] in other taxa is still awaiting. During the past decade, the first
plant NOS-like gene (AtNOS1) in higher plants is identified in Arabidopsis having
homology to a snail NOS. Additionally, the chlorotic symptoms of Atnos1 seedlings
disappeared by exogenous NO. Furthermore, the overexpression of AtNOS1
enhanced NO synthesis in Escherichia coli while proved to possess NOS activity
by converting L-arginine to L-citrulline using commercial kits. However, its orthologs from maize and rice failed to show NOS activity indicating its function more as
a regulator of NO rather than the actual gene coding for synthesis. Although few
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
597