subsequently contribute to the defensive reactions of the plant [15]. In silver ionexposed Brugmansia candida root culture, there is an increase in scopolamine due to
the downregulation of the enzyme hyoscyamine-6-β-hydroxylase responsible for
scopolamine release [65]. This can also be partly due to the metal-induced regulation
of signaling molecules such as ethylene which in turn can regulate the production of
tropane alkaloids such as scopolamine. Similarly heavy metals are also known to
stimulate the activity of ethylene biosynthesis genes, 1-aminocyclopropane-1-carboxylic acid synthase, and oxidase either directly or through the jasmonate-mediated
pathway [66]. In another study [67] also suggested a positive correlation between the
increase in signal molecules with an increase of secondary metabolites under Cu
2+
exposure. Similarly, the increase in the synthesis and metabolism of phenolic
compounds under Pb stress was reported in Phaseolus vulgaris. The Pb-induced
increase in phenolic content was thought to protect plants from oxidative damage
and membrane lipid per oxidation. Plants growing in aluminum-rich soils also
accumulate a lot of flavonoids in order to prevent oxidative stress [25]. It is clear
from these studies that alteration in secondary metabolism may be a strategy of the
plant to survive the phytotoxicity of heavy metals.
3
Abiotic Stress and Nitric Oxide
NO has gained significant attention in recent years due to its potentiality in enhancing
tolerance of plants to various environmental stresses [2, 12, 13]. As a redox molecules,
NO can function both as a positive and negative regulator of stress responses
depending on the local concentration. Being a free radicle, NO plays a powerful role
in activating ROS-scavenging enzyme activities and protecting from oxidative damage
under abiotic stress. Studies in the recent past have established the role of NO in
resistance to salt, drought, extreme temperature, UV-B, and heavy metal stress (Table
2; [12, 13, 74]). Although the complete mechanism by which NO reduces abiotic stress
is yet to be deciphered, a definite role of NO is suggested in several physiological
processes. In fact, an enhancement of endogenous NO accumulation has been
observed in several plant species exposed to wide variety of stress responses providing
evidence that endogenous NO could be actually involved in plant stress responses [2].
Although accumulation of NO during various stress conditions appears to be a general
response in diverse plant species and tissues, its specificity has been established by
using various inhibitors/scavengers such as c-PTIO or L-NAME which reversed these
NO-mediated effects in many such studies [2, 69]. Furthermore, exogenous supplementation of NO donors including SNP, SNAP, and diethylenediamine have reported
to offer protective actions against abiotic stress, while NO scavengers/inhibitors
reversed these actions [12, 84]. Most of the abiotic stresses lead to oxidative burst
that disrupt the cellular redox homeostasis. NO may act as a chain breaker and provide
protection against oxidative damage under given stress. Being lipolytic small molecules, NO can easily cross cell barriers and enhance cell communications under stress.
NO can directly or indirectly interact with a wide range of targets due to the number of
posttranslation modifications such as S-nitrosylation and nitration [79]. Several transcription factors involved in the regulation of abiotic stress responses in plants
604
P. Santisree et al.
the downregulation of the enzyme hyoscyamine-6-β-hydroxylase responsible for
scopolamine release [65]. This can also be partly due to the metal-induced regulation
of signaling molecules such as ethylene which in turn can regulate the production of
tropane alkaloids such as scopolamine. Similarly heavy metals are also known to
stimulate the activity of ethylene biosynthesis genes, 1-aminocyclopropane-1-carboxylic acid synthase, and oxidase either directly or through the jasmonate-mediated
pathway [66]. In another study [67] also suggested a positive correlation between the
increase in signal molecules with an increase of secondary metabolites under Cu
2+
exposure. Similarly, the increase in the synthesis and metabolism of phenolic
compounds under Pb stress was reported in Phaseolus vulgaris. The Pb-induced
increase in phenolic content was thought to protect plants from oxidative damage
and membrane lipid per oxidation. Plants growing in aluminum-rich soils also
accumulate a lot of flavonoids in order to prevent oxidative stress [25]. It is clear
from these studies that alteration in secondary metabolism may be a strategy of the
plant to survive the phytotoxicity of heavy metals.
3
Abiotic Stress and Nitric Oxide
NO has gained significant attention in recent years due to its potentiality in enhancing
tolerance of plants to various environmental stresses [2, 12, 13]. As a redox molecules,
NO can function both as a positive and negative regulator of stress responses
depending on the local concentration. Being a free radicle, NO plays a powerful role
in activating ROS-scavenging enzyme activities and protecting from oxidative damage
under abiotic stress. Studies in the recent past have established the role of NO in
resistance to salt, drought, extreme temperature, UV-B, and heavy metal stress (Table
2; [12, 13, 74]). Although the complete mechanism by which NO reduces abiotic stress
is yet to be deciphered, a definite role of NO is suggested in several physiological
processes. In fact, an enhancement of endogenous NO accumulation has been
observed in several plant species exposed to wide variety of stress responses providing
evidence that endogenous NO could be actually involved in plant stress responses [2].
Although accumulation of NO during various stress conditions appears to be a general
response in diverse plant species and tissues, its specificity has been established by
using various inhibitors/scavengers such as c-PTIO or L-NAME which reversed these
NO-mediated effects in many such studies [2, 69]. Furthermore, exogenous supplementation of NO donors including SNP, SNAP, and diethylenediamine have reported
to offer protective actions against abiotic stress, while NO scavengers/inhibitors
reversed these actions [12, 84]. Most of the abiotic stresses lead to oxidative burst
that disrupt the cellular redox homeostasis. NO may act as a chain breaker and provide
protection against oxidative damage under given stress. Being lipolytic small molecules, NO can easily cross cell barriers and enhance cell communications under stress.
NO can directly or indirectly interact with a wide range of targets due to the number of
posttranslation modifications such as S-nitrosylation and nitration [79]. Several transcription factors involved in the regulation of abiotic stress responses in plants
604
P. Santisree et al.