those target secondary metabolites such as shikonin [100], little effort has been put
onto revealing the signal transduction steps underlying NO activation of plant
secondary metabolism. PAL is the critical enzyme that mediates the conversion of
phenylalanine into trans-cinnamate, from which many plant phenolic compounds
originate [125]. In several studies, more plant phenolic compounds are produced
with increased PAL activity [120]. It has been clearly demonstrated that NO stimulates transcription of the PAL gene in plants [125]. Increased PAL production
means greater efficiency in converting phenylalanine into phenolic compounds;
and, therefore, in most cases, concentrations of plant phenolic compounds increase
following the use of NO. In T. chinensis cell cultures, NO enhanced PAL activity
while inhibiting the transcription of strictosidine synthase and tryptophan decarboxylase by inducing zinc finger-binding proteins [120]. Besides, exogenous NO donor
SNP is known to induce the expression levels of 4-hydroxybenzoate metageranyltransferase and 3-hydroxy-3-methylglutaryl CoA reductase involved in
shikonin biosynthesis in O. paniculatum cells [100].
Some abiotic stress-eliciting responses, including ROS production, lipid
peroxidation, the activation of PAL, and osmolyte production, were also mediated
by NO. For example, exogenous NO treatment enhanced the production of antioxidation-associated compounds, total phenolic content, proline, and flavonoids in
salt-stressed spinach and tomato [111–113]. Furthermore, exogenous NO application
has also increased the fresh and dry biomasses of edible parts compared to salt alone
treated plants. Given the results in spinach, the authors have proposed the application
of nitric oxide gas as an effective strategy for boosting biomass production and
nutrition quality in spinach under salt stress. NO donor has also been proven to exert
a protective effect against polyethylene glycol-induced drought stress in wheat
seedlings by enhancing growth, relative water content, and reducing oxidative
damage [2]. Similarly, heat shock in H. perforatum suspension cells induced NO
production subsequently resulting in hypericin production [105].
The most common protective mechanism against UV irradiation is the biosynthesis of UV-absorbing secondary metabolites [106]. Several reports indicated
the protective effect of NO against oxidative stress under UV-B irradiation [90,
91 ]. The role of NO in the regulation of flavonoid biosynthesis in G. biloba leaves
under the UV-B was elucidated by [106]. Additionally, the sequential occurrence of
NO production via increased NOS activity and increased chalcone synthase has been
suggested [94]. A similar observation was noted in pea seedlings, where UV-B
induced NR activity and NO production inhibited stem elongation due to the
inhibition of xyloglucan-degrading activity [107]. In Taxus chinensis, spraying
SNP and cPTIO had significant effect on the contents of photosynthetic pigments
and taxol production [108]. Interestingly high levels of flavonoids, condensed
tannins, total phenolics, and taxol were noted under UV-B+cPTIO treatment
suggesting the requirement of balanced levels of NO in the secondary metabolism.
Use of fungal elicitors is one of the most effective strategies for inducing
economically important secondary metabolites in plants. A study by [103] has
shown that NO mediates violate oil accumulation induced by the endophytic fungus
Gilmaniella sp. through salicylic acid and H 2 O 2 -dependent pathways in plantlets
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
611
onto revealing the signal transduction steps underlying NO activation of plant
secondary metabolism. PAL is the critical enzyme that mediates the conversion of
phenylalanine into trans-cinnamate, from which many plant phenolic compounds
originate [125]. In several studies, more plant phenolic compounds are produced
with increased PAL activity [120]. It has been clearly demonstrated that NO stimulates transcription of the PAL gene in plants [125]. Increased PAL production
means greater efficiency in converting phenylalanine into phenolic compounds;
and, therefore, in most cases, concentrations of plant phenolic compounds increase
following the use of NO. In T. chinensis cell cultures, NO enhanced PAL activity
while inhibiting the transcription of strictosidine synthase and tryptophan decarboxylase by inducing zinc finger-binding proteins [120]. Besides, exogenous NO donor
SNP is known to induce the expression levels of 4-hydroxybenzoate metageranyltransferase and 3-hydroxy-3-methylglutaryl CoA reductase involved in
shikonin biosynthesis in O. paniculatum cells [100].
Some abiotic stress-eliciting responses, including ROS production, lipid
peroxidation, the activation of PAL, and osmolyte production, were also mediated
by NO. For example, exogenous NO treatment enhanced the production of antioxidation-associated compounds, total phenolic content, proline, and flavonoids in
salt-stressed spinach and tomato [111–113]. Furthermore, exogenous NO application
has also increased the fresh and dry biomasses of edible parts compared to salt alone
treated plants. Given the results in spinach, the authors have proposed the application
of nitric oxide gas as an effective strategy for boosting biomass production and
nutrition quality in spinach under salt stress. NO donor has also been proven to exert
a protective effect against polyethylene glycol-induced drought stress in wheat
seedlings by enhancing growth, relative water content, and reducing oxidative
damage [2]. Similarly, heat shock in H. perforatum suspension cells induced NO
production subsequently resulting in hypericin production [105].
The most common protective mechanism against UV irradiation is the biosynthesis of UV-absorbing secondary metabolites [106]. Several reports indicated
the protective effect of NO against oxidative stress under UV-B irradiation [90,
91 ]. The role of NO in the regulation of flavonoid biosynthesis in G. biloba leaves
under the UV-B was elucidated by [106]. Additionally, the sequential occurrence of
NO production via increased NOS activity and increased chalcone synthase has been
suggested [94]. A similar observation was noted in pea seedlings, where UV-B
induced NR activity and NO production inhibited stem elongation due to the
inhibition of xyloglucan-degrading activity [107]. In Taxus chinensis, spraying
SNP and cPTIO had significant effect on the contents of photosynthetic pigments
and taxol production [108]. Interestingly high levels of flavonoids, condensed
tannins, total phenolics, and taxol were noted under UV-B+cPTIO treatment
suggesting the requirement of balanced levels of NO in the secondary metabolism.
Use of fungal elicitors is one of the most effective strategies for inducing
economically important secondary metabolites in plants. A study by [103] has
shown that NO mediates violate oil accumulation induced by the endophytic fungus
Gilmaniella sp. through salicylic acid and H 2 O 2 -dependent pathways in plantlets
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
611