Nitric oxide (NO) is an important endogenous plant signaling molecule that is responsible for several developmental and physiological processes (Neill et al. 2003, 2008; Tuteja et al. 2004; Delledonne 2005; Lamotte et al. 2005; Erdei and Kolbert 2008; Molassiotis et al. 2010). Extensive work on mammalian system has revealed that NO is a crucial signaling molecule in animals. The physiological functions in plants that are influenced by NO include reduction in seed dormancy (Libourel et al. 2006; Bethke et al. 2007), plant growth regulation and senescence (Mishina et al. 2007) , floral transition suppression (He et al. 2004) , stomatal movements (Bright et al. 2006; Garcia-Mata and Lamattina 2007), and tolerance to abiotic and biotic stress responses (Uchida et al. 2002; Modolo et al. 2005; Zhao et al. 2007; Floryszak-Wieczorek et al. 2007; Molassiotis et al. 2010). It has also been reported that externally applied NO or NO donors enhance plant tolerance to environmental stresses (Uchida et al. 2002; Garcia-Mata and Lamattina 2007; Zhao et al. 2007a, b).
In animals, NO is synthesized via a pathway where l-citrulline is formed from l-arginine with the help ofnitric oxide synthase (NOS) (Fig. 14.6). The electron donor in this reaction is NADPH and cofactors FMN, FADH, and tetrahydropterin are also used in this reaction. For the biosynthesis of NO in plants, two enzymes NOS and nitrate reductase (NR) are involved (Crawford 2006). There may be other sources for NO biosynthesis apart from these two enzymes (Arnaud et al. 2006). In higher plants, the genes responsible for the upregulation of NOS proteins are yet to be identified. A unique plant NOS (AtNOSl) was isolated from Arabidopsis which encodes a protein associated with NO synthesis (Guo et al. 2003). Overexpression of AtNOSl increases NO synthesis in E. coli; Reduced root growth and guard cell NO synthesis has been found in the Atnosl mutant of Arabidopsis plant in response to ABA (Guo et al. 2003) ; Low amount of NO accumulation in Atnosl mutants was also reported by many workers (Zhao et al. 2007b; Bright et al. 2006; Zottiniet al. 2007). Recently, AtNOSl was found not to have NOS activity and was not required for the normal synthesis of NO, and it now appears that it is not in fact an NOS at all (Crawford et al. 2006; Zemojtel et al. 2006).
Fig. 14.6 Synthesis of NO from l-arginine catalyzed by nitric oxide synthase o2 h2o o2 h2o
Fig. 14.6 Synthesis of NO from l-arginine catalyzed by nitric oxide synthase
AtNOS1 in the biosynthesis of NO is either indirect or regulatory and thus AtNOS1 was renamed as Arabidopsis thaliana nitric oxide-associated 1 (ATNOA1).
Another enzymatic source of NO generation is NAD(P)H-nitrate reductase (NR) that converts nitrite to nitric oxide (NO) (Yamasaki 2000; Rockel et al. 2002). The preliminary function of NR in plants is nitrogen assimilation, but in an NAD(P)H-dependent reaction NR can also convert nitrite to NO (Neill et al. 2003; Bright et al. 2006; Crawford 2006). Generation of NO by NR activity has been reported in different plants by different workers, e.g., cucumber (Haba et al. 2001), sunflower, maize (Rockel et al. 2002), and wheat (Xu and Zhao 2003) . NR is encoded by two genes NIA1 and NIA2 in Arabidopsis. Double mutants (nia1nia2) deficient in NIA genes showed low NR activity and low NO production in guard cells while their stomata do not close in response to ABA or nitrite (Desikan et al. 2002). Modolo et al. (2006) have reported that due to the lack of NR activity, the nia1nia2 double mutants showed reduced levels of l-arginine and the exogenous l-arginine can restore NO generation in this mutant. Reduction in NOS-mediated NO production may be due to reduced levels of argi-nine. It is still to be identified how cooperation of two pathways of NO generation controls production of NO in plants.
Another source of NO is a plasma membrane-bound root-specific enzyme, nitrite:NO oxi-doreductase (Ni-NOR). The electron donor in this enzyme is not NAD(P)H but cytochrome c and its optimum pH is more acidic than that of NR. The physiological role and genetic identity of this enzyme are not clear yet (Stohr and Stremlau 2006) .
(Durner et al. 1998) . NO activates the cGMP-dependent pathway leading to adventitious root formation in cucumber (Pagnussat et al. 2003). NO increases the cGMP content and inhibitors of cGMP synthesis through guanylate cyclase. It has also been observed that NO induces PCD in Arabidopsis and cGMP synthesis is also involved there. cGMP was suggested to be the likely target of NO signaling in guard cells. The inhibitor of cGMP synthesis ODQ attenuated ABA and NO-induced stomatal closure. Clarke et al. (2000) reported that the addition of cell-permeable cGMP analogue, 8-bromo cGMP relieved this inhibition. However, 8-bromo cGMP alone did not promote closing of stomata, suggesting that synthesis of cGMP is involved but is insufficient for stomotal closure (Neill et al. 2002) . Desikan et al. )2004) reported that stomatal closure by H2O2 was not suppressed by ODQ which indicates that H2 O2 and NO are different signaling pathways in terms of cGMP signaling. Neither guanylate cyclase nor a cGMP-dependent protein kinase has yet been isolated and cloned from plants (Neill et al. 2003) . NO activates intracel-lular Ca2+ channels through cGMP/cADPR-dependent signaling pathway. Besson-Bard et al. (2008) demonstrated that calcium and MAP kinase can mediate NO signaling. K/Na ratios in Populus eupharatica have been regulated by NO and H2O2 (Zhang et al. 2007). Activation of MAP kinases by NO has been reported in Arabidopsis (Clarke et al. 2000) and tobacco (Kumar and Klessig 2000). Other signaling molecules like H2O2 (Samuel et al. 2000) and SA (Kumar and Klessig 2000) have been reported to activate MAPK in tobacco which indicates that MAPK cascades should be a focal point of convergence of both H2O2 and NO signaling pathway activated in response to various stresses.
NO signaling involves cyclic GMP (cGMP)-dependent and -independent pathways such as protein nitrosylation. Pfeiffer et al. (1994) demonstrated that NO stimulated cGMP formation in spruce needles. It is also reported that in tobacco cGMP synthesis is required for NO signaling
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