Various data indicate that nitric oxide (NO) is an endogenous signal in plants that mediates responses to several stimuli. Experimental evidence in support of such signalling roles for NO has been obtained via the application of NO, usually in the form of NO donors, via the measurement of endogenous NO, and through the manipulation of endogenous NO content by chemical and genetic means. Stomatal closure, initiated by abscisic acid (ABA), is effected through a complex symphony of intracellular signalling in which NO appears to be one component. Exogenous NO induces stomatal closure, ABA triggers NO generation, removal of NO by scavengers inhibits stomatal closure in response to ABA, and ABA-induced stomatal closure is reduced in mutants that are impaired in NO generation. The data indicate that ABA-induced guard cell NO generation requires both nitric oxide synthase-like activity and, in Arabidopsis, the NIA1 isoform of nitrate reductase (NR). NO stimulates mitogen-activated protein kinase (MAPK) activity and cGMP production. Both these NO-stimulated events are required for ABA-induced stomatal closure. ABA also stimulates the generation of H2O2 in guard cells, and pharmacological and genetic data demonstrate that NO accumulation in these cells is dependent on such production. Recent data have extended this model to maize mesophyll cells where the induction of antioxidant defences by water stress and ABA required the generation of H2O2 and NO and the activation of a MAPK. Published data suggest that drought and salinity induce NO generation which activates cellular processes that afford some protection against the oxidative stress associated with these conditions. Exogenous NO can also protect cells against oxidative stress. Thus, the data suggest an emerging model of stress responses in which ABA has several ameliorative functions. These include the rapid induction of stomatal closure to reduce transpirational water loss and the activation of antioxidant defences to combat oxidative stress. These are two processes that both involve NO as a key signalling intermediate.

Nitric oxide, stomatal closure, and abiotic stress

The Arabidopsis thaliana protein nitric oxide synthase1 (NOS1) is needed for nitric oxide (NO) synthesis and signaling during defense responses, hormonal signaling, and flowering. The cellular localization of NOS1 was examined because it is predicted to be a mitochondrial protein. NOS1-green fluorescent protein fusions were localized by confocal microscopy to mitochondria in roots. Isolated mitochondria from leaves of wild-type plants supported Arg-stimulated NO synthesis that could be inhibited by NOS inhibitors and quenched by a NO scavenger; this NOS activity is absent in mitochondria isolated from nos1 mutant plants. Because mitochondria are a source of reactive oxygen species (ROS), which participate in senescence and programmed cell death, these parameters were examined in the nos1 mutant. Dark-induced senescence of detached leaves and intact plants progressed more rapidly in the mutant compared with the wild type. Hydrogen peroxide, superoxide anion, oxidized lipid, and oxidized protein levels were all higher in the mutant. These results demonstrate that NOS1 is a mitochondrial NOS that reduces ROS levels, mitigates oxidative damage, and acts as an antisenescence agent.

Arabidopsis Nitric Oxide Synthase1 Is Targeted to Mitochondria and Protects against Oxidative Damage and Dark-Induced Senescence

Nitric oxide as a bioactive signalling molecule in plant stress responses

Nitric oxide (NO), an endogenous signaling molecule in animals and plants, mediates responses to abiotic and biotic stresses. Our previous work demonstrated that 100 μM sodium nitroprusside (SNP, an NO donor) treatment of maize seedlings increased K+ accumulation in roots, leaves and sheathes, while decreasing Na+ accumulation (Zhang et al. in J Plant Physiol Mol Biol 30:455–459, 2004b). Here we investigate how NO regulates Na+, K+ ion homeostasis in maize. Pre-treatment with 100 μM SNP for 2 days improved later growth of maize plants under 100 mM NaCl stress, as indicated by increased dry matter accumulation, increased chlorophyll content, and decreased membrane leakage from leaf cells. An NO scavenger, methylene blue (MB-1), blocked the effect of SNP. These results indicated that SNP-derived NO enhanced maize tolerance to salt stress. Further analysis showed that NaCl induced a transient increase in the NO level in maize leaves. Both NO and NaCl treatment stimulated vacuolar H+-ATPase and H+-PPase activities, resulting in increased H+-translocation and Na+/H+ exchange. NaCl-induced H+-ATPase and H+-PPase activities were diminished by MB-1. 1-Butanol, an inhibitor of phosphatidic acid (PA) production by phospholipase D (PLD), reduced NaCl- and NO-induced H+-ATPase activation. In contrast, applied PA stimulated H+-ATPase activity. These results suggest that NO acts as a signal molecule in the NaCl response by increasing the activities of vacuolar H+-ATPase and H+-PPase, which provide the driving force for Na+/H+ exchange. PLD and PA play an important role in this process.

https://www.jstor.org/stable/pdfplus/23389434.pdf

Nitric oxide enhances salt tolerance in maize seedlings through increasing activities of proton-pump and Na+/H+ antiport in the tonoplast

Plant innate immune response to pathogen infection includes an elegant signaling pathway leading to reactive oxygen species generation and resulting hypersensitive response (HR); localized programmed cell death in tissue surrounding the initial infection site limits pathogen spread. A veritable symphony of cytosolic signaling molecules (including Ca2+, nitric oxide [NO], cyclic nucleotides, and calmodulin) have been suggested as early components of HR signaling. However, specific interactions among these cytosolic secondary messengers and their roles in the signal cascade are still unclear. Here, we report some aspects of how plants translate perception of a pathogen into a signal cascade leading to an innate immune response. We show that Arabidopsis thaliana CYCLIC NUCLEOTIDE GATED CHANNEL2 (CNGC2/DND1) conducts Ca2+ into cells and provide a model linking this Ca2+ current to downstream NO production. NO is a critical signaling molecule invoking plant innate immune response to pathogens. Plants without functional CNGC2 lack this cell membrane Ca2+ current and do not display HR; providing the mutant with NO complements this phenotype. The bacterial pathogen–associated molecular pattern elicitor lipopolysaccharide activates a CNGC Ca2+ current, which may be linked to NO generation due to buildup of cytosolic Ca2+/calmodulin.

/pdf/death-don-t-have-no-mercy-and-neither-does-calcium-55qek5yhh3.pdf

Death don't have no mercy and neither does calcium: Arabidopsis CYCLIC NUCLEOTIDE GATED CHANNEL2 and innate immunity

Nitric oxide (NO) is both a gaseous free radical and a versatile cell-signalling effector that plays important roles in diverse (patho)physiological processes. In ani mals, NO production is catalyzed predominantly by nitric oxide synthases (NOS), which are heme-contain ing proteins related to the cytochrome P450 family. These enzymes catalyze the conversion of L-arginine to L-citrulline and NO using NADPH and molecular oxy gen as cosubstrates, and employ FAD, FMN, tetrahy drobiopterin (BH4), and calmodulin (CaM) as cofactors (Bogdan 2001). The biological effects of NO are medi ated by posttranslational modification of cysteine resi dues and transition metal centers, a key process referred as nitrosylation (Stamler et al. 2001). Because of its high biological reactivity, NO production by NOS is tightly regulated to control the specificity of its signalling as well as to limit its toxicity (Kone et al. 2003). In recent years, NO has also become an increasingly popular target of investigation in plants. NO has been implicated in disease resistance, stomatal closure, re sponses to abiotic stress, iron homeostasis, and in vari ous developmental processes (Neill et al. 2002a, b; Wendehenne et al. 2004). A major advance in our understanding of NO functions in plants has been the identification of enzymes that catalyze NO synthesis. Nitrate reductase (NR) was the first enzymatic source of NO to be identified (Yamasaki and Sakihama 2000). In addition to its role in nitrate reduction, NR catalyzes the reduction of nitrite to NO using NAD(P)H as co-factor. As recently discussed by Meyer et al. (2004), it remains

Laurent Barnavon

Papers

Nitric oxide in plants: the biosynthesis and cell signalling properties of a fascinating molecule.