SummaryQuantitative data on nitric oxide (NO) production by plants, and knowledge of participating reactions and rate limiting factors are still rare. We quantified NO emission from tobacco (Nicotiana tabacum) wild-type leaves, from nitrate reductase (NR)-or nitrite reductase (NiR)-deficient leaves, from WT-or from NR-deficient cell suspensions and from mitochondria purified from leaves or cells, by following NO emission through chemiluminescence detection. In all systems, NO emission was exclusively due to the reduction of nitrite to NO, and the nitrite concentration was an important rate limiting factor. Using inhibitors and purified mitochondria, mitochondrial electron transport was identified as a major source for reduction of nitrite to NO, in addition to NR. NiR and xanthine dehydrogenase appeared to be not involved. At equal respiratory activity, mitochondria from suspension cells had a much higher capacity to produce NO than leaf mitochondria. NO emission in vivo by NiR-mutant leaves (which was not nitrite limited) was proportional to photosynthesis (high in light þCO 2 , low in light )CO 2 , or in the dark). With most systems including mitochondrial preparations, NO emission was low in air (and darkness for leaves), but high under anoxia (nitrogen). In contrast, NO emission by purified NR was not much different in air and nitrogen. The low aerobic NO emission of darkened leaves and cell suspensions was not due to low cytosolic NADH, and appeared only partly affected by oxygendependent NO scavenging. The relative contribution of NR and mitochondria to nitrite-dependent NO production is estimated.

Nitric oxide (NO) is a widespread signaling molecule, and numerous targets of its action exist in plants. Whereas the activity of NO in erythrocytes, microorganisms, and invertebrates has been shown to be regulated by several hemoglobins, the function of plant hemoglobins in NO detoxification has not yet been elucidated. Here, we show that Arabidopsis thaliana nonsymbiotic hemoglobin AHb1 scavenges NO through production of S-nitrosohemoglobin and reduces NO emission under hypoxic stress, indicating its role in NO detoxification. However, AHb1 does not affect NO-mediated hypersensitive cell death in response to avirulent Pseudomonas syringae, suggesting that it is not involved in the removal of NO bursts originated from acute responses when NO mediates crucial defense signaling functions.

The cyanobacterium Synechocystis sp. strain PCC 6803 possesses two CO 2 uptake systems and two HCO 3 ؊ transporters. We transformed a mutant impaired in CO 2 uptake and in cmpA-D encoding a HCO 3 ؊ transporter with a transposon inactivation library, and we recovered mutants unable to take up HCO 3 ؊ and grow in low CO 2 at pH 9.0. They are all tagged within slr1512 (designated sbtA). We show that SbtA-mediated transport is induced by low CO 2 , requires Na ؉ , and plays the major role in HCO 3 ؊ uptake in Synechocystis. Inactivation of slr1509 (homologous to ntpJ encoding a Na ؉ /K ؉ -translocating protein) abolished the ability of cells to grow at [Na ؉ ] higher than 100 mM and severely depressed the activity of the SbtA-mediated HCO 3 ؊ transport. We propose that the SbtA-mediated HCO 3 ؊ transport is driven by ⌬Na ؉ across the plasma membrane, which is disrupted by inactivating ntpJ. Phylogenetic analyses indicated that two types of sbtA exist in various cyanobacterial strains, all of which possess ntpJ. The sbtA gene is the first one identified as essential to Na ؉ -dependent HCO 3 ؊ transport in photosynthetic organisms and may play a crucial role in carbon acquisition when CO 2 supply is limited, or in Prochlorococcus strains that do not possess CO 2 uptake systems or Cmp-dependent HCO 3 ؊ transport.Growth of many photosynthetic microorganisms depends on the activity of a CO 2 -concentrating mechanism (CCM), 1 which raises the [CO 2 ] in close proximity to ribulose-1,5-bisphosphate carboxylase/oxygenase and thereby enables efficient CO 2 fixation despite the low affinity of the enzyme for CO 2 (1, 2). In the cyanobacterium Synechocystis sp. strain PCC 6803 (hereafter Synechocystis 6803), the CCM involves active CO 2 uptake and HCO 3 Ϫ transport. We have recently identified two systems for CO 2 uptake in Synechocystis 6803, one constitutive and the other inducible by low CO 2 (3). As deduced from phylogenetic analysis of proteins encoded by the genes involved, these CO 2 uptake systems are present in various cyanobacteria with the exception of Prochlorococcus marinus (3). The inducible system that depends on NdhD3/ NdhF3/CupA shows higher maximal activity and higher affinity for CO 2 than the constitutive, NdhD4/NdhF4/CupB-dependent system. Inactivation of two different genes, one encoding a component of the constitutive system and the other a constituent of the inducible system, abolished CO 2 uptake activity. The double mutants were unable to grow at pH 7.0 under air level of CO 2 (3, 4). In contrast, because the mutants possessed HCO 3 Ϫ transport capability, they could grow like the wild type (WT) at pH 9.0 in air.An ABC-type HCO 3 Ϫ transporter encoded by cmpABCD has been identified in Synechococcus sp. strain PCC 7942 (thereafter Synechococcus 7942) (5). Inactivation of cmp genes in Synechocystis 6803, however, had little effect on the HCO 3 Ϫ transport activity. This indicated that another HCO 3 Ϫ transporter, as yet unidentified, plays a central role in HCO 3 Ϫ uptake. Sodium ions are essential for ...

Recent experiments indicate that nitric oxide (NO) plays a pivotal role in disease resistance and several other physiological processes in plants. However, most of the current information about the function of NO in plants is based on pharmacological studies, and additional approaches are therefore required to ascertain the role of NO as an important signaling molecule in plants. We have expressed a bacterial nitric oxide dioxygenase (NOD) in Arabidopsis plants and/or avirulent Pseudomonas syringae pv tomato to study incompatible plant-pathogen interactions impaired in NO signaling. NOD expression in transgenic Arabidopsis resulted in decreased NO levels in planta and attenuated a pathogen-induced NO burst. Moreover, NOD expression in plant cells had very similar effects on plant defenses compared to NOD expression in avirulent Pseudomonas. The defense responses most affected by NO reduction during the incompatible interaction were decreased H 2 O 2 levels during the oxidative burst and a blockage of Phe ammonia lyase expression, the key enzyme in the general phenylpropanoid pathway. Expression of the NOD furthermore blocked UV light-induced Phe ammonia lyase and chalcone synthase gene expression, indicating a general signaling function of NO in the activation of the phenylpropanoid pathway. NO possibly functions in incompatible plant-pathogen interactions by inhibiting the plant antioxidative machinery, and thereby ensuring locally prolonged H 2 O 2 levels. Additionally, albeit to a lesser extent, we observed decreases in salicylic acid production, a diminished development of hypersensitive cell death, and a delay in pathogenesis-related protein 1 expression during these NO-deficient plant-pathogen interactions. Therefore, this genetic approach confirms that NO is an important regulatory component in the signaling network of plant defense responses.Plants have evolved several mechanisms to defend themselves from bacterial or fungal invasion. The rapid recognition of pathogenic microbes is based on the interaction of products from a pathogen-derived avirulence gene and a plant-derived resistance gene and represents a prerequisite to specific resistance in incompatible plant-pathogen interactions (Flor, 1956). The multicomponent defense responses associated with specific resistance include a burst of reactive oxygen intermediates (ROI; Lamb and Dixon, 1997), transcriptional activation of defense genes encoding phenylpropanoid pathway enzymes, lytic and antimicrobial pathogenesis-related (PR) proteins (Lamb et al., 1989), increase of intracellular levels of salicylic acid (SA; Malamy et al., 1990; Métraux et al., 1990), and development of the hypersensitive response (HR). The HR results in the rapid appearance of a dry, necrotic lesion at the infection site that is clearly delimited from surrounding healthy tissue and is thought to contribute to the limitation of pathogen spread (Keen, 1990).One of the earliest events following pathogen recognition is a burst of oxidative metabolism leading to the generation of su...

The mechanism of the post-translational modulation of nitrate reductase activity (NR, EC 1.6.6.1) is briefly summarized, and it is shown that by this mechanism nitric oxide production through NR is also rapidly modulated. New and partly unexpected details on the modulation mechanism have been obtained by using immunological techniques. The phosphorylation state of NR has been assessed with peptide antibodies raised against the serine phosphorylation motive of spinach NR. By co-immunoprecipitation experiments, 14-3-3 binding to phospho-NR and the function of Mg(2+) in that process has been elucidated. Conflicting data on the role of NR phosphorylation and 14-3-3 binding in controlling NR proteolysis are discussed. A possible role of other NR inactivating proteins is also briefly considered and the regulation of NR of Ricinus communis is described as an interesting special case that differs from the 'normal' mechanism in several important aspects.

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We examined the ratio of neutrophils to lymphocytes (N/L ratio) in the peripheral blood in patients with colorectal cancer. The ability to produce active oxygen and phagocytosis of neutrophils, G-CSF, sIL-2R and IAP (immunosuppressive acidic protein) were also measured. The N/L ratios were significantly higher in the advanced stages of cancer than in normal controls. The ability to produce active oxygen in the terminal stage was 33% lower than in the control group. The G-CSF levels had no relationship with the neutrophil counts. IAP levels increased with cancer stage, and were inversely related to the ability to produce active oxygen. The IAP levels correlated well with the sIL-2R levels and the N/L ratio. These findings suggest that the ability to produce active oxygen, N/L ratio and IAP reflect anticancer mechanisms and that they may be useful when considering treatment or prognosis of patients with advanced stages of cancer.

A hypersensitive response (HR) was induced in tobacco leaves and cell suspensions by the fungal elicitor cryptogein, and NO production was followed by chemiluminescence and occasionally by diaminofluorescein (DAF)-fluorescence. Results from both methods were at least partly consistent, but kinetics was different. NO emission was not induced by cryptogein in leaves, whereas in cell suspensions some weak NO emission was observed, which was nitrate reductase (NR)-dependent, but not required for cell death. Nitric oxide synthase (NOS) inhibitors did not prevent cell death, but PR-1 expression was weakened. In conclusion, neither NR nor NOS appear obligatory for the cryptogein-induced HR. However, a role for NO was still suggested by the fact that the NO scavenger cPTIO prevented the HR. Unexpectedly, cPTI, the reaction product of cPTIO and NO, also impaired the HR but without scavenging NO. Thus, prevention of the HR by cPTIO is not necessarily indicative for a role of NO. Further, even a 100-fold NO overproduction (over wild type) by a nitrite reductase-deficient mutant did not interfere with the cryptogein-induced HR. Accordingly, the role of NO in the HR should be reconsidered.