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Higher plant mitochondria produce nitric oxide (NO) by two separate systems. One is a mitochondrial nitric oxide synthase (NOS), which catalyzes the synthesis of NO and -citrulline from -arginine using NAD(P)H. The other one is the respiratory electron transport chain, with the terminal oxidases, CytOx and AOX, which both reduce nitrite to NO. While oxygen is obligatory for the former reaction, the latter activity appears very low in air but high under oxygen deficiency. However, even under anoxia, the rate of nitrite:NO reduction rarely reaches ±1% of respiratory electron transport. For as yet unknown reasons, nitrite:NO reduction appears absent in mitochondria from green leaves. The contribution of NOS and of nitrite reduction to overall NO production, and possible functions of nirite:NO reduction under hypoxia/anoxia are discussed.

Nitric oxide is an intermediate product of inorganic nitrogen assimilation. In plants, it can be formed either by reducing inorganic nitrogen by the nitrite-dependent pathway or by oxidation of organic nitrogen by the arginine-dependent pathway. Both pathways require adequate nitrogen supply to the plant and may not operate under nitrogen deficiency. However, the pathways are differently regulated in relation to oxygen availability and, therefore, have a different importance for underground organs like roots, than for above-ground organs like the shoot.

In animals, nitric oxide (NO) is an endogenously produced radical involved in cell communication and signal transduction. Its functions in plants are currently being discovered at an unprecedented pace, and insight into NO-derived mechanisms has mainly been gained from research on signal transduction. Numerous studies have firmly placed NO as one component of the signal perception–transduction network that connects plant responses to primary signals, including hormones, elicitors of defence responses or abiotic stresses. Protein kinases and the second messengers , cGMP, and cADPR convey part of the NO signal within cells. Furthermore, NO-based protein modifications are emerging as broad-based mechanisms for posttranslational regulation of protein function and might be implied in the regulation of numerous signaling pathways.

Nitric oxide (NO) has become recognized as a key signaling molecule in plants over the last few years, but still little is known about the way in which NO regulates different events in plants. Analyses of NO-dependent processes in animal systems have demonstrated protein S-nitrosylation – the covalent attachment of NO to the sulfhydryl group of cysteine residues – to be one of the dominant regulation mechanisms for many animal proteins. This reversible protein modification is an important posttranslational, redox-based regulation mechanism for many proteins of different classes in animals. For plants, however, the importance of protein S-nitrosylation remained to be elucidated.

This chapter will discuss the chemistry of S-nitrosothiol formation and the release of NO from S-nitrosylated cysteine residues, as well as the specificity and regulation of S-nitrosylation. Furthermore, the identification of plant proteins as candidates for this type of protein modification, and the physiological functions of protein S-nitrosylation in plants are described.

NO is an inorganic free radical gaseous molecule that has been shown to play an unprecedented range of roles in biological systems. Taking into account these numerous functions and the reports indicating that NO can regulate processes related to plant growth and development, endogenous sources of NO need to be clarified as well as the effect of the variations of NO levels upon the plant life cycle. We consider here the main endogenous sources of cellular NO in plant tissues, and the contribution of enzymatic sources upon seed germination. Non-enzymatic generation of NO from NO under conditions of low pH could be of considerable importance since significant amounts of NO can be found in plant tissues. However, at least under the reported experimental conditions, enzymatic activity seems to be more relevant to total NO generation, allowing a strict control of NO steady state concentration. The complexity of the overall scenario presented here shows the need for further studies into NO production and consumption pathways in germinating seeds exposed to fluctuating environmental conditions.

In the course of evolution, higher plants developed a special reproductive organ, the seed, which ensures their spatio-temporal distribution and perpetuation of the species. The fate of the future plant is almost completely determined when the seed “decides” to germinate. A number of dormancy mechanisms serve to detect surrounding conditions and define the appropriate point in time for germination. To ensure survival of the future seedling, environmental conditions have to be detected, integrated, and translated through different signaling molecules at the seed level, even before germination starts. One of the ten smallest molecular species known, nitric oxide, is now recognized as an endogenous mediator of seed germination, external dormancy-breaking agent, and outer information carrier that provides the seeds with integral information on the factors most important for plant growth and development.

Nitric oxide (NO) is a chemical messenger that actively operates in the plant kingdom. In recent years, NO has been shown to be involved in many and diverse growth, developmental, and physiological processes in plants. It has been shown that NO takes part in different hormone signaling pathways and also acts in concert with well-characterized second messengers. In this chapter, we discuss findings that contribute to the understanding of the role/s of NO during root organogenesis and stomatal movement, focusing on the interrelations between NO and the phytohormones auxin and abscisic acid. We emphasize the requirement of calcium as an essential intermediate present in the NO-mediated responses. Finally, novel data concerning the cross-talk between NO and phosphatidic acid signaling pathways in response to (a)biotic stresses are presented and discussed.

New roles are emerging for nitric oxide (NO) besides senescence and defense. While NO itself is probably used as a chemical weapon in defense, in several examples the biosynthesis of NO is up-regulated so rapidly that a function of NO as a second messenger in signal transduction seems likely. We investigated this postulate with the signal substances cytokinin and polyamines. After polyamine addition to seedlings, net NO biosynthesis increased with no apparent lag phase, and after zeatin addition, NO increased within 3 min. Thus, the up-regulation of NO levels is faster than activation of gene expression, consistent with our hypothesis of NO being a second messenger. A role for NO in signal transduction constitutes a new finding for both classes of signaling substances – polyamines and cytokinins. Cytokinin–polyamine crosstalk has already been indicated by known overlaps in the physiology of both signals, and the results reported here strengthen the case for this crosstalk. In addition, known multiple functions of polyamines in the physiology of pathogen defense, abiotic stress, hormones, and embryogenesis are already known to involve NO as a mediator. We discuss this new “input” into polyamine physiology.

Nitric oxide (NO) has profound effects on the regulation of ion channels in plants. Although direct evidence to date comes exclusively from electrophysiological studies of guard cells, there is good reason to expect similar patterns of action in other plant cell types as well. As in animals, NO appears to act through two distinct mechanisms. One mechanism is mediated via stimulation of guanylate cyclase, which leads to a rise in cyclic ADP-ribose and, in turn, an increase in the efficacy of Ca release triggered by Ca entry across the plasma membrane. This signal cascade underpins intracellular Ca release and the elevation of cytosolic-free [Ca] by the water-stress hormone abscisic acid and leads to profound changes in K and Cl channel activities, to facilitate the ion fluxes for stomatal closure. The second mechanism appears to arise from direct, covalent modification of ion channels by NO, notably of the outward-rectifying K channel at the guard cell plasma membrane. The physiological significance of this process of S-nitrosylation has yet to be explored in depth, but almost certainly is allied to plant cell responses to pathogen attack and apoptosis. Both processes, and ion transport in guard cells generally, are now sufficiently well-defined for a full description with accurate kinetics and flux equations in which all of the key parameters are constrained by experimental data. Thus, guard cells are now a prime focus for integrative (so-called systems biology) approaches. Applications of integrative analysis have already demonstrated the potential for accurately predicting physiological behaviours and signal interactions with membrane ion transport.

The establishment and functioning of nitrogen-fixing symbioses between legumes and rhizobia rely on a succession of infectious, developmental, and metabolic processes, which end in the formation of root nodules and the acquisition of nitrogen-fixing capacity by the bacteria. A tight regulation is required for the establishment of a successful interaction, and the identification of the regulatory network operating is therefore a major challenge for a better understanding of symbiotic associations. It is now well established that nitric oxide (NO) is a major signaling molecule controlling not only plant interactions with pathogens, but also plant development. This review presents our current knowledge on the metabolism and possible roles of NO during symbiotic interactions. In particular, the involvement of NO in nitrogen-fixation regulation, O-deprivation sensing, and nodule development are discussed.