Carbon monoxide-induced stomatal closure in Vicia faba is dependent on nitric oxide synthesis

Carbon monoxide promotes stomatal initiation by regulating the expression of two EPF genes in Arabidopsis cotyledons

The gaseous molecule carbon monoxide (CO) can freely pass through the cell membrane and participate in signal transduction in the cell to regulate physiological activities in plants. Here, we report that CO has a positive regulatory role in stomatal development. Exogenous CO donor CORM-2 [Tricarbonyldichlororuthenium (II) dimer] treatment resulted in an increase of stomatal index (SI) on the abaxial epidermis of cotyledons in wild-type, which can be reversed by the addition of the CO biosynthesis inhibitor ZnPPIX [Protoporphyrin IX zinc (II)]. Consistent with this result, mutation of the CO biosynthesis gene HY1 resulted in a decrease of SI in hy1-100 plants, while overexpression of HY1 led to an increase of SI. Further investigation revealed that CO acts upstream of SPCH and YDA in the stomatal development pathway, since the loss of function mutants spch-1 and yda-2 were insensitive to CORM-2. The expression of EPF2 was inhibited by CORM-2 treatment in wild type and is lower in hy1 than in wild-type plants. In contrast, the expression of STOMAGEN was promoted by CORM-2 treatment and is higher in HY1-overexpression lines. Loss of function mutants of both epf2 and stomagen are insensitive to CORM-2 treatment. These results indicated that CO positively regulates stomatal initiation and distribution by modulating the expression of EPF2 and STOMAGEN.

Phytomelatonin and gasotransmitters: a crucial combination for plant physiological functions

Melatonin, a molecule that was first identified in animal tissues, has been confirmed to be involved as a potential phytohormone in a variety of plant physiological responses. It is considered primarily as an antioxidant with important actions in controlling reactive oxygen and reactive nitrogen species. In addition to its role in regulating plant growth and development, phytomelatonin is involved in protection against abiotic and biotic stresses. The 'gasotransmitter'-that is, a gaseous signaling molecule-is a new concept that has been advanced in the past two decades, with functions in animal and plant physiological regulation. Gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide, methane, and, more recently identified, hydrogen gas are critical and indispensable in a wide range of biological processes. This review investigates the interrelationship between phytomelatonin and the above-mentioned gasotransmitters from the perspective of biosynthetic origin and functions. Moreover, the potential future research directions for phytomelatonin and gasotransmitters interactions are discussed.

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress

Abscisic acid (ABA) is a stress hormone that accumulates under different abiotic and biotic stresses. A typical effect of ABA on leaves is to reduce transpirational water loss by closing stomata and parallelly defend against microbes by restricting their entry through stomatal pores. ABA can also promote the accumulation of polyamines, sphingolipids, and even proline. Stomatal closure by compounds other than ABA also helps plant defense against both abiotic and biotic stress factors. Further, ABA can interact with other hormones, such as methyl jasmonate (MJ) and salicylic acid (SA). Such cross-talk can be an additional factor in plant adaptations against environmental stresses and microbial pathogens. The present review highlights the recent progress in understanding ABA's multifaceted role under stress conditions, particularly stomatal closure. We point out the importance of reactive oxygen species (ROS), reactive carbonyl species (RCS), nitric oxide (NO), and Ca²⁺ in guard cells as key signaling components during the ABA-mediated short-term plant defense reactions. The rise in ROS, RCS, NO, and intracellular Ca²⁺ triggered by ABA can promote additional events involved in long-term adaptive measures, including gene expression, accumulation of compatible solutes to protect the cell, hypersensitive response (HR), and programmed cell death (PCD). Several pathogens can counteract and try to reopen stomata. Similarly, pathogens attempt to trigger PCD of host tissue to their benefit. Yet, ABA-induced effects independent of stomatal closure can delay the pathogen spread and infection within leaves. Stomatal closure and other ABA influences can be among the early steps of defense and a crucial component of plants' innate immunity response. Stomatal guard cells are quite sensitive to environmental stress and are considered good model systems for signal transduction studies. Further research on the ABA-induced stomatal closure mechanism can help us design strategies for plant/crop adaptations to stress.

Crosstalk among hydrogen sulfide (H2S), nitric oxide (NO) and carbon monoxide (CO) in root-system development and its rhizosphere interactions: A gaseous interactome

Root development in higher plants is achieved by a precise intercellular communication which determines cell fate in the primary embryonic meristem where the gasotransmitters H₂S, NO and CO participate dynamically. Furthermore, the rhizosphere interaction of these molecules with microbial and soil metabolism also affects root development. NO regulates root growth and architecture in association with several other biomolecules like auxin indole-3-acetic acid (IAA), ethylene, jasmonic acid (JA), strigolactones, alkamides and melatonin. The CO-mediated signal transduction pathway in roots is closely linked to the NO-mediated signal cascades. Interestingly, H₂S acts also as an upstream component in IAA and NO-mediated crosstalk during root development. Heme oxygenase (HO) 1 generates CO and functions as a downstream component in H₂S-mediated adventitious rooting and H₂S-CO crosstalk. Likewise, reactive oxygen species (ROS), H₂S and NO crosstalk are important components in the regulation of root architecture. Deciphering these interactions will be a potential biotechnological tool which could provide benefits in crop management in soils, especially under adverse environmental conditions. This review aims to provide a comprehensive update of the complex networks of these gasotransmitters during the development of roots.

Methane Production and Bioactivity-A Link to Oxido-Reductive Stress

Biological methane formation is associated with anoxic environments and the activity of anaerobic prokaryotes (Archaea). However, recent studies have confirmed methane release from eukaryotes, including plants, fungi, and animals, even in the absence of microbes and in the presence of oxygen. Furthermore, it was found that aerobic methane emission in plants is stimulated by a variety of environmental stress factors, leading to reactive oxygen species (ROS) generation. Further research presented evidence that molecules with sulfur and nitrogen bonded methyl groups such as methionine or choline are carbon precursors of aerobic methane formation. Once generated, methane is widely considered to be physiologically inert in eukaryotes, but several studies have found association between mammalian methanogenesis and gastrointestinal (GI) motility changes. In addition, a number of recent reports demonstrated anti-inflammatory potential for exogenous methane-based approaches in model anoxia-reoxygenation experiments. It has also been convincingly demonstrated that methane can influence the downstream effectors of transiently increased ROS levels, including mitochondria-related pro-apoptotic pathways during ischemia-reperfusion (IR) conditions. Besides, exogenous methane can modify the outcome of gasotransmitter-mediated events in plants, and it appears that similar mechanism might be active in mammals as well. This review summarizes the relevant literature on methane-producing processes in eukaryotes, and the available results that underscore its bioactivity. The current evidences suggest that methane liberation and biological effectiveness are both linked to cellular redox regulation. The data collectively imply that exogenous methane influences the regulatory mechanisms and signaling pathways involved in oxidative and nitrosative stress responses, which suggests a modulator role for methane in hypoxia-linked pathologies.

Mechanism of Stomatal Closure in Plants Exposed to Drought and Cold Stress

Drought is one of the abiotic stresses which impairs the plant growth/development and restricts the yield of many crops throughout the world. Stomatal closure is a common adaptation response of plants to the onset of drought condition. Stomata are microscopic pores on the leaf epidermis, which regulate the transpiration/CO₂ uptake by leaves. Stomatal guard cells can sense various abiotic and biotic stress stimuli from the internal and external environment and respond quickly to initiate closure under unfavorable conditions. Stomata also limit the entry of pathogens into leaves, restricting their invasion. Drought is accompanied by the production and/or mobilization of the phytohormone, abscisic acid (ABA), which is well-known for its ability to induce stomatal closure. Apart from the ABA, various other factors that accumulate during drought and affect the stomatal function are plant hormones (auxins, MJ, ethylene, brassinosteroids, and cytokinins), microbial elicitors (salicylic acid, harpin, Flg 22, and chitosan), and polyamines . The role of various signaling components/secondary messengers during stomatal opening or closure has been a matter of intense investigation. Reactive oxygen species (ROS) , nitric oxide (NO) , cytosolic pH, and calcium are some of the well-documented signaling components during stomatal closure. The interrelationship and interactions of these signaling components such as ROS, NO, cytosolic pH, and free Ca²⁺ are quite complex and need further detailed examination.Low temperatures can have deleterious effects on plants. However, plants evolved protection mechanisms to overcome the impact of this stress. Cold temperature inhibits stomatal opening and causes stomatal closure. Cold-acclimated plants often exhibit marked changes in their lipid composition, particularly of the membranes. Cold stress often leads to the accumulation of ABA, besides osmolytes such as glycine betaine and proline. The role of signaling components such as ROS, NO, and Ca²⁺ during cold acclimation is yet to be established, though the effects of cold stress on plant growth and development are studied extensively. The information on the mitigation processes is quite limited. We have attempted to describe consequences of drought and cold stress in plants, emphasizing stomatal closure. Several of these factors trigger signaling components in roots, shoots, and atmosphere, all leading to stomatal closure. A scheme is presented to show the possible signaling events and their convergence and divergence of action during stomatal closure. The possible directions for future research are discussed.

Hydrogen sulfide may function downstream of hydrogen peroxide in mediating darkness-induced stomatal closure in Vicia faba

The relationship between hydrogen sulfide (H2S) and hydrogen peroxide (H2O2) during darkness-induced stomatal closure in Vicia faba L. was investigated by using pharmacological, spectrophotographic and lasers canning confocal microscopic approaches. Darkness-induced stomatal closure was inhibited by H2S scavenger hypotaurine (HT), H2S synthesis inhibitors aminooxy acetic acid (AOA) and hydroxylamine (NH2OH) and potassium pyruvate (N3H3KO3) and ammonia (NH3), which are the products of L-/D-cysteine desulfhydrase (L-/D-CDes). Moreover, darkness induced H2S generation and increased L-/D-CDes activity in leaves of V. faba. H2O2 scavenger and synthesis inhibitors suppressed darkness-induced increase of H2S levels and L-/D-CDes activity as well as stomatal closure in leaves of V. faba. However, H2S scavenger and synthesis inhibitors had no effect on darkness-induced H2O2 accumulation in guard cells of V. faba. From these data it can be deduced that H2S is involved in darkness-induced stomatal closure and acts downstream of H2O2 in V. faba.

Carbon Monoxide as a Signaling Molecule in Plants

Carbon monoxide (CO), a gaseous molecule, has emerged as a signaling molecule in plants, due to its ability to trigger a series of physiological reactions. This article provides a brief update on the synthesis of CO, its physiological functions in plant growth and development, as well as its roles in abiotic stress tolerance such as drought, salt, ultraviolet radiation, and heavy metal stress. CO has positive effects on seed germination, root development, and stomatal closure. Also, CO can enhance plant abiotic stress resistance commonly through the enhancement of antioxidant defense system. Moreover, CO shows cross talk with other signaling molecules including NO, phytohormones (IAA, ABA, and GA) and other gas signaling molecules (H2S, H2, CH4).

Role of hematin and sodium nitroprusside in regulating Brassica nigra seed germination under nanosilver and silver nitrate stresses

Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials, although the mechanisms of AgNP toxicity in terrestrial plants is still unclear. We compared the toxic effects of AgNPs and AgNO3 on Brassica nigra seed germination at physiological and molecular levels. Both AgNPs and AgNO3 inhibited seed germination, lipase activity, soluble and reducing sugar contents in germinating seeds and seedlings. These reductions were more pronounced in AgNP treatments than AgNO3 treatments. Application of 200-400mg/L both AgNPs and AgNO3 increased transcription of heme oxygenase-1. However, at 800, 1600 mg/L, AgNPs or AgNO3 suppressed HO-1 expression. At 400mg/L, AgNPs or AgNO3-induced inhibitory effects on seed germination and were ameliorated by the HO-1 inducer, hematin, or NO donor, sodium nitroprusside (SNP). Additionally, 4 μM hematin and 400 μM SNP were able to markedly boost the HO/NO system. However, the addition of the HO-1 inhibitor (ZnPPIX) or the specific scavenger of NO (cPTIO) not only reversed the protective effects conferred by hematin, but also blocked the up-regulation of HO activity. In addition, hematin-drived NO production in B. niger seeds under AgNPs was confirmed. Our results at physiological and molecular levels suggested that AgNPs were more toxic than AgNO3. Based on these results, for the first time, we suggest that endogenous HO is needed to alleviate AgNPs-induced germination inhibition, which might have a possible interaction with NO.

Carbon monoxide regulates the expression of the wound-inducible gene ipomoelin through antioxidation and MAPK phosphorylation in sweet potato

Carbon monoxide (CO), one of the haem oxygenase (HO) products, plays important roles in plant development and stress adaptation. However, the function of CO involved in wounding responses is seldom studied. A wound-inducible gene, ipomoelin (IPO), of sweet potato (Ipomoea batatas cv. Tainung 57) was used as a target to study the regulation of CO in wounding responses. After wounding for 1h, the endogenous CO content and IbHO expression level were significantly reduced in leaves. IPO expression upon wounding was prohibited by the HO activator hemin, whereas the HO inhibitor zinc protoporphyrin IX elevated IPO expression. The IPO expression induced by wounding, H2O2, or methyl jasmonate was inhibited by CO. CO also affected the activities of ascorbate peroxidase, catalase, and peroxidase, and largely decreased H2O2 content in leaves. CO inhibited the extracellular signal-regulated kinase (ERK) phosphorylation induced by wounding. IbMAPK, the ERK of sweet potato, was identified by immunoblotting, and the interaction with its upstream activator, IbMEK1, was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Conclusively, wounding in leaves repressed IbHO expression and CO production, induced H2O2 generation and ERK phosphorylation, and then stimulated IPO expression.

Nitric-oxide inhibits nyctinastic closure through cGMP in Albizia lophantha leaflets

Nitric oxide (NO) is a highly reactive radical that acts as a direct or indirect cellular signalling molecule in plant growth, development and environmental responses. Here we studied the contribution of NO to the control of leaflet movements during nyctinastic closure. For this purpose, we tested the effect of NO donors and an NO scavenger, all supplied in light, on Albizia lophantha leaflet closure after transferral to darkness. Exogenous NO, applied as four donors [sodium nitroprusside (SNP), diethylammonium (Z)-1-(N,N-diethylamino) diazen-1-ium-1,2-diolate (DEA-NONOate), S-nitroso-N-acetylpenicillamine (SNAP) and S-nitrosoglutathione (GS-NO)], inhibited nyctinastic leaflet closure while the application of an NO scavenger [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO)] plus SNP cancelled the effect of the latter. The inclusion of Nω-nitro-l-arginine methyl ester (l-NAME) or sodium tungstate in the incubation media enhanced nyctinastic closure and also resulted in a decrease in the nitrate plus nitrite released by leaflets into the incubation solution. These results support the notion that NO is involved in regulating the nyctinastic closure of A. lophantha leaflets. Cellular perception of NO did not appear to be mediated by calcium. Pharmacological application of inhibitors of soluble guanylate cyclase (sGC) [1H-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ) and 6-anilino-5,8-quinolinequinone (Ly83583)], phosphodiesterase type 5 (PDE5) (Sildenafil) and the cyclic guanosine monophosphate (cGMP) analogue 8-bromoguanosine-3',5'-cyclomonophosphate sodium salt (8-Br-cGMP) indicated that cGMP was downstream of the NO signalling cascade during nyctinastic closure.

Gasotransmitters are emerging as new guard cell signaling molecules and regulators of leaf gas exchange

Specialized guard cells modulate plant gas exchange through the regulation of stomatal aperture. The size of the stomatal pore is a direct function of the volume of the guard cells. The transport of solutes across channels in plasma membrane is a crucial process in the maintenance of guard cell water status. The fine tuned regulation of that transport requires an integrated convergence of multiple endogenous and exogenous signals perceived at both the cellular and the whole plant level. Gasotransmitters are novel signaling molecules with key functions in guard cell physiology. Three gasotransmitters, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S) are involved in guard cell regulatory processes. These molecules are endogenously produced by plant cells and are part of the guard cells responses to drought stress conditions through ABA-dependent pathways. In this review, we summarize the current knowledge of gasotransmitters as versatile molecules interacting with different components of guard cell signaling network and propose them as players in new paradigms to study ABA-independent guard cell responses to water deficit.

Carbon monoxide enhances the chilling tolerance of recalcitrant Baccaurea ramiflora seeds via nitric oxide-mediated glutathione homeostasis

Both carbon monoxide (CO) and nitric oxide (NO) play fundamental roles in plant responses to environmental stress. Glutathione (GSH) homeostasis through the glutathione-ascorbate cycle regulates the cellular redox status and protects the plant from damage due to reactive oxygen species (ROS) or reactive nitrogen species (RNS). Most recalcitrant seeds are sensitive to chilling stress, but the roles of and cross talk among CO, NO, ROS, and GSH in recalcitrant seeds under low temperature are not well understood. Here, we report that the germination of recalcitrant Baccaurea ramiflora seeds shows sensitivity to chilling stress, but application of exogenous CO or NO markedly increased GSH accumulation, enhanced the activities of antioxidant enzymes involved in the glutathione-ascorbate cycle, decreased the content of H(2)O(2) and RNS, and improved the tolerance of seeds to low-temperature stress. Compared to orthodox seeds such as maize, only transient accumulation of CO and NO was induced and only a moderate increase in GSH was shown in the recalcitrant B. ramiflora seeds. Exogenous CO or NO treatment further increased the GSH accumulation and S-nitrosoglutathione reductase (GSNOR) activity in B. ramiflora seeds under chilling stress. In contrast, suppressing CO or NO generation, removing GSH, or blocking GSNOR activity resulted in increases in ROS and RNS and impaired the germination of CO- or NO-induced seeds under chilling stress. Based on these results, we propose that CO acts as a novel regulator to improve the tolerance of recalcitrant seeds to low temperatures through NO-mediated glutathione homeostasis.

Ethylene inhibits darkness-induced stomatal closure by scavenging nitric oxide in guard cells of Vicia faba

The plant hormone ethylene regulates many aspects of plant growth and development. Despite the well-known relationship between ethylene and stress signalling, the involvement of ethylene in regulating stomatal movement is not completely explored. Here, the role and association between nitric oxide (NO) reduction and the inhibition of darkness-induced stomatal closure by ethylene was studied. Physiological data are provided that both ethylene-releasing compound 2-chloroethylene phosphonic acid (ethephon, ETH) and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, reduced the levels of NO in Vicia faba L. guard cells, and then induced stomatal opening in darkness. In addition, ACC and ETH not only reduced NO levels in guard cells caused by exogenous NO (derived from sodium nitroprusside, SNP) in light, but also abolished NO that had been generated during a dark period and promoted stomatal opening. Interestingly, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and hemoglobin (Hb), NO scavenger and the potent scavenger of NO/carbon monoxide (CO), respectively, also reduced NO levels by SNP and darkness. However, the above-mentioned effects of ACC and ETH were dissimilar to that of nitric oxide synthase (enzyme commission 1.14.13.39) inhibitor NG-nitro-L-Arg-methyl ester (L-NAME), which could neither reduce NO levels by SNP nor abolish NO that had been generated in the dark. Thus, it is concluded that ethylene reduces the levels of NO in V. faba guard cells via a pattern of NO scavenging, then induces stomatal opening in the dark.

Quantitating carbon monoxide production from heme by vascular plant preparations in vitro

Heme in animals is mainly degraded enzymatically, producing a predictable amount of carbon monoxide (CO). Under some conditions, alternative sources of CO production are important, such as lipid peroxidation and photo-oxidation. Less is known about CO production in plants as a reflection of enzymatic activity or coupled oxidation, but a sensitive assay for CO production in plants would be a valuable tool to explore the various sources in plants as the conditions of the reactions and mechanisms are defined. Using gas chromatography, we determined the requirements for heme-supported in vitro CO generation by exogenous reactants (NADPH, tissue supernatant, oxygen), optimum reaction conditions (time, temperature, pH, light), and effects of various cofactors and substrates using supernatants from Spinacia oleracea (spinach) leaf and Solanum tuberosa (potato) tuber homogenates. We then determined the CO production rate distribution between organ (root, stem, leaf, flower, fruit) supernatants in a number of commercially available plant species. CO production ranged from 4-65 nmol CO/h/g fresh weight and occurred in all vascular plant tissues examined, with the highest rates in chloroplast-containing tissues. In spinach leaves, CO production was concentrated (>2-fold) in the particulate fraction, whereas in potato tubers, the particulate fraction accounted for <50% of the rates in homogenates. We conclude that gas chromatography is uniquely suited for the determination of CO production in pigmented, heterogeneous plant tissue preparations.

Heme oxygenase/carbon monoxide system participates in regulating wheat seed germination under osmotic stress involving the nitric oxide pathway

To investigate the mechanism and signaling pathway of carbon monoxide (CO) and hematin in alleviating seed germination inhibition and lipid peroxidation, polyethylene glycol-6000 (PEG) was used to mimic osmotic stress in a series of experiments. The results showed that wheat seeds pretreated with a lower dose of PEG (12.5%) showed higher tolerance against osmotic stress as well as the up-regulation of heme oxygenase (HO, EC 1.14.99.3) and decreased lipid peroxidation during recuperation, compared to those with a higher dose of PEG (50%). Exposure of wheat seeds to 25% PEG, HO-1 inhibitor or specific scavenger of nitric oxide (NO) alone differentially led to seed germination inhibition. The PEG-induced inhibitory effects on seed germination were ameliorated by the HO-1 inducer hematin, CO or NO donor. Additionally, hematin was able to markedly boost the HO/CO system. However, the addition of the HO-1 inhibitor or the specific scavenger of NO not only reversed the protective effects conferred by hematin, but also blocked the up-regulation of HO/CO. In addition, hematin-driven NO production in wheat seeds under osmotic stress was confirmed. Based on these results, we conclude that the endogenous HO/CO signal system is required for the alleviation of osmotic stress-induced wheat seed germination inhibition and lipid peroxidation, which might have a possible interaction with NO.

Heme oxygenase is involved in cobalt chloride-induced lateral root development in tomato

In animals, heme oxygenase (HO), a rate-limiting enzyme responsible for carbon monoxide (CO) production, was regarded as a protective system maintaining cellular homeostasis. It was also established that metal ions are powerful HO-inducing agents and cobalt chloride (CoCl(2)) was the first metal ion identified with an inducing property. Previous study suggests that CoCl(2) stimulates adventitious root formation in tomato and cucumber cuttings. In this test, we discover that both CoCl(2) and an inducer of HO-1, hemin, could lead to the promotion of lateral root development, as well as the induction of HO-1 protein expression, HO activity, or LeHO-1/2 transcripts, in lateral root initiation zone of tomato seedlings. The effect is specific for HO since the potent HO-1 inhibitor zinc protoporphyrin IX (ZnPPIX) blocked the above actions of CoCl(2), and the inhibitory effect was reversed partially when 50% CO aqueous solution was added. However, the addition of ascorbic acid (AsA), a well-known antioxidant, exhibited no obvious effect on lateral root formation. Molecular evidence further showed that CoCl(2)-induced the up-regulation of target genes responsible for lateral root formation, including LeCDKA1, LeCYCA2;1, and LeCYCA3;1, was suppressed differentially by ZnPPIX. And these decreases were reversed further by the addition of CO. All together, these results suggest a novel role for HO in the CoCl(2)-induced tomato lateral root formation.

The beneficial effect of small toxic molecules on dormancy alleviation and germination of apple embryos is due to NO formation

Deep dormancy of apple (Malus domestica Borkh.) seeds is terminated by a 3-month-long cold stratification. It is expressed by rapid germination of seeds and undisturbed growth of seedlings. However, stimulation of germination of isolated apple embryos is also observed after applying inhibitors of cytochrome c oxidase: nitric oxide (NO) or hydrogen cyanide (HCN) during the first 3-6 h of imbibition of dormant embryos. The aim of this work was to compare the effect of yet another toxic gaseous molecule carbon monoxide (CO) with the effects of HCN and NO on germination of apple embryos and growth and development of young seedlings. We demonstrated that stimulation of germination after short-term pre-treatment with HCN, NO or CO was accompanied by enhanced NO emission from the embryo axes during their elongation. Moreover, similarly high NO production from non-dormant embryos, after cold stratification, was detected. Therefore, we propose that NO may act as signaling molecule in apple embryo dormancy break.