An ATP signalling pathway in plant cells: extracellular ATP triggers programmed cell death in Populus euphratica

We elucidated the extracellular ATP (eATP) signalling cascade active in programmed cell death (PCD) using cell cultures of Populus euphratica. Millimolar amounts of eATP induced a dose‐ and time‐dependent reduction in viability, and the agonist‐treated cells displayed hallmark features of PCD. eATP caused an elevation of cytosolic Ca²⁺ levels, resulting in Ca²⁺ uptake by the mitochondria and subsequent H₂O₂ accumulation. P. euphratica exhibited an increased mitochondrial transmembrane potential, and cytochrome c was released without opening of the permeability transition pore over the period of ATP stimulation. Moreover, the eATP‐induced increase of intracellular ATP, essential for the activation of caspase‐like proteases and subsequent PCD, was found to be related to increased mitochondrial transmembrane potential. NO is implicated as a downstream component of the cytosolic Ca²⁺ concentration but plays a negligible role in eATP‐stimulated cell death. We speculate that ATP binds purinoceptors in the plasma membrane, leading to the induction of downstream intermediate signals, as the proposed sequence of events in PCD signalling was terminated by the animal P2 receptor antagonist suramin.     

Extracellular ATP: an unexpected role as a signaler in plants

ATP and other nucleoside triphosphates not only drive energy-dependent reactions inside cells, but can also function outside the plasma membrane in the extracellular matrix, where they function as agonists that can induce diverse physiological responses without being hydrolyzed. This external role of ATP is well established in animal cells but only recently has it become apparent that extracellular ATP (eATP) can also function as a signaling agent in plants. Recent data have shown that eATP and other nucleotides can induce an increase in the cytosolic Ca(2+) concentration and diverse downstream changes that influence plant growth and defense responses. Ectoapyrase enzymes that regulate the eATP concentration also have an impact on plant growth. These results beg the question of whether there is a receptor that can bind to eATP and transduce this into signaling changes. Answering this will be key to understanding how eATP and ectoapyrases influence plant growth and development.     

The effects of extracellular adenosine 5′‐triphosphate on the tobacco proteome

Extracellular adenosine 5′‐triphosphate (eATP) is emerging as an important plant signalling compound capable of mobilising intracellular second messengers such as Ca²⁺, nitric oxide, and reactive oxygen species. However, the downstream molecular targets and the spectrum of physiological processes that eATP regulates are largely unknown. We used exogenous ATP and a non‐hydrolysable analogue as probes to identify the molecular and physiological effects of eATP‐mediated signalling in tobacco. 2‐DE coupled with MS/MS analysis revealed differential protein expression in response to perturbation of eATP signalling. These proteins are in several functional classes that included photosynthesis, mitochondrial ATP synthesis, and defence against oxidative stress, but the biggest response was in the pathogen defence‐related proteins. Consistent with this, impairment of eATP signalling induced resistance against the bacterial pathogen Erwinia carotovora subsp. carotovora. In addition, disease resistance activated by a fungal pathogen elicitor (xylanase from Trichoderma viride) was concomitant with eATP depletion. These results reveal several previously unknown putative molecular targets of eATP signalling, which pinpoint eATP as an important hub at which regulatory signals of some major primary metabolic pathways and defence responses are integrated.     

Extracellular ATP signaling and homeostasis in plant cells

Extracellular ATP (eATP) is now recognized as an important signaling agent in plant growth and defense response to environmental stimuli. eATP has dual functions in plant cell signaling, which is largely dependent on its concentration in the extracellular matrix (ECM). A lethal level of eATP (extremely low or high) causes cell death, whereas a moderate level of eATP benefits plant growth and development. Ecto-apyrases (Nucleoside Triphosphate-Diphosphohydrolase) help control the eATP concentrations in the ECM, and thus contributing to the mediation of plant growth and defense response upon environmental stress. In this review, we summarize eATP signaling in plants and highlight the correlation between eATP homeostasis control and programmed cell death.     

Porphyromonas gingivalis‐nucleoside‐diphosphate‐kinase inhibits ATP‐induced reactive‐oxygen‐species via P2X7 receptor/NADPH‐oxidase signalling and contributes to persistence

Ligation of P2X₇ receptors with a ‘danger signal’, extracellular ATP (eATP), has recently been shown to result in production of intracellular reactive‐oxygen‐species (ROS) in macrophages. We show that primary gingival epithelial cells (GECs) produce sustained, robust cellular ROS upon stimulation by eATP. The induction of ROS was mediated by P2X₇ receptor signalling coupled with NADPH‐oxidase activation, as determined by pharmacological inhibition and RNA interference. Furthermore, Porphyromonas gingivalis, an oral opportunistic pathogen, upregulated the antioxidant glutathione response, modulated eATP‐induced cytosolic and mitochondrial ROS generated through P2X₇/NADPH‐oxidase interactome, and subsequently blocked oxidative stress in GECs via temporal secretion of a P. gingivalis effector, nucleoside‐diphosphate‐kinase (Ndk). An ndk‐deficient P. gingivalis mutant lacked the ability to inhibit ROS production and persist intracellularly following eATP stimulation. Treatment with recombinant Ndk significantly diminished eATP‐evoked ROS production. P. gingivalis infection elicited a strong, time‐dependent increase in anti‐oxidativemitochondrial UCP₂ levels, whereas ndk‐deficient mutant did not cause any change. The results reveal a novel signalling cascade that is tightly coupled with eATP signalling and ROS regulation. Ndk by P. gingivalis counteracts these antimicrobial signalling activities by secreting Ndk, thus contributing to successful persistence of the pathogen.     

Proteomic Analysis of Extracellular ATP-Regulated Proteins Identifies ATP Synthase β-Subunit as a Novel Plant Cell Death Regulator

Extracellular ATP is an important signal molecule required to cue plant growth and developmental programs, interactions with other organisms, and responses to environmental stimuli. The molecular targets mediating the physiological effects of extracellular ATP in plants have not yet been identified. We developed a well characterized experimental system that depletes Arabidopsis cell suspension culture extracellular ATP via treatment with the cell death-inducing mycotoxin fumonisin B1. This provided a platform for protein profile comparison between extracellular ATP-depleted cells and fumonisin B1-treated cells replenished with exogenous ATP, thus enabling the identification of proteins regulated by extracellular ATP signaling. Using two-dimensional difference in-gel electrophoresis and matrix-assisted laser desorption-time of flight MS analysis of microsomal membrane and total soluble protein fractions, we identified 26 distinct proteins whose gene expression is controlled by the level of extracellular ATP. An additional 48 proteins that responded to fumonisin B1 were unaffected by extracellular ATP levels, confirming that this mycotoxin has physiological effects on Arabidopsis that are independent of its ability to trigger extracellular ATP depletion. Molecular chaperones, cellular redox control enzymes, glycolytic enzymes, and components of the cellular protein degradation machinery were among the extracellular ATP-responsive proteins. A major category of proteins highly regulated by extracellular ATP were components of ATP metabolism enzymes. We selected one of these, the mitochondrial ATP synthase β-subunit, for further analysis using reverse genetics. Plants in which the gene for this protein was knocked out by insertion of a transfer-DNA sequence became resistant to fumonisin B1-induced cell death. Therefore, in addition to its function in mitochondrial oxidative phosphorylation, our study defines a new role for ATP synthase β-subunit as a pro-cell death protein. More significantly, this protein is a novel target for extracellular ATP in its function as a key negative regulator of plant cell death.ATP is a ubiquitous, energy-rich molecule of fundamental importance in living organisms. It is a key substrate and vital cofactor in many biochemical reactions and is thus conserved by all cells. However, in addition to its localization and functions inside cells, ATP is actively secreted to the extracellular matrix where it forms a halo around the external cell surface. The existence of this extracellular ATP (eATP)¹ has been reported in several organisms including bacteria (1), primitive eukaryotes (2), animals (3), and plants (4–6). This eATP is not wasted, but harnessed at the cell surface as a potent signaling molecule enabling cells to communicate with their neighbors and regulate crucial growth and developmental processes.In animals, eATP is a crucial signal molecule in several physiological processes such as neurotransmission (7, 8), regulation of blood pressure (9), enhanced production of reactive oxygen species (ROS) (10), protein translocation (11), and apoptosis (12). Extracellular ATP signal perception at the animal cell surface is mediated by P2X and P2Y receptors, which bind ATP extracellularly and recruit intracellular second messengers (13, 14). P2X receptors are ligand-gated ion channels that provide extracellular Ca²⁺ a corridor for cell entry after binding eATP, facilitating a surge in cytosolic [Ca²⁺] that is essential in activating down-stream signaling. P2Y receptors transduce the eATP signal by marshalling heteromeric G-proteins on the cytosolic face of the plasma membrane and activating appropriate downstream effectors.Although eATP exists in plants, homologous P2X/P2Y receptors for eATP signal perception have not yet been identified, even in plant species with fully sequenced genomes. Notwithstanding the obscurity of plant eATP signal sensors, some of the key downstream messengers recruited by eATP-mediated signaling are known. For example, eATP triggers a surge in cytosolic Ca²⁺ concentration (15–17) and a heightened production of nitric oxide (18–20) and reactive oxygen species (17, 21, 22). Altering eATP levels is attended by activation of plant gene expression (16, 21) and changes in protein abundance (5, 23), indicating that eATP-mediated signaling impacts on plant physiology. Indeed eATP has been demonstrated to regulate plant growth (20, 24–26), gravitropic responses (27), xenobiotic resistance (4), plant-symbiont interactions (28), and plant-pathogen interactions (23, 29). However, the mechanism by which eATP regulates these processes remains unclear, largely because the eATP signal sensors and downstream signal regulatory genes and proteins have not been identified.We previously reported that eATP plays a central regulatory role in plant cell death processes (5). Therefore, an understanding of the signaling components galvanized by eATP in cell death regulation might serve a useful purpose in providing mechanistic detail of how eATP signals in plant physiological processes. We found that eATP-mediated signaling negatively regulates cell death as its removal by application of ATP-degrading enzymes to the apoplast activates plant cell death (5). Remarkably, fumonisin B1 (FB1), a pathogen-derived molecule that activates defense gene expression in Arabidopsis (30), commandeers this eATP-regulated signaling to trigger programmed cell death (5). FB1 is a mycotoxin secreted by fungi in the genus Fusarium and initiates programmed cell death in both animal and plant cells (31, 32). In Arabidopsis, FB1 inaugurates cell death by inactivating eATP-mediated signaling via triggering a drastic collapse in the levels of eATP (5). FB1-induced Arabidopsis programmed cell death is dependent on the plant signaling hormone salicylic acid (33), which is a key regulator of eATP levels (29). Because concurrent application of FB1 and exogenous ATP to remedy the FB1-induced eATP deficit blocks death, FB1 and exogenous ATP treatments can therefore be used as probes to identify the key signal regulators downstream of eATP in cell death control. This is vital for achieving the global objective of elucidating the mechanism of eATP signaling in plant physiology.Gel-based proteomic analyses have been previously applied to successfully identify the novel role of eATP in the regulation of plant defense gene expression and disease resistance (23, 29). We have now employed FB1 and ATP treatments together with two-dimensional difference in-gel electrophoresis (DIGE) and matrix-assisted laser desorption-time of flight MS (MALDI-TOF MS) to identify the changes in Arabidopsis protein profiles associated with a shift from normal to cell death-inception metabolism. Additional reverse genetic analyses enabled us to definitively identify a putative ATP synthase β-subunit as a target for eATP-mediated signaling with an unexpected function in the regulation of plant programmed cell death.     

Plant extracellular ATP signalling: new insight from proteomics

Complex signalling systems have evolved in multicellular organisms to enable cell-to-cell communication during growth and development. In plants, cell communication via the extracellular matrix (apoplast) controls many processes vital for plant survival. Secretion of ATP into the extracellular matrix is now recognised as a previously unknown stimulus for cell signalling with a role in many aspects of plant physiology. In the last decade, the secondary messenger molecules in extracellular ATP signalling were identified, but the downstream gene and protein networks that underpin plant responses to extracellular ATP are only beginning to be characterised. Here we review the current status of our knowledge of plant extracellular signalling and demonstrate how applying state-of-the art proteomic technologies is rapidly bringing new discoveries in extracellular ATP research. We discuss how monitoring of the global proteomic profile during responses to modulation of extracellular ATP signalling has led to novel insight into pathogen defence systems and plant programmed cell death regulation. On the basis of extensive proteomic, pharmacological, and reverse genetics data, extracellular ATP has been confirmed to constitute an important molecular switch that tightly controls organellar energy metabolism, reprogramming of primary metabolic pathways, and redirection of resources to protein networks that support adaptation of plants to stress.     

植物细胞外信号分子eATP研究进展

细胞外ATP(extracellular ATP, eATP)是目前公认的细胞外信号分子, 参与调控多种环境刺激下植物的生长、发育和防御反应。在植物细胞信号转导过程中, eATP具有双重功能, 其作用主要取决于细胞外基质中eATP的浓度。eATP含量过高或者过低都会导致细胞死亡, 适度水平的eATP则有助于植物的生长和发育。细胞外三磷酸腺苷双磷酸酶(Apyrase)严格控制细胞外基质中eATP的水平, 因此有助于调控植物在逆境条件下的生长和防御反应。该文总结了植物中eATP的发现、产生和清除以及受体和信号转导等研究进展, 重点论述eATP在逆境条件下的生理功能, 并对植物eATP的研究方向作了展望。     

Apyrases, extracellular ATP and the regulation of growth

Although no definitive receptor for extracellular ATP (eATP) has been identified in plants, there is now stronger physiological evidence that the effects of eATP on plant growth are mediated by a receptor, or, as in animals, by multiple receptors. Recent papers clarify how extracellular nucleotides induce changes in [Ca(2+)](cyt), and the production of nitric oxide (NO) and reactive oxygen species. They document links between eATP signaling and the synthesis or transport of hormones, and they reveal that applied nucleotides can regulate the aperture of stomates, which release ATP when stimulated by light and hormones. Ectoapyrases (ecto-nucleoside triphosphate-diphosphohydrolase) help control both the diverse signaling changes and downstream growth changes induced by extracellular nucleotides by limiting their concentration in the extracellular matrix (ECM).     

Extracellular ATP is a regulator of pathogen defence in plants

In healthy plants extracellular ATP (eATP) regulates the balance between cell viability and death. Here we show an unexpected critical regulatory role of eATP in disease resistance and defensive signalling. In tobacco, enzymatic depletion of eATP or competition with non-hydrolysable ATP analogues induced pathogenesis-related ( PR ) gene expression and enhanced resistance to tobacco mosaic virus and Pseudomonas syringae pv. tabaci . Artificially increasing eATP concentrations triggered a drop in levels of the important defensive signal chemical salicylic acid (SA) and compromised basal resistance to viral and bacterial infection. Inoculating tobacco leaf tissues with bacterial pathogens capable of activating PR gene expression triggered a rapid decline in eATP. Conversely, inoculations with mutant bacteria unable to induce defence gene expression failed to deplete eATP. Furthermore, a collapse in eATP concentration immediately preceded PR gene induction by SA. Our study reveals a previously unsuspected role for eATP as a negative regulator of defensive signal transduction and demonstrates its importance as a key signal integrating defence and cell viability in plants.     

Extracellular ATP in plants. Visualization, localization, and analysis of physiological significance in growth and signaling

Extracellular ATP (eATP) in animals is well documented and known to play an important role in cellular signaling (e.g. at the nerve synapse). The existence of eATP has been postulated in plants; however, there is no definitive experimental evidence for its presence or an explanation as to how such a polar molecule could exit the plant cell and what physiological role it may play in plant growth and development. The presence of eATP in plants (Medicago truncatula) was detected by constructing a novel reporter; i.e. fusing a cellulose-binding domain peptide to the ATP-requiring enzyme luciferase. Application of this reporter to plant roots allowed visualization of eATP in the presence of the substrate luciferin. Luciferase activity could be detected in the interstitial spaces between plant epidermal cells and predominantly at the regions of actively growing cells. The levels of eATP were closely correlated with regions of active growth and cell expansion. Pharmacological compounds known to alter cytoplasmic calcium levels revealed that ATP release is a calcium-dependent process and may occur through vesicular fusion, an important step in the polar growth of actively growing root hairs. Reactive oxygen species (ROS) activity at the root hair tip is not only essential for root hair growth, but also dependent on the cytoplasmic calcium levels. Whereas application of exogenous ATP and a chitin mixture increased ROS activity in root hairs, no changes were observed in response to adenosine, AMP, ADP, and nonhydrolyzable ATP (betagammameATP). However, application of exogenous potato (Solanum tuberosum) apyrase (ATPase) decreased ROS activity, suggesting that cytoplasmic calcium gradients and ROS activity are closely associated with eATP release.     

不同光强下细胞外三磷酸腺苷对菜豆叶片光化学反应特性的影响

三磷酸腺苷(ATP)不但分布在细胞内部, 而且广泛存在于动物和植物细胞的细胞外基质中。细胞外ATP (eATP)可与细胞膜表面相应的受体结合并激发细胞内的第二信使, 从而调节细胞的多种生理学功能。但目前对于eATP是否也能对植物的光合作用产生影响则研究较少。该文以菜豆(Phaseolus vulgaris)叶片为实验材料, 研究了在不同光强下eATP对菜豆叶片叶绿素荧光特性和光合放氧速率的影响。结果显示, 随着光强的增加, 叶片的光适应下最大光化学效率(F_v′/F_m′)、光系统II (PSII)实际光化学效率(Y(II))、光化学猝灭系数(q_P)均呈现下降趋势, 而电子传递速率(ETR)、非光化学猝灭系数(q_N)以及调节性能量耗散的量子产量(Y(NPQ))随着光强的增加呈上升趋势。与对照相比, eATP的处理可以显著提高菜豆叶片PSII的潜在最大光化学效率(F_v/F_m)、Y(II)、q_P、ETR和光合放氧速率; 但eATP的处理对F_v′/F_m′、q_N以及Y(NPQ)没有显著影响。AMP-PCP (β,γ-亚甲基三磷酸腺苷, eATP细胞外受体的抑制剂)的处理显著降低了F_v/F_m、F_v′/F_m′、Y(II)、ETR和光合放氧速率, 同时也显著增加了q_N以及Y(NPQ)的水平。以上结果显示, 植物eATP水平的变化对植物光合作用的光化学反应有着重要的影响。     

Beauvericin Decreases Cell Viability of Wheat

Recently, beauvericin (BEA) has been recognized as an important toxic compound synthesized by several Fusarium strains, infecting maize, wheat, and rice, worldwide. The effects of BEA on mammalian cells have been studied; however, its effects on the function of host plant cells are largely unknown. The purpose of our work was to assess whether BEA can affect the root and leaf cells of wheat cultivar (cv.) ‘Arina’ seedlings, using a cytotoxicity assay and fluorescence microscopy. Toxigenicity during wheat germination was higher in BEA‐treated wheat seedlings than in non‐treated seedlings (control). Leaf primordial, situated at the base and the tips of treated leaves, were more affected by BEA compared to the control when assayed in medium for cell viability measured by luminescent equipment. BEA‐Treated plant cells secrete adenosine triphosphate (ATP) to the extracellular matrix and invoke more luminescence by luciferase than the non‐treated seedlings. Our results were confirmed by fluorescence microscopy following ‘4′,6‐diamidino‐2‐phenylindole’ (DAPI) staining and by confocal microscopy. In addition, the bioluminescent protein luciferase was observed in the intracellular space indicating presence of ATP. The incidence of nuclear fragmentation increased significantly in cells of seedlings treated with BEA at 40 μM concentration implying that the intracellular phytotoxin BEA plays an important role, possibly as a mediator in cell‐death signalling.     

Characterization of ATP‐induced cell death in the GL261 mouse glioma

Gliomas have one of the worst prognosis among cancers. Their resistance to cell death induced by endogenous neurotoxic agents, such as extracellular ATP, seems to play an important role in their pathobiology since alterations in the degradation rate of extracellular ATP drastically affects glioma growth in rats. In the present work we characterized the mechanisms of cell death induced by extracellular ATP in a murine glioma cell line, GL261. ATP and BzATP, a P2X7 agonist, induced cell death at concentrations that are described to activate the P2X7 receptor in mouse. oATP, an antagonist of P2X7, blocked the ATP‐induced cell death. Agonists of purinergic receptors expressed in GL261 such as adenosine, ADP, UTP did not cause any cell death, even at mM concentrations. A sub‐population of cells more sensitive to ATP expressed more P2X7 when compared to a less sensitive subpopulation. Accordingly, RNA interference of the P2X7 receptor drastically reduced ATP‐induced cell death, suggesting that this receptor is necessary for this effect. The mechanism of ATP‐induced cell death is predominantly necrotic, since cells presented shrinkage accompanied by membrane permeabilization, but not apoptotic, since no phosphatidylserine externalization or caspase activity was observed. These data show the importance of P2X7 in ATP‐induced cell death and shed light on the importance of ATP‐induced cell death in glioma development. J. Cell. Biochem. 109: 983–991, 2010. © 2010 Wiley‐Liss, Inc.     

High dose of extracellular ATP switched autophagy to apoptosis in anchorage-dependent and anchorage-independent hepatoma cells

Extracellular adenosine triphosphate (eATP) transduces purinergic signal and plays an important regulatory role in many biological processes, including tumor cell growth and cell death. A large amount of eATP exists in the fast-growing tumor center and inflammatory tumor microenvironment. Tumor cells could acquire anoikis resistance and anchorage independence in tumor microenvironment and further cause metastatic lesion. Whether such a high amount of eATP has any effect on the anchored and non-anchored tumor cells in tumor microenvironment has not been elucidated and is investigated in this study. Our data showed that autophagy helped hepatoma cells to maintain survival under the treatment of no more than 1 mM of eATP. Only when eATP concentration reached a relatively high level (2.5 mM), cell organelle could not be further maintained by autophagy, and apoptosis and cell death occurred. In hepatoma cells under treatment of 2.5 mM of eATP, an AMP-activated protein kinase (AMPK) pathway was dramatically activated while mTOR signaling pathway was suppressed in coordination with apoptosis. Further investigation showed that the AMPK/mTOR axis played a key role in tipping the balance between autophagy-mediated cell survival and apoptosis-induced cell death under the treatment of eATP. This work provides evidence to explain how hepatoma cells escape from eATP-induced cytotoxicity as well as offers an important clue to consider effective manipulation of cancer.     

细胞外ATP通过刺激NADPH氧化酶缓解水杨酸诱导的细胞死亡

水杨酸（SA）是植物重要的信号分子,低浓度的SA能够诱导植物的抗病反应,而高浓度的SA导致植物细胞死亡。本文采用500μmol·L-1的SA处理烟草悬浮细胞BY-2,研究了细胞外ATP在SA诱导的细胞死亡中的作用及可能的机制。结果显示,外源ATP可缓解SA诱导的细胞死亡水平的上升。另外,SA导致NADPH氧化酶活性下降,而外源ATP则刺激其活性上升。外源ATP能缓解SA对NADPH氧化酶活性的抑制,且这种缓解作用可被NADPH氧化酶的抑制剂——二亚苯基碘（DPI）所消除。DPI还可部分消除外源ATP对SA所诱导的细胞死亡的缓解作用。上述结果表明,胞外ATP通过刺激NADPH氧化酶活性缓解SA诱导的细胞死亡。     

非生物胁迫对烟草悬浮细胞胞内和胞外ATP水平的影响

以烟草悬浮细胞BY-2(Nicotiana tabacum ‘Bright Yellow-2’)为材料,研究了NaCl、PEG(6000)、低温3种非生物胁迫对细胞内ATP(intracellular ATP,iATP)和细胞外ATP(extracellular ATP,eATP)水平的影响。结果显示：50～200 mmol·L^-1 NaCl处理导致烟草悬浮细胞膜通透性显著增加(P<0.05),100和200 mmol·L^-1 NaCl处理时iATP和eATP水平显著降低(P<0.05)。随着PEG质量浓度的增加(50、100、200 g·L^-1),烟草悬浮细胞膜通透性和eATP水平逐渐增加,其中在200 g·L^-1 PEG处理时eATP水平显著增加至对照的3.4倍(P<0.05),而iATP水平则在200 g·L^-1 PEG处理时显著降低至对照的0.5倍(P<0.05)。0～10℃低温处理后,烟草悬浮细胞膜通透性和iATP水平呈不同程度增加,其中0℃处...     

Extracellular ATP functions as an endogenous external metabolite regulating plant cell viability

ATP is a vital molecule used by living organisms as a universal source of energy required to drive the cogwheels of intracellular biochemical reactions necessary for growth and development. Animal cells release ATP to the extracellular milieu, where it functions as the primary signaling cue at the epicenter of a diverse range of physiological processes. Although recent findings revealed that intact plant tissues release ATP as well, there is no clearly defined physiological function of extracellular ATP in plants. Here, we show that extracellular ATP is essential for maintaining plant cell viability. Its removal by the cell-impermeant traps glucose-hexokinase and apyrase triggered death in both cell cultures and whole plants. Competitive exclusion of extracellular ATP from its binding sites by treatment with beta,gamma-methyleneadenosine 5'-triphosphate, a nonhydrolyzable analog of ATP, also resulted in death. The death response was observed in Arabidopsis thaliana, maize (Zea mays), bean (Phaseolus vulgaris), and tobacco (Nicotiana tabacum). Significantly, we discovered that fumonisin B1 (FB1) treatment of Arabidopsis triggered the depletion of extracellular ATP that preceded cell death and that exogenous ATP rescues Arabidopsis from FB1-induced death. These observations suggest that extracellular ATP suppresses a default death pathway in plants and that some forms of pathogen-induced cell death are mediated by the depletion of extracellular ATP.     

Nitric Oxide Signalling In Plants

Nitric oxide (NO) is an important molecule that acts in many tissues to regulate a diverse range of physiological processes. It is becoming apparent that NO is a ubiquitous signal in plants. Since the discovery of NO emission by plants in the 1970s, this gaseous compound has emerged as a major signalling molecule involved in multiple physiological functions. Research on NO in plants has gained significant awareness in recent years and there is increasing indication on the role of this molecule as a key-signalling molecule in plants. The investigations about NO in plants have been concentrated on three main fields: The search of NO or any source of NO generation, effects of exogenous NO treatments, NO transduction pathways. However we have limited information about signal transduction procedures by which NO interaction with cells results in altered cellular activities. This article reviews recent advances in NO synthesis and its signalling functions in plants. First, different sources and biosynthesis of NO in plants, then biological processes involving NO signalling are reviewed. NO signalling relation with cGMP, protein kinases and programmed cell death are also discussed. Besides, NO signalling in plant defense response is also examined. Especially NO signalling between animal and plant systems is compared.