Intersection of two signalling pathways: extracellular nucleotides regulate pollen germination and pollen tube growth via nitric oxide

Computational prediction and in vitro analysis of the potential ligand binding site within the extracellular ATP receptor, P2K2

The plant extracellular ATP (eATP) receptor, P2K2, binds eATP with strong ligand affinity through its extracellular lectin domain. Ligand binding activates the intracellular kinase domain of P2K2 resulting in a variety of intracellular responses and, ultimately, increased plant immunity to invading fungal and bacterial pathogens. Here, using a computational prediction approach, we developed a tertiary structure model of the P2K2 extracellular lectin domain. In silico target docking of ATP to the P2K2-binding site predicted interaction with several residues through hydrophobic interactions and hydrogen bonding. Our confirmation of the modeling was obtained by showing that H99, R144, and S256 are key residues essential for in vitro binding of ATP by P2K2.

The Plasma Membrane Purinoreceptor P2K1/DORN1 Is Essential in Stomatal Closure Evoked by Extracellular Diadenosine Tetraphosphate (Ap4A) in Arabidopsis thaliana

Dinucleoside polyphosphates (NpnNs) are considered novel signalling molecules involved in the induction of plant defence mechanisms. However, NpnN signal recognition and transduction are still enigmatic. Therefore, the aim of our research was the identification of the NpnN receptor and signal transduction pathways evoked by these nucleotides. Earlier, we proved that purine and pyrimidine NpnNs differentially affect the phenylpropanoid pathway in Vitis vinifera suspension-cultured cells. Here, we report, for the first time, that both diadenosine tetraphosphate (Ap4A) and dicytidine tetraphosphate (Cp4C)-induced stomatal closure in Arabidopsis thaliana. Moreover, we showed that plasma membrane purinoreceptor P2K1/DORN1 (does not respond to nucleotide 1) is essential for Ap4A-induced stomata movements but not for Cp4C. Wild-type Col-0 and the dorn1-3 A. thaliana knockout mutant were used. Examination of the leaf epidermis dorn1-3 mutant provided evidence that P2K1/DORN1 is a part of the signal transduction pathway in stomatal closure evoked by extracellular Ap4A but not by Cp4C. Reactive oxygen species (ROS) are involved in signal transduction caused by Ap4A and Cp4C, leading to stomatal closure. Ap4A induced and Cp4C suppressed the transcriptional response in wild-type plants. Moreover, in dorn1-3 leaves, the effect of Ap4A on gene expression was impaired. The interaction between P2K1/DORN1 and Ap4A leads to changes in the transcription of signalling hubs in signal transduction pathways.

Deep learning-based high-throughput detection of in vitro germination to assess pollen viability from microscopic images

In vitro pollen germination is considered the most efficient method to assess pollen viability. The pollen germination frequency and pollen tube length, which are key indicators of pollen viability, should be accurately measured during in vitro culture. In this study, a Mask R-CNN model trained using microscopic images of tree peony (Paeonia suffruticosa) pollen has been proposed to rapidly detect the pollen germination rate and pollen tube length. To reduce the workload during image acquisition, images of synthesized crossed pollen tubes were added to the training dataset, significantly improving the model accuracy in recognizing crossed pollen tubes. At an Intersection over Union threshold of 50%, a mean average precision of 0.949 was achieved. The performance of the model was verified using 120 testing images. The R2 value of the linear regression model using detected pollen germination frequency against the ground truth was 0.909 and that using average pollen tube length was 0.958. Further, the model was successfully applied to two other plant species, indicating a good generalizability and potential to be applied widely.

ATANN3 Is Involved in Extracellular ATP-Regulated Auxin Distribution in Arabidopsis thaliana Seedlings

Extracellular ATP (eATP) plays multiple roles in plant growth and development, and stress responses. It has been revealed that eATP suppresses growth and alters the growth orientation of the root and hypocotyl of Arabidopsis thaliana by affecting auxin transport and localization in these organs. However, the mechanism of the eATP-stimulated auxin distribution remains elusive. Annexins are involved in multiple aspects of plant cellular metabolism, while their role in response to apoplastic signals remains unclear. Here, by using the loss-of-function mutations, we investigated the role of AtANN3 in the eATP-regulated root and hypocotyl growth. Firstly, the inhibitory effects of eATP on root and hypocotyl elongation were weakened or impaired in the AtANN3 null mutants (atann3-1 and atann3-2). Meanwhile, the distribution of DR5-GUS and DR5-GFP indicated that the eATP-induced asymmetric distribution of auxin in the root tips or hypocotyl cells occurred in wild-type control plants, while in atann3-1 mutant seedlings, it was not observed. Further, the eATP-induced asymmetric distribution of PIN2-GFP in root-tip cells or that of PIN3-GFP in hypocotyl cells was reduced in atann3-1 seedlings. Finally, the eATP-induced asymmetric distribution of cytoplasmic vesicles in root-tip cells was impaired in atann3-1 seedlings. Based on these results, we suggest that AtANN3 may be involved in eATP-regulated seedling growth by regulating the distribution of auxin and auxin transporters in vegetative organs.

Advances in chilling injury of postharvest fruit and vegetable: Extracellular ATP aspects

Due to the global use of cold chain, the development of postharvest technology to reduce chilling injury (CI) in postharvest fruits and vegetables during storage and transport is needed urgently. Considerable evidence shows that maintaining intracellular adenosine triphosphate (iATP) in harvested fruits and vegetables is beneficial to inhibiting CI occurrence. Extracellular ATP (eATP) is a damage-associated signal molecule and plays an important role in CI of postharvest fruits and vegetables through its receptor and subsequent signal transduction under low-temperature stress. The development of new aptasensors for the simultaneous determination of eATP level allows for better understanding of the roles of eATP in a myriad of responses mediated by low-temperature stress in relation to the chilling tolerance of postharvest fruits and vegetables. The multiple biological functions of eATP and its receptors in postharvest fruits and vegetables were attributed to interactions with reactive oxygen species (ROS) and nitric oxide (NO) in coordination with phytohormones and other signaling molecules via downstream physiological activities. The complicated interconnection among eATP in relation to its receptors, eATP/iATP homeostasis, ROS, NO, and heat shock proteins triggered by eATP recognition has been emphasized. This paper reviews recent advances in the beneficial effects of energy handling, outlines the production and homeostasis of eATP, discusses the possible mechanism of eATP and its receptors in chilling tolerance, and provides future research directions for CI in postharvest fruits and vegetables during low-temperature storage.

Extracellular ATP targets Arabidopsis RIBONUCLEASE 1 to suppress mycotoxin stress-induced cell death

Extracellular ATP is a purinergic signal with important functions in regulating plant growth and stress-adaptive responses, including programmed cell death. While signalling events proximate to receptor activation at the plasma membrane have been characterised, downstream protein targets and the mechanism of cell death activation/regulation are unknown. We designed a proteomic screen to identify ATP-responsive proteins in Arabidopsis cell cultures exposed to mycotoxin stress via fumonisin B1 (FB1) application. Arabidopsis RIBONUCLEASE 1 (RNS1) was identified by the screen, and transgenic plants overexpressing native RNS1 showed greater susceptibility to FB1, while a gene knockout rns1 mutant and antisense RNS1 transgenic plants were resistant to FB1-induced cell death. Native RNS1 complemented rns1 mutants and restored the cell death response to FB1, while a catalytically inactive version of the ribonuclease could not. The FB1 resistance of salicylic acid (SA)-depleted nahG-expressing plants was abolished by transformation with native RNS1, but not the catalytically dead version. The mechanism of FB1-induced cell death is activation of RNS1-dependent RNA cleavage, which is blocked by ATP via RNS1 suppression, or enhanced by SA through induction of RNS1 expression. Our study reveals RNS1 as a previously unknown convergence point of ATP and SA signalling in the regulation of stress-induced cell death.

The legacy of kinesins in the pollen tube thirty years later

The pollen tube is fundamental in the reproduction of seed plants. Particularly in angiosperms, we now have much information about how it grows, how it senses extracellular signals, and how it converts them into a directional growth mechanism. The expansion of the pollen tube is also related to dynamic cytoplasmic processes based on the cytoskeleton (such as polymerization/depolymerization of microtubules and actin filaments) or motor activity along with the two cytoskeletal systems and is dependent on motor proteins. While a considerable amount of information is available for the actomyosin system in the pollen tube, the role of microtubules in the transport of organelles or macromolecular structures is still quite uncertain despite that 30 years ago the first work on the presence of kinesins in the pollen tube was published. Since then, progress has been made in elucidating the role of kinesins in plant cells. However, their role within the pollen tube is still enigmatic. In this review, I will postulate some roles of kinesins in the pollen tube 30 years after their initial discovery based on information obtained in other plant cells in the meantime. The most concrete hypotheses predict that kinesins in the pollen tube enable the short movement of specific organelles or contribute to generative cell or sperm cell transport, as well as mediate specific steps in the process of endocytosis.

Constitutive expression of a pea apyrase, psNTP9, increases seed yield in field-grown soybean

To address the demand for food by a rapidly growing human population, agricultural scientists have carried out both plant breeding and genetic engineering research. Previously, we reported that the constitutive expression of a pea apyrase (Nucleoside triphosphate, diphosphohydrolase) gene, psNTP9, under the control of the CaMV35S promoter, resulted in soybean plants with an expanded root system architecture, enhanced drought resistance and increased seed yield when they are grown in greenhouses under controlled conditions. Here, we report that psNTP9-expressing soybean lines also show significantly enhanced seed yields when grown in multiple different field conditions at multiple field sites, including when the gene is introgressed into elite germplasm. The transgenic lines have higher leaf chlorophyll and soluble protein contents and decreased stomatal density and cuticle permeability, traits that increase water use efficiency and likely contribute to the increased seed yields of field-grown plants. These altered properties are explained, in part, by genome-wide gene expression changes induced by the transgene.

Dissection and ultramicroscopic observation of an apical pollen tube of Pyrus

The pollen tube is ideal for studying cell polar growth, and observing the ultrastructure of the pollen tube tip using transmission electron microscopy (TEM) is the primary method for studying pollen tube growth. The preparation of ultrathin sections of the pollen tube tip sample is important for its successful microscopic observation. The direction of pollen tube growth in vitro is irregular, and it is difficult to dissect the tip of the pollen tube during ultrathin sectioning. Here, we used two methods to efficiently obtain an ultrathin section of the pollen tube tip of Pyrus. In the first method, laser micro-cutting was used to obtain the pollen tube tip, followed by ultrathin sectioning. In the other method, the pollen tubes were cultured in the same growth direction, followed by ultrathin sectioning. Ultrathin sections, which were observed via TEM, showed typical characteristics of the pollen tube tip, such as dense vesicles, numerous mitochondria, and secretory vesicles of the Golgi. We concluded that these two methods are effective in pollen tube tip sample preparation for ultrathin sectioning and provide the foundation for observing the ultrastructure of pollen tube tips.

Extracellular ATP is involved in regulating Arabidopsis seed germination

Extracellular ATP level induced a transient increase during germination of Arabidopsis seeds, and extracellular ATP could negatively regulate the seed germination by its receptor, DORN1. Extracellular ATP (exATP) acts as a signal molecule for regulating growth, development, and responses of plants to external environments. In this study, we investigated the possible involvement of exATP in regulating the seed germination of Arabidopsis thaliana. Treatments of Arabidopsis seeds with exogenous ATP delayed seed germination, suggesting that exATP could be a repressor for seed germination. During the germination of Arabidopsis seeds, the exATP level of the seeds presented a transient increase. When exogenous application of the glucose-hexokinase system effectively decreased the exATP level of the Arabidopsis seeds during germination, the percentage of germination was significantly enhanced, while the products of ATP hydrolysis had no effects on the germination. Further studies showed that the seeds of dorn 1-3 mutant plants (mutation in exATP receptor) showed a higher germination percentage, compared to the seeds of wide type (WT) plants. In addition, the dorn 1-3 mutant seeds were less sensitive to the delay-effect of exogenous ATP on seed germination than the WT seeds. The dorn 1-3 mutant seeds presented a higher GA (gibberellin) content, lower ABA (abscisic acid) content, and lower ratio of ABA/GA contents before the imbibition, compared to the WT seeds. The regulation of seed germination by exATP was dependent on the external temperature. These data suggest that exATP is involved in regulating Arabidopsis seed germination.

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.

P2K1 Receptor, Heterotrimeric Gα Protein and CNGC2/4 Are Involved in Extracellular ATP-Promoted Ion Influx in the Pollen of Arabidopsis thaliana

As an apoplastic signal, extracellular ATP (eATP) is involved in plant growth and development. eATP promotes tobacco pollen germination (PG) and pollen tube growth (PTG) by stimulating Ca²⁺ or K⁺ absorption. Nevertheless, the mechanisms underlying eATP-stimulated ion uptake and their role in PG and PTG are still unclear. Here, ATP addition was found to modulate PG and PTG in 34 plant species and showed a promoting effect in most of these species. Furthermore, by using Arabidopsis thaliana as a model, the role of several signaling components involved in eATP-promoted ion (Ca²⁺, K⁺) uptake, PG, and PTG were investigated. ATP stimulated while apyrase inhibited PG and PTG. Patch-clamping results showed that ATP promoted K⁺ and Ca²⁺ influx into pollen protoplasts. In loss-of-function mutants of P2K1 (dorn1-1 and dorn1-3), heterotrimeric G protein α subunit (gpa1-1, gpa1-2), or cyclic nucleotide gated ion channel (cngc2, cngc4), eATP-stimulated PG, PTG, and ion influx were all impaired. Our results suggest that these signaling components may be involved in eATP-promoted PG and PTG by regulating Ca²⁺ or K⁺ influx in Arabidopsis pollen grains.

MicroRNA Omics Analysis of Camellia sinesis Pollen Tubes in Response to Low-Temperature and Nitric Oxide

Nitric oxide (NO) as a momentous signal molecule participates in plant reproductive development and responds to various abiotic stresses. Here, the inhibitory effects of the NO-dominated signal network on the pollen tube growth of Camellia sinensis under low temperature (LT) were studied by microRNA (miRNA) omics analysis. The results showed that 77 and 71 differentially expressed miRNAs (DEMs) were induced by LT and NO treatment, respectively. Gene ontology (GO) analysis showed that DEM target genes related to microtubules and actin were enriched uniquely under LT treatment, while DEM target genes related to redox process were enriched uniquely under NO treatment. In addition, the target genes of miRNA co-regulated by LT and NO are only located on the cell membrane and cell wall, and most of them are enriched in metal ion binding and/or transport and cell wall organization. Furthermore, DEM and its target genes related to metal ion binding/transport, redox process, actin, cell wall organization and carbohydrate metabolism were identified and quantified by functional analysis and qRT-PCR. In conclusion, miRNA omics analysis provides a complex signal network regulated by NO-mediated miRNA, which changes cell structure and component distribution by adjusting Ca²⁺ gradient, thus affecting the polar growth of the C. sinensis pollen tube tip under LT.

Isolation of Arabidopsis extracellular ATP binding proteins by affinity proteomics and identification of PHOSPHOLIPASE C-LIKE 1 as an extracellular protein essential for fumonisin B1 toxicity

ATP is secreted to the extracellular matrix, where it activates plasma membrane receptors for controlling plant growth and stress-adaptive processes. DOES NOT RESPOND TO NUCLEOTIDES 1 (DORN1), was the first plant ATP receptor to be identified but key downstream proteins remain sought after. Here, we identified 120 proteins secreted by Arabidopsis cell cultures and screened them for putative stress-responsive proteins using ATP-affinity purification. We report three Arabidopsis proteins isolated by ATP-affinity: PEROXIDASE 52, SUBTILASE-LIKE SERINE PROTEASE 1.7 and PHOSPHOLIPASE C-LIKE 1. In wild-type Arabidopsis, the expression of genes encoding all three proteins responded to fumonisin B1, a cell death-activating mycotoxin. The expression of PEROXIDASE 52 and PHOSPHOLIPASE C-LIKE 1 was altered in fumonisin B1-resistant salicylic acid induction-deficient (sid2) mutants. Exposure to fumonisin B1 suppressed PHOSPHOLIPASE C-LIKE 1 expression in sid2 mutants, suggesting that the inactivation of this gene might provide mycotoxin tolerance. Accordingly, gene knockout mutants of PHOSPHOLIPASE C-LIKE 1 were resistant to fumonisin B1-induced death. The activation of PHOSPHOLIPASE C-LIKE 1 gene expression by exogenous ATP was not blocked in dorn1 loss-of-function mutants, indicating that DORN1 is not required. Furthermore, exogenous ATP rescued both the wild type and the dorn1 mutants from fumonisin-B1 toxicity, suggesting that different ATP receptor(s) are operational in this process. Our results point to the existence of additional plant ATP receptor(s) and provide crucial downstream targets for use in designing screens to identify these receptors. Finally, PHOSPHOLIPASE C-LIKE 1 serves as a convergence point for fumonisin B1 and extracellular ATP signalling, and functions in the Arabidopsis stress response to fumonisin B1.

Recent Advances Clarifying the Structure and Function of Plant Apyrases (Nucleoside Triphosphate Diphosphohydrolases)

Studies implicating an important role for apyrase (NTPDase) enzymes in plant growth and development began appearing in the literature more than three decades ago. After early studies primarily in potato, Arabidopsis and legumes, especially important discoveries that advanced an understanding of the biochemistry, structure and function of these enzymes have been published in the last half-dozen years, revealing that they carry out key functions in diverse other plants. These recent discoveries about plant apyrases include, among others, novel findings on its crystal structures, its biochemistry, its roles in plant stress responses and its induction of major changes in gene expression when its expression is suppressed or enhanced. This review will describe and discuss these recent advances and the major questions about plant apyrases that remain unanswered.

Nitric Oxide Signaling in Plants

Nitric oxide (NO) is an integral part of cell signaling mechanisms in animals and plants. In plants, its enzymatic generation is still controversial. Evidence points to nitrate reductase being important, but the presence of a nitric oxide synthase-like enzyme is still contested. Regardless, NO has been shown to mediate many developmental stages in plants, and to be involved in a range of physiological responses, from stress management to stomatal aperture closure. Downstream from its generation are alterations of the actions of many cell signaling components, with post-translational modifications of proteins often being key. Here, a collection of papers embraces the differing aspects of NO metabolism in plants.

Identification and characterization of Arabidopsis thaliana mitochondrial F1F0-ATPase inhibitor factor 1

Mitochondrial F1F0-ATP synthase (F1F0-ATPase) inhibitor factor 1 (IF1) has been extensively characterized as an endogenous inhibitor that prevents the hydrolysis of adenosine-5'-triphosphate (ATP) by mitochondrial ATPases in mammals and yeasts; however, IF1's functions in plants remain unclear. Here, a comprehensive bioinformatic analysis was performed to identify plant mitochondrial F1F0-ATPase IF1 orthologs. Plant IF1s contain a conserved F1F0-ATPase inhibitory domain, but lack the antiparallel α-helical coiled-coil structure compared with mammalian IF1s. A subcellular localization analysis in Arabidopsis thaliana revealed that AtIF1-green fluorescent protein was present only in mitochondria. Additionally, AtIF1 was widely expressed in diverse organs and intense β-glucuronidase staining was observed in reproductive tissues and germinating seeds. Compared with the wild-type and p35S:AtIF1-if1 etiolated seedlings, the ATP/ADP ratio was significantly lower in the AtIF1 T-DNA knockout seedlings (if1 mutant) growing under dark conditions, suggesting that AtIF1 can influence the energy state of cells. A significant reduction in seed yield and strong growth retardation under dark conditions were observed in the if1 mutant line. Furthermore, if1 plants exhibited a substantially decreased sensitivity to abscisic acid. Thus, the A. thaliana mitochondrial IF1, which is a conserved F1F0-ATPase inhibitor, is crucial for plant growth and responses to abscisic acid.

Arabidopsis Lectin Receptor Kinase P2K2 Is a Second Plant Receptor for Extracellular ATP and Contributes to Innate Immunity

In animals, extracellular ATP is a well-studied signaling molecule that is recognized by plasma membrane-localized P2-type purinergic receptors. However, in contrast, much less is known about purinergic signaling in plants. P2 receptors play critical roles in a variety of animal biological processes, including immune system regulation. The first plant purinergic receptor, Arabidopsis (Arabidopsis thaliana) P2K1 (L-type lectin receptor kinase-I.9), was shown to contribute to plant defense against bacterial, oomycete, and fungal pathogens. Here, we demonstrate the isolation of a second purinergic receptor, P2K2, by complementation of an Arabidopsis p2k1 mutant. P2K2 (LecRK-I.5) has 74% amino acid similarity to P2K1. The P2K2 extracellular lectin domain binds to ATP with higher affinity than P2K1 (dissociation constant [K d] = 44.47 ± 15.73 nm). Interestingly, p2k2 and p2k1 p2k2 mutant plants showed increased susceptibility to the pathogen Pseudomonas syringae, with the double mutant showing a stronger phenotype. In vitro and in planta studies demonstrate that P2K2 and P2K1 interact and cross-phosphorylate upon extracellular ATP treatment. Thus, similar to animals, plants possess multiple purinergic receptors.

Fertilization in flowering plants: an odyssey of sperm cell delivery

In light of the available discoveries in the field, this review manuscript discusses on plant reproduction mechanism and molecular players involved in the process. Sperm cells in angiosperms are immotile and are physically distant to the female gametophytes (FG). To secure the production of the next generation, plants have devised a clever approach by which the two sperm cells in each pollen are safely delivered to the female gametophyte where two fertilization events occur (by each sperm cell fertilizing an egg cell and central cell) to give rise to embryo and endosperm. Each of the successfully fertilized ovules later develops into a seed. Sets of macromolecules play roles in pollen tube (PT) guidance, from the stigma, through the transmitting tract and funiculus to the micropylar end of the ovule. Other sets of genetic players are involved in PT reception and in its rupture after it enters the ovule, and yet other sets of genes function in gametic fusion. Angiosperms have come long way from primitive reproductive structure development to today's sophisticated, diverse, and in most cases flamboyant organ. In this review, we will be discussing on the intricate yet complex molecular mechanism of double fertilization and how it might have been shaped by the evolutionary forces focusing particularly on the model plant Arabidopsis.

Redox-Responsive Transcription Factor 1 (RRFT1) Is Involved in Extracellular ATP-Regulated Arabidopsis thaliana Seedling Growth

Extracellular adenosine triphosphate (eATP) is an apoplastic signaling molecule that plays an essential role in the growth and development of plants. Arabidopsis seedlings have been reported to respond to eATP; however, the downstream signaling components are still not well understood. In this study, we report that an ethylene-responsive factor, Redox-Responsive Transcription Factor 1 (RRTF1), is involved in eATP-regulated Arabidopsis thaliana seedling growth. Exogenous adenosine triphosphate inhibited green seedling root growth and induced hypocotyl bending of etiolated seedlings. RRTF1 loss-of-function mutant (rrtf1) seedlings showed decreased responses to eATP, while its complementation or overexpression led to recovered or increased eATP responsiveness. RRTF1 was expressed rapidly after eATP stimulation and then migrated into the nuclei of root tip cells. eATP-induced auxin accumulation in root tip or hypocotyl cells was impaired in rrtf1. Chromatin immunoprecipitation and high-throughput sequencing results indicated that eATP induced some genes related to cell growth and development in wild type but not in rrtf1 cells. These results suggest that RRTF1 may be involved in eATP signaling by regulating functional gene expression and cell metabolism in Arabidopsis seedlings.

Quantification of Extracellular ATP in Plant Suspension Cell Cultures

Extracellular ATP functions as an important signaling molecule in both plants and animals. In plants, ATP is released in the extracellular region of cells in response to environmental perturbations, such as herbivory, cellular damage, or other abiotic and biotic stimuli, which is then perceived by the purinoceptor P2K1 as a damaged-self signal for activation of defense responses. Given its involvement in various physiological processes, quantification of extracellular ATP is important for further understanding of its molecular function. In this chapter, we describe a method for the accurate and reliable determination of extracellular ATP concentrations in plant cell culture media based on the luciferase-luciferin reaction, using either end-point or real-time detection assays. The protocol can be easily performed with any luminometer within 1 h after sample collection. Although we use Arabidopsis suspension cells, the protocol described can be optimized for any cell type.

Extracellular ATP released by copper stress could act as diffusible signal in alleviating the copper stress-induced cell death

In the present work, by using tobacco cell suspension and wheat seedlings, we studied that eATP (extracellular ATP) released by copper (Cu) stress could act as diffusible signal in alleviating the Cu stress-induced cell death. A semipermeable membrane was fixed in the middle of a plastic box to divide the box into two equal compartments (A and B, respectively). This semipermeable membrane can prevent direct cell-to-cell (or seedling-to-seedling) contact and the diffusion of the macromolecules [such as ATPase (adenosine 5'-triphosphatase)] between these two compartments. The cell suspension directly stressed with CuCl₂ was placed in compartment A and was incubated with the untreated cell suspension in compartment B. Such treatment significantly increased the levels of cell death and eATP content of the cell suspension in these two compartments. In contrast, addition of ATPase into the cell suspension directly stressed with CuCl₂ decreased the eATP level in these two compartments but further increased the level of cell death in compartment B, compared to no addition of ATPase. Similar results were obtained when tobacco cell suspension was replaced by wheat seedlings. These observations indicate that when Cu stress from compartment A induced the plant cell death in compartment B, ATP transferred from compartment A could play a role in alleviating this cell death. Thus, it is suggested that eATP released by copper stress could act as diffusible signal in alleviating the Cu stress-induced cell death.

Extracellular ATP promoted pollen germination and tube growth of Nicotiana tabacum through promoting K+ and Ca2+ absorption

Extracellular ATP (eATP) plays an essential role in plant growth, development, and stress tolerance. Here, we report that eATP participated in Nicotiana tabacum pollen germination (PG) and pollen tube growth (PTG) by regulating K+ and Ca2+ influx. Exogenous ATP or ADP effectively promoted PG and PTG in a dose-dependent manner; weakly hydrolysable ATP analog (ATPγS) showed a similar effect. AMP, adenosine, adenine, and phosphate did not affect PG or PTG. Within a certain range, higher concentrations of K+ or Ca2+ in the medium increased the effect of ATP in promoting PG and PTG. However, in mediums containing K+ or Ca2+ concentrations above this range, the effect of ATP was reversed, resulting in PG and PTG inhibition. Ca2+ chelators (EGTA), Ca2+ channel blockers, and K+ channel blockers suppressed ATP-promoted PG and PTG. Results from a patch clamp showed that ATP activated a K+ and Ca2+ influx in pollen protoplasts. These results suggest that, as an apoplastic signal, eATP may be involved in PG and PTG via regulating Ca2+ and K+ absorption.

Role of Ca2+ in Mediating Plant Responses to Extracellular ATP and ADP

Among the most recently discovered chemical regulators of plant growth and development are extracellular nucleotides, especially extracellular ATP (eATP) and extracellular ADP (eADP). Plant cells release ATP into their extracellular matrix under a variety of different circumstances, and this eATP can then function as an agonist that binds to a specific receptor and induces signaling changes, the earliest of which is an increase in the concentration of cytosolic calcium ([Ca2+]cyt). This initial change is then amplified into downstream-signaling changes that include increased levels of reactive oxygen species and nitric oxide, which ultimately lead to major changes in the growth rate, defense responses, and leaf stomatal apertures of plants. This review presents and discusses the evidence that links receptor activation to increased [Ca2+]cyt and, ultimately, to growth and diverse adaptive changes in plant development. It also discusses the evidence that increased [Ca2+]cyt also enhances the activity of apyrase (nucleoside triphosphate diphosphohydrolase) enzymes that function in multiple subcellular locales to hydrolyze ATP and ADP, and thus limit or terminate the effects of these potent regulators.

Molecular Mechanism of Plant Recognition of Extracellular ATP

Adenosine 5'-triphosphate (ATP), a ubiquitously dispersed biomolecule, is not only a major source of biochemical energy for living cells, but also acts as a critical signaling molecule through inter-cellular communication. Recent studies have clearly shown that extracellular ATP is involved in various physiological processes in plants, including root growth, stomata movement, pollen tube development, gravitropism, and abiotic/biotic stress responses. The first plant purinergic receptor for extracellular ATP, DORN1 (the founding member of the P2K family of purinergic receptors), was identified in Arabidopsis thaliana by a forward genetic screen. DORN1 consists of an extracellular lectin domain, transmembrane domain, and serine/threonine kinase, intracellular domain. The predicted structure of the DORN1 extracellular domain revealed putative key ATP binding residues but an apparent lack of sugar binding. In this chapter, we summarize recent studies on the molecular mechanism of plant recognition of extracellular ATP with specific reference to the role of DORN1.

Extracellular ATP Acts on Jasmonate Signaling to Reinforce Plant Defense

Damaged cells send various signals to stimulate defense responses. Recent identification and genetic studies of the plant purinoceptor, P2K1 (also known as DORN1), have demonstrated that extracellular ATP is a signal involved in plant stress responses, including wounding, perhaps to evoke plant defense. However, it remains largely unknown how extracellular ATP induces plant defense responses. Here, we demonstrate that extracellular ATP induces plant defense mediated through activation of the intracellular signaling of jasmonate (JA), a well-characterized defense hormone. In Arabidopsis (Arabidopsis thaliana) leaves, ATP pretreatment induced resistance against the necrotrophic fungus, Botrytis cinerea The induced resistance was enhanced in the P2K1 receptor overexpression line, but reduced in the receptor mutant, dorn1-3 Mining the transcriptome data revealed that ATP induces a set of JA-induced genes. In addition, the P2K1-associated coexpression network contains defense-related genes, including those encoding jasmonate ZIM-domain (JAZ) proteins, which play key roles as repressors of JA signaling. We examined whether extracellular ATP impacts the stability of JAZ1 in Arabidopsis. The results showed that the JAZ1 stability decreased in response to ATP addition in a proteasome-dependent manner. This reduction required intracellular signaling via second messengers-cytosolic calcium, reactive oxygen species, and nitric oxide. Interestingly, the ATP-induced JAZ1 degradation was attenuated in the JA receptor mutant, coi1, but not in the JA biosynthesis mutant, aos, or upon addition of JA biosynthesis inhibitors. Immunoprecipitation analysis demonstrated that ATP increases the interaction between COI1 and JAZ1, suggesting direct cross talk between extracellular ATP and JA in intracellular signaling events. Taken together, these results suggest that extracellular ATP signaling directly impacts the JA signaling pathway to maximize plant defense responses.

Generation of nitric oxide by olive (Olea europaea L.) pollen during in vitro germination and assessment of the S-nitroso- and nitro-proteomes by computational predictive methods

Nitric oxide is recognized as a signaling molecule involved in a broad range of physiological processes in plants including sexual reproduction. NO has been detected in the pollen grain at high levels and regulates pollen tube growth. Previous studies demonstrated that NO as well as ROS are produced in the olive reproductive tissues in a stage- and tissue-specific manner. The aim of this study was to assess the production of NO throughout the germination of olive (Olea europaea L.) pollen in vitro. The NO fluorescent probe DAF-2DA was used to image NO production in situ, which was correlated to pollen viability. Moreover, by means of a fluorimetric assay we showed that growing pollen tubes release NO. GSNO -a mobile reservoir of NO, formed by the S-nitrosylation of NO with reduced glutathione (GSH) - was for the first time detected and quantified at different stages of pollen tube growth using a LC-ES/MS analysis. Exogenous NO donors inhibited both pollen germination and pollen tube growth and these effects were partially reverted by the specific NO-scavenger c-PTIO. However, little is known about how NO affects the germination process. With the aim of elucidating the putative relevance of protein S-nitrosylation and Tyr-nitration as important post-translational modifications in the development and physiology of the olive pollen, a de novo assembled and annotated reproductive transcriptome from olive was challenged in silico for the putative capability of transcripts to become potentially modified by S-nitrosylation/Tyr-nitration according to well-established criteria. Numerous gene products with these characteristics were identified, and a broad discussion as regards to their potential role in plant reproduction was built after their functional classification. Moreover, the importance of both S-nitrosylation/Tyr-nitrations was experimentally assessed and validated by using Western blotting, immunoprecipitation and proteomic approaches.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Heterotrimeric G Protein-Regulated Ca2+ Influx and PIN2 Asymmetric Distribution Are Involved in Arabidopsis thaliana Roots' Avoidance Response to Extracellular ATP

Extracellular ATP (eATP) has been reported to be involved in plant growth as a primary messenger in the apoplast. Here, roots of Arabidopsis thaliana seedlings growing in jointed medium bent upon contact with ATP-containing medium to keep away from eATP, showing a marked avoidance response. Roots responded similarly to ADP and bz-ATP but did not respond to AMP and GTP. The eATP avoidance response was reduced in loss-of-function mutants of heterotrimeric G protein α subunit (Gα) (gpa1-1 and gpa1-2) and enhanced in Gα-over-expression (OE) lines (wGα and cGα). Ethylenebis(oxyethylenenitrilo) tetraacetic acid (EGTA) and Gd³⁺ remarkably suppressed eATP-induced root bending. ATP-stimulated Ca²⁺ influx was impaired in Gα null mutants and increased in its OE lines. DR5-GFP and PIN2 were asymmetrically distributed in ATP-stimulated root tips, this effect was strongly suppressed by EGTA and diminished in Gα null mutants. In addition, some eATP-induced genes' expression was also impaired in Gα null mutants. Based on these results, we propose that heterotrimeric Gα-regulated Ca²⁺ influx and PIN2 distribution may be key signaling events in eATP sensing and avoidance response in Arabidopsis thaliana roots.

Gene expression profile indicates involvement of NO in Camellia sinensis pollen tube growth at low temperature

BACKGROUND

Nitric oxide (NO) functions as a critical signaling molecule in the low-temperature stress responses in plants, including polarized pollen tube growth in Camellia sinensis. Despite this, the potential mechanisms underlying the participation of NO in pollen tube responses to low temperature remain unclear. Here, we investigate alterations to gene expression in C. sinensis pollen tubes exposed to low-temperature stress and NO using RNA-Seq technology, in order to find the potential candidate genes related to the regulation of pollen tube elongation by NO under low-temperature stress.

RESULTS

Three libraries were generated from C. sinensis cv. 'Longjingchangye' pollen tubes cultured at 25 °C (CsPT-CK) and 4 °C (CsPT-LT) or with 25 μM DEA NONOate (CsPT-NO). The number of unigenes found for the three biological replications were 39,726, 40,440 and 41,626 for CsPT-CK; 36,993, 39,070 and 39,439 for CsPT-LT; and 39,514, 38,298 and 39,061 for CsPT-NO. A total of 36,097 unique assembled and annotated sequences from C. sinensis pollen tube reads were found in a BLAST search of the following databases: NCBI non-redundant nucleotide, Swiss-prot protein, Kyoto Encyclopedia of Genes and Genomes, Cluster of Orthologous Groups of proteins, and Gene Ontology. The absolute values of log2Ratio > 1 and probability > 0.7 were used as the thresholds for significantly differential gene expression, and 766, 497 and 929 differentially expressed genes (DEGs) were found from the comparison analyses of the CK-VS-LT, CK-VS-NO and LT-VS-NO libraries, respectively. Genes related to metabolism and signaling pathways of plant hormones, transcription factors (TFs), vesicle polarized trafficking, cell wall biosynthesis, the ubiquitination machinery of the ubiquitin system and species-specific secondary metabolite pathways were mainly observed in the CK-VS-LT and CK-VS-NO libraries.

CONCLUSION

Differentially expressed unigenes related to the inhibition of C. sinensis pollen tube growth under low temperature and NO are identified in this study. The transcriptomic gene expression profiles present a valuable genomic tool to improve studying the molecular mechanisms underlying low-temperature tolerance in pollen tube.

The biosensor toolbox for plant developmental biology

Plant development is highly interconnected with the metabolic state of tissues and cells. Current research efforts focus on the identification of the links and mechanisms that govern the interplay between metabolic and gene-regulatory networks. Genetically encoded sensors that allow detection of small molecules in vivo and at high spatio-temporal resolution promise to be the tools of choice for quantifying and visualizing the dynamics of metabolite flux in plants. We provide an overview about current approaches to measure signaling molecules, such as hormones, calcium and sugars, as well as for monitoring the metabolic state via energy equivalents and pH. Biosensors show great potential to address questions of plant development but there are also limitations where alternative approaches are needed.

On the salty side of life: molecular, physiological and anatomical adaptation and acclimation of trees to extreme habitats

Saline and sodic soils that cannot be used for agriculture occur worldwide. Cultivating stress-tolerant trees to obtain biomass from salinized areas has been suggested. Various tree species of economic importance for fruit, fibre and timber production exhibit high salinity tolerance. Little is known about the mechanisms enabling tree crops to cope with high salinity for extended periods. Here, the molecular, physiological and anatomical adjustments underlying salt tolerance in glycophytic and halophytic model tree species, such as Populus euphratica in terrestrial habitats, and mangrove species along coastlines are reviewed. Key mechanisms that have been identified as mediating salt tolerance are discussed at scales from the genetic to the morphological level, including leaf succulence and structural adjustments of wood anatomy. The genetic and transcriptomic bases for physiological salt acclimation are salt sensing and signalling networks that activate target genes; the target genes keep reactive oxygen species under control, maintain the ion balance and restore water status. Evolutionary adaptation includes gene duplication in these pathways. Strategies for and limitations to tree improvement, particularly transgenic approaches for increasing salt tolerance by transforming trees with single and multiple candidate genes, are discussed.

Extracellular ATP: a potential regulator of plant cell death

Adenosine 5'-triphosphate (ATP) has been regarded as an intracellular energy currency molecule for many years. In recent decades, it has been determined that ATP is released into the extracellular milieu by animal, plant and microbial cells. In animal cells, this extracellular ATP (eATP) functions as a signalling compound to mediate many cellular processes through its interaction with membrane-associated receptor proteins. It has also been reported that eATP is a signalling molecule required for the regulation of plant growth, development and responses to environmental stimuli. Recently, the first plant receptor for eATP was identified in Arabidopsis thaliana. Interestingly, some studies have shown that eATP is of particular importance in the control of plant cell death. In this review article, we summarize and discuss the theoretical and experimental advances that have been made with regard to the roles and mechanisms of eATP in plant cell death. We also make an attempt to address some speculative aspects to help develop and expand future research in this area.

Evidence for Extracellular ATP as a Stress Signal in a Single-Celled Organism

ATP is omnipresent in biology and acts as an extracellular signaling molecule in mammals. Information regarding the signaling function of extracellular ATP in single-celled eukaryotes is lacking. Here, we explore the role of extracellular ATP in cell volume recovery during osmotic swelling in the amoeba Dictyostelium. Release of micromolar ATP could be detected during cell swelling and regulatory cell volume decrease (RVD) phases during hypotonic challenge. Scavenging ATP with apyrase caused profound cell swelling and loss of RVD. Apyrase-induced swelling could be rescued by 100 μM βγ-imidoATP. N-Ethylmalemide (NEM), an inhibitor of vesicular exocytosis, caused heightened cell swelling, loss of RVD, and inhibition of ATP release. Amoebas with impaired contractile vacuole (CV) fusion (drainin knockout [KO] cells) displayed increased swelling but intact ATP release. One hundred micromolar Gd(3+) caused cell swelling while blocking any recovery by βγ-imidoATP. ATP release was 4-fold higher in the presence of Gd(3+). Cell swelling was associated with an increase in intracellular nitric oxide (NO), with NO-scavenging agents causing cell swelling. Swelling-induced NO production was inhibited by both apyrase and Gd(3+), while NO donors rescued apyrase- and Gd(3+)-induced swelling. These data suggest extracellular ATP released during cell swelling is an important signal that elicits RVD. Though the cell surface receptor for ATP in Dictyostelium remains elusive, we suggest ATP operates through a Gd(3+)-sensitive receptor that is coupled with intracellular NO production.

Extracellular ATP, a danger signal, is recognized by DORN1 in Arabidopsis

ATP, the universal energy currency of all organisms, is released into the extracellular matrix and serves as a signal among cells, where it is referred to as an extracellular ATP. Although a signalling role for extracellular ATP has been well studied in mammals over the last 40 years, investigations of such a role in plants are at an early stage. Recently, the first plant receptor for extracellular ATP, DOes not Respond to Nucleotides (DORN1), was identified in Arabidopsis thaliana by mutant screening. DORN1 encodes a legume-type lectin receptor kinase that is structurally distinct from the mammalian extracellular ATP receptors. In the present review, we highlight the genetic and biochemical evidence for the role of DORN1 in extracellular ATP signalling, placing this within the wider context of extracellular ATP signalling during plant stress responses.

Hyperpolization-activated Ca(2+) channels in guard cell plasma membrane are involved in extracellular ATP-promoted stomatal opening in Vicia faba

Extracellular ATP (eATP) plays essential roles in plant growth, development, and stress tolerance. Extracellular ATP-regulated stomatal movement of Arabidopsis thaliana has been reported. Here, ATP was found to promote stomatal opening of Vicia faba in a dose-dependent manner. Three weakly hydrolysable ATP analogs (adenosine 5'-O-(3-thio) triphosphate (ATPγS), 3'-O-(4-benzoyl) benzoyl adenosine 5'-triphosphate (Bz-ATP) and 2-methylthio-adenosine 5'-triphosphate (2meATP)) showed similar effects, indicating that ATP acts as a signal molecule rather than an energy charger. ADP promoted stomatal opening, while AMP and adenosine did not affect stomatal movement. An ATP-promoted stomatal opening was blocked by the NADPH oxidase inhibitor diphenylene iodonium (DPI), the reductant dithiothreitol (DTT) or the Ca(2+) channel blockers GdCl3 and LaCl3. A hyperpolarization-activated Ca(2+) channel was detected in plasma membrane of guard cell protoplast. Extracellular ATP and weakly hydrolyzable ATP analogs activated this Ca(2+) channel significantly. Extracellular ATP-promoted Ca(2+) channel activation was markedly inhibited by DPI or DTT. These results indicated that eATP may promote stomatal opening via reactive oxygen species that regulate guard cell plasma membrane Ca(2+) channels.

Extracellular ATP is a central signaling molecule in plant stress responses

Because of their sessile nature, plants have developed a number of sophisticated signaling systems to adapt to environmental changes. Previous research has shown that extracellular ATP is an important signaling molecule used by plants and functions in a variety of processes, including growth, development, and stress responses. Recently, DORN1 was identified as the first plant purinoceptor, essential for the plant response to ATP. The identification of the receptor is a milestone for our overall understanding of various physiological events regulated by extracellular ATP. In this review, we will discuss the possible roles of DORN1 providing future direction for research into the role of extracellular ATP in plants.

Breakthroughs spotlighting roles for extracellular nucleotides and apyrases in stress responses and growth and development

Animal and plant cells release nucleotides into their extracellular matrix when touched, wounded, and when their plasma membranes are stretched during delivery of secretory vesicles and growth. These released nucleotides then function as signaling agents that induce rapid increases in the concentration of cytosolic calcium, nitric oxide and superoxide. These, in turn, are transduced into downstream physiological changes. These changes in plants include changes in the growth of diverse tissues, in gravitropism, and in the opening and closing of stomates. The concentration of extracellular nucleotides is controlled by various phosphatases, prominent among which are apyrases EC 3.6.1.5 (nucleoside triphosphate diphosphohydrolases, NTPDases). This review provides phylogenetic and pHMM analyses of plant apyrases as well as analysis of predicted post-translational modifications for Arabidopsis apyrases. This review also summarizes and discusses recent advances in research on the roles of apyrases and extracellular nucleotides in controlling plant growth and development. These include new findings that document how apyrases and extracellular nucleotides control auxin transport, modulate stomatal aperture, and mediate biotic and abiotic stress responses, and on how apyrase suppression leads to growth inhibition.

Identification of a plant receptor for extracellular ATP

Extracellular adenosine 5'-triphosphate (ATP) is an essential signaling molecule that is perceived in mammals by plasma membrane P2-type purinoceptors. Similar ATP receptors do not exist in plants, although extracellular ATP has been shown to play critical roles in plant growth, development, and stress responses. Here, we identify an ATP-insensitive Arabidopsis mutant, dorn1 (Does not Respond to Nucleotides 1), defective in lectin receptor kinase I.9 (Arabidopsis Information Resource accession code At5g60300). DORN1 binds ATP with high affinity (dissociation constant of 45.7 ± 3.1 nanomolar) and is required for ATP-induced calcium response, mitogen-activated protein kinase activation, and gene expression. Ectopic expression of DORN1 increased the plant response to physical wounding. We propose that DORN1 is essential for perception of extracellular ATP and likely plays a variety of roles in plant stress resistance.

Current status and proposed roles for nitric oxide as a key mediator of the effects of extracellular nucleotides on plant growth

Recent data indicate that nucleotides are released into the extracellular matrix during plant cell growth, and that these extracellular nucleotides induce signaling changes that can, in a dose-dependent manner, increase or decrease the cell growth. After activation of a presumed receptor, the earliest signaling change induced by extracellular nucleotides is an increase in the concentration of cytosolic Ca(2+), but rapidly following this change is an increase in the cellular level of nitric oxide (NO). In Arabidopsis, mutants deficient in nitrate reductase activity (nia1nia2) have drastically reduced nitric oxide production and cannot transduce the effects of applied nucleotides into growth changes. Both increased levels of extracellular nucleotides and increased NO production inhibit auxin transport and inhibit growth, and these effects are potentially due to disruption of the localization and/or function of auxin transport facilitators. However, because NO- and auxin-induced signaling pathways can intersect at multiple points, there may be diverse ways by which the induction of NO by extracellular ATP could modulate auxin signaling and thus influence growth. This review will discuss these optional mechanisms and suggest possible regulatory routes based on current experimental data and predictive computational analyses.

Extracellular ATP signaling is mediated by H₂O₂ and cytosolic Ca²⁺ in the salt response of Populus euphratica cells

Extracellular ATP (eATP) has been implicated in mediating plant growth and antioxidant defense; however, it is largely unknown whether eATP might mediate salinity tolerance. We used confocal microscopy, a non-invasive vibrating ion-selective microelectrode, and quantitative real time PCR analysis to evaluate the physiological significance of eATP in the salt resistance of cell cultures derived from a salt-tolerant woody species, Populus euphratica. Application of NaCl (200 mM) shock induced a transient elevation in [eATP]. We investigated the effects of eATP by blocking P2 receptors with suramin and PPADS and applying an ATP trap system of hexokinase-glucose. We found that eATP regulated a wide range of cellular processes required for salt adaptation, including vacuolar Na⁺ compartmentation, Na⁺/H⁺ exchange across the plasma membrane (PM), K⁺ homeostasis, reactive oxygen species regulation, and salt-responsive expression of genes related to K⁺/Na⁺ homeostasis and PM repair. Furthermore, we found that the eATP signaling was mediated by H₂O₂ and cytosolic Ca²⁺ released in response to high salt in P. euphratica cells. We concluded that salt-induced eATP was sensed by purinoceptors in the PM, and this led to the induction of downstream signals, like H₂O₂ and cytosolic Ca²⁺, which are required for the up-regulation of genes linked to K⁺/Na⁺ homeostasis and PM repair. Consequently, the viability of P. euphratica cells was maintained during a prolonged period of salt stress.

Nitric oxide participates in cold-inhibited Camellia sinensis pollen germination and tube growth partly via cGMP in vitro

Nitric oxide (NO) plays essential roles in many biotic and abiotic stresses in plant development procedures, including pollen tube growth. Here, effects of NO on cold stress inhibited pollen germination and tube growth in Camellia sinensis were investigated in vitro. The NO production, NO synthase (NOS)-like activity, cGMP content and proline (Pro) accumulation upon treatment with NO scavenger cPTIO, NOS inhibitor L-NNA, NO donor DEA NONOate, guanylate cyclase (GC) inhibitor ODQ or phosphodiesterase (PDE) inhibitor Viagra at 25°C (control) or 4°C were analyzed. Exposure to 4°C for 2 h reduced pollen germination and tube growth along with increase of NOS-like activity, NO production and cGMP content in pollen tubes. DEA NONOate treatment inhibited pollen germination and tube growth in a dose-dependent manner under control and reinforced the inhibition under cold stress, during which NO production and cGMP content promoted in pollen tubes. L-NNA and cPTIO markedly reduced the generation of NO induced by cold or NO donor along with partly reverse of cold- or NO donor-inhibited pollen germination and tube growth. Furthermore, ODQ reduced the cGMP content under cold stress and NO donor treatment in pollen tubes. Meanwhile, ODQ disrupted the reinforcement of NO donor on the inhibition of pollen germination and tube growth under cold condition. Additionally, Pro accumulation of pollen tubes was reduced by ODQ compared with that receiving NO donor under cold or control condition. Effects of cPTIO and L-NNA in improving cold-treated pollen germination and pollen tube growth could be lowered by Viagra. Moreover, the inhibitory effects of cPTIO and L-NNA on Pro accumulation were partly reversed by Viagra. These data suggest that NO production from NOS-like enzyme reaction decreased the cold-responsive pollen germination, inhibited tube growth and reduced Pro accumulation, partly via cGMP signaling pathway in C. sinensis.

Role for apyrases in polar auxin transport in Arabidopsis

Recent evidence indicates that extracellular nucleotides regulate plant growth. Exogenous ATP has been shown to block auxin transport and gravitropic growth in primary roots of Arabidopsis (Arabidopsis thaliana). Cells limit the concentration of extracellular ATP in part through the activity of ectoapyrases (ectonucleoside triphosphate diphosphohydrolases), and two nearly identical Arabidopsis apyrases, APY1 and APY2, appear to share this function. These findings, plus the fact that suppression of APY1 and APY2 blocks growth in Arabidopsis, suggested that the expression of these apyrases could influence auxin transport. This report tests that hypothesis. The polar movement of [(3)H]indole-3-acetic acid in both hypocotyl sections and primary roots of Arabidopsis seedlings was measured. In both tissues, polar auxin transport was significantly reduced in apy2 null mutants when they were induced by estradiol to suppress the expression of APY1 by RNA interference. In the hypocotyl assays, the basal halves of APY-suppressed hypocotyls contained considerably lower free indole-3-acetic acid levels when compared with wild-type plants, and disrupted auxin transport in the APY-suppressed roots was reflected by their significant morphological abnormalities. When a green fluorescent protein fluorescence signal encoded by a DR5:green fluorescent protein construct was measured in primary roots whose apyrase expression was suppressed either genetically or chemically, the roots showed no signal asymmetry following gravistimulation, and both their growth and gravitropic curvature were inhibited. Chemicals that suppress apyrase activity also inhibit gravitropic curvature and, to a lesser extent, growth. Taken together, these results indicate that a critical step connecting apyrase suppression to growth suppression is the inhibition of polar auxin transport.

Extracellular ATP promotes stomatal opening of Arabidopsis thaliana through heterotrimeric G protein α subunit and reactive oxygen species

In recent years, adenosine tri-phosphate (ATP) has been reported to exist in apoplasts of plant cells as a signal molecule. Extracellular ATP (eATP) plays important roles in plant growth, development, and stress tolerance. Here, extracellular ATP was found to promote stomatal opening of Arabidopsis thaliana in light and darkness. ADP, GTP, and weakly hydrolyzable ATP analogs (ATPγS, Bz-ATP, and 2meATP) showed similar effects, whereas AMP and adenosine did not affect stomatal movement. Apyrase inhibited stomatal opening. ATP-promoted stomatal opening was blocked by an NADPH oxidase inhibitor (diphenylene iodonium) or deoxidizer (dithiothreitol), and was impaired in null mutant of NADPH oxidase (atrbohD/F). Added ATP triggered ROS generation in guard cells via NADPH oxidase. ATP also induced Ca(2+) influx and H(+) efflux in guard cells. In atrbohD/F, ATP-induced ion flux was strongly suppressed. In null mutants of the heterotrimeric G protein α subunit, ATP-promoted stomatal opening, cytoplasmic ROS generation, Ca(2+) influx, and H(+) efflux were all suppressed. These results indicated that eATP-promoted stomatal opening possibly involves the heterotrimeric G protein, ROS, cytosolic Ca(2+), and plasma membrane H(+)-ATPase.

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 Ca(2+) levels, resulting in Ca(2+) uptake by the mitochondria and subsequent H(2) O(2) 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 Ca(2+) 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 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.