NADPH oxidase AtrbohD and AtrbohF genes function in ROS-dependent ABA signaling in Arabidopsis

Hydrogen peroxide as a signal controlling plant programmed cell death

Hydrogen peroxide (H₂O₂) has established itself as a key player in stress and programmed cell death responses, but little is known about the signaling pathways leading from H₂O₂ to programmed cell death in plants. Recently, identification of key regulatory mutants and near-full genome coverage microarray analysis of H₂O₂-induced cell death have begun to unravel the complexity of the H₂O₂ network. This review also describes a novel link between H₂O₂ and sphingolipids, two signals that can interplay and regulate plant cell death.

Guard cell metabolism and CO2 sensing

In this review we concentrate on guard cell metabolism and CO2 sensing. Although a matter of some controversy, it is generally accepted that the Calvin cycle plays a minor role in stomatal movements. Recent data emphasise the importance of guard cell starch degradation and of carbon import from the guard cell apoplast in promoting and maintaining stomatal opening. Chloroplast maltose and glucose transporters appear to be crucial to the export of carbon from both guard and mesophyll cells. The way guard cells sense CO2 remains an unresolved question. However, a better understanding of the cellular events downstream from CO2 sensing is emerging. We now recognise that there are common as well as unique steps in abscisic acid (ABA) and CO2 signalling pathways. For example, while ABA and CO2 both trigger increases in cytoplasmic free calcium, unlike ABA, CO2 does not promote a cytoplasmic pH change. Future advances in this area are likely to result from the increased use of techniques and resources, such as, reverse genetics, novel mutants, confocal imaging, and microarray analyses of the guard cell transcriptome.

Different signaling and cell death roles of heterotrimeric G protein alpha and beta subunits in the Arabidopsis oxidative stress response to ozone

Arabidopsis thaliana plants with null mutations in the genes encoding the alpha and beta subunits of the single heterotrimeric G protein are less and more sensitive, respectively, to O3 damage than wild-type Columbia-0 plants. The first peak of the bimodal oxidative burst elicited by O3 in wild-type plants is almost entirely missing in both mutants. The late peak is normal in plants lacking the Gbeta protein but missing in plants lacking the Galpha protein. Endogenous reactive oxygen species (ROS) are first detectable in chloroplasts of leaf epidermal guard cells. ROS production in adjacent cells is triggered by extracellular ROS signals produced by guard cell membrane-associated NADPH oxidases encoded by the AtrbohD and AtrbohF genes. The late, tissue damage-associated component of the oxidative burst requires only the Galpha protein and arises from multiple cellular sources. The early component of the oxidative burst, arising primarily from chloroplasts, requires signaling through the heterotrimer (or the Gbetagamma complex) and is separable from Galpha-mediated activation of membrane-bound NADPH oxidases necessary for both intercellular signaling and cell death.

Modulation of plant ion channels by oxidizing and reducing agents

Ion channels are proteins forming hydrophilic pathways through the membranes of all living organisms. They play important roles in the electrogenic transport of ions and metabolites. Because of biophysical properties such as high selectivity for the permeant ion, high turnover rate, and modulation by physico-chemical parameters (e.g., membrane potential, calcium concentration), they are involved in several physiological processes in plant cells (e.g., maintenance of the turgor pressure, stomatal movements, and nutrient absorption by the roots). As plants cannot move, plant metabolism must be flexible and dynamic, to cope with environmental changes, to compete with other living species and to prevent pathogen invasion. An example of this flexibility and dynamic behavior is represented by their handling of the so-called reactive oxygen species, inevitable by-products of aerobic metabolism. Plants cope with these species on one side avoiding their toxic effects, on the other utilizing them as signalling molecules and as a means of defence against pathogens. In this review, we present the state-of-the-art of the modulation of plant ion channels by oxidizing and reducing agents.

Auxin-induced reactive oxygen species production requires the activation of phosphatidylinositol 3-kinase

We recently reported that production of reactive oxygen species (ROS) is essential for auxin-induced gravitropic signaling. Here, we investigated the role of phosphatidylinositol 3-kinase and its product, PtdIns(3)P, in auxin-mediated ROS production and the root gravitropic response. Pretreatment with LY294002, an inhibitor of PtdIns 3-kinase activity, blocked auxin-mediated ROS generation, and reduced the sensitivity of root tissue to gravistimulation. The amount of PtdIns(3)P increased in response to auxin, and this effect was abolished by pretreatment with LY294002. In addition, sequestration of PtdIns(3)P by transient expression of the endosome binding domain in protoplasts abrogated IAA-induced ROS accumulation. These results indicate that activation of PtdIns 3-kinase and its product PtdIns(3)P are required for auxin-induced production of ROS and root gravitropism.

Regulatory Networks of the Phytohormone Abscisic Acid

Structurally similar to retinoic acid (RA), the phytohormone abscisic acid (ABA) controls many developmental and physiological processes via complicated signaling networks that are composed of receptors, secondary messengers, protein kinase/phosphatase cascades, transcription factors, and chromatin-remodeling factors. In addition, ABA signaling is further modulated by mRNA maturation and stability, microRNA (miRNA) levels, nuclear speckling, and protein degradation. This chapter highlights the identified regulators of ABA signaling and reports their homologues in dicotyledonous and monocotyledonous plants.

Cytosolic ascorbate peroxidase 1 is a central component of the reactive oxygen gene network of Arabidopsis

Reactive oxygen species (ROS), such as O2- and H2O2, play a key role in plant metabolism, cellular signaling, and defense. In leaf cells, the chloroplast is considered to be a focal point of ROS metabolism. It is a major producer of O2- and H2O2 during photosynthesis, and it contains a large array of ROS-scavenging mechanisms that have been extensively studied. By contrast, the function of the cytosolic ROS-scavenging mechanisms of leaf cells is largely unknown. In this study, we demonstrate that in the absence of the cytosolic H2O2-scavenging enzyme ascorbate peroxidase 1 (APX1), the entire chloroplastic H2O2-scavenging system of Arabidopsis thaliana collapses, H2O2 levels increase, and protein oxidation occurs. We further identify specific proteins oxidized in APX1-deficient plants and characterize the signaling events that ensue in knockout-Apx1 plants in response to a moderate level of light stress. Using a dominant-negative approach, we demonstrate that heat shock transcription factors play a central role in the early sensing of H2O2 stress in plants. Using knockout plants for the NADPH oxidase D protein (knockout-RbohD), we demonstrate that RbohD might be required for ROS signal amplification during light stress. Our study points to a key role for the cytosol in protecting the chloroplast during light stress and provides evidence for cross-compartment protection of thylakoid and stromal/mitochondrial APXs by cytosolic APX1.

Inhibition of blue light-dependent H+ pumping by abscisic acid through hydrogen peroxide-induced dephosphorylation of the plasma membrane H+-ATPase in guard cell protoplasts

Blue light (BL)-dependent H+ pumping by guard cells, which drives stomatal opening, is inhibited by abscisic acid (ABA). We investigated this response with respect to the activity of plasma membrane H+-ATPase using Vicia guard cell protoplasts. ATP hydrolysis by the plasma membrane H+-ATPase, phosphorylation of the H+-ATPase, and the binding of 14-3-3 protein to the H+-ATPase stimulated by BL were inhibited by ABA at 10 microm. All of these responses were similarly inhibited by hydrogen peroxide (H2O2) at 1 mm. The ABA-induced inhibitions of BL-dependent H+ pumping and phosphorylation of the H+-ATPase were partially restored by ascorbate, an intracellular H2O2 scavenger. A single-cell analysis of the cytosolic H2O2 using 2',7'-dichlorofluorescin revealed that H2O2 was generated by ABA in guard cell protoplasts. We also indicated that H+ pumping induced by fusicoccin and the binding of 14-3-3 protein to the H+-ATPase were inhibited slightly (approximately 20%) by both ABA and H2O2. By contrast, H2O2 at 1 mm did not affect H+ pumping by the H+-ATPase in microsomal membranes. From these results, we concluded that inhibition of BL-dependent H+ pumping by ABA was due to a decrease in the phosphorylation levels of H+-ATPase and that H2O2 might be involved in this response. Moreover, there are at least two inhibition sites by ABA in the BL signaling pathway of guard cells.

Extracellular calmodulin-induced stomatal closure is mediated by heterotrimeric G protein and H2O2

Extracellular calmodulin (ExtCaM) exerts multiple functions in animals and plants, but the mode of ExtCaM action is not well understood. In this paper, we provide evidence that ExtCaM stimulates a cascade of intracellular signaling events to regulate stomatal movement. Analysis of the changes of cytosolic free Ca2+ ([Ca2+]cyt) and H2O2 in Vicia faba guard cells combined with epidermal strip bioassay suggests that ExtCaM induces an increase in both H2O2 levels and [Ca2+]cyt, leading to a reduction in stomatal aperture. Pharmacological studies implicate heterotrimeric G protein in transmitting the ExtCaM signal, acting upstream of [Ca2+]cyt elevation, and generating H2O2 in guard cell responses. To further test the role of heterotrimeric G protein in ExtCaM signaling in stomatal closure, we checked guard cell responses in the Arabidopsis (Arabidopsis thaliana) Galpha-subunit-null gpa1 mutants and cGalpha overexpression lines. We found that gpa1 mutants were insensitive to ExtCaM stimulation of stomatal closure, whereas cGalpha overexpression enhanced the guard cell response to ExtCaM. Furthermore, gpa1 mutants are impaired in ExtCaM induction of H2O2 generation in guard cells. Taken together, our results strongly suggest that ExtCaM activates an intracellular signaling pathway involving activation of a heterotrimeric G protein, H2O2 generation, and changes in [Ca2+]cyt in the regulation of stomatal movements.

Guard cells: a dynamic signaling model

The year 2003 has provided a continuing accretion of knowledge concerning the diverse ways in which guard cells sense and respond to abscisic acid. A deeper understanding of the biochemical mechanisms governing the response of guard cells to blue light has been gained, and new insights have been garnered regarding roles of the extracellular matrix in stomatal regulation.

Hydrogen peroxide is a common signal for darkness- and ABA-induced stomatal closure in Pisum sativum

The requirement for hydrogen peroxide (H₂O₂) generation and action during stomatal closure induced by darkness and abscisic acid (ABA) was investigated in pea (Pisum sativum L.). Stomatal closure induced by darkness or ABA was inhibited by the H₂O₂-scavenging enzyme catalase or the antioxidant N-acetyl cysteine (NAC), or by diphenylene iodonium (DPI), an inhibitor of the H₂O₂-generating enzyme NADPH oxidase. Exogenous H₂O₂ induced stomatal closure in a dose- and time-dependent manner, and H₂O₂ was also required for ABA-inhibition of stomatal opening in the light. H₂O₂ accumulation in guard cells was increased by darkness or ABA, as assessed with the fluorescent dye dichlorodihydrofluorescein diacetate (H₂-DCFDA) and confocal microscopy. Such increases were inhibited by catalase, NAC or DPI, consistent with the effects of these compounds on stomatal apertures. Employing polymerase chain reaction (PCR) with degenerate oligonucleotide primers, several NADPH oxidase homologues were identified from pea genomic DNA that had substantial identity to the Arabidopsis thaliana (L.) Heynh. rboh (respiratory burst oxidase homologue) genes. Furthermore, an antibody raised against the tomato rboh identified immunoreactive proteins in epidermal, mesophyll and guard cells.

ROP/RAC GTPase: an old new master regulator for plant signaling

The ROP family of small GTPases has emerged as a versatile and pivotal regulator in plant signal transduction. Recent studies have implicated ROP signaling in diverse processes ranging from cytoskeletal organization to hormone and stress responses. Acting as a switch early in signaling cascades, ROPs are also capable of orchestrating several downstream pathways to amplify a specific signal.

Signalling by tips

New molecules, including protein kinases, lipids and molecules that have neurotransmitter activities in animals have emerged as important players in tip-growing cells. Transcriptomics analysis reveals that the largest single class of genes expressed in pollen tubes encode signal transducers, reflecting the necessity to decode complex and diverse pathways that are associated with tip growth. Many of these pathways may use common intracellular second messengers, with ions and reactive oxygen species emerging as two major common denominators in many of the processes involved in tip growth. These second messengers might influence the actin cytoskeleton through known interactions with actin-binding proteins. In turn, changes in the dynamic properties of the cytoskeleton would define the basic polarity events needed to shape and modify tip-growing cells.

Monitoring the expression profiles of genes induced by hyperosmotic, high salinity, and oxidative stress and abscisic acid treatment in Arabidopsis cell culture using a full-length cDNA microarray

Transcriptional regulation in response to hyperosmotic, high-salinity and oxidative stress, and abscisic acid (ABA) treatment in Arabidopsis suspension-cultured cell line T87 was investigated with a cDNA microarray containing 7000 independent full-length Arabidopsis cDNAs. The transcripts of 102, 11, 84 and 73 genes were increased more than 5-fold within 5h after treatment with 0.5M mannitol, 0.1M NaCl, 50 microM ABA and 10mM H2O2, respectively. On the other hand, the transcripts of 44, 57, 25 and 34 genes were down-regulated to less than one-third within 5h after treatment with 0.5M mannitol, 0.1M NaCl, 50 microM ABA and 10mM H2O2, respectively. Venn diagram analysis revealed 11 genes were induced significantly by mannitol, NaCl, and ABA, indicating crosstalk among these signaling pathways. Comparison of the genes induced by each stress revealed that 32%, 17% and 33% of mannitol-, NaCl- and ABA-inducible genes were also induced by H2O2, indicating the crosstalk between the signaling pathways for osmotic stress and oxidative stress. Although the expression profiles revealed that the T87 cells had most of the regulatory systems seen in Arabidopsis seedlings, the T87 cells did not have one of ABA-dependent signaling pathways.

The AoPR10 promoter and certain endogenous PR10 genes respond to oxidative signals in Arabidopsis

SUMMARY The PR10 class of genes has been associated with plant defence. Previous studies with an asparagus PR10 gene (AoPR1) promoter in heterologous plants suggested that the AoPR10-GUS transgene was responsive to oxidative signals/stresses. Arabidopsis thaliana AoPR10-GUS transgenics allowed expression to be compared with that of a close homologue from the large family of PR10 class genes within the Arabidopsis genome. AoPR10-GUS was induced developmentally at sites of phenylpropanoid accumulation and by wounding, pathogen challenge and treatment with H(2)O(2) but not with salicylic acid (SA), ethylene, methyljasmonate or NO donors. Both wound- and pathogen-associated AoPR10-GUS expression could be suppressed by superoxide dismutase and the NADPH oxidase inhibitor, diphenylene iodonium. Northern blotting using an Arabidopsis PR10 homologue as a probe revealed transcript up-regulation by oxidative species generated by glucose oxidase and xanthine oxidase. In Arabidopsis, the AoPR10-GUS transgene was potentiated by SA and expressed systemically following wounding or challenge with avirulent bacteria. AoPR10-GUS x npr1-1 crosses revealed that potentiation and systemic expression was NPR1-independent. Systemic AoPR10-GUS expression following elicitation of a hypersensitive response but not wounding was abolished in NahG crosses, suggesting an SA-mediated potentiating action during SAR (systemic acquired resistance). These data suggest that the AoPR10 promoter reports the expression of reactive oxygen species-responsive PR10 genes and may indicate systemic changes in oxidative status following either wounding and/or the elicitation of a hypersensitive response in Arabidopsis.

LESION SIMULATING DISEASE 1 is required for acclimation to conditions that promote excess excitation energy

The lsd1 mutant of Arabidopsis fails to limit the boundaries of hypersensitive cell death response during avirulent pathogen infection and initiates unchecked lesions in long day photoperiod giving rise to the runaway cell death (rcd) phenotype. We link here the initiation and propagation of rcd to the activity of photosystem II, stomatal conductance and ultimately to photorespiratory H(2)O(2). A cross of lsd1 with the chlorophyll a/b binding harvesting-organelle specific (designated cao) mutant, which has a reduced photosystem II antenna, led to reduced lesion formation in the lsd1/cao double mutant. This lsd1 mutant also had reduced stomatal conductance and catalase activity in short-day permissive conditions and induced H(2)O(2) accumulation followed by rcd when stomatal gas exchange was further impeded. All of these traits depended on the defense regulators EDS1 and PAD4. Furthermore, nonphotorespiratory conditions retarded propagation of lesions in lsd1. These data suggest that lsd1 failed to acclimate to light conditions that promote excess excitation energy (EEE) and that LSD1 function was required for optimal catalase activity. Through this regulation LSD1 can influence the effectiveness of photorespiration in dissipating EEE and consequently may be a key determinant of acclimatory processes. Salicylic acid, which induces stomatal closure, inhibits catalase activity and triggers the rcd phenotype in lsd1, also impaired acclimation of wild-type plants to conditions that promote EEE. We propose that the roles of LSD1 in light acclimation and in restricting pathogen-induced cell death are functionally linked.

Direct involvement of hydrogen peroxide in curvature of wheat coleoptile in blue-light-treated and dark-grown coleoptiles

Blue-light-induced photomorphogenesis is the sum total of a sequence of phenomena involving absorption of light by specific receptors, generation of a signal, processing transmembrane transport of signal, and the activation of a cascade of reactions in the cell interior. Though four blue-light receptors cryptochrome1, cryptochrome2, phototropin1, and phototropin2 have been identified, the signal transduction events associated with blue-light receptor activation are not understood. In this report, we demonstrate the generation and spatiotemporal distribution of H(2)O(2) in wheat coleoptile in response to blue light. Interception of the free-radical generation pathways dithiothreitol and propyl gallate rendered wheat coleoptile tips phototropically non-responsive. Unilateral application of H(2)O(2) onto the sub-apical region of a growing coleoptile brought about curvature in dark. Blue light also caused lipid peroxidation and augmented membrane rigidity of coleoptile cell membranes. We conclude that H(2)O(2) can act as a translocating second messenger that could bring about coleoptile curvature, and the signaling events may trigger Ca(2+) signaling cascades, changes in gene expression, and protein modifications.

Arabidopsis RADICAL-INDUCED CELL DEATH1 belongs to the WWE protein-protein interaction domain protein family and modulates abscisic acid, ethylene, and methyl jasmonate responses

Experiments with several Arabidopsis thaliana mutants have revealed a web of interactions between hormonal signaling. Here, we show that the Arabidopsis mutant radical-induced cell death1 (rcd1), although hypersensitive to apoplastic superoxide and ozone, is more resistant to chloroplastic superoxide formation, exhibits reduced sensitivity to abscisic acid, ethylene, and methyl jasmonate, and has altered expression of several hormonally regulated genes. Furthermore, rcd1 has higher stomatal conductance than the wild type. The rcd1-1 mutation was mapped to the gene At1g32230 where it disrupts an intron splice site resulting in a truncated protein. RCD1 belongs to the (ADP-ribosyl)transferase domain-containing subfamily of the WWE protein-protein interaction domain protein family. The results suggest that RCD1 could act as an integrative node in hormonal signaling and in the regulation of several stress-responsive genes.

Arabidopsis RADICAL-INDUCED CELL DEATH1 belongs to the WWE protein-protein interaction domain protein family and modulates abscisic acid, ethylene, and methyl jasmonate responses

Experiments with several Arabidopsis thaliana mutants have revealed a web of interactions between hormonal signaling. Here, we show that the Arabidopsis mutant radical-induced cell death1 (rcd1), although hypersensitive to apoplastic superoxide and ozone, is more resistant to chloroplastic superoxide formation, exhibits reduced sensitivity to abscisic acid, ethylene, and methyl jasmonate, and has altered expression of several hormonally regulated genes. Furthermore, rcd1 has higher stomatal conductance than the wild type. The rcd1-1 mutation was mapped to the gene At1g32230 where it disrupts an intron splice site resulting in a truncated protein. RCD1 belongs to the (ADP-ribosyl)transferase domain-containing subfamily of the WWE protein-protein interaction domain protein family. The results suggest that RCD1 could act as an integrative node in hormonal signaling and in the regulation of several stress-responsive genes.

NAD - new roles in signalling and gene regulation in plants

The pyridine nucleotides, NAD⁺ , NADH, NADP⁺ , and NADPH have long-established and well-characterised roles as redox factors in processes such as oxidative phosphorylation, the TCA cycle, and as electron acceptors in photosynthesis. Recent years have seen an increase in the number of signalling and gene regulatory processes where NAD⁺ or NADP⁺ are metabolised. Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are metabolites of NAD⁺ and NADP⁺ , respectively, and now have widely accepted roles as potent intracellular calcium releasing agents in animals, but are less well characterised in plants. NAD kinases catalyse the transfer of a phosphate group from ATP to NAD to form NADP and are well characterised in plants in their requirement for the calcium binding protein calmodulin, thereby putatively linking their regulation to stress-induced intracellular calcium release. A second group of proteins unrelated to those above, the sirtuins (Sir2) and poly ADP-ribose polymerases (PARPs), cleave NAD and transfer the ADP-ribose group to acetyl groups and proteins, respectively. These have roles in transcriptional control and DNA repair in eukaryotes. Contents Summary I. Introduction 32 II. NAD synthesis and breakdown 32 III. cADPR in plants 34 IV. NAADP in plants 35 V. NAD kinases 35 VI. NAD and gene regulation 37 VII. Sir2 is an NAD dependant histone deacetylase 37 VIII. Nicotinamidases 38 IX. Poly ADP-ribosylation 39 X. Poly(ADP-ribose) glycohydrolase (PARG) 40 XI. Subcellular compartmentation of NAD and NADP in plants 41 XII. Conclusions 41 Acknowledgements 41 References 41.

Positioning Arabidopsis in plant biology. A key step toward unification of plant research

One of the major challenges in biological investigation involves developing a robust predictive framework in which biological outputs can be predicted from input data and knowledge of the state of the system. Currently, genomics-based strategies provide a strong framework for integrating biological knowledge within a species and linking knowledge between diverse organisms, as DNA sequence is a durable, accurate, and complete record of biological information. As such, it provides the best source of information upon which predictive rules can start to be built, tested, and generalized. Generalization is a key component of predictive biology because it defines the extent to which we can accurately predict from one instance to another. In plant science, several important research themes are concerned with generalization, and progress in these areas is reviewed here. The importance of developing a framework for predictive biology that includes a much wider variety of plant species is also emphasized.

Reactive oxygen signalling: the latest news

During the past two years, a wide range of plant responses have been found to be triggered by hydrogen peroxide that is generated in a genetically controlled manner by NADPH oxidases. Several studies have revealed examples of how changes in the concentrations of reactive oxygen species (ROS) are perceived and transferred into signals that change the transcription of genes. Moreover, both the chemical identity of a given ROS and the intracellular site of its production seem to affect the specificity of its biological activity, further increasing the complexity of ROS signalling within plants.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.

Plasma membrane depolarization induced by abscisic acid in Arabidopsis suspension cells involves reduction of proton pumping in addition to anion channel activation, which are both Ca2+ dependent

In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H(+)-ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H+ pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H(+)-ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca2+]cyt increase induced by ABA since increase in [Ca(2+)](cyt) enhanced anion channels and impaired H(+)-ATPases.

Plasma membrane depolarization induced by abscisic acid in Arabidopsis suspension cells involves reduction of proton pumping in addition to anion channel activation, which are both Ca2+ dependent

In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H(+)-ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H+ pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H(+)-ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca2+]cyt increase induced by ABA since increase in [Ca(2+)](cyt) enhanced anion channels and impaired H(+)-ATPases.

Reactive oxygen species generated by microbial NADPH oxidase NoxA regulate sexual development in Aspergillus nidulans

NADPH oxidases (Nox) have been characterized as higher eukaryotic enzymes used deliberately to produce reactive oxygen species (ROS). The recent discovery of new functional members of the Nox family in plants and animals has led to the recognition of the increasing importance of ROS as signals involved in regulation of diverse cellular processes such as defence, growth and signalling. Here, we address the role of NADPH oxidase-generated ROS in the biology of the filamentous fungus Aspergillus nidulans. We characterize the noxA gene and show that it encodes a member of a novel NADPH oxidase subfamily ubiquitous in lower eukaryotes. Deletion of noxA specifically blocks differentiation of sexual fruit bodies (cleistothecia), without affecting hyphal growth or asexual development. Accordingly, the noxA gene is induced during sexual development, peaking at the time of cleistothecia differentiation and in parallel with the hülle cell-associated catalase peroxidase gene cpeA. This expression pattern is not dependent on transcription factors SteA and StuA, which are essential for cleistothecia formation. In contrast, noxA-dependent premature sexual development correlates with noxA derepression in DeltasakA null mutants, connecting stress MAPK signalling to the regulated production of ROS. Using a nitroblue tetrazolium (NBT) assay to detect superoxide, we found that hülle cells and cleistothecia initials produce superoxide in a process inhibited by NADPH oxidase inhibitor DPI and markedly reduced in DeltanoxA mutants. Furthermore, using H2DCFDA, we detected that H2O2 and possibly other ROS are generated in a NoxA-dependent fashion, mainly in the external walls from cleistothecia initials. The essential role of NoxA-generated ROS in A. nidulans sexual differentiation and the presence of one or two noxA homologues in all analysed filamentous fungi suggest that NADPH oxidase-generated ROS play important roles in fungal physiology and differentiation.

Cytoplasmic alkalization precedes reactive oxygen species production during methyl jasmonate- and abscisic acid-induced stomatal closure

Signaling events during abscisic acid (ABA) or methyl jasmonate (MJ)-induced stomatal closure were examined in Arabidopsis wild type, ABA-insensitive (ost1-2), and MJ-insensitive mutants (jar1-1) in order to examine a crosstalk between ABA and MJ signal transduction. Some of the experiments were performed on epidermal strips of Pisum sativum. Stomata of jar1-1 mutant plants are insensitive to MJ but are able to close in response to ABA. However, their sensitivity to ABA is less than that of wild-type plants. Reciprocally, the stomata of ost1-2 are insensitive to ABA but are able to close in response to MJ to a lesser extent compared to wild-type plants. Both MJ and ABA promote H(2)O(2) production in wild-type guard cells, while exogenous application of diphenylene iodonium (DPI) chloride, an inhibitor of NAD(P)H oxidases, results in the suppression of ABA- and MJ-induced stomatal closure. ABA elevates H(2)O(2) production in wild-type and jar1-1 guard cells but not in ost1-2, whereas MJ induces H(2)O(2) production in both wild-type and ost1-2 guard cells, but not in jar1-1. MJ-induced stomatal closing is suppressed in the NAD(P)H oxidase double mutant atrbohD/F and in the outward potassium channel mutant gork1. Furthermore, MJ induces alkalization in guard cell cytosol, and MJ-induced stomatal closing is inhibited by butyrate. Analyses of the kinetics of cytosolic pH changes and reactive oxygen species (ROS) production show that the alkalization of cytoplasm precedes ROS production during the stomatal response to both ABA and MJ. Our results further indicate that JAR1, as OST1, functions upstream of ROS produced by NAD(P)H oxidases and that the cytoplasmic alkalization precedes ROS production during MJ or ABA signal transduction in guard cells.

Microarray expression analyses of Arabidopsis guard cells and isolation of a recessive abscisic acid hypersensitive protein phosphatase 2C mutant

Oligomer-based DNA Affymetrix GeneChips representing about one-third of Arabidopsis (Arabidopsis thaliana) genes were used to profile global gene expression in a single cell type, guard cells, identifying 1309 guard cell-expressed genes. Highly pure preparations of guard cells and mesophyll cells were isolated in the presence of transcription inhibitors that prevented induction of stress-inducible genes during cell isolation procedures. Guard cell expression profiles were compared with those of mesophyll cells, resulting in identification of 64 transcripts expressed preferentially in guard cells. Many large gene families and gene duplications are known to exist in the Arabidopsis genome, giving rise to redundancies that greatly hamper conventional genetic and functional genomic analyses. The presented genomic scale analysis identifies redundant expression of specific isoforms belonging to large gene families at the single cell level, which provides a powerful tool for functional genomic characterization of the many signaling pathways that function in guard cells. Reverse transcription-PCR of 29 genes confirmed the reliability of GeneChip results. Statistical analyses of promoter regions of abscisic acid (ABA)-regulated genes reveal an overrepresented ABA responsive motif, which is the known ABA response element. Interestingly, expression profiling reveals ABA modulation of many known guard cell ABA signaling components at the transcript level. We further identified a highly ABA-induced protein phosphatase 2C transcript, AtP2C-HA, in guard cells. A T-DNA disruption mutation in AtP2C-HA confers ABA-hypersensitive regulation of stomatal closing and seed germination. The presented data provide a basis for cell type-specific genomic scale analyses of gene function.

Plant respiratory burst oxidase homologs impinge on wound responsiveness and development in Lycopersicon esculentum

Plant respiratory burst oxidase homologs (Rboh) are homologs of the human neutrophil pathogen-related gp91(phox). Antisense technology was employed to ascertain the biological function of Lycopersicon esculentum (tomato) Rboh. Lines with diminished Rboh activity showed a reduced level of reactive oxygen species (ROS) in the leaf, implying a role for Rboh in establishing the cellular redox milieu. Surprisingly, the antisense plants acquired a highly branched phenotype, switched from indeterminate to determinate growth habit, and had fasciated reproductive organs. Wound-induced systemic expression of proteinase inhibitor II was compromised in the antisense lines, indicating that ROS intermediates supplied by Rboh are required for this wound response. Extending these observations by transcriptome analysis revealed ectopic leaf expression of homeotic MADS box genes that are normally expressed only in reproductive organs. In addition, both Rboh-dependent and -independent wound-induced gene induction was detected as well as transcript changes related to redox maintenance. The results provide novel insights into how the steady state cellular level of ROS is controlled and portrays the role of Rboh as a signal transducer of stress and developmental responses.

Microarray expression analyses of Arabidopsis guard cells and isolation of a recessive abscisic acid hypersensitive protein phosphatase 2C mutant

Oligomer-based DNA Affymetrix GeneChips representing about one-third of Arabidopsis (Arabidopsis thaliana) genes were used to profile global gene expression in a single cell type, guard cells, identifying 1309 guard cell-expressed genes. Highly pure preparations of guard cells and mesophyll cells were isolated in the presence of transcription inhibitors that prevented induction of stress-inducible genes during cell isolation procedures. Guard cell expression profiles were compared with those of mesophyll cells, resulting in identification of 64 transcripts expressed preferentially in guard cells. Many large gene families and gene duplications are known to exist in the Arabidopsis genome, giving rise to redundancies that greatly hamper conventional genetic and functional genomic analyses. The presented genomic scale analysis identifies redundant expression of specific isoforms belonging to large gene families at the single cell level, which provides a powerful tool for functional genomic characterization of the many signaling pathways that function in guard cells. Reverse transcription-PCR of 29 genes confirmed the reliability of GeneChip results. Statistical analyses of promoter regions of abscisic acid (ABA)-regulated genes reveal an overrepresented ABA responsive motif, which is the known ABA response element. Interestingly, expression profiling reveals ABA modulation of many known guard cell ABA signaling components at the transcript level. We further identified a highly ABA-induced protein phosphatase 2C transcript, AtP2C-HA, in guard cells. A T-DNA disruption mutation in AtP2C-HA confers ABA-hypersensitive regulation of stomatal closing and seed germination. The presented data provide a basis for cell type-specific genomic scale analyses of gene function.

Cryptogein-induced initial events in tobacco BY-2 cells: pharmacological characterization of molecular relationship among cytosolic Ca(2+) transients, anion efflux and production of reactive oxygen species

Ion fluxes and the production of reactive oxygen species (ROS) are early events that follow elicitor treatment or microbial infection. However, molecular mechanisms for these responses as well as their relationship have been controversial and still largely unknown. We here simultaneously monitored the temporal sequence of initial events at the plasma membrane in suspension-cultured tobacco cells (cell line BY-2) in response to a purified proteinaceous elicitor, cryptogein, which induced hypersensitive cell death. The elicitor induced transient rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) showing two distinct peaks, followed by biphasic (rapid/transient and slow/prolonged) Cl(-) efflux and H(+) influx. Pharmacological analyses suggested that the two phases of the [Ca(2+)](cyt) response correspond to Ca(2+) influx through the plasma membrane and an inositol 1,4,5-trisphophate-mediated release of Ca(2+) from intracellular Ca(2+) stores, respectively, and the [Ca(2+)](cyt) transients and the Cl(-) efflux were mutually dependent events regulated by protein phosphorylation. The elicitor also induced production of ROS including (*)O(2)(-) and H(2)O(2), which initiated after the [Ca(2+)](cyt) rise and required Ca(2+) influx, Cl(-) efflux and protein phosphorylation. An inhibitor of NADPH oxidase, diphenylene iodonium, completely inhibited the elicitor-induced production of (*)O(2)(-) and H(2)O(2), but did not affect the [Ca(2+)](cyt) transients. These results suggest that cryptogein-induced plasma membrane Ca(2+) influx is independent of ROS, and NADPH oxidase dependent ROS production is regulated by these series of ion fluxes.

Cell expansion in roots

Cell expansion in roots is crucial for the exploration and exploitation of the soil substrate and the plethora of activities that roots engage in. Expansion requires the coordinated activities of many cell processes. Central to this is the control of ion transport during vacuolar growth, which mediates the increase in cell size and the concomitant production of new wall and membrane at the surface of growing cells. The cytoskeleton plays an important role in growth and the control of growth direction. Evidence is accumulating to show that plant hormones also coordinate cell expansion throughout the plant by controlling the activities of growth-regulating DELLA proteins.

The generation of Ca(2+) signals in plants

The calcium ion is firmly established as a ubiquitous intracellular second messenger in plants. At their simplest, Ca(2+)-based signaling systems are composed of a receptor, a system for generating the increase in [Ca(2+)]cyt, downstream components that are capable of reacting to the increase in [Ca(2+)]cyt, and other cellular systems responsible for returning [Ca(2+)]cyt to its prestimulus level. Here we review the various mechanisms responsible for generating the stimulus-induced increases in [Ca(2+)]cyt known as Ca(2+) signals. We focus particularly on the mechanisms responsible for generating [Ca(2+)]cyt oscillations and transients and use Nod Factor signaling in legume root hairs and stimulus-response coupling in guard cells to assess the physiological significance of these classes of Ca(2+) signals.

Purification and characterization of a barley aleurone abscisic acid-binding protein

A protein designated ABAP1 and encoded by a novel gene (GenBank accession number AF127388) was purified and shown to specifically bind abscisic acid (ABA). ABAP1 protein is a 472-amino acid polypeptide containing a WW protein interaction domain and is induced by ABA in barley aleurone layers. Polyclonal antiidiotypic antibodies (AB2) cross-reacted with purified ABAP1 and with a corresponding 52-kDa protein associated with membrane fractions of ABA-treated barley aleurones. ABAP1 genes were detected in diverse monocot and dicot species, including wheat, tobacco, alfalfa, garden pea, and oilseed rape. The recombinant ABAP1 protein optimally bound (3)H-(+)-ABA at neutral pH. Denatured ABAP1 protein did not bind (3)H-(+)-ABA, nor did bovine serum albumin. The maximum specific binding as shown by Scatchard plot analysis was 0.8 mol of ABA mol(-1) protein with a linear function of r(2) = 0.94, an indication of one ABA-binding site with a dissociation constant (K(d)) of 28 x 10(-9) m. ABA binding in aleurone plasma membranes showed a maximum binding capacity of 330 nmol of ABA g(-1) protein with a K(d) of 26.5 x 10(-9) m. The similarities in the dissociation constants for ABA binding of the recombinant protein and that of the plasma membranes suggest that the protein within the plasma membrane fraction is the native form of ABAP1. The stereospecificity of ABAP1 was established by the incapability of ABA analogs and metabolites, including (-)-ABA, trans-ABA, phaseic acid, dihydrophaseic acid, and (+)-abscisic acid-glucose ester, to displace (3)H-(+)-ABA bound to ABAP1. However, two ABA precursors, (+)-ABA aldehyde and (+)-ABA alcohol, were able to displace (3)H-(+)-ABA, an indication that the structural requirement of ABAP1 at the C-1 position is not strict. Our data show that ABAP1 exerts high binding affinity for ABA. The interaction is reversible, follows saturation kinetics, and has stereospecificity, thus meeting the criteria for an ABA-binding protein.

Relay and control of abscisic acid signaling

Insights into the signal transduction of the phytohormone abscisic acid (ABA) have unfolded dramatically in the past few years and reveal an unanticipated complexity. Knockout lines and RNA-interference technology, together with protein interaction analyses, have been used to identify many of the cellular components that regulate or modulate ABA responses. ABA signaling is characterized by a plethora of intracellular messengers. This may reflect the function of ABA in integrating several stress responses and antagonizing pathways via cross-talk, but it hampers the establishment of a unifying concept. Transcriptome analyses have unraveled more than a thousand genes that are differentially regulated by ABA, and these ABA-mediated changes in gene expression translate to major changes in proteome expression. ABA-induced mechanisms that re-adjust cellular protein expression are just surfacing. ABA-response-specific transcription factors have a well-established function in that process and, recently, it has also become clear that phytohormone signaling enforces a sophisticated interference with protein expression at the posttranscriptional level. This interference includes both targeted proteolysis and the regulation of the translation of specific mRNAs by RNA-binding proteins.

The oleate-stimulated phospholipase D, PLDdelta, and phosphatidic acid decrease H2O2-induced cell death in Arabidopsis

Hydrolysis of common membrane phospholipids occurs in response to various environmental stresses, but the control and cellular function of this hydrolysis are not fully understood. Hydrogen peroxide (H2O2) is a pivotal signaling molecule involved in various stress responses. Here, we show that the plasma membrane-bound phospholipase D, PLDdelta, is activated in response to H2O2 and that the resulting phosphatidic acid (PA) functions to decrease H2O2-promoted programmed cell death. The Arabidopsis genome has 12 PLD genes, and knockout of PLDdelta abolishes specifically the oleate-stimulated PLD activity. H2O2 treatment of Arabidopsis cells activates PLD enzyme activity, and ablation of PLDdelta abolishes that activation. PLDdelta-null cells display increased sensitivity to H2O2-induced cell death. The addition of PA to PLDdelta-null cells mitigates the H2O2 effect, whereas suppression of the H2O2-induced PA formation in wild-type cells increases the effect. PLDdelta-ablated plants exhibit increased susceptibility to stress. These results demonstrate that activation of oleate-stimulated PLDdelta constitutes an important step in the plant response to H2O2 and increasing plant stress tolerance.