Spatial variation in H2O2 response of Arabidopsis thaliana root epidermal Ca2+ flux and plasma membrane Ca2+ channels

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.

Salt-sensitive and salt-tolerant barley varieties differ in the extent of potentiation of the ROS-induced K(+) efflux by polyamines

Generation of high levels of polyamines and reactive oxygen species (ROS) is common under stress conditions. Our recent study on a salt-sensitive pea species revealed an interaction between natural polyamines and hydroxyl radicals in inducing non-selective conductance and stimulating Ca(2+)-ATPase pumps at the root plasma membrane (I. Zepeda-Jazo, A.M. Velarde-Buendía, R. Enríquez-Figueroa, B. Jayakumar, S. Shabala, J. Muñiz, I. Pottosin, Polyamines interact with hydroxyl radicals in activating Ca2+ and K+ transport across the root epidermal plasma membranes, Plant Phys. 157 (2011) 1-14). In this work, we extended that study to see if interaction between polyamines and ROS may determine the extent of genotypic variation in salinity tolerance. This work was conducted using barley genotypes contrasting in salinity tolerance. Similar to our findings in pea, application of hydroxyl radicals-generating Cu(2+)/ascorbate mixture induced transient Ca(2+) and K(+) fluxes in barley roots. Putrescine and spermine alone induced only transient Ca(2+) efflux and negligible K(+) flux. However, both putrescine and spermine strongly potentiated hydroxyl radicals-induced K(+) efflux and respective non-selective current. This synergistic effect was much more pronounced in a salt-sensitive cultivar Franklin as compared to a salt-tolerant TX9425. As retention of K(+) under salt stress is a key determinant of salinity tolerance in barley, we suggest that the alteration of cytosolic K(+) homeostasis, caused by interaction between polyamines and ROS, may have a substantial contribution to genetic variability in salt sensitivity in this species.

Ion Channels in Plants

Since the first recordings of single potassium channel activities in the plasma membrane of guard cells more than 25 years ago, patch-clamp studies discovered a variety of ion channels in all cell types and plant species under inspection. Their properties differed in a cell type- and cell membrane-dependent manner. Guard cells, for which the existence of plant potassium channels was initially documented, advanced to a versatile model system for studying plant ion channel structure, function, and physiology. Interestingly, one of the first identified potassium-channel genes encoding the Shaker-type channel KAT1 was shown to be highly expressed in guard cells. KAT1-type channels from Arabidopsis thaliana and its homologs from other species were found to encode the K+-selective inward rectifiers that had already been recorded in early patch-clamp studies with guard cells. Within the genome era, additional Arabidopsis Shaker-type channels appeared. All nine members of the Arabidopsis Shaker family are localized at the plasma membrane, where they either operate as inward rectifiers, outward rectifiers, weak voltage-dependent channels, or electrically silent, but modulatory subunits. The vacuole membrane, in contrast, harbors a set of two-pore K+ channels. Just very recently, two plant anion channel families of the SLAC/SLAH and ALMT/QUAC type were identified. SLAC1/SLAH3 and QUAC1 are expressed in guard cells and mediate Slow- and Rapid-type anion currents, respectively, that are involved in volume and turgor regulation. Anion channels in guard cells and other plant cells are key targets within often complex signaling networks. Here, the present knowledge is reviewed for the plant ion channel biology. Special emphasis is drawn to the molecular mechanisms of channel regulation, in the context of model systems and in the light of evolution.

Manipulation of monoubiquitin improves salt tolerance in transgenic tobacco

Ubiquitin (Ub) is regarded as a stress protein involved in many stress responses. In this paper, sense and antisense transgenic tobacco plants, as well as the wild type and vector control, were used to study the role of Ub in salt tolerance of plants. In sense Ta-Ub2 transgenic tobacco plants, there was higher expression of Ub protein conjugates than in the wild type and vector control, but the reverse trend was observed in antisense Nt-Ub1 transgenic plants. The germination rate of tobacco seed, growth status and photosynthesis of the tobacco plants suggested that over-expressing Ub promoted the growth of transgenic tobacco plants and enhanced their salt tolerance, but the opposite effect was seen in plants with repressed Ub expression. Changes in antioxidant capacity may be one of the mechanisms underlying Ub-regulated salt tolerance. Furthermore, improved tolerance to a combination of stresses was also observed in the sense transgenic tobacco plants. These findings imply that Ub is involved in the tolerance of plants to abiotic stress.

Calcium efflux systems in stress signaling and adaptation in plants

Transient cytosolic calcium ([Ca(2+)](cyt)) elevation is an ubiquitous denominator of the signaling network when plants are exposed to literally every known abiotic and biotic stress. These stress-induced [Ca(2+)](cyt) elevations vary in magnitude, frequency, and shape, depending on the severity of the stress as well the type of stress experienced. This creates a unique stress-specific calcium "signature" that is then decoded by signal transduction networks. While most published papers have been focused predominantly on the role of Ca(2+) influx mechanisms to shaping [Ca(2+)](cyt) signatures, restoration of the basal [Ca(2+)](cyt) levels is impossible without both cytosolic Ca(2+) buffering and efficient Ca(2+) efflux mechanisms removing excess Ca(2+) from cytosol, to reload Ca(2+) stores and to terminate Ca(2+) signaling. This is the topic of the current review. The molecular identity of two major types of Ca(2+) efflux systems, Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers, is described, and their regulatory modes are analyzed in detail. The spatial and temporal organization of calcium signaling networks is described, and the importance of existence of intracellular calcium microdomains is discussed. Experimental evidence for the role of Ca(2+) efflux systems in plant responses to a range of abiotic and biotic factors is summarized. Contribution of Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers in shaping [Ca(2+)](cyt) signatures is then modeled by using a four-component model (plasma- and endo-membrane-based Ca(2+)-permeable channels and efflux systems) taking into account the cytosolic Ca(2+) buffering. It is concluded that physiologically relevant variations in the activity of Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers are sufficient to fully describe all the reported experimental evidence and determine the shape of [Ca(2+)](cyt) signatures in response to environmental stimuli, emphasizing the crucial role these active efflux systems play in plant adaptive responses to environment.

Polyamines interact with hydroxyl radicals in activating Ca(2+) and K(+) transport across the root epidermal plasma membranes

Reactive oxygen species (ROS) are integral components of the plant adaptive responses to environment. Importantly, ROS affect the intracellular Ca(2+) dynamics by activating a range of nonselective Ca(2+)-permeable channels in plasma membrane (PM). Using patch-clamp and noninvasive microelectrode ion flux measuring techniques, we have characterized ionic currents and net K(+) and Ca(2+) fluxes induced by hydroxyl radicals (OH(•)) in pea (Pisum sativum) roots. OH(•), but not hydrogen peroxide, activated a rapid Ca(2+) efflux and a more slowly developing net Ca(2+) influx concurrent with a net K(+) efflux. In isolated protoplasts, OH(•) evoked a nonselective current, with a time course and a steady-state magnitude similar to those for a K(+) efflux in intact roots. This current displayed a low ionic selectivity and was permeable to Ca(2+). Active OH(•)-induced Ca(2+) efflux in roots was suppressed by the PM Ca(2+) pump inhibitors eosine yellow and erythrosine B. The cation channel blockers gadolinium, nifedipine, and verapamil and the anionic channel blockers 5-nitro-2(3-phenylpropylamino)-benzoate and niflumate inhibited OH(•)-induced ionic currents in root protoplasts and K(+) efflux and Ca(2+) influx in roots. Contrary to expectations, polyamines (PAs) did not inhibit the OH(•)-induced cation fluxes. The net OH(•)-induced Ca(2+) efflux was largely prolonged in the presence of spermine, and all PAs tested (spermine, spermidine, and putrescine) accelerated and augmented the OH(•)-induced net K(+) efflux from roots. The latter effect was also observed in patch-clamp experiments on root protoplasts. We conclude that PAs interact with ROS to alter intracellular Ca(2+) homeostasis by modulating both Ca(2+) influx and efflux transport systems at the root cell PM.

Plant responses to water stress: role of reactive oxygen species

Responses of plants to water stress may be assigned as either injurious change or tolerance index. One of the primary and cardinal changes in response to drought stress is the generation of reactive oxygen species (ROS), which is being considered as the cause of cellular damage. However, recently a signaling role of such ROS in triggering the ROS scavenging system that may confer protection or tolerance against stress is emerging. Such scavenging system consists of antioxidant enzymes like SOD, catalase and peroxidases, and antioxidant compounds like ascorbate, reduced glutathione; a balance between ROS generation and scavenging ultimately determines the oxidative load. As revealed in case of defence against pathogen, signaling via ROS is initiated by NADPH oxidase-catalyzed superoxide generation in the apoplastic space (cell wall) followed by conversion to hydrogen peroxide by the activity of cell wall-localized SOD. Wall peroxidase may also play role in ROS generation for signaling. Hydrogen peroxide may use Ca2+ and MAPK pathway as downstream signaling cascade. Plant hormones associated with stress responses like ABA and ethylene play their role possibly via a cross talk with ROS towards stress tolerance, thus projecting a dual role of ROS under drought stress.

Ion transport and osmotic adjustment in plants and bacteria

Plants and bacteria respond to hyperosmotic stress by an increase in intracellular osmolality, adjusting their cell turgor to altered growth conditions. This can be achieved either by increased uptake orde novosynthesis of a variety of organic osmolytes (so-called ‘compatible solutes’), or by controlling fluxes of ions across cellular membranes. The relative contributions of each of these mechanisms have been debated in literature for many years and remain unresolved. This paper summarises all the arguments and reopens a discussion on the efficiency and strategies of osmotic adjustment in plants and bacteria. We show that the bulk of osmotic adjustment in both plants and bacteria is achieved by increased accumulation of inorganic osmolytes such as K+, Na+and Cl-. This is applicable to both halophyte and glycophyte species. At the same time,de novosynthesis of compatible solutes is an energetically expensive and slow option and can be used only for the fine adjustment of the cell osmotic potential. The most likely role the organic osmolytes play in osmotic adjustment is in osmoprotection of key membrane transport proteins and reactive oxygen species (ROS) scavenging. The specific mechanisms by which compatible solutes regulate activity of ion transporters remain elusive and require more thorough investigation. It is concluded that creating transgenic species with increased levels of organic osmolytes by itself is counterproductive due to high yield penalties; all these attempts should be complemented by a concurrent increase in the accumulation of inorganic ions directly used for osmotic adjustment.

Receptor-like activity evoked by extracellular ADP in Arabidopsis root epidermal plasma membrane

Extracellular purine nucleotides are implicated in the control of plant development and stress responses. While extracellular ATP is known to activate transcriptional pathways via plasma membrane (PM) NADPH oxidase and calcium channel activation, very little is known about signal transduction by extracellular ADP. Here, extracellular ADP was found to activate net Ca(2+) influx in roots of Arabidopsis (Arabidopsis thaliana) and transiently elevate cytosolic free Ca(2+) in root epidermal protoplasts. An inward Ca(2+)-permeable conductance in root epidermal PM was activated within 1 s of ADP application and repeated application evoked a smaller current. Such response speed and densitization are consistent with operation of equivalents to animal ionotropic purine receptors, although to date no equivalent genes for such receptors have been identified in higher plants. In contrast to ATP, extracellular ADP did not evoke accumulation of intracellular reactive oxygen species. While high concentrations of ATP caused net Ca(2+) efflux from roots, equivalent concentrations of ADP caused net influx. Overall the results point to a discrete ADP signaling pathway, reliant on receptor-like activity at the PM.

Effect of cold and menthol on membrane potential in plants

In animals, cooling substances such as menthol are perceived as cold sensation because they bind to the same receptor TRPM8 (transient receptor potential melastatin) that activates upon temperature drops. We investigated the effect of menthol on the plant membrane potential to search for analogies between animal and plant perception systems. The study was conducted on the liverwort Conocephalum conicum- a non-vascular plant generating action potentials (APs) in response to different stimuli including cold. (+)Menthol, (-)menthol and (+/-)menthol induced one or more APs, depending on the concentration. In contrast to animal reactions to menthol, threshold concentrations of these isomers were the same (1 mM). The presence of menthol in medium shortened cold-induced APs, whereas low temperature prolonged the repolarization phase of AP evoked by menthol. Cells of C. conicum with anion and potassium channels blocked by anthracene-9-carboxylic acid (A9C) and tetraethylammonium chloride (TEACl) generate short spike-like voltage transients (VTs) in response to cold and light stimulation. Membrane potential changes evoked by menthol in A9C- and TEACl-treated plants differed significantly from VTs - lasted much longer and frequently occurred in series. 5 mM LaCl(3) , 1 mM EGTA (ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) (0 Ca(2+) ) but not 0.2 mM verapamil blocked the putative calcium component of AP induced by menthol. Similar inhibitory effect was observed after the application of proton pump inhibitors: 0.05 mM N,N-dicyclohexylcarbodiimide (DCCD), 0.05 mM diethylstilbestrol (DES) or 0.01 mM carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP). Our results indicate that cold and menthol act independently, activating different membrane transporters in C. conicum cells.

Plasma membrane Ca²+ transporters mediate virus-induced acquired resistance to oxidative stress

This paper reports the phenomenon of acquired cross-tolerance to oxidative stress in plants and investigates the activity of specific Ca²+ transport systems mediating this phenomenon. Nicotiana benthamiana plants were infected with Potato virus X (PVX) and exposed to oxidative [either ultraviolet (UV-C) or H₂O₂] stress. Plant adaptive responses were assessed by the combined application of a range of electrophysiological (non-invasive microelectrode ion flux measurements), biochemical (Ca²+- and H+-ATPase activity), imaging (fluorescence lifetime imaging measurements of changes in intracellular Ca²+ concentrations), pharmacological and cytological transmission electrone microscopy techniques. Virus-infected plants had a better ability to control UV-induced elevations in cytosolic-free Ca²+ and prevent structural and functional damage of chloroplasts. Taken together, our results suggest a high degree of crosstalk between UV and pathogen-induced oxidative stresses, and highlight the crucial role of Ca²+ efflux systems in acquired resistance to oxidative stress in plants.

Sodium transport in plants: a critical review

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

Root hair systems biology

Plant functional genomic studies have largely measured the response of whole plants, organs and tissues, resulting in the dilution of the signal from individual cells. Methods are needed where the full repertoire of functional genomic tools can be applied to a single plant cell. Root hair cells are an attractive model to study the biology of a single, differentiated cell type because of their ease of isolation, polar growth, and role in water and nutrient uptake, as well as being the site of infection by nitrogen-fixing bacteria. This review highlights the recent advances in our understanding of plant root hair biology and examines whether the root hair has potential as a model for plant cell systems biology.

Spermidine oxidase-derived H₂O₂ regulates pollen plasma membrane hyperpolarization-activated Ca(2+) -permeable channels and pollen tube growth

Spermidine (Spd) has been correlated with various physiological and developmental processes in plants, including pollen tube growth. In this work, we show that Spd induces an increase in the cytosolic Ca(2+) concentration that accompanies pollen tube growth. Using the whole-cell patch clamp and outside-out single-channel patch clamp configurations, we show that exogenous Spd induces a hyperpolarization-activated Ca(2+) current: the addition of Spd cannot induce the channel open probability increase in excised outside-out patches, indicating that the effect of Spd in the induction of Ca(2+) currents is exerted via a second messenger. This messenger is hydrogen peroxide (H₂O₂), and is generated during Spd oxidation, a reaction mediated by polyamine oxidase (PAO). These reactive oxygen species trigger the opening of the hyperpolarization-activated Ca(2+) -permeable channels in pollen. To provide further evidence that PAO is in fact responsible for the effect of Spd on the Ca(2+) -permeable channels, two Arabidopsis mutants lacking expression of the peroxisomal-encoding AtPAO3 gene, were isolated and characterized. Pollen from these mutants was unable to induce the opening of the Ca(2+) -permeable channels in the presence of Spd, resulting in reduced pollen tube growth and seed number. However, a high Spd concentration triggers a Ca(2+) influx beyond the optimal, which has a deleterious effect. These findings strongly suggest that the Spd-derived H₂O₂ signals Ca(2+) influx, thereby regulating pollen tube growth.

Cross-talk between ROS and calcium in regulation of nuclear activities

Calcium and Reactive Oxygen Species (ROS) are acknowledged as crucial second messengers involved in the response to various biotic and abiotic stresses. However, it is still not clear how these two compounds can play a role in different signaling pathways leading the plant to a variety of processes such as root development or defense against pathogens. Recently, it has been shown that the concept of calcium and ROS signatures, initially discovered in the cytoplasm, can also be extended to the nucleus of plant cells. In addition, it has been clearly proved that both ROS and calcium signals are intimately interconnected. How this cross-talk can finally modulate the translocation and/or the activity of nuclear proteins leading to the control of specific genes expression is the main focus of this review. We will especially focus on how calcium and ROS interact at the molecular level to modify their targets.

Extracellular ATP and nitric oxide signaling pathways regulate redox-dependent responses associated to root hair growth in etiolated Arabidopsis seedlings

Extracellular ATP (eATP) and nitric oxide (NO) have emerged as crucial players in plant development, stress responses and cell viability. Glutathione (GSH) is an abundant reducing agent with proposed roles in plant growth, development and stress physiology. In a recent publication, we demonstrated that eATP and NO restore hypocotyl elongation of etiolated Arabidopsis seedlings treated with GSH. Here it is reported that exogenous ATP also restore root hair growth suggesting a role for ATP and NO in the regulation of redox balance associated to specific processes of plant morphogenesis. A tentative model integrating redox-, eATP- and NO- signaling pathways during root hair growth in Arabidopsis seedlings is presented.

H2O2 and cytosolic Ca2+ signals triggered by the PM H-coupled transport system mediate K+/Na+ homeostasis in NaCl-stressed Populus euphratica cells

Using confocal microscopy, X-ray microanalysis and the scanning ion-selective electrode technique, we investigated the signalling of H(2)O(2), cytosolic Ca(2+) ([Ca(2+)](cyt)) and the PM H(+)-coupled transport system in K(+)/Na(+) homeostasis control in NaCl-stressed calluses of Populus euphratica. An obvious Na(+)/H(+) antiport was seen in salinized cells; however, NaCl stress caused a net K(+) efflux, because of the salt-induced membrane depolarization. H(2)O(2) levels, regulated upwards by salinity, contributed to ionic homeostasis, because H(2)O(2) restrictions by DPI or DMTU caused enhanced K(+) efflux and decreased Na(+)/H(+) antiport activity. NaCl induced a net Ca(2+) influx and a subsequent rise of [Ca(2+)](cyt), which is involved in H(2)O(2)-mediated K(+)/Na(+) homeostasis in salinized P. euphratica cells. When callus cells were pretreated with inhibitors of the Na(+)/H(+) antiport system, the NaCl-induced elevation of H(2)O(2) and [Ca(2+)](cyt) was correspondingly restricted, leading to a greater K(+) efflux and a more pronounced reduction in Na(+)/H(+) antiport activity. Results suggest that the PM H(+)-coupled transport system mediates H(+) translocation and triggers the stress signalling of H(2)O(2) and Ca(2+), which results in a K(+)/Na(+) homeostasis via mediations of K(+) channels and the Na(+)/H(+) antiport system in the PM of NaCl-stressed cells. Accordingly, a salt stress signalling pathway of P. euphratica cells is proposed.

Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Reactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K(+) channel responsible for K(+) efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca(2+), tetraethylammonium, Ba(2+), Cs(+) and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K(+) channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K(+) efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K(+)-channel activation. In animals, activation of K(+)-efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K(+)-channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K(+) channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K(+) channels.

ROS in plant development

Reactive oxygen species (ROS) are now recognized as important regulators of plant developmental programs and recent work on tip-growing systems has revealed a central role for the NADPH oxidases in generating such developmentally important ROS. Tip-growing cells have also shown that the functions of cytosolic ROS, acting as regulators of activities such as ion channel gating, are closely linked to those of ROS produced to the apoplast, where they act to modulate cell wall properties. Thus, coordination of ROS production and their activities between compartments is emerging as an important theme in our understanding of how growth and developmental programs are integrated.

Disruption of actin filaments induces mitochondrial Ca2+ release to the cytoplasm and [Ca2+]c changes in Arabidopsis root hairs

BACKGROUND

Mitochondria are dynamic organelles that move along actin filaments, and serve as calcium stores in plant cells. The positioning and dynamics of mitochondria depend on membrane-cytoskeleton interactions, but it is not clear whether microfilament cytoskeleton has a direct effect on mitochondrial function and Ca2+ storage. Therefore, we designed a series of experiments to clarify the effects of actin filaments on mitochondrial Ca2+ storage, cytoplasmic Ca2+ concentration ([Ca2+]c), and the interaction between mitochondrial Ca2+ and cytoplasmic Ca2+ in Arabidopsis root hairs.

RESULTS

In this study, we found that treatments with latrunculin B (Lat-B) and jasplakinolide (Jas), which depolymerize and polymerize actin filaments respectively, decreased membrane potential and Ca2+ stores in the mitochondria of Arabidopsis root hairs. Simultaneously, these treatments induced an instantaneous increase of cytoplasmic Ca2+, followed by a continuous decrease. All of these effects were inhibited by pretreatment with cyclosporin A (Cs A), a representative blocker of the mitochondrial permeability transition pore (mPTP). Moreover, we found there was a Ca2+ concentration gradient in mitochondria from the tip to the base of the root hair, and this gradient could be disrupted by actin-acting drugs.

CONCLUSIONS

Based on these results, we concluded that the disruption of actin filaments caused by Lat-B or Jas promoted irreversible opening of the mPTP, resulting in mitochondrial Ca2+ release into the cytoplasm, and consequent changes in [Ca2+]c. We suggest that normal polymerization and depolymerization of actin filaments are essential for mitochondrial Ca2+ storage in root hairs.

Making sense out of Ca(2+) signals: their role in regulating stomatal movements

Plant cells maintain high Ca(2+) concentration gradients between the cytosol and the extracellular matrix, as well as intracellular compartments. During evolution, the regulatory mechanisms, maintaining low cytosolic free Ca(2+) concentrations, most likely provided the backbone for the development of Ca(2+)-dependent signalling pathways. In this review, the current understanding of molecular mechanisms involved in Ca(2+) homeostasis of plants cells is evaluated. The question is addressed to which extent the mechanisms, controlling the cytosolic Ca(2+) concentration, are linked to Ca(2+)-based signalling. A large number of environmental stimuli can evoke Ca(2+) signals, but the Ca(2+)-induced responses are likely to differ depending on the stimulus applied. Two mechanisms are put forward to explain signal specificity of Ca(2+)-dependent responses. A signal may evoke a specific Ca(2+) signature that is recognized by downstream signalling components. Alternatively, Ca(2+) signals are accompanied by Ca(2+)-independent signalling events that determine the specificity of the response. The existence of such parallel-acting pathways explains why guard cell responses to abscisic acid (ABA) can occur in the absence, as well as in the presence, of Ca(2+) signals. Future research may shed new light on the relation between parallel acting Ca(2+)-dependent and -independent events, and may provide insights in their evolutionary origin.

Plant sensing and signaling in response to K+-deficiency

Potassium (K(+)) is one of the essential macronutrients for plant growth and development. However, K(+) content in soils is usually limited so that the crop yields are restricted. Plants may adapt to K(+)-deficient environment by adjusting their physiological and morphological status, indicating that plants may have evolved their sensing and signaling mechanisms in response to K(+)-deficiency. This short review particularly discusses some components as possible sensors or signal transducers involved in plant sensing and signaling in response to K(+)-deficiency, such as K(+) channels and transporters, H(+)-ATPase, some cytoplasmic enzymes, etc. Possible involvement of Ca²(+) and ROS signals in plant responses to K(+)-deficiency is also discussed.

Plant extracellular ATP signalling by plasma membrane NADPH oxidase and Ca2+ channels

Extracellular ATP regulates higher plant growth and adaptation. The signalling events may be unique to higher plants, as they lack animal purinoceptor homologues. Although it is known that plant cytosolic free Ca2+ can be elevated by extracellular ATP, the mechanism is unknown. Here, we have studied roots of Arabidopsis thaliana to determine the events that lead to the transcriptional stress response evoked by extracellular ATP. Root cell protoplasts were used to demonstrate that signalling to elevate cytosolic free Ca2+ is determined by ATP perception at the plasma membrane, and not at the cell wall. Imaging revealed that extracellular ATP causes the production of reactive oxygen species in intact roots, with the plasma membrane NADPH oxidase AtRBOHC being the major contributor. This resulted in the stimulation of plasma membrane Ca2+-permeable channels (determined using patch-clamp electrophysiology), which contribute to the elevation of cytosolic free Ca2+. Disruption of this pathway in the AtrbohC mutant impaired the extracellular ATP-induced increase in reactive oxygen species (ROS), the activation of Ca2+ channels, and the transcription of the MAP kinase3 gene that is known to be involved in stress responses. This study shows that higher plants, although bereft of purinoceptor homologues, could have evolved a distinct mechanism to transduce the ATP signal at the plasma membrane.

Plant ion channels: gene families, physiology, and functional genomics analyses

Distinct potassium, anion, and calcium channels in the plasma membrane and vacuolar membrane of plant cells have been identified and characterized by patch clamping. Primarily owing to advances in Arabidopsis genetics and genomics, and yeast functional complementation, many of the corresponding genes have been identified. Recent advances in our understanding of ion channel genes that mediate signal transduction and ion transport are discussed here. Some plant ion channels, for example, ALMT and SLAC anion channel subunits, are unique. The majority of plant ion channel families exhibit homology to animal genes; such families include both hyperpolarization- and depolarization-activated Shaker-type potassium channels, CLC chloride transporters/channels, cyclic nucleotide-gated channels, and ionotropic glutamate receptor homologs. These plant ion channels offer unique opportunities to analyze the structural mechanisms and functions of ion channels. Here we review gene families of selected plant ion channel classes and discuss unique structure-function aspects and their physiological roles in plant cell signaling and transport.

Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization

The water uptake capacity of plant roots (i.e. their hydraulic conductivity, Lp(r)) is determined in large part by aquaporins of the plasma membrane intrinsic protein (PIP) subfamily. In the present work, we investigated two stimuli, salicylic acid (SA) and salt, because of their ability to induce an accumulation of reactive oxygen species (ROS) and an inhibition of Lp(r) concomitantly in the roots of Arabidopsis plants. The inhibition of Lp(r) by SA was partially counteracted by preventing the accumulation of hydrogen peroxide (H(2)O(2)) with exogenous catalase. In addition, exogenous H(2)O(2) was able to reduce Lp(r) by up to 90% in <15 min. Based on the lack of effects of H(2)O(2) on the activity of individual aquaporins in Xenopus oocytes, and on a pharmacological dissection of the action of H(2)O(2) on Lp(r), we propose that ROS do not gate Arabidopsis root aquaporins through a direct oxidative mechanism, but rather act through cell signalling mechanisms. Expression in transgenic roots of PIP-GFP fusions and immunogold labelling indicated that external H(2)O(2) enhanced, in <15 min, the accumulation of PIPs in intracellular structures tentatively identified as vesicles and small vacuoles. Exposure of roots to SA or salt also induced an intracellular accumulation of the PIP-GFP fusion proteins, and these effects were fully counteracted by co-treatment with exogenous catalase. In conclusion, the present work identifies SA as a novel regulator of aquaporins, and delineates an ROS-dependent signalling pathway in the roots of Arabidopsis. Several abiotic and biotic stress-related stimuli potentially share this path, which involves an H(2)O(2)-induced internalization of PIPs, to downregulate root water transport.

Two voltage-dependent calcium channels co-exist in the apical plasma membrane of Arabidopsis thaliana root hairs

Calcium (Ca2+)-permeable plasma membrane ion channels are critical to root hair elongation and signalling. Arabidopsis thaliana root hair plasma membrane contains a hyperpolarization-activated Ca2+ channel (HACC) conductance. Here, the co-residence of HACC with a depolarization-activated Ca2+ channel (DACC) conductance has been investigated. Whole-cell patch-clamping of apical plasma membrane has been used to study Ca2+ conductances and reveal the negative slope conductance typical of DACCs. Specific voltage protocols, Ba(2+)-permeation and inhibition by the cation channel blocker Gd3+ have been used to identify the DACC conductance. The Gd3+ sensitive DACC conductance was identified in only a minority of cells. DACC activity was quickly masked by the development of the HACC conductance. However, in the period between the disappearance of the negative slope conductance and the predominance of HACC, DACC activity could still be detected. A DACC conductance coexists with HACC in the root hair apical plasma membrane and could provide Ca2+ influx over a wide voltage range, consistent with a role in signalling.

Transcriptome analyses give insights into selenium-stress responses and selenium tolerance mechanisms in Arabidopsis

Selenate is chemically similar to sulfate and can be taken up and assimilated by plants via the same transporters and enzymes. In contrast to many other organisms, selenium (Se) has not been shown to be essential for higher plants. In excess, Se is toxic and restricts development. Both Se deficiency and toxicity pose problems worldwide. To obtain better insights into the effects of Se on plant metabolism and into plant mechanisms involved in Se tolerance, the transcriptome of Arabidopsis plants grown with or without selenate was studied and Se-responsive genes identified. Roots and shoots exhibited different Se-related changes in gene regulation and metabolism. Many genes involved in sulfur (S) uptake and assimilation were upregulated. Accordingly, Se treatment enhanced sulfate levels in plants, but the quantity of organic S metabolites decreased. Transcripts regulating the synthesis and signaling of ethylene and jasmonic acid were also upregulated by Se. Arabidopsis mutants defective in ethylene or jasmonate response pathways exhibited reduced tolerance to Se, suggesting an important role for these two stress hormones in Se tolerance. Selenate upregulated a variety of transcripts that were also reportedly induced by salt and osmotic stress. Selenate appeared to repress plant development, as suggested by the downregulation of genes involved in cell wall synthesis and auxin-regulated proteins. The Se-responsive genes discovered in this study may help create plants that can better tolerate and accumulate Se, which may enhance the effectiveness of Se phytoremediation or serve as Se-fortified food.

Extracellular ATP-induced NO production and its dependence on membrane Ca2+ flux in Salvia miltiorrhiza hairy roots

Extracellular ATP (eATP) is a novel signalling agent, and nitric oxide (NO) is a well-established signal molecule with diverse functions in plant growth and development. This study characterizes NO production induced by exogenous ATP and examines its relationship with other important signalling agents, Ca(2+) and H(2)O(2) in Salvia miltiorrhiza hairy root culture. Exogenous ATP was applied at 10-500 microM to the hairy root cultures and stimulated NO production was detectable within 30 min. The NO level increased with ATP dose from 10-100 microM but decreased from 100-200 muM or higher. The ATP-induced NO production was mimicked by a non-hydrolysable ATP analogue ATPgammaS, but only weakly by ADP, AMP or adenosine. The ATP-induced NO production was blocked by Ca(2+) antagonists, but not affected by a protein kinase inhibitor. ATP also induced H(2)O(2) production, which was dependent on both Ca(2+) and protein kinases, and also on NO biosynthesis. On the other hand, ATP induced a rapid increase in the intracellular Ca(2+) level, which was dependent on NO but not H(2)O(2). The results suggest that NO is implicated in ATP-induced responses and signal transduction in plant cells, and ATP signalling is closely related to Ca(2+) and ROS signalling.

Disruption of arabinogalactan proteins disorganizes cortical microtubules in the root of Arabidopsis thaliana

The cortical array of microtubules inside the cell and arabinogalactan proteins on the external surface of the cell are each implicated in plant morphogenesis. To determine whether the cortical array is influenced by arabinogalactan proteins, we first treated Arabidopsis roots with a Yariv reagent that binds arabinogalactan proteins. Cortical microtubules were markedly disorganized by 1 microM beta-D-glucosyl (active) Yariv but not by up to 10 microM beta-D-mannosyl (inactive) Yariv. This was observed for 24-h treatments in wild-type roots, fixed and stained with anti-tubulin antibodies, as well as in living roots expressing a green fluorescent protein (GFP) reporter for microtubules. Using the reporter line, microtubule disorganization was evident within 10 min of treatment with 5 microM active Yariv and extensive by 30 min. Active Yariv (5 microM) disorganized cortical microtubules after gadolinium pre-treatment, suggesting that this effect is independent of calcium influx across the plasma membrane. Similar effects on cortical microtubules, over a similar time scale, were induced by two anti-arabinogalactan-protein antibodies (JIM13 and JIM14) but not by antibodies recognizing pectin or xyloglucan epitopes. Active Yariv, JIM13, and JIM14 caused arabinogalactan proteins to aggregate rapidly, as assessed either in fixed wild-type roots or in the living cells of a line expressing a plasma membrane-anchored arabinogalactan protein from tomato fused to GFP. Finally, electron microscopy of roots prepared by high-pressure freezing showed that treatment with 5 microM active Yariv for 2 h significantly increased the distance between cortical microtubules and the plasma membrane. These findings demonstrate that cell surface arabinogalactan proteins influence the organization of cortical microtubules.

Compatible solutes reduce ROS-induced potassium efflux in Arabidopsis roots

Reactive oxygen species (ROS) are known to be primarily responsible for the impairment of cellular function under numerous abiotic and biotic stress conditions. In this paper, using non-invasive microelectrode ion flux measuring (MIFE) system, we show that the application of a hydroxyl radical (OH*)-generating Cu2+/ascorbate (Cu/a) mixture to Arabidopsis roots results in a massive, dose-dependent efflux of K+ from epidermal cells in the elongation zone. Pharmacological experiments suggest that both outward-rectifying K+ channels and non-selective cation channels (NSCCs) mediate such effluxes. Low (5 mM) concentrations of compatible solutes (glycine betaine, proline, mannitol, trehalose or myo-inositol) significantly reduces OH*-induced K+ efflux, similar to our previous reports for NaCl-induced K+ efflux. Importantly, a significant reduction in K+ efflux was found using osmolytes with no reported free radical scavenging activity, as well as those for which a role in free radical scavenging has been demonstrated. This indicates that compatible solutes must play other (regulatory) roles, in addition to free radical scavenging, in mitigating the damaging effects of oxidative stress.

Heterotrimeric G-protein participation in Arabidopsis pollen germination through modulation of a plasmamembrane hyperpolarization-activated Ca2+-permeable channel

The role of heterotrimeric G proteins in pollen germination and tube growth was investigated using Arabidopsis thaliana plants in which the gene (GPA) encoding the G-protein a subunit (Galpha) was null or overexpressed. Pollen germination, free cytosolic calcium concentration ([Ca(2+)](cyt)) and Ca(2+) channel activity in the plasma membrane (PM) of pollen cells were investigated. Results showed that, compared with pollen grains of the wild type (ecotype Wassilewskija, ws), in vitro germinated pollen of Galpha null mutants (gpa1-1 and gpa1-2) had lower germination percentages and shorter pollen tubes, while pollen from Galpha overexpression lines (wGalpha and cGalpha) had higher germination percentages and longer pollen tubes. Compared with ws pollen cells, [Ca(2+)](cyt) was lower in gpa1-1 and gpa1-2 and higher in wGalpha and cGalpha. In whole-cell patch clamp recordings, a hyperpolarization-activated Ca(2+)-permeable conductance was identified in the PM of pollen protoplasts. The conductance was suppressed by trivalent cations but insensitive to organic blockers; its permeability to divalent cations was Ba(2+) > Ca(2+) > Mg(2+) > Sr(2+) > Mn(2+). The activity of the Ca(2+)-permeable channel conductance was down-regulated in pollen protoplasts of gpa1-1 and gpa1-2, and up-regulated in wGalpha and cGalpha. The results suggest that Galpha may participate in pollen germination through modulation of the hyperpolarization-activated Ca(2+) channel in the PM of pollen cells.

Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development

Nonselective cation channels (NSCCs) catalyse passive fluxes of cations through plant membranes. NSCCs do not, or only to a small extent, select between monovalent cations, and several are also permeable to divalent cations. Although a number of NSCC genes has been identified in plant genomes, a direct correlation between gene products and in vivo observed currents is still largely absent for most NSCCs. In this review, physiological functions and molecular properties of NSCCs are critically discussed. Recent studies have demonstrated that NSCCs are directly involved in a multitude of stress responses, growth and development, uptake of nutrients and calcium signalling. NSCCs can also function in the perception of external stimuli and as signal transducers for reactive oxygen species, pathogen elicitors, cyclic nucleotides, membrane stretch, amino acids and purines.