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

Unravelling the plant signalling machinery: an update on the cellular and genetic basis of plant signal transduction

Plant signalling is a set of phenomena that serves the transduction of external and internal signals into physiological responses such as modification of enzyme activity, cytoskeleton structure or gene expression. It operates at the level of cell compartments, whole cells, tissues, organs or even plant communities. To achieve this, plants have evolved a network of signalling proteins including plasma membrane receptors and ion transporters, cascades of kinases and other enzymes as well as several second messengers such as cytosolic calcium (Ca2+), reactive oxygen/nitrogen species (ROS/RNS), cyclic nucleotides (cAMP and cGMP) and others. Overall, these systems recognise and decode environmental signals and co-ordinate ontogeny programs. This paper summarises recent progress in the field of plant signalling, which was a major theme of the 4th International Symposium on Plant Signalling and Behaviour, 2016, in Saint Petersburg, Russia. Several novel hypotheses and concepts were proposed during this meeting. First, the concept of ROS-Ca2+ hubs has found further evidence and acceptance. This concept is based on reciprocal activation of NADPH oxidases by cytosolic Ca2+ on the one hand, and Ca2+-permeable channels that are activated by NADPH-produced ROS. ROS-Ca2+ hubs enhance the intensity and duration of originally weak Ca2+ and ROS signals. Hubs are directly involved in ROS- and Ca2+-mediated physiological reactions, such as stress response, growth, programmed cell death, autophagy and long-distance signalling. Second, recent findings have widened the list of cyclic nucleotide-regulated processes and strengthened the biochemical basis of cyclic nucleotide biochemistry by exploring cyclase activities of new receptors such as the Phytosulfokine Receptor 1, the pathogen peptide 1 receptor (atPepR1), the brassinosteroid BRI1 receptor and the cell wall-associated kinase like 10. cGMP and cAMP signalling has demonstrated strong link to Ca2+ signalling, via cyclic nucleotide-gated Ca2+-permeable ion channels (CNGCs), and to ROS and RNS via their nitrosylated forms. Third, a novel role for cytosolic K+ as a regulator of plant autophagy and programmed cell death has emerged. The cell death-associated proteases and endonucleases were demonstrated to be activated by a decrease of cytosolic K+ via ROS-induced stimulation of the plasma membrane K+ efflux channel GORK. Importantly, the origin of ROS for induction of autophagy and cell death varies in different tissues and comprises several pools, including NADPH oxidases, peroxidases, photosynthetic and respiratory electron-transporting chains and peroxisomal enzymes. The peroxisome pool is the 'latest' addition to established cellular ROS-producing machineries and is dependent on the state and abundance of catalase in this compartment. Finally, new aspects of phytohormone signalling, such as regulation of root hydraulic pressure by abscisic acid and rate of mitosis by cytokinins, as well as localising cytokinin receptors in endoplasmic reticulum, are reported. Other observations suggest that melatonin is a hormone-like substance in plants, because it targets Ca2+, ROS and RNS.

The role of ion disequilibrium in induction of root cell death and autophagy by environmental stresses

Environmental stresses such as salinity, drought, oxidants, heavy metals, hypoxia, extreme temperatures and others can induce autophagy and necrosis-type programmed cell death (PCD) in plant roots. These reactions are accompanied by the generation of reactive oxygen species (ROS) and ion disequilibrium, which is induced by electrolyte/K+ leakage through ROS-activated ion channels, such as the outwardly-rectifying K+ channel GORK and non-selective cation channels. Here, we discuss mechanisms of the stress-induced ion disequilibrium and relate it with ROS generation and onset of morphological, biochemical and genetic symptoms of autophagy and PCD in roots. Based on our own data and that in the literature, we propose a hypothesis on the induction of autophagy and PCD in roots by loss of cytosolic K+. To support this, we present data showing that in conditions of salt stress-induced autophagy, gork1-1 plants lacking root K+ efflux channel have fewer autophagosomes compared with the wild type. Overall, literature analyses and presented data strongly suggest that stress-induced root autophagy and PCD are controlled by the level of cytosolic potassium and ROS.

Mechanisms of cytosolic calcium elevation in plants: the role of ion channels, calcium extrusion systems and NADPH oxidase-mediated 'ROS-Ca2+ Hub'

Elevation in the cytosolic free calcium is crucial for plant growth, development and adaptation. Calcium influx into plant cells is mediated by Ca2+ depolarisation-activated, hyperpolarisation-activated and voltage-independent Ca2+-permeable channels (DACCs, HACCs and VICCs respectively). These channels are encoded by the following gene families: (1) cyclic nucleotide-gated channels (CNGCs), (2) ionotropic glutamate receptors (GLRs), (3) annexins, (4) 'mechanosensitive channels of small (MscS) conductance'-like channels (MSLs), (5) 'mid1-complementing activity' channels (MCAs), Piezo channels, and hyperosmolality-induced [Ca2+]cyt. channel 1 (OSCA1). Also, a 'tandem-pore channel1' (TPC1) catalyses Ca2+ efflux from the vacuole in response to the plasma membrane-mediated Ca2+ elevation. Recent experimental data demonstrated that Arabidopsis thaliana (L.) Heynh. CNGCs 2, 5-10, 14, 16 and 18, GLRs 1.2, 3.3, 3.4, 3.6 and 3.7, TPC1, ANNEXIN1, MSL9 and MSL10,MCA1 and MCA2, OSCA1, and some their homologues counterparts in other species, are responsible for Ca2+ currents and/or cytosolic Ca2+ elevation. Extrusion of Ca2+ from the cytosol is mediated by Ca2+-ATPases and Ca2+/H+ exchangers which were recently examined at the level of high resolution crystal structure. Calcium-activated NADPH oxidases and reactive oxygen species (ROS)-activated Ca2+ conductances form a self-amplifying 'ROS-Ca2+hub', enhancing and transducing Ca2+ and redox signals. The ROS-Ca2+ hub contributes to physiological reactions controlled by ROS and Ca2+, demonstrating synergism and unity of Ca2+ and ROS signalling mechanisms.

Constitutive expression of Brassica juncea annexin, AnnBj2 confers salt tolerance and glucose and ABA insensitivity in mustard transgenic plants

Annexins belong to a plasma membrane binding (in a calcium dependent manner), multi-gene family of proteins, which play ameliorating roles in biotic and abiotic stresses. The expression of annexin AnnBj2 of Indian mustard is tissue specific with higher expression in roots and under treatments with sodium chloride and abscisic acid (ABA) at seedling stage. The effect of constitutive expression of AnnBj2 in mustard was analyzed in detail. AnnBj2 OE (over expression) plants exhibited insensitivity to ABA, glucose and sodium chloride. The insensitivity/tolerance of the transgenic plants was associated with enhanced total chlorophylls, relative water content, proline, calcium and potassium with reduced thiobarbituric acid reactive substances and sodium ion accumulation. The altered ABA insensitivity of AnnBj2 OE lines is linked to downregulation of ABI4 and ABI5 transcription factors and upregulation of ABA catabolic gene CYP707A2. Furthermore, we found that overexpression of AnnBj2 upregulated the expression of ABA-dependent RAB18 and ABA-independent DREB2B stress marker genes suggesting that the tolerance phenotype exhibited by AnnBj2 OE lines is probably controlled by both ABA-dependent and -independent mechanisms.

Anionic Cerium Oxide Nanoparticles Protect Plant Photosynthesis from Abiotic Stress by Scavenging Reactive Oxygen Species

Plant abiotic stress leads to accumulation of reactive oxygen species (ROS) and a consequent decrease in photosynthetic performance. We demonstrate that a plant nanobionics approach of localizing negatively charged, sub-11 nm, spherical cerium oxide nanoparticles (nanoceria) inside chloroplasts in vivo augments ROS scavenging and photosynthesis of Arabidopsis thaliana plants under excess light (2000 μmol m^-2 s^-1, 1.5 h), heat (35 °C, 2.5 h), and dark chilling (4 °C, 5 days). Poly(acrylic acid) nanoceria (PNC) with a hydrodynamic diameter (10.3 nm)-lower than the maximum plant cell wall porosity-and negative ζ-potential (-16.9 mV) exhibit significantly higher colocalization (46%) with chloroplasts in leaf mesophyll cells than aminated nanoceria (ANC) (27%) of similar size (12.6 nm) but positive charge (9.7 mV). Nanoceria are transported into chloroplasts via nonendocytic pathways, influenced by the electrochemical gradient of the plasma membrane potential. PNC with a low Ce³⁺/Ce⁴⁺ ratio (35.0%) reduce leaf ROS levels by 52%, including hydrogen peroxide, superoxide anion, and hydroxyl radicals. For the latter ROS, there is no known plant enzyme scavenger. Plants embedded with these PNC that were exposed to abiotic stress exhibit an increase up to 19% in quantum yield of photosystem II, 67% in carbon assimilation rates, and 61% in Rubisco carboxylation rates relative to plants without nanoparticles. In contrast, PNC with high Ce³⁺/Ce⁴⁺ ratio (60.8%) increase overall leaf ROS levels and do not protect photosynthesis from oxidative damage during abiotic stress. This study demonstrates that anionic, spherical, sub-11 nm PNC with low Ce³⁺/Ce⁴⁺ ratio can act as a tool to study the impact of oxidative stress on plant photosynthesis and to protect plants from abiotic stress.

Expressing Arabidopsis thaliana V-ATPase subunit C in barley (Hordeum vulgare) improves plant performance under saline condition by enabling better osmotic adjustment

Salinity is a global problem affecting agriculture that results in an estimated US$27 billion loss in revenue per year. Overexpression of vacuolar ATPase subunits has been shown to be beneficial in improving plant performance under saline conditions. Most studies, however, have not shown whether overexpression of genes encoding ATPase subunits results in improvements in grain yield, and have not investigated the physiological mechanisms behind the improvement in plant growth. In this study, we constitutively expressed Arabidopsis Vacuolar ATPase subunit C (AtVHA-C) in barley. Transgenic plants were assessed for agronomical and physiological characteristics, such as fresh and dry biomass, leaf pigment content, stomatal conductance, grain yield, and leaf Na+ and K+ concentration, when grown in either 0 or 300mM NaCl. When compared with non-transformed barley, AtVHA-C expressing barley lines had a smaller reduction in both biomass and grain yield under salinity stress. The transgenic lines accumulated Na+ and K+ in leaves for osmotic adjustment. This in turn saves energy consumed in the synthesis of organic osmolytes that otherwise would be needed for osmotic adjustment.

Root phonotropism: Early signalling events following sound perception in Arabidopsis roots

Sound is a fundamental form of energy and it has been suggested that plants can make use of acoustic cues to obtain information regarding their environments and alter and fine-tune their growth and development. Despite an increasing body of evidence indicating that it can influence plant growth and physiology, many questions concerning the effect of sound waves on plant growth and the underlying signalling mechanisms remains unknown. Here we show that in Arabidopsis thaliana, exposure to sound waves (200Hz) for 2 weeks induced positive phonotropism in roots, which grew towards to sound source. We found that sound waves triggered very quickly (within  minutes) an increase in cytosolic Ca²⁺, possibly mediated by an influx through plasma membrane and a release from internal stock. Sound waves likewise elicited rapid reactive oxygen species (ROS) production and K⁺ efflux. Taken together these results suggest that changes in ion fluxes (Ca²⁺ and K⁺) and an increase in superoxide production are involved in sound perception in plants, as previously established in animals.

Epidermal bladder cells confer salinity stress tolerance in the halophyte quinoa and Atriplex species

Epidermal bladder cells (EBCs) have been postulated to assist halophytes in coping with saline environments. However, little direct supporting evidence is available. Here, Chenopodium quinoa plants were grown under saline conditions for 5 weeks. One day prior to salinity treatment, EBCs from all leaves and petioles were gently removed by using a soft cosmetic brush and physiological, ionic and metabolic changes in brushed and non-brushed leaves were compared. Gentle removal of EBC neither initiated wound metabolism nor affected the physiology and biochemistry of control-grown plants but did have a pronounced effect on salt-grown plants, resulting in a salt-sensitive phenotype. Of 91 detected metabolites, more than half were significantly affected by salinity. Removal of EBC dramatically modified these metabolic changes, with the biggest differences reported for gamma-aminobutyric acid (GABA), proline, sucrose and inositol, affecting ion transport across cellular membranes (as shown in electrophysiological experiments). This work provides the first direct evidence for a role of EBC in salt tolerance in halophytes and attributes this to (1) a key role of EBC as a salt dump for external sequestration of sodium; (2) improved K⁺ retention in leaf mesophyll and (3) EBC as a storage space for several metabolites known to modulate plant ionic relations.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

Physiological and molecular mechanisms mediating xylem Na+ loading in barley in the context of salinity stress tolerance

Time-dependent kinetics of xylem Na⁺ loading was investigated using a large number of barley genotypes contrasting in their salinity tolerance. Salt-sensitive varieties were less efficient in controlling xylem Na⁺ loading and showed a gradual increase in the xylem Na⁺ content over the time. To understand underlying ionic and molecular mechanisms, net fluxes of Ca²⁺ , K⁺ and Na⁺ were measured from the xylem parenchyma tissue in response to H₂ O₂ and ABA; both of them associated with salinity stress signalling. Our results indicate that NADPH oxidase-mediated apoplastic H₂ O₂ production acts upstream of the xylem Na⁺ loading and is causally related to ROS-inducible Ca²⁺ uptake systems in the root stelar tissue. It was also found that ABA regulates (directly or indirectly) the process of Na⁺ retrieval from the xylem and the significant reduction of Na⁺ and K⁺ fluxes induced by bumetanide are indicative of a major role of chloride cation co-transporter (CCC) on xylem ion loading. Transcript levels of HvHKT1;5_like and HvSOS1_like genes in the root stele were observed to decrease after salt stress, while there was an increase in HvSKOR_like gene, indicating that these ion transporters are involved in primary Na⁺ /K⁺ movement into/out of xylem.

Salinity stress induced alterations in antioxidant metabolism and nitrogen assimilation in wheat (Triticum aestivum L) as influenced by potassium supplementation

Experiments were conducted on two wheat (Triticum aestivum L) cultivars exposed to NaCl stress with and without potassium (K) supplementation. Salt stress induced using NaCl caused oxidative stress resulting into enhancement in lipid peroxidation and altered growth as well as yield. Added potassium led to significant improvement in growth having positive effects on the attributes including nitrogen and antioxidant metabolism. NaCl-induced stress triggered the antioxidant defence system nevertheless, the activity of antioxidant enzymes and the content of non-enzymatic antioxidants increased in K fed plants. Enhancement in the accumulation of osmolytes comprising free proline, sugars and amino acids was observed at both the developmental stages with K supplementation associated with improvement of the relative water content and ultimately yield. Potassium significantly increased uptake and assimilation of nitrogen with concomitant reduction in the Na ions and consequently Na/K ratio. Optimal K can be used as a potential tool for alleviating NaCl stress in wheat to some extent.

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Revealing the roles of GORK channels and NADPH oxidase in acclimation to hypoxia in Arabidopsis

Regulation of root cell K+ is essential for acclimation to low oxygen stress. The potential roles of GORK (depolarization-activated guard cell outward-rectifying potassium) channels and RBOHD (respiratory burst oxidase homologue D) in plant adaptive responses to hypoxia were investigated in the context of tissue specificity (epidermis versus stele; elongation versus mature zone) in roots of Arabidopsis. The expression of GORK and RBOHD was down-regulated by 2- to 3-fold within 1 h and 24 h of hypoxia treatment in Arabidopsis wild-type (WT) roots. Interestingly, a loss of the functional GORK channel resulted in a waterlogging-tolerant phenotype, while rbohD knockout was sensitive to waterlogging. To understand their functions under hypoxia stress, we studied K+, Ca2+, and reactive oxygen species (ROS) distribution in various root cell types. gork1-1 plants had better K+ retention ability in both the elongation and mature zone compared with the WT and rbohD under hypoxia. Hypoxia induced a Ca2+ increase in each cell type after 72 h, and the increase was much less pronounced in rbohD than in the WT. In most tissues except the elongation zone in rbohD, the H2O2 concentration had decreased after 1 h of hypoxia, but then increased significantly after 24 h of hypoxia in each zone and tissue, further suggesting that RBOHD may shape hypoxia-specific Ca2+ signatures via the modulation of apoplastic H2O2 production. Taken together, our data suggest that plants lacking functional GORK channels are more capable of retaining K+ for their better performance under hypoxia, and that RBOHD is crucial in hypoxia-induced Ca2+ signalling for stress sensing and acclimation mechanism.

In vivo inhibition of polyamine oxidase by a spermine analogue, MDL-72527, in tomato exposed to sublethal and lethal salt stress

The spermine analogue N1,N4-bis-(2,3-butadienyl)-1,4-butanediamine (MDL-72527), an effective inhibitor of polyamine oxidases (PAOs), triggers a systemic response in tomato (Solanum lycopersicum L.) exposed to sublethal (100mM) and lethal (250mM) NaCl concentrations. The accumulation of free polyamines (PAs), the terminal oxidation of PAs by diamine oxidases (DAOs) and PAOs, and the production of H2O2 by PA oxidases depends on the intensity of salt stress. Spermidine and spermine content increased significantly under sublethal salt concentrations, but remained low under lethal salt stress. Along with increased expression of the selected SlDAO1 and SlPAO1 genes in the leaves and roots, respectively, DAO and PAO activities and their product, H2O2, increased and initiated cell death by irreversible loss of electrolytes at 250mM NaCl. MDL-72527 significantly increased spermine, spermidine and/or putrescine contents as a result of reduced activity of PA oxidases; furthermore, it inhibited H2O2 and NO production during salt treatment. These results indicate that PAO contributed to H2O2 and NO production under salt stress, and the terminal activities of DAO and PAO play a role in cell death induction at 250mM NaCl. However, the inhibition of PAO by MDL-72527 does not increase the salt tolerance of plants, since electrolyte leakage increased significantly in the presence of the inhibitor.

Melatonin and nitric oxide regulate sunflower seedling growth under salt stress accompanying differential expression of Cu/Zn SOD and Mn SOD

Salinity results in significant reduction in sunflower (Helianthus annuus L.) seedling growth and excessive generation of reactive oxygen species (ROS). Present work highlights the possible role of melatonin as an antioxidant through its interaction with nitric oxide (NO), and as an early and long distance NaCl-stress sensing signaling molecule in seedling cotyledons. Exogenous melatonin (15µM)±NaCl (120mM) inhibit seedling growth, which is also correlated with NO availability, accumulation of potential superoxide anion (O₂^•-) and peroxynitrite anion (ONOO^-), extent of tyrosine-nitration of proteins, spatial localization and activity of superoxide dismutase (SOD) isoforms. NO acts as a positive modulator of melatonin accumulation in seedling cotyledons as a long-distance signaling response. Modulation of superoxide anion and peroxynitrite anion content by melatonin highlights its crucial role in combating deleterious effects of ROS and reactive nitrogen species (RNS). Present findings provide evidence for an interaction between melatonin and NO in their effect on seedling growth under salt stress accompanying differential modulation of two SOD isoforms, i.e. Cu/Zn SOD and Mn SOD.

Genomics, Physiology, and Molecular Breeding Approaches for Improving Salt Tolerance

Salt stress reduces land and water productivity and contributes to poverty and food insecurity. Increased salinization caused by human practices and climate change is progressively reducing agriculture productivity despite escalating calls for more food. Plant responses to salt stress are well understood, involving numerous critical processes that are each controlled by multiple genes. Knowledge of the critical mechanisms controlling salt uptake and exclusion from functioning tissues, signaling of salt stress, and the arsenal of protective metabolites is advancing. However, little progress has been made in developing salt-tolerant varieties of crop species using standard (but slow) breeding approaches. The genetic diversity available within cultivated crops and their wild relatives provides rich sources for trait and gene discovery that has yet to be sufficiently utilized. Transforming this knowledge into modern approaches using genomics and molecular tools for precision breeding will accelerate the development of tolerant cultivars and help sustain food production.

Antioxidant responses under salinity and drought in three closely related wild monocots with different ecological optima

Some deleterious effects of drought, soil salinity and other abiotic stresses are mediated by the generation of oxidative stress through an increase in reactive oxygen species (ROS) that damage cellular membranes, proteins and DNA. In response to increased ROS, plants activate an array of enzymatic and non-enzymatic antioxidant defences. We have correlated the activation of these responses with the contrasting tolerance to salinity and drought of three species of the genus Juncus, viz. J. maritimus, J. acutus (both halophytes) and J. articulatus (salt-sensitive). Both stresses were given for 8 weeks to 6-week-old seedlings in a controlled environment chamber. Each stress inhibited growth and degraded photosynthetic pigments in the three species with the most pronounced effects being in J. articulatus. Salt and water stress also generated oxidative stress in all three taxa with J. articulatus being the most affected in terms of accumulation of malondialdehyde (a reliable oxidative stress marker). The apparent lower oxidative stress in halophytic J. maritimus and J. acutus compared with salt-sensitive J. articulatus is explained by a more efficient activation of antioxidant systems since salt or water deficiency induced a stronger accumulation of antioxidant phenolic compounds and flavonoids in J. maritimus and J. acutus than in J. articulatus. Qualitative and quantitative differences in antioxidant enzymes were also detected when comparing the three species and the two stress treatments. Accordingly, glutathione reductase and superoxide dismutase activities increased in the two halophytes under both stresses, but only in response to drought in J. articulatus. In contrast, ascorbate peroxidase activity varied between and within species according to treatment. These results show the relative importance of different antioxidant responses for stress tolerance in species with distinct ecological requirements. The salt-sensitive J. articulatus, contrary to the tolerant taxa, did not activate enzymatic antioxidant responses to salinity-induced oxidative stress.

Potassium retention in leaf mesophyll as an element of salinity tissue tolerance in halophytes

Soil salinity remains a major threat to global food security, and the progress in crop breeding for salinity stress tolerance may be achieved only by pyramiding key traits mediating plant adaptive responses to high amounts of dissolved salts in the rhizosphere. This task may be facilitated by studying natural variation in salinity tolerance among plant species and, specifically, exploring mechanisms of salinity tolerance in halophytes. The aim of this work was to establish the causal link between mesophyll ion transport activity and plant salt tolerance in a range of evolutionary contrasting halophyte and glycophyte species. Plants were grown under saline conditions in a glasshouse, followed by assessing their growth and photosynthetic performance. In a parallel set of experiments, net K⁺ and H⁺ transport across leaf mesophyll and their modulation by light were studied in control and salt-treated mesophyll segments using vibrating non-invasive ion selective microelectrode (the MIFE) technique. The reported results show that mesophyll cells in glycophyte species loses 2-6 fold more K⁺ compared with their halophyte counterparts. This decline was reflected in a reduced maximum photochemical efficiency of photosystem II, chlorophyll content and growth observed in the glasshouse experiments. In addition to reduced K⁺ efflux, the more tolerant species also exhibited reduced H⁺ efflux, which is interpreted as an energy-saving strategy allowing more resources to be redirected towards plant growth. It is concluded that the ability of mesophyll to retain K⁺ without a need to activate plasma membrane H⁺-ATPase is an essential component of salinity tolerance in halophytes and halophytic crop plants.

Hyperactive mutant of a wheat plasma membrane Na+/H+ antiporter improves the growth and salt tolerance of transgenic tobacco

Wheat SOS1 (TaSOS1) activity could be relieved upon deletion of the C-terminal 168 residues (the auto-inhibitory domain). This truncated form of wheat SOS1 (TaSOS1-974) was shown to increase compensation (compared to wild-type TaSOS1) for the salt sensitivity of a yeast mutant strain, AXT3K, via increased Na+ transportation out of cells during salinity stress. Expression of the plasma membrane proteins TaSOS1-974 or TaSOS1 improved the growth of transgenic tobacco plants compared with wild-type plants under normal conditions. However, plants expressing TaSOS1-974 grew better than TaSOS1-transformed plants. Upon salinity stress, Na+ efflux and K+ influx rates in the roots of transgenic plants expressing TaSOS1-974 or TaSOS1 were greater than those of wild-type plants. Furthermore, compared to TaSOS1-transgenic plants, TaSOS1-974-expressing roots showed faster Na+ efflux and K+ influx, resulting in less Na+ and more K+ accumulation in TaSOS1-974-transgenic plants compared to TaSOS1-transgenic and wild-type plants. TaSOS1-974-expressing plants had the lowest MDA content and electrolyte leakage among all tested plants, indicating that TaSOS1-974 might protect the plasma membrane against oxidative damage generated by salt stress. Overall, TaSOS1-974 conferred higher salt tolerance in transgenic plants compared to TaSOS1. Consistent with this result, transgenic plants expressing TaSOS1-974 showed a better growth performance than TaSOS1-expressing and wild-type plants under saline conditions.

Natural variation in primary root growth and K+ retention in roots of habanero pepper (Capsicum chinense) under salt stress

In this work, we analysed the natural variation in mechanisms for protection against salt stress in pepper varieties (Capsicum chinense Jacq. cv. Rex, Chichen-Itza and Naranja and Capsicum annuum L. cv. Padron), considering primary root growth and viability of the post-stressed seedlings. NaCl-induced K+ and H+ efflux in roots was also studied by ion-selective microelectrodes under application of pharmacological agents. In these pepper varieties, the magnitude of the K+ leakage in the roots positively correlated with growth inhibition of the primary root in the presence of NaCl, with Rex variety showing a higher level of tolerance than Chichen-Itza. The K+ leakage and the activity of the H+ pump in the roots were dependent on the NaCl concentration. Pharmacological analysis indicated that the NaCl-induced K+ leakage was mediated by TEA+-sensitive KOR channels but not by NSCC channels. In addition, we present evidence for the possible participation of proline, and a Na+-insensitive HAK K+ transporter expressed in habanero pepper roots for maintaining K+ homeostasis under salt stress conditions.

New Insights on Plant Salt Tolerance Mechanisms and Their Potential Use for Breeding

Soil salinization is a major threat to agriculture in arid and semi-arid regions, where water scarcity and inadequate drainage of irrigated lands severely reduce crop yield. Salt accumulation inhibits plant growth and reduces the ability to uptake water and nutrients, leading to osmotic or water-deficit stress. Salt is also causing injury of the young photosynthetic leaves and acceleration of their senescence, as the Na+ cation is toxic when accumulating in cell cytosol resulting in ionic imbalance and toxicity of transpiring leaves. To cope with salt stress, plants have evolved mainly two types of tolerance mechanisms based on either limiting the entry of salt by the roots, or controlling its concentration and distribution. Understanding the overall control of Na+ accumulation and functional studies of genes involved in transport processes, will provide a new opportunity to improve the salinity tolerance of plants relevant to food security in arid regions. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress. Moreover, associations of cultures with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi could serve as an alternative and sustainable strategy to increase crop yields in salt-affected fields.

Potassium fluxes and reactive oxygen species production as potential indicators of salt tolerance in Cucumis sativus

Salt stress, among other abiotic stresses, has a high impact on crop yield. Salt tolerance is a multifactorial trait that involves the ability of cells to retain K ions, regulate reactive O species (ROS) production, and synthesise new molecules to cope with osmotic stress. In the present work, two different cultivars of Cucumis sativus L. (cv. Parys, sensitive; cv. Polan, tolerant) were selected based on their germination capabilities under 100mM NaCl. The capacity of these two cultivars to tolerate salt stress was analysed using several different physiological and genetic approaches. K+ fluxes from roots, as an immediate response to salinity, showed the higher ability of cv. Polan to maintain K+ compared with cv. Parys, according to the expression level of inward rectifying potassium channel 1 (AKT1). ROS production was also investigated in both cultivars and a higher basal ROS level was observed in cv. Polan than in cv. Parys. Concurrently, an increased basal level of respiratory burst oxidase homologue F (RBOHF) gene was also found, as well as a strong induction of the ethylene responsive factor 109 (ERF109) transcription factor after salt treatment in cv. Polan. Our data suggest that roots' ability to retain K+, a higher level of RBOHF and a strong induction of ERF109 should all be considered important components for salt tolerance in C. sativus.

Salinity effects on chloroplast PSII performance in glycophytes and halophytes

The effects of NaCl stress and K+ nutrition on photosynthetic parameters of isolated chloroplasts were investigated using PAM fluorescence. Intact mesophyll cells were able to maintain optimal photosynthetic performance when exposed to salinity for more than 24h whereas isolated chloroplasts showed declines in both the relative electron transport rate (rETR) and the maximal photochemical efficiency of PSII (Fv/Fm) within the first hour of treatment. The rETR was much more sensitive to salt stress compared with Fv/Fm, with 40% inhibition of rETR observed at apoplastic NaCl concentration as low as 20mM. In isolated chloroplasts, absolute K+ concentrations were more essential for the maintenance of the optimal photochemical performance (Fv/Fm values) rather than sodium concentrations per se. Chloroplasts from halophyte species of quinoa (Chenopodium quinoa Willd.) and pigface (Carpobrotus rosii (Haw.) Schwantes) showed less than 18% decline in Fv/Fm under salinity, whereas the Fv/Fm decline in chloroplasts from glycophyte pea (Pisum sativum L.) and bean (Vicia faba L.) species was much stronger (31 and 47% respectively). Vanadate (a P-type ATPase inhibitor) significantly reduced Fv/Fm in both control and salinity treated chloroplasts (by 7 and 25% respectively), whereas no significant effects of gadolinium (blocker of non-selective cation channels) were observed in salt-treated chloroplasts. Tetraethyl ammonium (TEA) (K+ channel inhibitor) and amiloride (inhibitor of the Na+/H+ antiporter) increased the Fv/Fm of salinity treated chloroplasts by 16 and 17% respectively. These results suggest that chloroplasts' ability to regulate ion transport across the envelope and thylakoid membranes play a critical role in leaf photosynthetic performance under salinity.

Oxidative defense metabolites induced by salinity stress in roots of Salicornia herbacea

High salinity is a major abiotic stress that affects the growth and development of plants. This type of stress can influence flowering, the production of crops, defense mechanisms and other physiological processes. Previous studies have attempted to elucidate salt-tolerance mechanisms to improve plant growth and productivity in the presence of sodium chloride. One such plant that has been studied in detail is Salicornia, a well-known halophyte, which has adapted to grow in the presence of high salt. To further the understanding of how Salicornia grows and develops under high saline conditions, Salicornia herbacea (S. herbacea) was grown under varying saline concentrations (0, 50, 100, 200, 300, and 400mM), and the resulting phenotype, ion levels, and metabolites were investigated. The optimal condition for the growth of S. herbacea was determined to be 100mM NaCl, and increased salt concentrations directly decreased the internal concentrations of other inorganic ions including Ca²⁺, K⁺, and Mg²⁺. Metabolomics were performed on the roots of the plant as a systematic metabolomics study has not yet been reported for Salicornia roots. Using ethylacetate and methanol extraction followed by high resolution ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS), 1793 metabolites were identified at different NaCl levels. Structural and functional analyses demonstrated that the concentration of 53 metabolites increased as the concentration of NaCl increased. These metabolites have been linked to stress responses, primarily oxidative stress responses, which increase under saline stress. Most metabolites can be classified as polyols, alkaloids, and steroids. Functional studies of these metabolites show that shikimic acid, vitamin K1, and indole-3-carboxylic acid are generated as a result of defense mechanisms, including the shikimate pathway, to protect against reactive oxygen species (ROS) generated by salt stress. This metabolite profiling provides valuable information on the salt-tolerance mechanisms of S. herbacea and may be applied to bioengineer plants with improved salt tolerance.

Ethylene responsive transcription factor ERF109 retards PCD and improves salt tolerance in plant

BACKGROUND

The ultimate goal of this work was to detect the role of transcription factors (TFs) concordantly expressed with genes related to programmed cell death (PCD) during PCD and salt stress. This work was based on the hypothesis that TFs and their driven genes likely co-express under different stimuli. The conserved superfamily ethylene responsive factor (AP2/ERF) draw attention of the present study as it participates in the response to biotic and abiotic stimuli as well as to program cell death (PCD).

RESULTS

RNA-Seq analysis was done for tobacco (N. benthamiana) leaves exposed to oxalic acid (OA) at 20 mM for 0, 2, 6, 12 and 24 h to induce PCD. Genes up-regulated after 2 h of OA treatment with known function during PCD were utilized as landmarks to select TFs with concordant expression. Knockdown mutants of these TFs were generated in tobacco via virus induced gene silencing (VIGS) in order to detect their roles during PCD. Based on the results of PCD assay, knockout (KO) T-DNA insertion mutants of Arabidopsis as well as over-expression lines of two selected TFs, namely ERF109 and TFIID5, analogs to those in tobacco, were tested under salt stress (0, 100, 150 and 200 mM NaCl).

CONCLUSIONS

Results of knockdown mutant tobacco cells confirmed the influence of these two TFs during PCD. Knockout insertion mutants and over-expression lines indicated the role of ERF109 in conferring salt tolerance in Arabidopsis.

Evaluating relative contribution of osmotolerance and tissue tolerance mechanisms toward salinity stress tolerance in three Brassica species

Three different species of Brassica, with differential salt sensitivity were used to understand physiological mechanisms of salt tolerance operating in these species and to evaluate the relative contribution of different strategies to cope with salt load. Brassica napus was the most tolerant species in terms of the overall performance, with Brassica juncea and Brassica oleracea being much more sensitive to salt stress with no obvious difference between them. While prominent reduction in net CO2 assimilation was observed in both sensitive species, physiological mechanisms beyond this reduction differed strongly. Brassica juncea plants possessed high osmotolerance and were able to maintain high transpiration rate but showed a significant reduction in leaf chlorophyll content and efficiency of leaf photochemistry. On the contrary, B. oleracea plants possessed the highest (among the three species) tissue tolerance but showed a very significant stomatal limitation of photosynthesis. Electrophysiological experiments revealed that the high tissue tolerance in B. oleracea was related to the ability of leaf mesophyll cells to maintain highly negative membrane potential in the presence of high apoplastic Na(+) . In addition to high osmotolerance, the most tolerant B. napus showed also lesser accumulation of toxic Na(+) and Cl(-) in the leaf, possessed moderate tissue tolerance and had a superior K(+) retention ability. Taken together, the results from this study indicate that the three Brassica species employ very different mechanisms to cope with salinity and, despite its overall sensitivity to salinity, B. oleracea could be recommended as a valuable 'donor' of tissue tolerance genes to confer this trait for marker-assisted breeding programs.

Hydrogen peroxide affects ion channels in lily pollen grain protoplasts

Ion homeostasis plays a central role in polarisation and polar growth. In several cell types ion channels are controlled by reactive oxygen species (ROS). One of the most important cells in the plant life cycle is the male gametophyte, which grows under the tight control of both ion fluxes and ROS balance. The precise relationship between these two factors in pollen tubes has not been completely elucidated, and in pollen grains it has never been studied to date. In the present study we used a simple model - protoplasts obtained from lily pollen grains at the early germination stage - to reveal the effect of H2 O2 on cation fluxes crucial for pollen germination. Here we present direct evidence for two ROS-sensitive currents on the pollen grain plasma membrane: the hyperpolarisation-activated calcium current, which is strongly enhanced by H2 O2 , and the outward potassium current, which is modestly enhanced by H2 O2 . We used low concentrations of H2 O2 that do not cause an intracellular oxidative burst and do not damage cells, as demonstrated with fluorescent staining.

Calcium-Mediated Abiotic Stress Signaling in Roots

Roots are subjected to a range of abiotic stresses as they forage for water and nutrients. Cytosolic free calcium is a common second messenger in the signaling of abiotic stress. In addition, roots take up calcium both as a nutrient and to stimulate exocytosis in growth. For calcium to fulfill its multiple roles must require strict spatio-temporal regulation of its uptake and efflux across the plasma membrane, its buffering in the cytosol and its sequestration or release from internal stores. This prompts the question of how specificity of signaling output can be achieved against the background of calcium's other uses. Threats to agriculture such as salinity, water availability and hypoxia are signaled through calcium. Nutrient deficiency is also emerging as a stress that is signaled through cytosolic free calcium, with progress in potassium, nitrate and boron deficiency signaling now being made. Heavy metals have the capacity to trigger or modulate root calcium signaling depending on their dose and their capacity to catalyze production of hydroxyl radicals. Mechanical stress and cold stress can both trigger an increase in root cytosolic free calcium, with the possibility of membrane deformation playing a part in initiating the calcium signal. This review addresses progress in identifying the calcium transporting proteins (particularly channels such as annexins and cyclic nucleotide-gated channels) that effect stress-induced calcium increases in roots and explores links to reactive oxygen species, lipid signaling, and the unfolded protein response.

Acclimation improves salt stress tolerance in Zea mays plants

Plants exposure to low level salinity activates an array of processes leading to an improvement of plant stress tolerance. Although the beneficial effect of acclimation was demonstrated in many herbaceous species, underlying mechanisms behind this phenomenon remain poorly understood. In the present study we have addressed this issue by investigating ionic mechanisms underlying the process of plant acclimation to salinity stress in Zea mays. Effect of acclimation were examined in two parallel sets of experiments: a growth experiment for agronomic assessments, sap analysis, stomatal conductance, chlorophyll content, and confocal laser scanning imaging; and a lab experiment for in vivo ion flux measurements from root tissues. Being exposed to salinity, acclimated plants (1) retain more K(+) but accumulate less Na(+) in roots; (2) have better vacuolar Na(+) sequestration ability in leaves and thus are capable of accumulating larger amounts of Na(+) in the shoot without having any detrimental effect on leaf photochemistry; and (3) rely more on Na(+) for osmotic adjustment in the shoot. At the same time, acclimation affect was not related in increased root Na(+) exclusion ability. It appears that even in a such salt-sensitive species as maize, Na(+) exclusion from uptake is of a much less importance compared with the efficient vacuolar Na(+) sequestration in the shoot.

Difference in root K+ retention ability and reduced sensitivity of K+-permeable channels to reactive oxygen species confer differential salt tolerance in three Brassica species

Brassica species are known to possess significant inter and intraspecies variability in salinity stress tolerance, but the cell-specific mechanisms conferring this difference remain elusive. In this work, the role and relative contribution of several key plasma membrane transporters to salinity stress tolerance were evaluated in three Brassica species (B. napus, B. juncea, and B. oleracea) using a range of electrophysiological assays. Initial root growth assay and viability staining revealed that B. napus was most tolerant amongst the three species, followed by B. juncea and B. oleracea At the mechanistic level, this difference was conferred by at least three complementary physiological mechanisms: (i) higher Na(+) extrusion ability from roots resulting from increased expression and activity of plasma membrane SOS1-like Na(+)/H(+) exchangers; (ii) better root K(+) retention ability resulting from stress-inducible activation of H(+)-ATPase and ability to maintain more negative membrane potential under saline conditions; and (iii) reduced sensitivity of B. napus root K(+)-permeable channels to reactive oxygen species (ROS). The last two mechanisms played the dominant role and conferred most of the differential salt sensitivity between species. Brassica napus plants were also more efficient in preventing the stress-induced increase in GORK transcript levels and up-regulation of expression of AKT1, HAK5, and HKT1 transporter genes. Taken together, our data provide the mechanistic explanation for differential salt stress sensitivity amongst these species and shed light on transcriptional and post-translational regulation of key ion transport systems involved in the maintenance of the root plasma membrane potential and cytosolic K/Na ratio as a key attribute for salt tolerance in Brassica species.

OsCHX14 is Involved in the K+ Homeostasis in Rice (Oryza sativa) Flowers

Previously we showed in the osjar1 mutants that the lodicule senescence which controls the closing of rice flowers was delayed. This resulted in florets staying open longer when compared with the wild type. The gene OsJAR1 is silenced in osjar1 mutants and is a key member of the jasmonic acid (JA) signaling pathway. We found that K concentrations in lodicules and flowers of osjar1-2 were significantly elevated compared with the wild type, indicating that K⁺ homeostasis may play a role in regulating the closure of rice flowers. The cation/H⁺ exchanger (CHX) family from rice was screened for potential K⁺ transporters involved as many members of this family in Arabidopsis were exclusively or preferentially expressed in flowers. Expression profiling confirmed that among 17 CHX genes in rice, OsCHX14 was the only member that showed an expression polymorphism, not only in osjar1 mutants but also in RNAi (RNA interference) lines of OsCOI1, another key member of the JA signaling pathway. This suggests that the expression of OsCHX14 is regulated by the JA signaling pathway. Green fluorescent protein (GFP)-tagged OsCHX14 protein was preferentially localized to the endoplasmic reticulum. Promoter-β-glucuronidase (GUS) analysis of transgenic rice revealed that OsCHX14 is mainly expressed in lodicules and the region close by throughout the flowering process. Characterization in yeast and Xenopus laevis oocytes verified that OsCHX14 is able to transport K⁺, Rb⁺ and Cs⁺ in vivo. Our data suggest that OsCHX14 may play an important role in K⁺ homeostasis during flowering in rice.

Tissue-specific root ion profiling reveals essential roles of the CAX and ACA calcium transport systems in response to hypoxia in Arabidopsis

Waterlogging is a major abiotic stress that limits the growth of plants. The crucial role of Ca(2+) as a second messenger in response to abiotic and biotic stimuli has been widely recognized in plants. However, the physiological and molecular mechanisms of Ca(2+) distribution within specific cell types in different root zones under hypoxia is poorly understood. In this work, whole-plant physiological and tissue-specific Ca(2+) changes were studied using several ACA (Ca(2+)-ATPase) and CAX (Ca(2+)/proton exchanger) knock-out Arabidopsis mutants subjected to waterlogging treatment. In the wild-type (WT) plants, several days of hypoxia decreased the expression of ACA8, CAX4, and CAX11 by 33% and 50% compared with the control. The hypoxic treatment also resulted in an up to 11-fold tissue-dependent increase in Ca(2+) accumulation in root tissues as revealed by confocal microscopy. The increase was much higher in stelar cells in the mature zone of Arabidopsis mutants with loss of function for ACA8, ACA11, CAX4, and CAX11 In addition, a significantly increased Ca(2+) concentration was found in the cytosol of stelar cells in the mature zone after hypoxic treatment. Three weeks of waterlogging resulted in dramatic loss of shoot biomass in cax11 plants (67% loss in shoot dry weight), while in the WT and other transport mutants this decline was only 14-22%. These results were also consistent with a decline in leaf chlorophyll fluorescence (F v/F m). It is suggested that CAX11 plays a key role in maintaining cytosolic Ca(2+) homeostasis and/or signalling in root cells under hypoxic conditions.

A highly versatile and easily configurable system for plant electrophysiology

In this study we present a highly versatile and easily configurable system for measuring plant electrophysiological parameters and ionic flow rates, connected to a computer-controlled highly accurate positioning device. The modular software used allows easy customizable configurations for the measurement of electrophysiological parameters. Both the operational tests and the experiments already performed have been fully successful and rendered a low noise and highly stable signal. Assembly, programming and configuration examples are discussed. The system is a powerful technique that not only gives precise measuring of plant electrophysiological status, but also allows easy development of ad hoc configurations that are not constrained to plant studies. •We developed a highly modular system for electrophysiology measurements that can be used either in organs or cells and performs either steady or dynamic intra- and extracellular measurements that takes advantage of the easiness of visual object-oriented programming.•High precision accuracy in data acquisition under electrical noisy environments that allows it to run even in a laboratory close to electrical equipment that produce electrical noise.•The system makes an improvement of the currently used systems for monitoring and controlling high precision measurements and micromanipulation systems providing an open and customizable environment for multiple experimental needs.

Cellular and molecular insight into the inhibition of primary root growth of Arabidopsis induced by peptaibols, a class of linear peptide antibiotics mainly produced by Trichoderma spp

Trichoderma spp. are well known biocontrol agents that produce a variety of antibiotics. Peptaibols are a class of linear peptide antibiotics mainly produced by Trichoderma Alamethicin, the most studied peptaibol, is reported as toxic to plants at certain concentrations, while the mechanisms involved are unclear. We illustrated the toxic mechanisms of peptaibols by studying the growth-inhibitory effect of Trichokonin VI (TK VI), a peptaibol from Trichoderma longibrachiatum SMF2, on Arabidopsis primary roots. TK VI inhibited root growth by suppressing cell division and cell elongation, and disrupting root stem cell niche maintenance. TK VI increased auxin content and disrupted auxin response gradients in root tips. Further, we screened the Arabidopsis TK VI-resistant mutant tkr1. tkr1 harbors a point mutation in GORK, which encodes gated outwardly rectifying K(+)channel proteins. This mutation alleviated TK VI-induced suppression of K(+)efflux in roots, thereby stabilizing the auxin gradient. The tkr1 mutant also resisted the phytotoxicity of alamethicin. Our results indicate that GORK channels play a key role in peptaibol-plant interaction and that there is an inter-relationship between GORK channels and maintenance of auxin homeostasis. The cellular and molecular insight into the peptaibol-induced inhibition of plant root growth advances our understanding of Trichoderma-plant interactions.

Arabidopsis NRT1.5 Mediates the Suppression of Nitrate Starvation-Induced Leaf Senescence by Modulating Foliar Potassium Level

Nitrogen deficiency induces leaf senescence. However, whether or how nitrate might affect this process remains to be investigated. Here, we report an interesting finding that nitrate-instead of nitrogen-starvation induced early leaf senescence in nrt1.5 mutant, and present genetic and physiological data demonstrating that nitrate starvation-induced leaf senescence is suppressed by NRT1.5. NRT1.5 suppresses the senescence process dependent on its function from roots, but not the nitrate transport function. Further analyses using nrt1.5 single and nia1 nia2 nrt1.5-4 triple mutant showed a negative correlation between nitrate concentration and senescence rate in leaves. Moreover, when exposed to nitrate starvation, foliar potassium level decreased in nrt1.5, but adding potassium could essentially restore the early leaf senescence phenotype of nrt1.5 plants. Nitrate starvation also downregulated the expression of HAK5, RAP2.11, and ANN1 in nrt1.5 roots, and appeared to alter potassium level in xylem sap from nrt1.5. These data suggest that NRT1.5 likely perceives nitrate starvation-derived signals to prevent leaf senescence by facilitating foliar potassium accumulation.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

Silicon-mediated changes in polyamines participate in silicon-induced salt tolerance in Sorghum bicolor L

Silicon (Si) is generally considered a beneficial element for the growth of higher plants, especially under stress conditions, but the mechanisms remain unclear. Here, we tested the hypothesis that Si improves salt tolerance through mediating important metabolism processes rather than acting as a mere mechanical barrier. Seedlings of sorghum (Sorghum bicolor L.) growing in hydroponic culture were treated with NaCl (100 mm) combined with or without Si (0.83 mm). The result showed that supplemental Si enhanced sorghum salt tolerance by decreasing Na(+) accumulation. Simultaneously, polyamine (PA) levels were increased and ethylene precursor (1-aminocyclopropane-1-carboxylic acid: ACC) concentrations were decreased. Several key PA synthesis genes were up-regulated by Si under salt stress. To further confirm the role of PA in Si-mediated salt tolerance, seedlings were exposed to spermidine (Spd) or a PA synthesis inhibitor (dicyclohexylammonium sulphate, DCHA) combined with salt and Si. Exogenous Spd showed similar effects as Si under salt stress whereas exogenous DCHA eliminated Si-enhanced salt tolerance and the beneficial effect of Si in decreasing Na(+) accumulation. These results indicate that PAs and ACC are involved in Si-induced salt tolerance in sorghum and provide evidence that Si plays an active role in mediating salt tolerance.

On a quest for stress tolerance genes: membrane transporters in sensing and adapting to hostile soils

Abiotic stresses such as salinity, drought, and flooding severely limit food and fibre production and result in penalties of in excess of US$100 billion per annum to the agricultural sector. Improved abiotic stress tolerance to these environmental constraints via traditional or molecular breeding practices requires a good understanding of the physiological and molecular mechanisms behind roots sensing of hostile soils, as well as downstream signalling cascades to effectors mediating plant adaptive responses to the environment. In this review, we discuss some common mechanisms conferring plant tolerance to these three major abiotic stresses. Central to our discussion are: (i) the essentiality of membrane potential maintenance and ATP production/availability and its use for metabolic versus adaptive responses; (ii) reactive oxygen species and Ca(2+) 'signatures' mediating stress signalling; and (iii) cytosolic K(+) as the common denominator of plant adaptive responses. We discuss in detail how key plasma membrane and tonoplast transporters are regulated by various signalling molecules and processes observed in plants under stress conditions (e.g. changes in membrane potential; cytosolic pH and Ca(2+); reactive oxygen species; polyamines; abscisic acid) and how these stress-induced changes are related to expression and activity of specific ion transporters. The reported results are then discussed in the context of strategies for breeding crops with improved abiotic stress tolerance. We also discuss a classical trade-off between tolerance and yield, and possible avenues for resolving this dilemma.

Mechanisms of thaxtomin A-induced root toxicity revealed by a thaxtomin A sensitive Arabidopsis mutant (ucu2-2/gi-2)

The Arabidopsis mutant ( ucu2 - 2/gi - 2 ) is thaxtomin A, isoxaben and NPA-sensitive indicated by root growth and ion flux responses providing new insights into these compounds mode of action and interactions. Thaxtomin A (TA) is a cellulose biosynthetic inhibitor (CBI) that promotes plant cell hypertrophy and cell death. Electrophysiological analysis of steady-state K(+) and Ca(2+) fluxes in Arabidopsis thaliana roots pretreated with TA for 24 h indicated a disturbance in the regulation of ion movement across the plant cell membrane. The observed inability to control solute movement, recorded in rapidly growing meristematic and elongation root zones, may partly explain typical root toxicity responses to TA treatment. Of note, the TA-sensitive mutant (ucu2-2/gi-2) was more susceptible with K(+) and Ca(2+) fluxes altered between 1.3 and eightfold compared to the wild-type control where fluxes altered between 1.2 and threefold. Root growth inhibition assays showed that the ucu2-2/gi-2 mutant had an increased sensitivity to the auxin 2,4-D, but not IAA or NAA; it also had increased sensitivity to the auxin efflux transport inhibitor, 1-naphthylphthalamic acid (NPA), but not 2,3,5- Triiodobenzoic acid (TIBA), when compared to the WT. The NPA sensitivity data were supported by electrophysiological analysis of H(+) fluxes in the mature (but not elongation) root zone. Increased sensitivity to the CBI, isoxaben (IXB), but not dichlobenil was recorded. Increased sensitivity to both TA and IXB corresponded with higher levels of accumulation of these toxins in the root tissue, compared to the WT. Further root growth inhibition assays showed no altered sensitivity of ucu2-2/gi-2 to two other plant pathogen toxins, alternariol and fusaric acid. Identification of a TA-sensitive Arabidopsis mutant provides further insight into how this CBI toxin interacts with plant cells.

PDH45 transgenic rice maintain cell viability through lower accumulation of Na(+), ROS and calcium homeostasis in roots under salinity stress

Salinity severely affects the growth/productivity of rice, which is utilized as major staple food crop worldwide. PDH45 (pea DNA helicase 45), a member of the DEAD-box helicase family, actively provides salinity stress tolerance, but the mechanism behind this is not well known. Therefore, in order to understand the mechanism of stress tolerance, sodium ion (Na(+)), reactive oxygen species (ROS), cytosolic calcium [Ca(2+)]cyt and cell viability were analyzed in roots of PDH45 transgenic-IR64 rice lines along with wild-type (WT) IR64 rice under salinity stress (100mM and 200 mM NaCl). In addition, the roots of salinity-tolerant (FL478) and susceptible (Pusa-44) rice varieties were also analyzed under salinity stress for comparative analysis. The results reveal that, under salinity stress (100mM and 200 mM NaCl), roots of PDH45 transgenic lines accumulate lower levels of Na(+), ROS and maintain [Ca(2+)]cyt and exhibit higher cell viability as compared with roots of WT (IR64) plants. Similar results were also obtained in the salinity-tolerant FL478 rice. However, the roots of WT and salinity-susceptible Pusa-44 rice accumulated higher levels of Na(+), ROS and [Ca(2+)]cyt imbalance and lower cell viability during salinity stress, which is in contrast to the overexpressing PDH45 transgenic lines and salinity-tolerant FL478 rice. Further, to understand the mechanism of PDH45 at molecular level, comparative expression profiling of 12 cation transporters/genes was also conducted in roots of WT (IR64) and overexpressing PDH45 transgenic lines (L1 and L2) under salt stress (24h of 200 mM NaCl). The expression analysis results show altered and differential gene expression of cation transporters/genes in salt-stressed roots of WT (IR64) and overexpressing transgenic lines (L1 and L2). These observations collectively suggest that, under salinity stress conditions, PDH45 is involved in the regulation of Na(+) level, ROS production, [Ca(2+)]cyt homeostasis, cell viability and cation transporters in roots of PDH45 transgenic-IR64 rice and consequently provide salinity tolerance. Elucidating the detailed regulatory mechanism of PDH45 will provide a better understanding of salinity stress tolerance and further open new ways to manipulate genome to achieve higher agricultural production under stress.

Engineered silver nanoparticles are sensed at the plasma membrane and dramatically modify the physiology of Arabidopsis thaliana plants

Silver nanoparticles (Ag NPs) are the world's most important nanomaterial and nanotoxicant. The aim of this study was to determine the early stages of interactions between Ag NPs and plant cells, and to investigate their physiological roles. We have shown that the addition of Ag NPs to cultivation medium, at levels above 300 mg L(-1) , inhibited Arabidopsis thaliana root elongation and leaf expansion. This also resulted in decreased photosynthetic efficiency and the extreme accumulation of Ag in tissues. Acute application of Ag NPs induced a transient elevation of [Ca(2+) ]cyt and the accumulation of reactive oxygen species (ROS; partially generated by NADPH oxidase). Whole-cell patch-clamp measurements on root cell protoplasts demonstrated that Ag NPs slightly inhibited plasma membrane K(+) efflux and Ca(2+) influx currents, or caused membrane breakdown; however, in excised outside-out patches, Ag NPs activated Gd(3+) -sensitive Ca(2+) influx channels with unitary conductance of approximately 56 pS. Bulk particles did not modify the plasma membrane currents. Tests with electron paramagnetic resonance spectroscopy showed that Ag NPs were not able to catalyse hydroxyl radical generation, but that they directly oxidized the major plant antioxidant, l-ascorbic acid. Overall, the data presented shed light on mechanisms of the impact of nanosilver on plant cells, and show that these include the induction of classical stress signalling reactions (mediated by [Ca(2+) ]cyt and ROS) and a specific effect on the plasma membrane conductance and the reduced ascorbate.

Salt stress sensing and early signalling events in plant roots: Current knowledge and hypothesis

Soil salinity is a major environmental constraint to crop production. While the molecular identity and functional expression of Na(+) transport systems mediating Na(+) exclusion from the cytosol has been studied in detail, far less is known about the mechanisms by which plants sense high Na(+) levels in the soil and the rapid signalling events that optimise plant performance under saline conditions. This review aims to fill this gap. We first discuss the nature of putative salt stress sensors, candidates which include Na(+) transport systems, mechanosensory proteins, proteins with regulatory Na(+) binding sites, sensing mediated by cyclic nucleotide-gated channels, purine receptors, annexin and voltage gating. We suggest that several transport proteins may be clustered together to form a microdomain in a lipid raft, allowing rapid changes in the activity of an individual protein to be translated into stress-induced Ca(2+) and H2O2 signatures. The pathways of stress signalling to downstream targets are discussed, and the kinetics and specificity of salt stress signalling between glycophytes and halophytes is compared. We argue that these sensing mechanisms operate in parallel, providing plants with a robust system for decoding information about the specific nature and severity of the imposed salt stress.

Salt stress-induced production of reactive oxygen- and nitrogen species and cell death in the ethylene receptor mutant Never ripe and wild type tomato roots

The salt stress triggered by sublethal, 100 mM and lethal, 250 mM NaCl induced ethylene production as well as rapid accumulation of superoxide radical and H2O2 in the root tips of tomato (Solanum lycopersicum cv. Ailsa Craig) wild type and ethylene receptor mutant, Never ripe (Nr/Nr) plants. In the wild type plants superoxide accumulation confined to lethal salt concentration while H2O2 accumulated more efficiently under sublethal salt stress. However, in Nr roots the superoxide production was higher and unexpectedly, H2O2 level was lower than in the wild type under sublethal salt stress. Nitric oxide production increased significantly under sublethal and lethal salt stress in both genotypes especially in mutant plants, while peroxynitrite accumulated significantly under lethal salt stress. Thus, the nitro-oxidative stress may be stronger in Nr roots, which leads to the programmed death of tissues, characterized by the DNA and protein degradation and loss of cell viability under moderate salt stress. In Nr mutants the cell death was induced in the absence of ethylene perception. Although wild type roots could maintain their potassium content under moderate salt stress, K(+) level significantly declined leading to small K(+)/Na(+) ratio in Nr roots. Thus Nr mutants were more sensitive to salt stress than the wild type and the viability of root cells decreased significantly under moderate salt stress. These changes can be attributed to a stronger ionic stress due to the K(+) loss from the root tissues.

Modulation of Potassium Channel Activity in the Balance of ROS and ATP Production by Durum Wheat Mitochondria-An Amazing Defense Tool Against Hyperosmotic Stress

In plants, the existence of a mitochondrial potassium channel was firstly demonstrated about 15 years ago in durum wheat as an ATP-dependent potassium channel (PmitoKATP). Since then, both properties of the original PmitoKATP and occurrence of different mitochondrial potassium channels in a number of plant species (monocotyledonous and dicotyledonous) and tissues/organs (etiolated and green) have been shown. Here, an overview of the current knowledge is reported; in particular, the issue of PmitoKATP physiological modulation is addressed. Similarities and differences with other potassium channels, as well as possible cross-regulation with other mitochondrial proteins (Plant Uncoupling Protein, Alternative Oxidase, Plant Inner Membrane Anion Channel) are also described. PmitoKATP is inhibited by ATP and activated by superoxide anion, as well as by free fatty acids (FFAs) and acyl-CoAs. Interestingly, channel activation increases electrophoretic potassium uptake across the inner membrane toward the matrix, so collapsing membrane potential (ΔΨ), the main component of the protonmotive force (Δp) in plant mitochondria; moreover, cooperation between PmitoKATP and the K(+)/H(+) antiporter allows a potassium cycle able to dissipate also ΔpH. Interestingly, ΔΨ collapse matches with an active control of mitochondrial reactive oxygen species (ROS) production. Fully open channel is able to lower superoxide anion up to 35-fold compared to a condition of ATP-inhibited channel. On the other hand, ΔΨ collapse by PmitoKATP was unexpectedly found to not affect ATP synthesis via oxidative phosphorylation. This may probably occur by means of a controlled collapse due to ATP inhibition of PmitoKATP; this brake to the channel activity may allow a loss of the bulk phase Δp, but may preserve a non-classically detectable localized driving force for ATP synthesis. This ability may become crucial under environmental/oxidative stress. In particular, under moderate hyperosmotic stress (mannitol or NaCl), PmitoKATP was found to be activated by ROS, so inhibiting further large-scale ROS production according to a feedback mechanism; moreover, a stress-activated phospholipase A2 may generate FFAs, further activating the channel. In conclusion, a main property of PmitoKATP is the ability to keep in balance the control of harmful ROS with the mitochondrial/cellular bioenergetics, thus preserving ATP for energetic needs of cell defense under stress.

Mechanistic study of programmed cell death of root border cells of cucumber (Cucumber sativus L.) induced by copper

Programmed cell death (PCD) in root border cells (RBCs) induced by Copper (Cu) has been little studied. This study explored whether Cu induced PCD in RBCs of cucumber or not and investigated the possible mechanisms. The results showed that the percentage of apoptotic and necrotic RBCs increased with increasing concentration of Cu treatment. A quick burst of ROS in RBCs was detected, while mitochondrial membrane potential (ΔΨm) decreased sharply with Cu treatment. Caspase-3 like protease activity showed a tendency of increase with Cu treatment. The potential of Cu to induce PCD in RBCs of cucumber was first proved. Our results showed that ROS generation and mitochondrial membrane potential loss played important roles in Cu-induced caspase-3-like activation and PCD in RBCs of cucumber, which provided new insight into the signaling cascades that modulate Cu phytotoxicity mechanism.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant's ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant's priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Salinity tolerance in plants. Quantitative approach to ion transport starting from halophytes and stepping to genetic and protein engineering for manipulating ion fluxes

Ion transport is the fundamental factor determining salinity tolerance in plants. The Review starts from differences in ion transport between salt tolerant halophytes and salt-sensitive plants with an emphasis on transport of potassium and sodium via plasma membranes. The comparison provides introductory information for increasing salinity tolerance. Effects of salt stress on ion transport properties of membranes show huge opportunities for manipulating ion fluxes. Further steps require knowledge about mechanisms of ion transport and individual genes of ion transport proteins. Initially, the Review describes methods to measure ion fluxes, the independent set of techniques ensures robust and reliable basement for quantitative approach. The Review briefly summarizes current data concerning Na(+) and K(+) concentrations in cells, refers to primary thermodynamics of ion transport and gives special attention to individual ion channels and transporters. Simplified scheme of a plant cell with known transport systems at the plasma membrane and tonoplast helps to imagine the complexity of ion transport and allows choosing specific transporters for modulating ion transport. The complexity is enhanced by the influence of cell size and cell wall on ion transport. Special attention is given to ion transporters and to potassium and sodium transport by HKT, HAK, NHX, and SOS1 proteins. Comparison between non-selective cation channels and ion transporters reveals potential importance of ion transporters and the balance between the two pathways of ion transport. Further on the Review describes in detail several successful attempts to overexpress or knockout ion transporters for changing salinity tolerance. Future perspectives are questioned with more attention given to promising candidate ion channels and transporters for altered expression. Potential direction of increasing salinity tolerance by modifying ion channels and transporters using single point mutations is discussed and questioned. An alternative approach from synthetic biology is to create new regulation networks using novel transport proteins with desired properties for transforming agricultural crops. The approach had not been widely used earlier; it leads also to theoretical and pure scientific aspects of protein chemistry, structure-function relations of membrane proteins, systems biology and physiology of stress and ion homeostasis. Summarizing, several potential ways are aimed at required increase in salinity tolerance of plants of interest.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Both the concentration and redox state of glutathione and ascorbate influence the sensitivity of arabidopsis to cadmium

BACKGROUND AND AIMS

Cadmium (Cd) is a non-essential trace element that elicits oxidative stress. Plants respond to Cd toxicity via increasing their Cd-chelating and antioxidative capacities. They predominantly chelate Cd via glutathione (GSH) and phytochelatins (PCs), while antioxidative defence is mainly based on the use and recycling of both GSH and ascorbate (AsA), complemented by superoxide dismutase (SOD) and catalase (CAT). In addition, both metabolites act as a substrate for the regeneration of other essential antioxidants, which neutralize and regulate reactive oxygen species (ROS). Together, these functions influence the concentration and cellular redox state of GSH and AsA. In this study, these two parameters were examined in plants of Arabidopsis thaliana exposed to sub-lethal Cd concentrations.

METHODS

Wild-type plants and mutant arabidopsis plants containing 30-45 % of wild-type levels of GSH (cad2-1) or 40-50 % of AsA (vtc1-1), together with the double-mutant (cad2-1 vtc1-1) were cultivated in a hydroponic system and exposed to sub-lethal Cd concentrations. Cadmium detoxification was investigated at different levels including gene expression and metabolite concentrations.

KEY RESULTS

In comparison with wild-type plants, elevated basal thiol levels and enhanced PC synthesis upon exposure to Cd efficiently compensated AsA deficiency in vtc1-1 plants and contributed to decreased sensitivity towards Cd. Glutathione-deficient (cad2-1 and cad2-1 vtc1-1) mutants, however, showed a more oxidized GSH redox state, resulting in initial oxidative stress and a higher sensitivity to Cd. In order to cope with the Cd stress to which they were exposed, GSH-deficient mutants activated multiple alternative pathways.

CONCLUSIONS

Our observations indicate that GSH and AsA deficiency differentially alter plant GSH homeostasis, resulting in opposite Cd sensitivities relative to wild-type plants. Upon Cd exposure, GSH-deficient mutants were hampered in chelation. They experienced phenotypic disturbances and even more oxidative stress, and therefore activated multiple alternative pathways such as SOD, CAT and ascorbate peroxidase, indicating a higher Cd sensitivity. Ascorbate deficiency, however, was associated with enhanced PC synthesis in comparison with wild-type plants after Cd exposure, which contributed to decreased sensitivity towards Cd.