Outward-rectifying potassium channels GORK and SKOR function in regulation of root growth under salt stress in Arabidopsis thaliana

Plants often face high salinity as a significant environmental challenge with roots being the first responders to this stress. Maintaining K+/Na+ ratio within plant cells is crucial for survival, as the intracellular K+ level decreases and the intracellular Na+ level increases under saline conditions. However, knowledge about the molecular regulatory mechanisms of K+ loss in response to salt stress through outward-rectifying K+ channels in plants is largely unknown. In this study, we found that the Arabidopsis double mutant gorkskor, in which the GORK and SKOR genes are disrupted, showed an improved primary root growth under salt stress compared to wild-type (WT) and the gork and skor single-mutant plants. No significant differences in the sensitivity to mannitol stress between the WT and gorkskor mutant were observed. Accumulation of ROS induced by salt stress was reduced in the gorkskor roots. The gorkskor mutant seedlings had significantly higher K+ content, lower Na+ content, and a greater resultant K+/Na+ ratio than the WT under salt stress. Moreover, salt-stress-induced elevation of cytosolic free Ca2+ concentration was reduced in the gorkskor roots. Taken together, these results suggest that Arabidopsis Shaker-type outward-rectifying K+ channels GORK and SKOR may redundantly function in regulation of primary root growth under salt stress and are involved in not only the late-stage response (e.g. K+ leakage) but also the early response including ROS production and [Ca2+]cyt elevation.

Exogenous zinc application mitigates negative effects of salinity on barley (Hordeum vulgare) growth by improving root ionic homeostasis

Detrimental effects of salinity could be mitigated by exogenous zinc (Zn) application; however, the mechanisms underlying this amelioration are poorly understood. This study demonstrated the interaction between Zn and salinity by measuring plant biomass, photosynthetic performance, ion concentrations, ROS accumulation, antioxidant activity and electrophysiological parameters in barley (Hordeum vulgare L.). Salinity stress (200mM NaCl for 3weeks) resulted in a massive reduction in plant biomass; however, both fresh and dry weight of shoots were increased by ~30% with adequate Zn supply. Zinc supplementation also maintained K+ and Na+ homeostasis and prevented H2 O2 toxicity under salinity stress. Furthermore, exposure to 10mM H2 O2 resulted in massive K+ efflux from root epidermal cells in both the elongation and mature root zones, and pre-treating roots with Zn reduced ROS-induced K+ efflux from the roots by 3-4-fold. Similar results were observed for Ca2+ . The observed effects may be causally related to more efficient regulation of cation-permeable non-selective channels involved in the transport and sequestration of Na+ , K+ and Ca2+ in various cellular compartments and tissues. This study provides valuable insights into Zn protective functions in plants and encourages the use of Zn fertilisers in barley crops grown on salt-affected soils.

Root and Leaf Anatomy, Ion Accumulation, and Transcriptome Pattern under Salt Stress Conditions in Contrasting Genotypes of Sorghum bicolor

Roots from salt-susceptible ICSR-56 (SS) sorghum plants display metaxylem elements with thin cell walls and large diameter. On the other hand, roots with thick, lignified cell walls in the hypodermis and endodermis were noticed in salt-tolerant CSV-15 (ST) sorghum plants. The secondary wall thickness and number of lignified cells in the hypodermis have increased with the treatment of sodium chloride stress to the plants (STN). Lignin distribution in the secondary cell wall of sclerenchymatous cells beneath the lower epidermis was higher in ST leaves compared to the SS genotype. Casparian thickenings with homogenous lignin distribution were observed in STN roots, but inhomogeneous distribution was evident in SS seedlings treated with sodium chloride (SSN). Higher accumulation of K⁺ and lower Na⁺ levels were noticed in ST compared to the SS genotype. To identify the differentially expressed genes among SS and ST genotypes, transcriptomic analysis was carried out. Both the genotypes were exposed to 200 mM sodium chloride stress for 24 h and used for analysis. We obtained 70 and 162 differentially expressed genes (DEGs) exclusive to SS and SSN and 112 and 26 DEGs exclusive to ST and STN, respectively. Kyoto Encyclopaedia of Genes and Genomes (KEGG) and Gene Ontology (GO) enrichment analysis unlocked the changes in metabolic pathways in response to salt stress. qRT-PCR was performed to validate 20 DEGs in each SSN and STN sample, which confirms the transcriptomic results. These results surmise that anatomical changes and higher K⁺/Na⁺ ratios are essential for mitigating salt stress in sorghum apart from the genes that are differentially up- and downregulated in contrasting genotypes.

Over-expression of spermidine synthase 2 (SlSPDS2) in tomato plants improves saline-alkali stress tolerance by increasing endogenous polyamines content to regulate antioxidant enzyme system and ionic homeostasis

Endogenous spermidine can improve the resistance of plants to saline-alkali stress. SlSPDS1 and SlSPDS2 are the main spermidine synthase (SPDS) genes in tomatoes. In comparison with SlSPDS1, SlSPDS2 plays an important role in wild-type tomato seedling under saline-alkali stress. However, limited research has focused on the role of SlSPDS2 in saline-alkali stress. Wild-type (WT) and SPDS gene (SlSPDS2) transgenic over-expression tomato seedlings were used to explore the function of endogenous spermidine on the saline-alkali resistance of tomato seedlings. The results show that SlSPDS2 overexpression under normal conditions and saline-alkali stress increased the content of endogenous free polyamines and the expression levels of polyamine synthesis-related genes in tomato seedlings. Under saline-alkali stress, SlSPDS2 overexpression significantly reduced Na+/K+ ratio, relative electrical conductivity, O2·-, H2O2, and malondialdehyde content, increased Seedling index, relative water content, antioxidant enzyme activities (peroxidase, superoxide dismutase, and catalase), and the contents of proline and soluble sugar in tomato leaf, and mitigated the adverse effect of saline-alkali stress on tomato seedlings. In summary, the overexpression of SlSPDS2 tomato seedlings regulated the ionic homeostasis, antioxidant enzyme system, and osmotic regulatory substances of tomato seedlings living in saline-alkali environment by increasing endogenous free polyamine content, thereby improving the resistance of tomato seedlings against saline-alkali stress.

Male Fertility under Environmental Stress: Do Polyamines Act as Pollen Tube Growth Protectants?

Although pollen structure and morphology evolved toward the optimization of stability and fertilization efficiency, its performance is affected by harsh environmental conditions, e.g., heat, cold, drought, pollutants, and other stressors. These phenomena are expected to increase in the coming years in relation to predicted environmental scenarios, contributing to a rapid increase in the interest of the scientific community in understanding the molecular and physiological responses implemented by male gametophyte to accomplish reproduction. Here, after a brief introduction summarizing the main events underlying pollen physiology with a focus on polyamine involvement in its development and germination, we review the main effects that environmental stresses can cause on pollen. We report the most relevant evidence in the literature underlying morphological, cytoskeletal, metabolic and signaling alterations involved in stress perception and response, focusing on the final stage of pollen life, i.e., from when it hydrates, to pollen tube growth and sperm cell transport, with these being the most sensitive to environmental changes. Finally, we hypothesize the molecular mechanisms through which polyamines, well-known molecules involved in plant development, stress response and adaptation, can exert a protective action against environmental stresses in pollen by decoding the essential steps and the intersection between polyamines and pollen tube growth mechanisms.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Lipid kinases PIP5K7 and PIP5K9 are required for polyamine-triggered K+ efflux in Arabidopsis roots

Polyamines, such as putrescine, spermidine and spermine (Spm), are low-molecular-weight polycationic molecules present in all living organisms. Despite their implication in plant cellular processes, little is known about their molecular mode of action. Here, we demonstrate that polyamines trigger a rapid increase in the regulatory membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ), and that this increase is required for polyamine effects on K⁺ efflux in Arabidopsis roots. Using in vivo ³² P_i -labelling of Arabidopsis seedlings, low physiological (μm) concentrations of Spm were found to promote a rapid PIP₂ increase in roots that was time- and dose-dependent. Confocal imaging of a genetically encoded PIP₂ biosensor revealed that this increase was triggered at the plasma membrane. Differential ³² P_i -labelling suggested that the increase in PIP₂ was generated through activation of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) activity rather than inhibition of a phospholipase C or PIP₂ 5-phosphatase activity. Systematic analysis of transfer DNA insertion mutants identified PIP5K7 and PIP5K9 as the main candidates involved in the Spm-induced PIP₂ response. Using non-invasive microelectrode ion flux estimation, we discovered that the Spm-triggered K⁺ efflux response was strongly reduced in pip5k7 pip5k9 seedlings. Together, our results provide biochemical and genetic evidence for a physiological role of PIP₂ in polyamine-mediated signalling controlling K⁺ flux in plants.

The role of NO in plant response to salt stress: interactions with polyamines

Soil salinity is a major abiotic stress that limits plant growth and productivity. High concentrations of sodium chloride can cause osmotic and ionic effects. This stress minimises a plant's ability to uptake water and minerals, and increases Na+ accumulation in the cytosol, thereby disturbing metabolic processes. Prolonged plant exposure to salt stress can lead to oxidative stress and increased production of reactive oxygen species (ROS). Higher plants developed some strategies to cope with salt stress. Among these, mechanisms involving nitric oxide (NO) and polyamines (PAs) are particularly important. NO is a key signalling molecule that mediates a variety of physiological functions and defence responses against abiotic stresses in plants. Under salinity conditions, NO donors increase growth parameters, reduce Na+ toxicity, maintain ionic homeostasis, stimulate osmolyte accumulation and prevent damages caused by ROS. NO enhances salt tolerance of plants via post-translational protein modifications through S-nitrosylation of thiol groups, nitration of tyrosine residues and modulation of multiple gene expression. Several reviews have reported on the role of polyamines in modulating salt stress plant response and the capacity to enhance PA synthesis upon salt stress exposure, and it is known that NO and PAs interact under salinity. In this review, we focus on the role of NO in plant response to salt stress, paying particular attention to the interaction between NO and PAs.

Identification and characterization of differentially expressed genes in the rice root following exogenous application of spermidine during salt stress

Salinity is a major limiting factor in crop production. Exogenous spermidine (spd) effectively ameliorates salt injury, though the underlying molecular mechanism is poorly understood. We have used a suppression subtractive hybridization method to construct a cDNA library that has identified up-regulated genes from rice root under the treatment of spd and salt. Total 175 high-quality ESTs of about 100-500 bp in length with an average size of 200 bp are isolated, clustered and assembled into a collection of 62 unigenes. Gene ontology analysis using the KEGG pathway annotation database has classified the unigenes into 5 main functional categories and 13 subcategories. The transcripts abundance has been validated using Real-Time PCR. We have observed seven different types of post-translational modifications in the DEPs. 44 transmembrane helixes are predicted in 6 DEPs. This above information can be used as first-hand data for dissecting the administrative role of spd during salinity.

Ectopic expression of CsTGase enhances salt tolerance by regulating polyamine biosynthesis, antioxidant activities and Na+/K+ homeostasis in transgenic tobacco

Transglutaminases (TGases), mediators of the transamidation of specific proteins by polyamines (PA), play critical roles in PA metabolism in animals, but their functions and regulatory mechanisms are largely unknown in plants. In this study, we demonstrated that TGase from cucumber played a protective role in the regulation of PA metabolism under salt stress. The expression of TGase was induced by salt stress in cucumber. Ectopic overexpression of cucumber TGase in tobacco conferred enhanced tolerance to salt stress based on both external symptoms and membrane integrity. Overexpression lines maintained high levels of PAs under salt stress, suggesting that PAs played a vital role in TGase-induced salt tolerance. In contrast, the levels of Na+ content in the wild-type (WT) plants increased, while they decreased in the overexpression plants. The expression levels of several genes related to ion exchange enhanced, and the Na+/K+ ratio decreased by increased TGase activity under salt stress. The activities of the proton-pump ATPase (H+-ATPase), vacuolar H+-ATPase (V-ATPase) and vacuolar H+-pyrophosphatase (PPase) were higher in the overexpression lines than in WT plants under salt stress. Moreover, the malondialdehyde (MDA) and H2O2 contents were significantly lower in the overexpression lines than in WT plants, accompanied by increased antioxidant enzyme activity. Taken together, these findings demonstrate that TGase plays protective roles in response to salt stress, which may promote plant survival by regulating PA metabolism and the Na+/K+ balance under salt stress.

What is the role of putrescine accumulated under potassium deficiency?

Biomarker metabolites are of increasing interest in crops since they open avenues for precision agriculture, whereby nutritional needs and stresses can be monitored optimally. Putrescine has the potential to be a useful biomarker to reveal potassium (K⁺ ) deficiency. In fact, although this diamine has also been observed to increase during other stresses such as drought, cold or heavy metals, respective changes are comparably low. Due to its multifaceted biochemical properties, several roles for putrescine under K⁺ deficiency have been suggested, such as cation balance, antioxidant, reactive oxygen species mediated signalling, osmolyte or pH regulator. However, the specific association of putrescine build-up with low K⁺ availability in plants remains poorly understood, and possible regulatory roles must be consistent with putrescine concentration found in plant tissues. We hypothesize that the massive increase of putrescine upon K⁺ starvation plays an adaptive role. A distinction of putrescine function from that of other polyamines (spermine, spermidine) may be based either on its specificity or (which is probably more relevant under K⁺ deficiency) on a very high attainable concentration of putrescine, which far exceeds those for spermidine and spermine. putrescine and its catabolites appear to possess a strong potential in controlling cellular K⁺ and Ca²⁺ , and mitochondria and chloroplasts bioenergetics under K⁺ stress.

Enzymes contributing to the hydrogen peroxide signal dynamics that regulate wall labyrinth formation in transfer cells

Transfer cells are characterized by an amplified plasma membrane area supported on a wall labyrinth composed of a uniform wall layer (UWL) from which wall ingrowth (WI) papillae arise. Adaxial epidermal cells of developing Vicia faba cotyledons, when placed in culture, undergo a rapid (hours) trans-differentiation to a functional epidermal transfer cell (ETC) phenotype. The trans-differentiation event is controlled by a signalling cascade comprising auxin, ethylene, apoplasmic reactive oxygen species (apoROS), and cytosolic Ca2+. Apoplasmic hydrogen peroxide (apoH2O2) was confirmed as the apoROS regulating UWL and WI papillae formation. Informed by an ETC-specific transcriptome, a pharmacological approach identified a temporally changing cohort of H2O2 biosynthetic enzymes. The cohort contained a respiratory burst oxidase homologue, polyamine oxidase, copper amine oxidase, and a suite of class III peroxidases. Collectively these generated two consecutive bursts in apoH2O2 production. Spatial organization of biosynthetic/catabolic enzymes was deduced from responses to pharmacologically blocking their activities on the cellular and subcellular distribution of apoH2O2. The findings were consistent with catalase activity constraining the apoH2O2 signal to the outer periclinal wall of the ETCs. Strategic positioning of class III peroxidases in this outer domain shaped subcellular apoH2O2 signatures that differed during assembly of the UWL and WI papillae.

Differences of endogenous polyamines and putative genes associated with paraquat resistance in goosegrass (Eleusine indica L.)

Background

Paraquat is one of the most effective herbicides used to control weeds in agricultural management, while the pernicious weed goosegrass (Eleusine indica) has evolved resistance to herbicides, including paraquat. Polyamines provide high-level paraquat resistance in many plants. In the present study, we selected three polyamines, namely, putrescine, spermidine, and spermine, as putative genes to investigate their correlation with paraquat resistance by using paraquat-resistant (R) and paraquat-susceptible (S) goosegrass populations.

Results

There was no significant difference in the putrescine nor spermine content between the R and S biotypes. However, 30 and 90 min after paraquat treatment, the spermidine concentration was 346.14-fold and 421.04-fold (P < 0.001) higher in the R biotype than in the S biotype, but the spermidine concentration was drastically reduced to a marginal level after 90 min. Since the transcript level of PqE was low while the spermidine concentration showed a transient increase, the PqE gene was likely involved in the synthesis of the paraquat resistance mechanism, regulation of polyamine content, and synthesis of spermidine and spermine. PqTS1, PqTS2, and PqTS3 encode transporter proteins involved in the regulation of paraquat concentration but showed different transcription patterns with synchronous changes in polyamine content.

Conclusion

Endogenous polyamines (especially spermidine) play a vital role in paraquat resistance in goosegrass. PqE, PqTS1, PqTS2, and PqTS3 were speculated on the relationship between polyamine metabolism and paraquat resistance. To validate the roles of PqE, PqTS1, PqTS2, and PqTS3 in polyamine transport systems, further research is needed.

GABA operates upstream of H+-ATPase and improves salinity tolerance in Arabidopsis by enabling cytosolic K+ retention and Na+ exclusion

The non-protein amino acid γ-aminobutyric acid (GABA) rapidly accumulates in plant tissues in response to salinity. However, the physiological rationale for this elevation remains elusive. This study compared electrophysiological and whole-plant responses of salt-treated Arabidopsis mutants pop2-5 and gad1,2, which have different abilities to accumulate GABA. The pop2-5 mutant, which was able to overaccumulate GABA in its roots, showed a salt-tolerant phenotype. On the contrary, the gad1,2 mutant, lacking the ability to convert glutamate to GABA, showed oversensitivity to salinity. The greater salinity tolerance of the pop2-5 line was explained by: (i) the role of GABA in stress-induced activation of H+-ATPase, thus leading to better membrane potential maintenance and reduced stress-induced K+ leak from roots; (ii) reduced rates of net Na+ uptake; (iii) higher expression of SOS1 and NHX1 genes in the leaves, which contributed to reducing Na+ concentration in the cytoplasm by excluding Na+ to apoplast and sequestering Na+ in the vacuoles; (iv) a lower rate of H2O2 production and reduced reactive oxygen species-inducible K+ efflux from root epidermis; and (v) better K+ retention in the shoot associated with the lower expression level of GORK channels in plant leaves.

Silicon enhances the salt tolerance of cucumber through increasing polyamine accumulation and decreasing oxidative damage

Silicon can increase salt tolerance, but the underlying mechanism has remained unclear. Here, we investigated the effect of silicon on polyamine metabolism and the role of polyamine accumulation in silicon-mediated salt tolerance in cucumber. Seedlings of cucumber 'JinYou 1' were subjected to salt stress (75 mM NaCl) in the presence or absence of added 0.3 mM silicon. Plant growth, polyamine metabolism and effects of exogenous polyamines and polyamine synthesis inhibitor dicyclohexylammonium sulphate on oxidative damage were investigated. The results showed that salt stress inhibited plant growth and decreased leaf chlorophyll levels and the maximum quantum yield of PSII, and added silicon ameliorated these negative effects. Salt stress increased polyamine accumulation in the leaves and roots. Compared with salt stress alone, overall, silicon addition decreased free putrescine concentrations, but increased spermidine and spermine concentrations in both leaves and roots under salt stress. Silicon application resulted in increased polyamine levels under salt stress by promoting the activities of S-adenosylmethionine decarboxylase and arginine decarboxylase while inhibiting the activity of diamine oxidase. Exogenous application of spermidine and spermine alleviated salt-stress-induced oxidative damage, whereas polyamine synthesis inhibitor eliminated the silicon-mediated decrease in oxidative damage. The results suggest that silicon-enhanced polyamine accumulation in cucumber under salt stress may play a role in decreasing oxidative damage and therefore increase the salt tolerance.

Developing a high-throughput phenotyping method for oxidative stress tolerance in barley roots

BACKGROUND

More than 20% of the world's agricultural land is affected by salinity, resulting in multibillion-dollar penalties and jeopardising food security. While the recent progress in molecular technologies has significantly advanced plant breeding for salinity stress tolerance, accurate plant phenotyping remains a bottleneck of many breeding programs. We have recently shown the existence of a strong causal link between salinity and oxidative stress tolerance in cereals (wheat and barley). Using the microelectrode ion flux estimation (MIFE) method, we have also found a major QTL conferring ROS control of ion flux in roots that coincided with the major QTL for the overall salinity stress tolerance. These findings open new (previously unexplored) prospects of improving salinity tolerance by pyramiding this trait alongside with other (traditional) mechanisms.

RESULTS

In this work, two high-throughput phenotyping methods-viability assay and root growth assay-were tested and assessed as a viable alternative to the (technically complicated) MIFE method using barley as a check species. In viability staining experiments, a dose-dependent H2O2-triggered loss of root cell viability was observed, with salt sensitive varieties showing significantly more damage to root cells. In the root growth assays, relative root length (RRL) was measured in plants grown in paper rolls under different H2O2 concentrations. The biggest difference in RRL between contrasting varieties was observed for 1 mM H2O2 treatment. Under these conditions, a significant negative correlation in the reduction in RRL and the overall salinity tolerance is reported.

CONCLUSIONS

These findings offer plant breeders a convenient high throughput method to screen germplasm for oxidative stress tolerance, targeting root-based genes regulating ion homeostasis and thus conferring salinity stress tolerance in barley (and potentially other species).

Extracellular Spermine Triggers a Rapid Intracellular Phosphatidic Acid Response in Arabidopsis, Involving PLDδ Activation and Stimulating Ion Flux

Polyamines, such as putrescine (Put), spermidine (Spd), and spermine (Spm), are low-molecular-weight polycationic molecules found in all living organisms. Despite the fact that they have been implicated in various important developmental and adaptative processes, their mode of action is still largely unclear. Here, we report that Put, Spd, and Spm trigger a rapid increase in the signaling lipid, phosphatidic acid (PA) in Arabidopsis seedlings but also mature leaves. Using time-course and dose-response experiments, Spm was found to be the most effective; promoting PA responses at physiological (low μM) concentrations. In seedlings, the increase of PA occurred mainly in the root and partly involved the plasma membrane polyamine-uptake transporter (PUT), RMV1. Using a differential ³²P_i-labeling strategy combined with transphosphatidylation assays and T-DNA insertion mutants, we found that phospholipase D (PLD), and in particular PLDδ was the main contributor of the increase in PA. Measuring non-invasive ion fluxes (MIFE) across the root plasma membrane of wild type and pldδ-mutant seedlings, revealed that the formation of PA is linked to a gradual- and transient efflux of K⁺. Potential mechanisms of how PLDδ and the increase of PA are involved in polyamine function is discussed.

Cellular mechanisms to survive salt in the halophyte Cakile maritima

We recently identified two behaviours in cultured cells of the salt accumulating halophyte Cakile maritima: one related to a sustained depolarization due to Na⁺ influx through the non-selective cation channels leading to programmed cell death of these cells, a second one related to a transient depolarization allowing cells to survive (Ben Hamed-Laouti, 2016). In this study, we considered at the cellular level mechanisms that could participate to the exclusion of Na⁺ out of the cell and thus participate in the regulation of the internal contents of Na⁺ and cell survival. Upon addition of NaCl in the culture medium of suspension cells of C. maritima, we observed a rapid influx of Na⁺ followed by an efflux dependent of the activity of plasma membrane H⁺-ATPases, in accordance with the functioning of a Na⁺/H⁺ antiporter and the ability of some cells to repolarize. The Na⁺ efflux was shown to be dependent on Na⁺-dependent on Ca²⁺ influx like the SOS1 Na⁺/H⁺ antiporter. We further could observe in response to salt addition, an early production of singlet oxygen (¹O₂) probably due to peroxidase activities. This early ¹O₂ production seemed to be a prerequisite to the Na⁺ efflux. Our findings suggest that in addition to the pathway leading to PCD (Ben Hamed-Laouti, 2016), a second pathway comprising an SOS-like system could participate to the survival of a part of the C. maritima cultured cells challenged by salt stress.

The effects of putrescine are partly overlapping with osmotic stress processes in wheat

Polyamine metabolism is in relation with several metabolic pathways and linked with plant hormones or signalling molecules; in addition polyamines may modulate the up- or down-regulation of gene expression. However the precise mechanism by which polyamines act at the transcription level is still unclear. In the present study the modifying effect of putrescine pre-treatment has been investigated using the microarray transcriptome profile analysis under the conditions where exogenous putrescine alleviated osmotic stress in wheat plants. Pre-treatment with putrescine induced the unique expression of various general stress-related genes. Although there were obvious differences between the effects of putrescine and polyethylene glycol treatments, there was also a remarkable overlap between the effects of putrescine and osmotic stress responses in wheat plants, suggesting that putrescine has already induced acclimation processes under control conditions. The fatty acid composition in certain lipid fractions and the antioxidant enzyme activities have also been specifically changed under osmotic stress conditions or after treatment with putrescine.

Hydrogen Peroxide-Induced Root Ca2+ and K⁺ Fluxes Correlate with Salt Tolerance in Cereals: Towards the Cell-Based Phenotyping

Salinity stress-induced production of reactive oxygen species (ROS) and associated oxidative damage is one of the major factors limiting crop production in saline soils. However, the causal link between ROS production and stress tolerance is not as straightforward as one may expect, as ROS may also play an important signaling role in plant adaptive responses. In this study, the causal relationship between salinity and oxidative stress tolerance in two cereal crops-barley (Hordeum vulgare) and wheat (Triticum aestivum)-was investigated by measuring the magnitude of ROS-induced net K⁺ and Ca2+ fluxes from various root tissues and correlating them with overall whole-plant responses to salinity. We have found that the association between flux responses to oxidative stress and salinity stress tolerance was highly tissue specific, and was also dependent on the type of ROS applied. No correlation was found between root responses to hydroxyl radicals and the salinity tolerance. However, when oxidative stress was administered via H₂O₂ treatment, a significant positive correlation was found for the magnitude of ROS-induced K⁺ efflux and Ca2+ uptake in barley and the overall salinity stress tolerance, but only for mature zone and not the root apex. The same trends were found for wheat. These results indicate high tissue specificity of root ion fluxes response to ROS and suggest that measuring the magnitude of H₂O₂-induced net K⁺ and Ca2+ fluxes from mature root zone may be used as a tool for cell-based phenotyping in breeding programs aimed to improve salinity stress tolerance in cereals.

Spermine modulates fungal morphogenesis and activates plasma membrane H+-ATPase during yeast to hyphae transition

Polyamines play a regulatory role in eukaryotic cell growth and morphogenesis. Despite many molecular advances, the underlying mechanism of action remains unclear. Here, we investigate a mechanism by which spermine affects the morphogenesis of a dimorphic fungal model of emerging relevance in plant interactions, Yarrowia lipolytica, through the recruitment of a phytohormone-like pathway involving activation of the plasma membrane P-type H⁺-ATPase. Morphological transition was followed microscopically, and the H⁺-ATPase activity was analyzed in isolated membrane vesicles. Proton flux and acidification were directly probed at living cell surfaces by a non-invasive selective ion electrode technique. Spermine and indol-3-acetic acid (IAA) induced the yeast-hypha transition, influencing the colony architecture. Spermine induced H⁺-ATPase activity and H⁺ efflux in living cells correlating with yeast-hypha dynamics. Pharmacological inhibition of spermine and IAA pathways prevented the physio-morphological responses, and indicated that spermine could act upstream of the IAA pathway. This study provides the first compelling evidence on the fungal morphogenesis and colony development as modulated by a spermine-induced acid growth mechanism analogous to that previously postulated for the multicellular growth regulation of plants.

Summary: This study presents a new mechanistic model for the integrative role of the polyamine spermine and hormone auxin in the signaling of yeast-to-hypha transition, filling an important gap in fungal morphogenesis.

Methods Related to Polyamine Control of Cation Transport Across Plant Membranes

Polyamines (PAs) are unique polycationic metabolites, which modulate plants' growth, development, and stress responses. As polycations, PAs interfere with cationic transport systems as ion channels and ionotropic pumps. Here, we describe the application of two techniques, MIFE to study the effects of PAs on cation fluxes in vivo and conventional patch-clamp to evaluate the PA blockage of ion currents in isolated plant vacuoles. Preparation of vacuoles for patch-clamp assays is described and solutions and voltage protocols are given, which allow separate recordings of major vacuolar channel currents and quantify their blockage by PAs.

Differing metabolic responses to salt stress in wheat-barley addition lines containing different 7H chromosomal fragments

Salinity-induced osmotic, ionic and oxidative stress responses were investigated on Asakaze/Manas wheat/barley addition lines 7H, 7HL and 7HS, together with their barley (salt-tolerant) and wheat (relatively salt-sensitive) parents. Growth, photosynthetic activity, chlorophyll degradation, proline, glycine betaine accumulation, sugar metabolism, Na+ and K+ uptake and transport processes and the role of polyamines and antioxidants were studied in young plants grown in hydroponic culture with or without salt treatment. Changes in plant growth and photosynthetic activity of plants demonstrated that the salt tolerance of the addition lines 7H and 7HL was similar to that of barley parent cv. Manas, while the sensitivity of the addition line 7HS was similar to that of the wheat parent cv. Asakaze. The Na accumulation in the roots and shoots did not differ between the addition lines and wheat parent. The activation of various genes related to Na uptake and transport was not correlated with the salt tolerance of the genotypes. These results indicated that the direct regulation of Na transport processes is not the main reason for the salt tolerance of these genotypes. Salt treatment induced a complex metabolic rearrangement in both the roots and shoots of all the genotypes. Elevated proline accumulation in the roots and enhanced sugar metabolism in the shoots were found to be important for salt tolerance in the 7H and 7HL addition lines and in barley cv. Manas. In wheat cv. Asakaze and the 7HS addition line the polyamine metabolism was activated. It seems that osmotic adjustment is a more important process in the improvement of salt tolerance in 7H addition lines than the direct regulation of Na transport processes or antioxidant defence.

Inhibition of ethylene synthesis reduces salt-tolerance in tomato wild relative species Solanum chilense

Exposure to salinity induces a burst in ethylene synthesis in the wild tomato halophyte plant species Solanum chilense. In order to gain information on the role of ethylene in salt adaptation, plants of Solanum chilense (accession LA4107) and of cultivated glycophyte Solanum lycopersicum (cv. Ailsa Craig) were cultivated for 7days in nutrient solution containing 0 or 125mM NaCl in the presence or absence of the inhibitor of ethylene synthesis (aminovinylglycine (AVG) 2μM). Salt-induced ethylene synthesis in S. chilense occurred concomitantly with an increase in stomatal conductance, an efficient osmotic adjustment and the maintenance of carbon isotope discrimination value (Δ¹³C). In contrast, in S. lycopersicum, salt stress decreased stomatal conductance and Δ¹³C values while osmotic potential remained higher than in S. chilense. Inhibition of stress-induced ethylene synthesis by AVG decreased stomatal conductance and Δ¹³C in S. chilense and compromised osmotic adjustment. Solanum chilense behaved as an includer and accumulated high amounts of Na in the shoot but remained able to maintain K nutrition in the presence of NaCl. This species however did not stimulate the expression of genes coding for high-affinity K transport but genes coding for ethylene responsive factor ERF5 and JREF1 were constitutively more expressed in S. chilense than in S. lycopersicum. It is concluded that ethylene plays a key role in salt tolerance of S. chilense.

Comparative analysis of polyamine metabolism in wheat and maize plants

In the present work changes in polyamine contents were investigated after various hydroponic polyamine treatments (putrescine, spermidine and spermine at 0.1, 0.3 and 0.5 mM concentrations) in two different crop species, wheat and maize. In contrast to putrescine, higher polyamines (spermidine and spermine) induced concentration-dependent oxidative damage in both crops, resulting in decreased biomass. The unfavourable effects of polyamines were more pronounced in the roots, and maize was more sensitive than wheat. The adverse effects of polyamine treatment were proportional to the accumulation of polyamine and the plant hormone salicylic acid in the leaves and roots of both plant species. Changes in polyamine content and catabolism during osmotic stress conditions were also studied after beneficial pre-treatment with putrescine. The greater positive effect of putrescine in wheat than in maize can be explained by differences in the polyamine metabolism under normal and osmotic stress conditions, and by relationship between polyamines and salicylic acid. The results demonstrated that changes in the polyamine pool are important for fine tuning of polyamine signalling, which influences the hormonal balance required if putrescine is to exert a protective effect under stress conditions.

Short Term Effect of Salt Shock on Ethylene and Polyamines Depends on Plant Salt Sensitivity

In the present manuscript the short term effect (3-24 h) of a saline shock (NaCl 100 mM) on fresh weight, water content, respiration rate, ethylene production and Na⁺, Cl^-, ACC and polyamine concentration was studied in four plant species with different salt sensitivity, pepper, lettuce, spinach, and beetroot. Higher reduction in fresh weight and water content as a consequence of saline shock was found in pepper and lettuce plants than in spinach and beetroot, the latter behaving as more salinity tolerant. In general, salinity led to rapid increases in respiration rate, ethylene production and ACC and polyamine (putrescine, spermidine, and spermine) concentrations in shoot and root. These increases were related to plant salinity sensitivity, since they were higher in the most sensitive species and vice versa. However, ethylene and respiration rates in salt stressed plants recovered similar values to controls after 24 h of treatment in salt tolerant plants, while still remaining high in the most sensitive. On the other hand, sudden increases in putrescine, spermidine, and spermine concentration were higher and occurred earlier in pepper and lettuce, the most sensitive species, than in spinach and beetroot, the less sensitive ones. These increases tended to disappear after 24 h, except in lettuce. These changes would support the conclusion that ethylene and polyamine increases could be considered as a plant response to saline shock and related to the plant species sensitivity to this stress. In addition, no competition between polyamines and ethylene biosynthesis for their common precursor was observed.

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.

Comparison of NaCl-induced programmed cell death in the obligate halophyte Cakile maritima and the glycophyte Arabidopsis thaliana

Salinity represents one of the most important constraints that adversely affect plants growth and productivity. In this study, we aimed at determining possible differences between salt tolerant and salt sensitive species in early salt stress response. To this purpose, we subjected suspension-cultured cells from the halophyte Cakile maritima and the glycophyte Arabidopsis thaliana, two Brassicaceae, to salt stress and compared their behavior. In both species we could observe a time and dose dependent programmed cell death requiring an active metabolism, a dysfunction of mitochondria and caspase-like activation although C. maritima cells appeared less sensitive than A. thaliana cells. This capacity to mitigate salt stress could be due to a higher ascorbate pool that could allow C. maritima reducing the oxidative stress generated in response to NaCl. It further appeared that a higher number of C. maritima cultured cells when compared to A. thaliana could efficiently manage the Na(+) accumulation into the cytoplasm through non selective cation channels allowing also reducing the ROS generation and the subsequent cell death.

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.

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.

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

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

The effect of exogenous calcium on mitochondria, respiratory metabolism enzymes and ion transport in cucumber roots under hypoxia

Hypoxia induces plant stress, particularly in cucumber plants under hydroponic culture. In plants, calcium is involved in stress signal transmission and growth. The ultimate goal of this study was to shed light on the mechanisms underlying the effects of exogenous calcium on the mitochondrial antioxidant system, the activity of respiratory metabolism enzymes, and ion transport in cucumber (Cucumis sativus L. cv. Jinchun No. 2) roots under hypoxic conditions. Our experiments revealed that exogenous calcium reduces the level of reactive oxygen species (ROS) and increases the activity of antioxidant enzymes in mitochondria under hypoxia. Exogenous calcium also enhances the accumulation of enzymes involved in glycolysis and the tricarboxylic acid (TCA) cycle. We utilized fluorescence and ultrastructural cytochemistry methods to observe that exogenous calcium increases the concentrations of Ca(2+) and K(+) in root cells by increasing the activity of plasma membrane (PM) H(+)-ATPase and tonoplast H(+)-ATPase and H(+)-PPase. Overall, our results suggest that hypoxic stress has an immediate and substantial effect on roots. Exogenous calcium improves metabolism and ion transport in cucumber roots, thereby increasing hypoxia tolerance in cucumber.

Mechanisms underlying turgor regulation in the estuarine alga Vaucheria erythrospora (Xanthophyceae) exposed to hyperosmotic shock

Aquatic organisms are often exposed to dramatic changes in salinity in the environment. Despite decades of research, many questions related to molecular and physiological mechanisms mediating sensing and adaptation to salinity stress remain unanswered. Here, responses of Vaucheria erythrospora, a turgor-regulating xanthophycean alga from an estuarine habitat, have been investigated. The role of ion uptake in turgor regulation was studied using a single cell pressure probe, microelectrode ion flux estimation (MIFE) technique and membrane potential (Em ) measurements. Turgor recovery was inhibited by Gd(3+) , tetraethylammonium chloride (TEA), verapamil and orthovanadate. A NaCl-induced shock rapidly depolarized the plasma membrane while an isotonic sorbitol treatment hyperpolarized it. Turgor recovery was critically dependent on the presence of Na(+) but not K(+) and Cl(-) in the incubation media. Na(+) uptake was strongly decreased by amiloride and changes in net Na(+) and H(+) fluxes were oppositely directed. This suggests active uptake of Na(+) in V. erythrospora mediated by an antiport Na(+) /H(+) system, functioning in the direction opposite to that of the SOS1 exchanger in higher plants. The alga also retains K(+) efficiently when exposed to high NaCl concentrations. Overall, this study provides insights into mechanisms enabling V. erythrospora to regulate turgor via ion movements during hyperosmotic stress.

Regulation of potassium transport in plants under hostile conditions: implications for abiotic and biotic stress tolerance

Intracellular potassium homeostasis is a prerequisite for the optimal operation of plant metabolic machinery and plant's overall performance. It is controlled by K(+) uptake, efflux and intracellular and long-distance relocation, mediated by a large number of K(+) -selective and non-selective channels and transporters located at both plasma and vacuolar membranes. All abiotic and biotic stresses result in a significant disturbance to intracellular potassium homeostasis. In this work, we discuss molecular mechanisms and messengers mediating potassium transport and homeostasis focusing on four major environmental stresses: salinity, drought, flooding and biotic factors. We argue that cytosolic K(+) content may be considered as one of the 'master switches' enabling plant transition from the normal metabolism to 'hibernated state' during first hours after the stress exposure and then to a recovery phase. We show that all these stresses trigger substantial disturbance to K(+) homeostasis and provoke a feedback control on K(+) channels and transporters expression and post-translational regulation of their activity, optimizing K(+) absorption and usage, and, at the extreme end, assisting the programmed cell death. We discuss specific modes of regulation of the activity of K(+) channels and transporters by membrane voltage, intracellular Ca(2+) , reactive oxygen species, polyamines, phytohormones and gasotransmitters, and link this regulation with plant-adaptive responses to hostile environments.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plant's life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.

Identification of polyamine transporters in plants: paraquat transport provides crucial clues

Polyamine (PA) transport as well as PA biosynthesis, degradation and conjugation plays a vital role in the regulation of intracellular PA levels, which are essential for cell growth. Generally, PA uptake activity is elevated in rapidly proliferating cells. Previous studies showed that PA uptake in plant cells occurred via energy-dependent, protein-mediated transport systems. Numerous lines of evidence suggest that paraquat (PQ), one of the most widely used herbicides, is transported by the PA transport system in diverse organisms including plants. The PA/PQ transport interactions are proposed to be due to specific structural similarities between PA and PQ. The understanding of PA transport mechanisms has progressed in parallel with that of PQ transport, but the molecular identity of the plant PA/PQ transporter has remained an enigma. Recently, independent studies identified the L-type amino acid transporter (LAT) family transmembrane proteins as transporters of both PA and PQ. Arabidopsis LAT family proteins showed different subcellular localization properties, which suggested that these transporters were involved in intracellular PA trafficking and PA uptake across the plasma membrane. The identification of plant PA transporters is an important step in understanding the mechanism of PA homeostasis in plant cells. In this review, we highlight recent advances in the study of PA transport systems that are linked to the understanding of PQ translocation.

Polyamines control of cation transport across plant membranes: implications for ion homeostasis and abiotic stress signaling

Polyamines are unique polycationic metabolites, controlling a variety of vital functions in plants, including growth and stress responses. Over the last two decades a bulk of data was accumulated providing explicit evidence that polyamines play an essential role in regulating plant membrane transport. The most straightforward example is a blockage of the two major vacuolar cation channels, namely slow (SV) and fast (FV) activating ones, by the micromolar concentrations of polyamines. This effect is direct and fully reversible, with a potency descending in a sequence Spm(4+) > Spd(3+) > Put(2+). On the contrary, effects of polyamines on the plasma membrane (PM) cation and K(+)-selective channels are hardly dependent on polyamine species, display a relatively low affinity, and are likely to be indirect. Polyamines also affect vacuolar and PM H(+) pumps and Ca(2+) pump of the PM. On the other hand, catabolization of polyamines generates H2O2 and other reactive oxygen species (ROS), including hydroxyl radicals. Export of polyamines to the apoplast and their oxidation there by available amine oxidases results in the induction of a novel ion conductance and confers Ca(2+) influx across the PM. This mechanism, initially established for plant responses to pathogen attack (including a hypersensitive response), has been recently shown to mediate plant responses to a variety of abiotic stresses. In this review we summarize the effects of polyamines and their catabolites on cation transport in plants and discuss the implications of these effects for ion homeostasis, signaling, and plant adaptive responses to environment.

Kinetics of xylem loading, membrane potential maintenance, and sensitivity of K(+) -permeable channels to reactive oxygen species: physiological traits that differentiate salinity tolerance between pea and barley

Salt sensitive (pea) and salt tolerant (barley) species were used to understand the physiological basis of differential salinity tolerance in crops. Pea plants were much more efficient in restoring otherwise depolarized membrane potential thereby effectively decreasing K(+) efflux through depolarization-activated outward rectifying potassium channels. At the same time, pea root apex was 10-fold more sensitive to physiologically relevant H2 O2 concentration and accumulated larger amounts of H2 O2 under saline conditions. This resulted in a rapid loss of cell viability in the pea root apex. Barley plants rapidly loaded Na(+) into the xylem; this increase was only transient, and xylem and leaf Na(+) concentration remained at a steady level for weeks. On the contrary, pea plants restricted xylem Na(+) loading during the first few days of treatment but failed to prevent shoot Na(+) elevation in the long term. It is concluded that superior salinity tolerance of barley plants compared with pea is conferred by at least three different mechanisms: (1) efficient control of xylem Na(+) loading; (2) efficient control of H2 O2 accumulation and reduced sensitivity of non-selective cation channels to H2 O2 in the root apex; and (3) higher energy saving efficiency, with less ATP spent to maintain membrane potential under saline conditions.

Deciphering early events involved in hyperosmotic stress-induced programmed cell death in tobacco BY-2 cells

Hyperosmotic stresses represent one of the major constraints that adversely affect plants growth, development, and productivity. In this study, the focus was on early responses to hyperosmotic stress- (NaCl and sorbitol) induced reactive oxygen species (ROS) generation, cytosolic Ca(2+) concentration ([Ca(2+)]cyt) increase, ion fluxes, and mitochondrial potential variations, and on their links in pathways leading to programmed cell death (PCD). By using BY-2 tobacco cells, it was shown that both NaCl- and sorbitol-induced PCD seemed to be dependent on superoxide anion (O2·(-)) generation by NADPH-oxidase. In the case of NaCl, an early influx of sodium through non-selective cation channels participates in the development of PCD through mitochondrial dysfunction and NADPH-oxidase-dependent O2·(-) generation. This supports the hypothesis of different pathways in NaCl- and sorbitol-induced cell death. Surprisingly, other shared early responses, such as [Ca(2+)]cyt increase and singlet oxygen production, do not seem to be involved in PCD.

Cross-talk between reactive oxygen species and polyamines in regulation of ion transport across the plasma membrane: implications for plant adaptive responses

Many stresses are associated with increased accumulation of reactive oxygen species (ROS) and polyamines (PAs). PAs act as ROS scavengers, but export of putrescine and/or PAs to the apoplast and their catabolization by amine oxidases gives rise to H2O2 and other ROS, including hydroxyl radicals ((•)OH). PA catabolization-based signalling in apoplast is implemented in plant development and programmed cell death and in plant responses to a variety of biotic and abiotic stresses. Central to ROS signalling is the induction of Ca(2+) influx across the plasma membrane. Different ion conductances may be activated, depending on ROS, plant species, and tissue. Both H2O2 and (•)OH can activate hyperpolarization-activated Ca(2+)-permeable channels. (•)OH is also able to activate both outward K(+) current and weakly voltage-dependent conductance (ROSIC), with a variable cation-to-anion selectivity and sensitive to a variety of cation and anion channel blockers. Unexpectedly, PAs potentiated (•)OH-induced K(+) efflux in vivo, as well as ROSIC in isolated protoplasts. This synergistic effect is restricted to the mature root zone and is more pronounced in salt-sensitive cultivars compared with salt-tolerant ones. ROS and PAs suppress the activity of some constitutively expressed K(+) and non-selective cation channels. In addition, both (•)OH and PAs activate plasma membrane Ca(2+)-ATPase and affect H(+) pumping. Overall, (•)OH and PAs may provoke a substantial remodelling of cation and anion conductance at the plasma membrane and affect Ca(2+) signalling.

An Arabidopsis soil-salinity-tolerance mutation confers ethylene-mediated enhancement of sodium/potassium homeostasis

High soil Na concentrations damage plants by increasing cellular Na accumulation and K loss. Excess soil Na stimulates ethylene-induced soil-salinity tolerance, the mechanism of which we here define via characterization of an Arabidopsis thaliana mutant displaying transpiration-dependent soil-salinity tolerance. This phenotype is conferred by a loss-of-function allele of ethylene overproducer1 (ETO1; mutant alleles of which cause increased production of ethylene). We show that lack of ETO1 function confers soil-salinity tolerance through improved shoot Na/K homeostasis, effected via the ethylene resistant1-constitutive triple response1 ethylene signaling pathway. Under transpiring conditions, lack of ETO1 function reduces root Na influx and both stelar and xylem sap Na concentrations, thereby restricting root-to-shoot delivery of Na. These effects are associated with increased accumulation of respiratory burst oxidase homolog F (RBOHF)-dependent reactive oxygen species in the root stele. Additionally, lack of ETO1 function leads to significant enhancement of tissue K status by an RBOHF-independent mechanism associated with elevated high-affinity K(+) TRANSPORTER5 transcript levels. We conclude that ethylene promotes soil-salinity tolerance via improved Na/K homeostasis mediated by RBOHF-dependent regulation of Na accumulation and RBOHF-independent regulation of K accumulation.

Salicylic acid improves salinity tolerance in Arabidopsis by restoring membrane potential and preventing salt-induced K+ loss via a GORK channel

Despite numerous reports implicating salicylic acid (SA) in plant salinity responses, the specific ionic mechanisms of SA-mediated adaptation to salt stress remain elusive. To address this issue, a non-invasive microelectrode ion flux estimation technique was used to study kinetics of NaCl-induced net ion fluxes in Arabidopsis thaliana in response to various SA concentrations and incubation times. NaCl-induced K(+) efflux and H(+) influx from the mature root zone were both significantly decreased in roots pretreated with 10-500 μM SA, with strongest effect being observed in the 10-50 μM SA range. Considering temporal dynamics (0-8-h SA pretreatment), the 1-h pretreatment was most effective in enhancing K(+) retention in the cytosol. The pharmacological, membrane potential, and shoot K(+) and Na(+) accumulation data were all consistent with the model in which the SA pretreatment enhanced activity of H(+)-ATPase, decreased NaCl-induced membrane depolarization, and minimized NaCl-induced K(+) leakage from the cell within the first hour of salt stress. In long-term treatments, SA increased shoot K(+) and decreased shoot Na(+) accumulation. The short-term NaCl-induced K(+) efflux was smallest in the gork1-1 mutant, followed by the rbohD mutant, and was highest in the wild type. Most significantly, the SA pretreatment decreased the NaCl-induced K(+) efflux from rbohD and the wild type to the level of gork1-1, whereas no effect was observed in gork1-1. These data provide the first direct evidence that the SA pretreatment ameliorates salinity stress by counteracting NaCl-induced membrane depolarization and by decreasing K(+) efflux via GORK channels.

Cloning and characterization of TaSnRK2.3, a novel SnRK2 gene in common wheat

Environmental stresses such as drought, salinity, and cold are major adverse factors that significantly affect agricultural productivity. Protein phosphorylation/dephosphorylation is a major signalling event induced by osmotic stress in higher plants. Sucrose non-fermenting 1-related protein kinase 2 (SnRK2) family members play essential roles in the response to hyperosmotic stresses in plants. In this study, the TaSnRK2.3 gene, a novel SnRK2 member was cloned, and three copies located on chromosomes 1A, 1B, and 1D were identified in common wheat. TaSnRK2.3 was strongly expressed in leaves, and responded to polyethylene glycol, NaCl, abscisic acid, and cold stresses. To characterize its function, transgenic Arabidopsis overexpressing TaSnRK2.3-GFP controlled by the cauliflower mosaic virus 35S promoter was generated and subjected to severe abiotic stresses. Overexpression of TaSnRK2.3 resulted in an improved root system and significantly enhanced tolerance to drought, salt, and freezing stresses, simultaneously demonstrated by enhanced expression of abiotic stress-responsive genes and ameliorative physiological indices, including a decreased rate of water loss, enhanced cell membrane stability, improved photosynthetic potential, and significantly increased osmotic potential and free proline content under normal and/or stressed conditions. These results demonstrate that TaSnRK2.3 is a multifunctional regulator, with potential for utilization in transgenic breeding for improved abiotic stress tolerance in crop plants.

Ion flux measurements using the MIFE technique

Noninvasive microelectrode ion flux measurements (the MIFE™ technique) allow the concurrent quantification of net fluxes of several ions with high spatial (several μm) and temporal (ca 5 s) resolution. The MIFE technique has become a popular tool for studying the adaptive responses of plant cells and tissues to a large number of abiotic and biotic stresses. This chapter briefly summarizes some key findings on spatial and temporal organization of plant nutrient acquisition obtained by the MIFE technique, as well as the MIFE contribution towards elucidating the mechanisms behind a plant's perception and signaling of major abiotic stresses. The full protocols for microelectrode fabrication, calibration, and use are then given, and two basic routines for mapping root ion flux profiles and studying transient ion flux kinetics are given.

Synergism between polyamines and ROS in the induction of Ca ( 2+) and K (+) fluxes in roots

Stress conditions cause increases in ROS and polyamines levels, which are not merely collateral. There is increasing evidence for the ROS participation in signaling as well as for polyamine protective roles under stress. Polyamines and ROS, respectively, inhibit cation channels and induce novel cation conductance in the plasma membrane. Our new results indicate that polyamines and OH (•) also stimulate Ca ( 2+) pumping across the root plasma membrane. Besides, polyamines potentiate the OH (•) -induced non-selective current and respective passive K (+) and Ca ( 2+) fluxes. In roots this synergism, however, is restricted to the mature zone, whereas in the distal elongation zone only the Ca ( 2+) pump activation is observed. Remodeling the plasma membrane ion conductance by OH (•) and polyamines would impact K (+) homeostasis and Ca ( 2+) signaling under stress.

Natural variation in a polyamine transporter determines paraquat tolerance in Arabidopsis

Polyamines (PAs) are ubiquitous, polycationic compounds that are essential for the growth and survival of all organisms. Although the PA-uptake system plays a key role in mammalian cancer and in plant survival, the underlying molecular mechanisms are not well understood. Here, we identified an Arabidopsis L-type amino acid transporter (LAT) family transporter, named RMV1 (resistant to methyl viologen 1), responsible for uptake of PA and its analog paraquat (PQ). The natural variation in PQ tolerance was determined in 22 Arabidopsis thaliana accessions based on the polymorphic variation of RMV1. An RMV1-GFP fusion protein localized to the plasma membrane in transformed cells. The Arabidopsis rmv1 mutant was highly resistant to PQ because of the reduction of PQ uptake activity. Uptake studies indicated that RMV1 mediates proton gradient-driven PQ transport. RMV1 overexpressing plants were hypersensitive to PA and PQ and showed elevated PA/PQ uptake activity, supporting the notion that PQ enters plant cells via a carrier system that inherently functions in PA transport. Furthermore, we demonstrated that polymorphic variation in RMV1 controls PA/PQ uptake activity. Our identification of a molecular entity for PA/PQ uptake and sensitivity provides an important clue for our understanding of the mechanism and biological significance of PA uptake.

Quantifying kinetics of net ion fluxes from plant tissues by non-invasive microelectrode measuring MIFE technique

Non-invasive microelectrode ion flux measuring (the MIFE system) allows concurrent quantification of net fluxes of several ions with high spatial (several μm) and temporal (ca 5 s) resolution. Over the last 10 years, the MIFE system has been widely used to study various aspects of salt stress signaling and adaptation in plants. This chapter summarizes some major findings in the area such as using MIFE for deciphering the specific and non-specific components of salinity stress, resolving the role of the plasma membrane H(+)-pump in salinity responses, proving K(+) homeostasis as a key feature of salinity tolerance, and discovering the mechanisms behind the ameliorative effects of Ca(2+) and other mitigating factors (such as polyamines or compatible solutes). The full protocols for microelectrode fabrication, calibration, and use are then given, and two basic routines for measuring net K(+) and Na(+) fluxes from salinity stressed roots are described in the context of plant screening for salt stress tolerance.

Calcium-sensing receptor regulates stomatal closure through hydrogen peroxide and nitric oxide in response to extracellular calcium in Arabidopsis

The Arabidopsis calcium-sensing receptor CAS is a crucial regulator of extracellular calcium-induced stomatal closure. Free cytosolic Ca(2+) (Ca(2+)(i)) increases in response to a high extracellular calcium (Ca(2+)(o)) level through a CAS signalling pathway and finally leads to stomatal closure. Multidisciplinary approaches including histochemical, pharmacological, fluorescent, electrochemical, and molecular biological methods were used to discuss the relationship of hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) signalling in the CAS signalling pathway in guard cells in response to Ca(2+)(o). Here it is shown that Ca(2+)(o) could induce H(2)O(2) and NO production from guard cells but only H(2)O(2) from chloroplasts, leading to stomatal closure. In addition, the CASas mutant, the atrbohD/F double mutant, and the Atnoa1 mutant were all insensitive to Ca(2+)(o)-stimulated stomatal closure, as well as H(2)O(2) and NO elevation in the case of CASas. Furthermore, it was found that the antioxidant system might function as a mediator in Ca(2+)(o) and H(2)O(2) signalling in guard cells. The results suggest a hypothetical model whereby Ca(2+)(o) induces H(2)O(2) and NO accumulation in guard cells through the CAS signalling pathway, which further triggers Ca(2+)(i) transients and finally stomatal closure. The possible cross-talk of Ca(2+)(o) and abscisic acid signalling as well as the antioxidant system are discussed.

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.