Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition

Salinity tolerance of Arabidopsis: a good model for cereals?

Arabidopsis is a glycophyte species that is sensitive to moderate levels of NaCl. Arabidopsis offers unique benefits to genetic and molecular research and has provided much information about both Na(+) transport processes and Na(+) tolerance. A compilation of data available on Na(+) accumulation and Na(+) tolerance in Arabidopsis is presented, and comparisons are made with several crop plant species. The relationship between Na(+) tolerance and Na(+) accumulation is different in Arabidopsis and cereals, with an inverse relationship often found within cereal species that is not as evident in Arabidopsis ecotypes. Results on salinity tolerance obtained in Arabidopsis should therefore be extrapolated to cereals with caution. Arabidopsis remains a useful model to study and discover plant Na(+) transport processes.

Overexpression of a putative maize calcineurin B-like protein in Arabidopsis confers salt tolerance

The calcineurin B-like proteins (CBLs) represent a unique family of calcium sensors in plants. Although extensive studies and remarkable progress have been made in Arabidopsis (Arabidopsis thaliana) CBLs, their functions in other plant species are still quite limited. Here, we report the cloning and functional characterization of ZmCBL4, a novel CBL gene from maize (Zea mays). ZmCBL4 encodes a putative homolog of the Arabidopsis CBL4/SOS3 protein, with novel properties. ZmCBL4 has one copy in maize genome and harbors seven introns in its coding region. ZmCBL4 expressed differentially in various organs of the maize plants at a low level under normal condition, and its expression was regulated by NaCl, LiCl, ABA and PEG treatments. Expression of 35S::ZmCBL4 not only complemented the salt hypersensitivity in Arabidopsis sos3 mutant, but also enhanced the salt tolerance in Arabidopsis wild type at the germination and seedling stages. Moreover, the LiCl tolerance in all of the ZmCBL4-expressing lines increased more significantly as compared with the NaCl tolerance, and in consistent with this, it was found that the expression of Arabidopsis AtNHX8, a putative plasma membrane Li+/H+ antiporter gene identified recently, was induced in these transgenic lines under LiCl stress. The ZmCBL4-expressing Arabidopsis lines accumulated less Na+ and Li+ as compared with the control plants. This study has identified a putative maize CBL gene which functions in the salt stress-elicited calcium signaling and thus in the tolerance to salinity.

NADPH oxidase-dependent hydrogen peroxide production, induced by salinity stress, may be involved in the regulation of total calcium in roots of wheat

Hydrogen peroxide (H(2)O(2)) is often generated by cells and tissues under environmental stress. In this work, we provide evidence that plasma membrane (PM) NADPH oxidase-dependent H(2)O(2) production might act as an intermediate step in the NaCl-induced elevation of calcium (Ca) in roots of wheat. Remarkable increases in the content of total Ca were observed not only in roots exposed to NaCl but also in roots of seedlings exposed to exogenous H(2)O(2). In roots, H(2)O(2) production increased upon exposure to salt stress. PM vesicles were isolated from roots, and NADPH oxidase activity was determined by measuring superoxide anion (O(2)(-)) production. NADPH oxidase-dependent O(2)(-) production was 11.6nmolmg(-1)proteinmin(-1) in control vesicles, but 19.6nmol after NaCl treatment (24h), indicating that salt stress resulted in the activation of the PM NADPH oxidase. Furthermore, the NaCl-induced increase in total Ca was partially abolished by the addition of 150U/mL catalase (CAT), a H(2)O(2) scavenger, and also by 10microM diphenylane iodonium (DPI), a NADPH oxidase inhibitor. This data suggest that NADPH oxidase-dependent H(2)O(2) production might be involved in the modulation of the Ca content in wheat roots. In conclusion, our results show that salinity stress increases the total Ca content of wheat roots, which is partly due to PM NADPH oxidase-dependent H(2)O(2) generation.

Isolation and characterization of shs1, a sugar-hypersensitive and ABA-insensitive mutant with multiple stress responses

To identify salt tolerance determinants, we screened for double mutants from a T-DNA tagged sos3-1 mutant population in the Arabidopsis Col-0 gl1 background. The shs1-1 (sodium hypersensitive) sos3-1 mutant was isolated as more sensitive to NaCl than sos3-1 plants. TAIL-PCR revealed that the introduced T-DNA was located 62 bp upstream of the initiation codon of an adenylate translocator-like protein gene on chromosome IV. SHS1 mRNA did not accumulate in shs1-1 sos3-1 plants although it accumulated in shoots of both sos3-1 and the wild type plants, indicating that this gene is inactive in the mutant. Genetic co-linkage analysis revealed that the mutation causing the phenotype segregated as a recessive, single gene mutation. This mutant showed altered sensitive responses to salt as well as to cold stress. It also demonstrated sugar sensitive and ABA insensitive phenotypes including enhanced germination, reduced growth, altered leaf morphology, and necrosis on leaves at an early growth stage. Sensitivity of sos3-1 shs1-1 root growth to LiCl, KCl, and mannitol was not significantly different from growth of sos3-1 roots. Further, expression of 35S::SHS1 in sos3-1 shs1-1 plants complemented NaCl and sugar sensitivity and partially restored the leaf morphology.

SOS2 promotes salt tolerance in part by interacting with the vacuolar H+-ATPase and upregulating its transport activity

The salt overly sensitive (SOS) pathway is critical for plant salt stress tolerance and has a key role in regulating ion transport under salt stress. To further investigate salt tolerance factors regulated by the SOS pathway, we expressed an N-terminal fusion of the improved tandem affinity purification tag to SOS2 (NTAP-SOS2) in sos2-2 mutant plants. Expression of NTAP-SOS2 rescued the salt tolerance defect of sos2-2 plants, indicating that the fusion protein was functional in vivo. Tandem affinity purification of NTAP-SOS2-containing protein complexes and subsequent liquid chromatography-tandem mass spectrometry analysis indicated that subunits A, B, C, E, and G of the peripheral cytoplasmic domain of the vacuolar H+-ATPase (V-ATPase) were present in a SOS2-containing protein complex. Parallel purification of samples from control and salt-stressed NTAP-SOS2/sos2-2 plants demonstrated that each of these V-ATPase subunits was more abundant in NTAP-SOS2 complexes isolated from salt-stressed plants, suggesting that the interaction may be enhanced by salt stress. Yeast two-hybrid analysis showed that SOS2 interacted directly with V-ATPase regulatory subunits B1 and B2. The importance of the SOS2 interaction with the V-ATPase was shown at the cellular level by reduced H+ transport activity of tonoplast vesicles isolated from sos2-2 cells relative to vesicles from wild-type cells. In addition, seedlings of the det3 mutant, which has reduced V-ATPase activity, were found to be severely salt sensitive. Our results suggest that regulation of V-ATPase activity is an additional key function of SOS2 in coordinating changes in ion transport during salt stress and in promoting salt tolerance.

Interaction of SOS2 with nucleoside diphosphate kinase 2 and catalases reveals a point of connection between salt stress and H2O2 signaling in Arabidopsis thaliana

SOS2, a class 3 sucrose-nonfermenting 1-related kinase, has emerged as an important mediator of salt stress response and stress signaling through its interactions with proteins involved in membrane transport and in regulation of stress responses. We have identified additional SOS2-interacting proteins that suggest a connection between SOS2 and reactive oxygen signaling. SOS2 was found to interact with the H2O2 signaling protein nucleoside diphosphate kinase 2 (NDPK2) and to inhibit its autophosphorylation activity. A sos2-2 ndpk2 double mutant was more salt sensitive than a sos2-2 single mutant, suggesting that NDPK2 and H2O2 are involved in salt resistance. However, the double mutant did not hyperaccumulate H2O2 in response to salt stress, suggesting that it is altered signaling rather than H2O2 toxicity alone that is responsible for the increased salt sensitivity of the sos2-2 ndpk2 double mutant. SOS2 was also found to interact with catalase 2 (CAT2) and CAT3, further connecting SOS2 to H2O2 metabolism and signaling. The interaction of SOS2 with both NDPK2 and CATs reveals a point of cross talk between salt stress response and other signaling factors including H2O2.

Characterization of stress-responsive CIPK genes in rice for stress tolerance improvement

Plants respond to adverse environments by initiating a series of signaling processes that often involves diverse protein kinases, including calcineurin B-like protein-interacting protein kinases (CIPKs). In this study, putative CIPK genes (OsCIPK01-OsCIPK30) in the rice (Oryza sativa) genome were surveyed for their transcriptional responses to various abiotic stresses. The results showed that 20 OsCIPK genes were differentially induced by at least one of the stresses, including drought, salinity, cold, polyethylene glycol, and abscisic acid treatment. Most of the genes induced by drought or salt stress were also induced by abscisic acid treatment but not by cold. A few CIPK genes containing none of the reported stress-responsive cis-elements in their promoter regions were also induced by multiple stresses. To prove that some of these stress-responsive OsCIPK genes are potentially useful for stress-tolerance improvement, three CIPK genes (OsCIPK03, OsCIPK12, and OsCIPK15) were overexpressed in japonica rice 'Zhonghua 11'. Transgenic plants overexpressing the transgenes OsCIPK03, OsCIPK12, and OsCIPK15 showed significantly improved tolerance to cold, drought, and salt stress, respectively. Under cold and drought stresses, OsCIPK03- and OsCIPK12-overexpressing transgenic plants accumulated significantly higher contents of proline and soluble sugars than the wild type. Putative proline synthetase and transporter genes had significantly higher expression level in the transgenic plants than in the wild type. The differentially induced expression of OsCIPK genes by different stresses and the examples of improved stress tolerance of the OsCIPK transgenic rice suggest that rice CIPK genes have diverse roles in different stress responses and some of them may possess potential usefulness in stress-tolerance improvement of rice.

Potential Role of Annexin AnnAt1 from Arabidopsis thaliana in pH-Mediated Cellular Response to Environmental Stimuli

Plant annexins, Ca(2+)- and membrane-binding proteins, are probably implicated in the cellular response to stress resulting from acidification of cytosol. To understand how annexins can contribute to cellular ion homeostasis, we investigated the pH-induced changes in the structure and function of recombinant annexin AnnAt1 from Arabidopsis thaliana. The decrease of pH from 7.0 to 5.8 reduced the time of the formation of ion channels by AnnAt1 in artificial lipid membranes from 3.5 h to 15-20 min and increased their unitary conductance from 32 to 63 pS. These changes were accompanied by an increase in AnnAt1 hydrophobicity as revealed by hydrophobicity predictions, by an increase in fluorescence of 2-(p-toluidino)naphthalene-6-sulfonic acid (TNS) bound to AnnAt1 and fluorescence resonance energy transfer from AnnAt1 tryptophan residues to TNS. Concomitant lipid partition of AnnAt1 at acidic pH resulted in its partial protection from proteolytic digestion. Secondary structures of AnnAt1 determined by circular dichroism and infrared spectroscopy were also affected by lowering the pH from 7.2 to 5.2. These changes were characterized by an increase in beta-sheet content at the expense of alpha-helical structures, and were accompanied by reversible formation of AnnAt1 oligomers as probed by ultracentrifugation in a sucrose gradient. A further decrease of pH from 5.2 to 4.5 or lower led to the formation of irreversible aggregates and loss of AnnAt1 ionic conductance. Our findings suggest that AnnAt1 can sense changes of the pH milieu over the pH range from 7 to 5 and respond by changes in ion channel conductance, hydrophobicity, secondary structure of the protein and formation of oligomers. Further acidification irreversibly inactivated AnnAt1. We suggest that the pH-sensitive ion channel activity of AnnAt1 may play a role in intracellular ion homeostasis.

SCABP8/CBL10, a putative calcium sensor, interacts with the protein kinase SOS2 to protect Arabidopsis shoots from salt stress

The SOS (for Salt Overly Sensitive) pathway plays essential roles in conferring salt tolerance in Arabidopsis thaliana. Under salt stress, the calcium sensor SOS3 activates the kinase SOS2 that positively regulates SOS1, a plasma membrane sodium/proton antiporter. We show that SOS3 acts primarily in roots under salt stress. By contrast, the SOS3 homolog SOS3-LIKE CALCIUM BINDING PROTEIN8 (SCABP8)/CALCINEURIN B-LIKE10 functions mainly in the shoot response to salt toxicity. While root growth is reduced in sos3 mutants in the presence of NaCl, the salt sensitivity of scabp8 is more prominent in shoot tissues. SCABP8 is further shown to bind calcium, interact with SOS2 both in vitro and in vivo, recruit SOS2 to the plasma membrane, enhance SOS2 activity in a calcium-dependent manner, and activate SOS1 in yeast. In addition, sos3 scabp8 and sos2 scabp8 display a phenotype similar to sos2, which is more sensitive to salt than either sos3 or scabp8 alone. Overexpression of SCABP8 in sos3 partially rescues the sos3 salt-sensitive phenotype. However, overexpression of SOS3 fails to complement scabp8. These results suggest that SCABP8 and SOS3 are only partially redundant in their function, and each plays additional and unique roles in the plant salt stress response.

Potassium and sodium relations in salinised barley tissues as a basis of differential salt tolerance

A large-scale glasshouse trial, including nearly 70 barley cultivars (5300 plants in total), was conducted over 2 consecutive years to investigate plant physiological responses to salinity. In a parallel set of experiments, plant salt tolerance was assessed by non-invasive microelectrode measurements of net K⁺ flux from roots of 3-day-old seedlings of each cultivar after 1 h treatment in 80 mm NaCl as described in our previous publication (Chen et al. 2005). K⁺ flux from the root in response to NaCl treatment was highly (P < 0.001) inversely correlated with relative grain yield, shoot biomass, plant height, net CO₂ assimilation, survival rate and thousand-seed weight measured in glasshouse experiments after 4-5 months of salinity treatment. No significant correlation with relative germination rate or tillering was found. In general, 62 out of 69 cultivars followed an inverse relationship between K⁺ efflux and salt tolerance. In a few cultivars, however, high salt tolerance (measured as grain yield at harvest) was observed for plants showing only modest ability to retain K⁺ in the root cells. Tissue elemental analysis showed that these plants had a much better ability to prevent Na⁺ accumulation in plant leaves and, thus, to maintain a higher K⁺/Na⁺ ratio. Taken together, our results show that a plant's ability to maintain high K⁺/Na⁺ ratio (either retention of K⁺ or preventing Na⁺ from accumulating in leaves) is a key feature for salt tolerance in barley.

A plastid-localized glycogen synthase kinase 3 modulates stress tolerance and carbohydrate metabolism

Glycogen synthase kinase 3 (GSK-3) was originally identified as a regulator of glycogen synthesis in mammals. Like starch in plants, glycogen is a polymer of glucose, and serves as an energy and carbon store. Starch is the main carbohydrate store in plants. Regulation of starch metabolism, in particular in response to environmental cues, is of primary importance for carbon and energy flow in plants but is still obscure. Here, we provide evidence that MsK4, a novel Medicago sativa GSK-3-like kinase, connects stress signalling with carbon metabolism. MsK4 was found to be a plastid-localized protein kinase that is associated with starch granules. High-salt stress rapidly induced the in vivo kinase activity of MsK4. Metabolic profiling of MsK4 over-expressor lines revealed changes in sugar metabolism, including increased amounts of maltose, the main degradation product of starch in leaves. Plants over-expressing MsK4 showed improved tolerance to salt stress. Moreover, under high-salinity conditions, MsK4-over-expressing plants accumulated significantly more starch and showed modified carbohydrate content compared with wild-type plants. Overall, these data indicate that MsK4 is an important regulator that adjusts carbohydrate metabolism to environmental stress.

Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots

The pivotal role of glucose-6-phosphate dehydrogenase (G-6-PDH)-mediated nitric oxide (NO) production in the tolerance to oxidative stress induced by 100 mM NaCl in red kidney bean (Phaseolus vulgaris) roots was investigated. The results show that the G-6-PDH activity was enhanced rapidly in the presence of NaCl and reached a maximum at 100 mM. Western blot analysis indicated that the increase of G-6-PDH activity in the red kidney bean roots under 100 mM NaCl was mainly due to the increased content of the G-6-PDH protein. NO production and nitrate reductase (NR) activity were also induced by 100 mM NaCl. The NO production was reduced by NaN(3) (an NR inhibitor), but not affected by N(omega)-nitro-L-arginine (L-NNA) (an NOS inhibitor). Application of 2.5 mM Na(3)PO(4), an inhibitor of G-6-PDH, blocked the increase of G-6-PDH and NR activity, as well as NO production in red kidney bean roots under 100 mM NaCl. The activities of antioxidant enzymes in red kidney bean roots increased in the presence of 100 mM NaCl or sodium nitroprusside (SNP), an NO donor. The increased activities of all antioxidant enzymes tested at 100 mM NaCl were completely inhibited by 2.5 mM Na(3)PO(4). Based on these results, we conclude that G-6-PDH plays a pivotal role in NR-dependent NO production, and in establishing tolerance of red kidney bean roots to salt stress.

AtNHX8, a member of the monovalent cation: proton antiporter-1 family in Arabidopsis thaliana, encodes a putative Li/H antiporter

The Arabidopsis monovalent cation:proton antiporter-1 (CPA1) family includes eight members, AtNHX1-8. AtNHX1 and AtNHX7/SOS1 have been well characterized as tonoplast and plasma membrane Na+/H+ antiporters, respectively. The proteins AtNHX2-6 have been phylogenetically linked to AtNHX1, while AtNHX8 appears to be related to AtNHX7/SOS1. Here we report functional characterization of AtNHX8. AtNHX8 T-DNA insertion mutants are hypersensitive to lithium ions (Li+) relative to wild-type plants, but not to the other metal ions such as sodium (Na+), potassium (K+) and caesium (Cs+). AtNHX8 overexpression in a triple-deletion yeast mutant AXT3 that exhibits defective Na+/Li+ transport specifically suppresses sensitivity to Li+, but does not affect Na+ sensitivity. Likewise, AtNHX8 overexpression complemented sensitivity to Li+, but not Na+, in sos1-1 mutant seedlings, and increased Li+ tolerance of both the sos1-1 mutant and wild-type seedlings. Results of Li+ and K+ measurement of loss-of-function and gain-of-function mutants indicate that AtNHX8 may be responsible for Li+ extrusion, and may be able to maintain K+ acquisition and intracellular ion homeostasis. Subcellular localization of the AtNHX8-enhanced green fluorescent protein (EGFP) fusion protein suggested that AtNHX8 protein is targeted to the plasma membrane. Taken together, our findings suggest that AtNHX8 encodes a putative plasma membrane Li+/H+ antiporter that functions in Li detoxification and ion homeostasis in Arabidopsis.

Antioxidative response mechanisms in halophytes: Their role in stress defence

Normal growth and development of plants is greatly dependent on the capacity to overcome environmental stresses. Environmental stress conditions like high salinity, drought, high incident light and low or high temperature cause major crop losses worldwide. A common denominator in all these adverse conditions is the production of reactive oxygen species (ROS) within different cellular compartments of the plant cell. Plants have developed robust mechanisms including enzymatic or nonenzymatic scavenging pathways to counter the deleterious effects of ROS production. There are a number of general reviews on oxidative stress in plants and few on the role of ROS scavengers during stress conditions. Here we review the regulation of antioxidant enzymes during salt stress in halophytes, especially mangroves. We conclude that (i) antioxidant enzymes protect halophytes from deleterious ROS production during salt stress, and (ii) genetic information from mangroves and other halophytes would be helpful in defining the roles of individual isoforms. This information would be critical in using the appropriate genes for oxidative stress defence for genetic engineering of enhanced stress tolerance in crop systems.

The cytosolic Na+ : K+ ratio does not explain salinity-induced growth impairment in barley: a dual-tracer study using 42K+ and 24Na+

It has long been believed that maintenance of low Na+ : K+ ratios in the cytosol of plant cells is critical to the plant's ability to tolerate salinity stress. Direct measurements of such ratios, however, have been few. Here we apply the non-invasive technique of compartmental analysis, using the short-lived radiotracers 42K+ and 22Na+, in intact seedlings of barley (Hordeum vulgare L.), to evaluate unidirectional plasma membrane fluxes and cytosolic concentrations of K+ and Na+ in root tissues, under eight nutritional conditions varying in levels of salinity and K+ supply. We show that Na+ : K+ ratios in the cytosol of root cells adjust significantly across the conditions tested, and that these ratios are poor predictors of the plant's growth response to salinity. Our study further demonstrates that Na+ is subject to rapid and futile cycling at the plasma membrane at all levels of Na+ supply, independently of external K+, while K+ influx is reduced by Na+, from a similar baseline, and to a similar extent, at both low and high K+ supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na+ : K+ ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na+ and K+.

Deciphering the regulatory mechanisms of abiotic stress tolerance in plants by genomic approaches

Environmental constraints that include abiotic stress factors such as salt, drought, cold and extreme temperatures severely limit crop productivity. Improvement of crop plants with traits that confer tolerance to these stresses was practiced using traditional and modern breeding methods. Molecular breeding and genetic engineering contributed substantially to our understanding of the complexity of stress response. Mechanisms that operate signal perception, transduction and downstream regulatory factors are now being examined and an understanding of cellular pathways involved in abiotic stress responses provide valuable information on such responses. This review presents genomic-assisted methods which have helped to reveal complex regulatory networks controlling abiotic stress tolerance mechanisms by high-throughput expression profiling and gene inactivation techniques. Further, an account of stress-inducible regulatory genes which have been transferred into crop plants to enhance stress tolerance is discussed as possible modes of integrating information gained from functional genomics into knowledge-based breeding programs. In addition, we envision an integrative genomic and breeding approach to reveal developmental programs that enhance yield stability and improve grain quality under unfavorable environmental conditions of abiotic stresses.

Functional analysis of a calcium-binding transcription factor involved in plant salt stress signaling

Calcium is known to serve as a secondary messenger to mediate salt stress signaling pathway. We found a calcium-binding basic/helix-loop-helix-type transcription factor (AtNIG1) as a salt stress-responsive gene by using the suppression subtractive hybridization. The AtNIG1 was targeted into nucleus and bound to (45)Ca(2+), suggesting that AtNIG1 is a nuclear calcium-binding protein. In addition, AtNIG1 bound specifically to the E-box-DNA sequence (CANNTG), which is found in the promoter regions of many salt stress-related genes. Functional analyses with an atnig1-1 knockout mutant revealed that the mutant plants show enhanced sensitivity to salt stress. Further analyses indicated that the atnig1-1 plants have reduced survival rate, fresh weight, chlorophyll content, and protein content upon salt stress, suggesting that the AtNIG1 plays a critical role in plant salt stress signaling. Therefore, this study represents that AtNIG1 is the first known calcium-binding transcription factor involved in plant salt stress signaling.

Molecular cloning and stress-dependent regulation of potassium channel gene in Chinese cabbage (Brassica rapa ssp. Pekinensis)

Potassium channels are important for many physiological functions in plants, one of which is to regulate plant adaption to stress conditions. In this study, KCT2, the gene encoding a membrane-bound protein potassium channel (GenBank accession number: ), was isolated from Chinese cabbage (Brassica rapa ssp. Pekinensis) by RACE-PCR technique. Bioinformatics methods were performed for the gene structure and molecular similarity analysis. The KCT2 expression patterns under various stress conditions were studied by semi-quantitative RT-PCR. DNA gel blot was used to analyze genomic organization. The putative KCT2 was found to contain five membrane-spanning segments, a pore-forming domain (P-domain) between the last two transmembrane spans, a TxxTxGYGD motif in the P-domain and a putative cyclic nucleotide-binding-like domain within a long C-terminal region. KCT2 is closest to KAT2 in Arabidopsis. KCT2 could be a one-copy gene with different isoforms or belong to a small gene family with four or five members. KCT2 was expressed more strongly in leaves than in shoots and roots. KCT2 transcription products were up-regulated by a 4-h-incubation in abscisic acid (ABA) and various stress treatment including cold stress (4 degrees C) for 24 h, drought stress for 1h, and salt stress for 12 h. KCT2 transcription was not affected by anoxia stress for 8h and was down-regulated with cold stress for 48 h. KCT2 was cloned for the first time from the genus Brassica. Expression analysis indicated that in the early stage of plant adaption to stress conditions KCT2 is up-regulated, which results in a stimulation of potassium transport.

Expression and functional analysis of the rice plasma-membrane intrinsic protein gene family

Plasma membrane intrinsic proteins (PIPs) are a subfamily of aquaporins that enable fast and controlled translocation of water across the membrane. In this study, we systematically identified and cloned ten PIP genes from rice. Based on the similarity of the amino acid sequences they encoded, these rice PIP genes were classified into two groups and designated as OsPIP1-1 to OsPIP1-3 and OsPIP2-1 to OsPIP2-7 following the nomenclature of PIP genes in maize. Quantitative RT-PCR analysis identified three root-specific and one leaf-specific OsPIP genes. Furthermore, the expression profile of each OsPIP gene in response to salt, drought and ABA treatment was examined in detail. Analysis on transgenic plants over-expressing of either OsPIP1 (OsPIP1-1) or OsPIP2 (OsPIP2-2) in wild-type Arabidopsis, showed enhanced tolerance to salt (100 mM of NaCl) and drought (200 mM of mannitol), but not to salt treatment of higher concentration (150 mM of NaCl). Taken together, these data suggest a distinct role of each OsPIP gene in response to different stresses, and should add a new layer to the understanding of the physiological function of rice PIP genes.

CASAR82A, a pathogen-induced pepper SAR8.2, exhibits an antifungal activity and its overexpression enhances disease resistance and stress tolerance

Pepper SAR8.2 gene (CASAR82A) was previously reported to be locally or systemically induced in pepper plants by biotic and abiotic stresses. In this study, the physiological and molecular functions of the pepper SAR8.2 protein in the plant defense responses were investigated by generating Arabidopsis transgenic lines overexpressing the CASAR82A gene. The transgenic Arabidopsis plants grew faster than the wild-type plants, indicating that the CASAR82A gene was involved in plant development. The ectopic expression of CASAR82A in Arabidopsis was accompanied by the expression of the Arabidopsis pathogenesis-related (PR)-genes including AtPR-1, AtPR-4 and AtPR-5. CASAR82A overexpression enhanced the resistance against infections by Pseudomonas syringae pv. tomato, Fusarium oxysporum f.sp. matthiolae or Botrytis cinerea. The transgenic plants also exhibited increased NaCl and drought tolerance during all growth stages. Moreover, the methyl viologen test showed that the transgenic plants were tolerant to oxidative stress. The purified recombinant CASAR82A protein and crude protein extracts of the transgenic plants exhibited antifungal activity against some phytopathogenic fungi, indicating that the enhanced resistance of the transgenic plants to fungal pathogen infection may be due to the antifungal effect of SAR8.2 protein.

Calcium Regulation of Sodium Hypersensitivities of sos3 and athkt1 Mutants

T-DNA disruption mutations in the AtHKT1 gene have previously been shown to suppress the salt sensitivity of the sos3 mutant. However, both sos3 and athkt1 single mutants show sodium (Na+) hypersensitivity. In the present study we further analyzed the underlying mechanisms for these non-additive and counteracting Na+ sensitivities by characterizing athkt1-1 sos3 and athkt1-2 sos3 double mutant plants. Unexpectedly, mature double mutant plants grown in soil clearly showed an increased Na+ hypersensitivity compared with wild-type plants when plants were subjected to salinity stress. The salt sensitive phenotype of athkt1 sos3 double mutant plants was similar to that of athkt1 plants, which showed chlorosis in leaves and stems. The Na+ content in xylem sap samples of soil-grown athkt1 sos3 double and athkt1 single mutant plants showed dramatic Na+ overaccumulation in response to salinity stress. Salinity stress analyses using basic minimal nutrient medium and Murashige-Skoog (MS) medium revealed that athkt1 sos3 double mutant plants show a more athkt1 single mutant-like phenotype in the presence of 3 mM external Ca2+, but show a more sos3 single mutant-like phenotype in the presence of 1 mM external Ca2+. Taken together multiple analyses demonstrate that the external Ca2+ concentration strongly impacts the Na+ stress response of athkt1 sos3 double mutants. Furthermore, the presented findings show that SOS3 and AtHKT1 are physiologically distinct major determinants of salinity resistance such that sos3 more strongly causes Na+ overaccumulation in roots, whereas athkt1 causes an increase in Na+ levels in the xylem sap and shoots and a concomitant Na+ reduction in roots.

Methods and concepts in quantifying resistance to drought, salt and freezing, abiotic stresses that affect plant water status

The abiotic stresses of drought, salinity and freezing are linked by the fact that they all decrease the availability of water to plant cells. This decreased availability of water is quantified as a decrease in water potential. Plants resist low water potential and related stresses by modifying water uptake and loss to avoid low water potential, accumulating solutes and modifying the properties of cell walls to avoid the dehydration induced by low water potential and using protective proteins and mechanisms to tolerate reduced water content by preventing or repairing cell damage. Salt stress also alters plant ion homeostasis, and under many conditions this may be the predominant factor affecting plant performance. Our emphasis is on experiments that quantify resistance to realistic and reproducible low water potential (drought), salt and freezing stresses while being suitable for genetic studies where a large number of lines must be analyzed. Detailed protocols for the use of polyethylene glycol-infused agar plates to impose low water potential stress, assay of salt tolerance based on root elongation, quantification of freezing tolerance and the use of electrolyte leakage experiments to quantify cellular damage induced by freezing and low water potential are also presented.

Salinity-induced ion flux patterns from the excised roots of Arabidopsis sos mutants

The SOS signal-transduction pathway is known to be important for ion homeostasis and salt tolerance in plants. However, there is a lack of in planta electrophysiological data about how the changes in signalling and ion transport activity are integrated at the cellular and tissue level. In this study, using the non-invasive ion flux MIFE technique, we compared net K+, H+ and Na+ fluxes from elongation and mature root zones of Arabidopsis wild type Columbia and sos mutants. Our results can be summarised as follows: (1) SOS mutations affect the function of the entire root, not just the root apex; (2) SOS signalling pathway is highly branched; (3) Na+ effects on SOS1 may by-pass the SOS2/SOS3 complex in the root apex; (4) SOS mutation affects H+ transport even in the absence of salt stress; (5) SOS1 mutation affects intracellular K+ homeostasis with a plasma membrane depolarisation-activated outward-rectifying K+ channel being a likely target; (6) H+ pump also may be a target of SOS signalling. We provide an improved model of SOS signalling and discuss physiological mechanisms underlying salt stress perception and signalling in plants. Our work shows that in planta studies are essential for understanding the functional genomics of plant salt tolerance.

Cold, salinity and drought stresses: An overview

World population is increasing at an alarming rate and is expected to reach about six billion by the end of year 2050. On the other hand food productivity is decreasing due to the effect of various abiotic stresses; therefore minimizing these losses is a major area of concern for all nations to cope with the increasing food requirements. Cold, salinity and drought are among the major stresses, which adversely affect plants growth and productivity; hence it is important to develop stress tolerant crops. In general, low temperature mainly results in mechanical constraint, whereas salinity and drought exerts its malicious effect mainly by disrupting the ionic and osmotic equilibrium of the cell. It is now well known that the stress signal is first perceived at the membrane level by the receptors and then transduced in the cell to switch on the stress responsive genes for mediating stress tolerance. Understanding the mechanism of stress tolerance along with a plethora of genes involved in stress signaling network is important for crop improvement. Recently, some genes of calcium-signaling and nucleic acid pathways have been reported to be up-regulated in response to both cold and salinity stresses indicating the presence of cross talk between these pathways. In this review we have emphasized on various aspects of cold, salinity and drought stresses. Various factors pertaining to cold acclimation, promoter elements, and role of transcription factors in stress signaling pathway have been described. The role of calcium as an important signaling molecule in response to various stress signals has also been covered. In each of these stresses we have tried to address the issues, which significantly affect the gene expression in relation to plant physiology.

Salinity-induced ion flux patterns from the excised roots of Arabidopsis sos mutants

The SOS signal-transduction pathway is known to be important for ion homeostasis and salt tolerance in plants. However, there is a lack of in planta electrophysiological data about how the changes in signalling and ion transport activity are integrated at the cellular and tissue level. In this study, using the non-invasive ion flux MIFE technique, we compared net K+, H+ and Na+ fluxes from elongation and mature root zones of Arabidopsis wild type Columbia and sos mutants. Our results can be summarised as follows: (1) SOS mutations affect the function of the entire root, not just the root apex; (2) SOS signalling pathway is highly branched; (3) Na+ effects on SOS1 may by-pass the SOS2/SOS3 complex in the root apex; (4) SOS mutation affects H+ transport even in the absence of salt stress; (5) SOS1 mutation affects intracellular K+ homeostasis with a plasma membrane depolarisation-activated outward-rectifying K+ channel being a likely target; (6) H+ pump also may be a target of SOS signalling. We provide an improved model of SOS signalling and discuss physiological mechanisms underlying salt stress perception and signalling in plants. Our work shows that in planta studies are essential for understanding the functional genomics of plant salt tolerance.

Uptake of sodium in protoplasts of salt-sensitive and salt-tolerant cultivars of rice, Oryza sativa L. determined by the fluorescent dye SBFI

In this study, the uptake of Na+ into the cytosol of rice (Oryza sativa L. cvs Pokkali and BRRI Dhan29) protoplasts was measured using the acetoxy methyl ester of the fluorescent sodium-binding benzofuran isopthalate, SBFI-AM, and fluorescence microscopy. By means of inhibitor analyses the mechanisms for uptake and sequestration of Na+ in the salt-sensitive indica rice cv. BRRI Dhan29 and in the salt-tolerant indica rice cv. Pokkali were detected. Less Na+ was taken up into the cytosol of Pokkali than into BRRI Dhan29. The results indicate that K+-selective channels do not contribute to the Na+ uptake in Pokkali, whereas they are the major pathways for Na+ uptake in BRRI Dhan29 along with non-selective cation channels. However, non-selective cation channels seem to be the main pathways for Na+ uptake in Pokkali. Protoplasts from Pokkali leaves took up Na+ only transiently in the presence of extracellular Na+ at 5-100 mM. Therefore, it is likely that the protoplasts have a mechanism for fast extrusion of Na+ out of the cytoplasm. Experiments with protoplasts pretreated with NH4NO3 and NH4VO3 suggest that the salt-tolerant Pokkali extrudes Na+ mainly into the vacuole. After cultivation of both cultivars in the presence of 10 or 50 mM NaCl for 72 h, the isolated protoplasts from Pokkali took up less Na+ than the control protoplasts. The results suggest that the salt-tolerance in Pokkali depends on reduced uptake through K+-selective channels and a fast extrusion of Na+ into the vacuoles.

Transcriptome analysis of salinity stress responses in common wheat using a 22k oligo-DNA microarray

In this study, we constructed a 22k wheat oligo-DNA microarray. A total of 148,676 expressed sequence tags of common wheat were collected from the database of the Wheat Genomics Consortium of Japan. These were grouped into 34,064 contigs, which were then used to design an oligonucleotide DNA microarray. Following a multistep selection of the sense strand, 21,939 60-mer oligo-DNA probes were selected for attachment on the microarray slide. This 22k oligo-DNA microarray was used to examine the transcriptional response of wheat to salt stress. More than 95% of the probes gave reproducible hybridization signals when targeted with RNAs extracted from salt-treated wheat shoots and roots. With the microarray, we identified 1,811 genes whose expressions changed more than 2-fold in response to salt. These included genes known to mediate response to salt, as well as unknown genes, and they were classified into 12 major groups by hierarchical clustering. These gene expression patterns were also confirmed by real-time reverse transcription-PCR. Many of the genes with unknown function were clustered together with genes known to be involved in response to salt stress. Thus, analysis of gene expression patterns combined with gene ontology should help identify the function of the unknown genes. Also, functional analysis of these wheat genes should provide new insight into the response to salt stress. Finally, these results indicate that the 22k oligo-DNA microarray is a reliable method for monitoring global gene expression patterns in wheat.

Early effects of salinity on water transport in Arabidopsis roots. Molecular and cellular features of aquaporin expression

Aquaporins facilitate the uptake of soil water and mediate the regulation of root hydraulic conductivity (Lp(r)) in response to a large variety of environmental stresses. Here, we use Arabidopsis (Arabidopsis thaliana) plants to dissect the effects of salt on both Lp(r) and aquaporin expression and investigate possible molecular and cellular mechanisms of aquaporin regulation in plant roots under stress. Treatment of plants by 100 mm NaCl was perceived as an osmotic stimulus and induced a rapid (half-time, 45 min) and significant (70%) decrease in Lp(r), which was maintained for at least 24 h. Macroarray experiments with gene-specific tags were performed to investigate the expression of all 35 genes of the Arabidopsis aquaporin family. Transcripts from 20 individual aquaporin genes, most of which encoded members of the plasma membrane intrinsic protein (PIP) and tonoplast intrinsic protein (TIP) subfamilies, were detected in nontreated roots. All PIP and TIP aquaporin transcripts with a strong expression signal showed a 60% to 75% decrease in their abundance between 2 and 4 h following exposure to salt. The use of antipeptide antibodies that cross-reacted with isoforms of specific aquaporin subclasses revealed that the abundance of PIP1s decreased by 40% as early as 30 min after salt exposure, whereas PIP2 and TIP1 homologs showed a 20% to 40% decrease in abundance after 6 h of treatment. Expression in transgenic plants of aquaporins fused to the green fluorescent protein revealed that the subcellular localization of TIP2;1 and PIP1 and PIP2 homologs was unchanged after 45 min of exposure to salt, whereas a TIP1;1-green fluorescent protein fusion was relocalized into intracellular spherical structures tentatively identified as intravacuolar invaginations. The appearance of intracellular structures containing PIP1 and PIP2 homologs was occasionally observed after 2 h of salt treatment. In conclusion, this work shows that exposure of roots to salt induces changes in aquaporin expression at multiple levels. These changes include a coordinated transcriptional down-regulation and subcellular relocalization of both PIPs and TIPs. These mechanisms may act in concert to regulate root water transport, mostly in the long term (> or =6 h).

Variable interactions between sucrose non-fermented 1-related protein kinases and regulatory proteins in higher plants

WPK4 is a sucrose non-fermented 1 (SNF1)-related wheat protein kinase, and was previously reported to interact with 14-3-3 proteins. We identified four Arabidopsis thaliana WPK4-like genes, and designated them AtWL1 through AtWL4. Yeast two-hybrid analysis, however, indicated that none of the AtWLs interacted with any of A. thaliana 14-3-3 (At14-3-3) proteins, although WPK4 itself interacted with six of them. Structurally, AtWLs were classified into a subfamiliy of AtCIPK, which generally interacts with calucineurin B-like proteins (CBL). This was also the case for AtWL1 and AtWL2, showing an efficient interaction with AtCBL2. In contrast, WPK4 interacted with none of the CBLs. In addition, to ascertain the possible interaction in vivo, expression of those genes was examined with a promoter-GUS assay. These results suggested that the interacting partner of SNF1-related protein kinases varies among plant species, and that, in the case of A. thaliana, it was CBLs, some of which were predicted to broadly regulate multiple CIPKs.

Athb-12, a homeobox-leucine zipper domain protein from Arabidopsis thaliana, increases salt tolerance in yeast by regulating sodium exclusion

An Arabidopsis cDNA clone that encodes Athb-12, a homeobox-leucine zipper domain protein (HD-Zip), was isolated by functional complementation of the NaCl-sensitive phenotype of a calcineurin (CaN)-deficient yeast mutant (cnbDelta, regulatory subunit null). CaN, a Ca2+/calmodulin-dependent protein phosphatase, regulates Na+ ion homeostasis in yeast. Expression of Athb-12 increased NaCl tolerance but not osmotic stress tolerance of these cnbDelta cells. Furthermore, expression of two other HD-Zip from Arabidopsis, Athb-1 and -7, did not suppress NaCl sensitivity of cnbDelta cells. These results suggest that Athb-12 specifically functions in Na+ ion homeostasis in yeast. Consistent with these observations, expression of Athb-12 in yeast turned on transcription of the NaCl stress-inducible PMR2A, which encodes a Na+/Li+ translocating P-type ATPase, and decreased Na+ levels in yeast cells. To investigate the biological function of Athb-12 in Arabidopsis, we performed Northern blot analysis. Expression of Athb-12 was dramatically induced by NaCl and ABA treatments, but not by KCl. In vivo targeting experiments using a green fluorescent protein reporter indicated that Athb-12 was localized to the nucleus. These results suggest that Athb-12 is a putative transcription factor that may be involved in NaCl stress responses in plants.

Integration and channeling of calcium signaling through the CBL calcium sensor/CIPK protein kinase network

Plant development and reproduction depend on a precise recognition of environmental conditions and the integration of this information with endogenous metabolic and developmental cues. Calcium ions have been firmly established as ubiquitous second messengers functioning in these processes. Calcium signal deciphering and signal-response coupling often involve calcium-binding proteins as responders or relays in this information flow. Here we review the calcineurin B-like protein (CBL) calcium sensor/CBL-interacting protein kinase (CIPK) network as a newly emerging signaling system mediating a complex array of environmental stimuli. We focus particularly on the mechanisms generating signaling specificity. Moreover, we emphasize the functional implications that are emerging from the analyses of CBL and CIPK loss-of-function mutants.

Protection of plasma membrane K+ transport by the salt overly sensitive1 Na+-H+ antiporter during salinity stress

Physicochemical similarities between K(+) and Na(+) result in interactions between their homeostatic mechanisms. The physiological interactions between these two ions was investigated by examining aspects of K(+) nutrition in the Arabidopsis salt overly sensitive (sos) mutants, and salt sensitivity in the K(+) transport mutants akt1 (Arabidopsis K(+) transporter) and skor (shaker-like K(+) outward-rectifying channel). The K(+)-uptake ability (membrane permeability) of the sos mutant root cells measured electrophysiologically was normal in control conditions. Also, growth rates of these mutants in Na(+)-free media displayed wild-type K(+) dependence. However, mild salt stress (50 mm NaCl) strongly inhibited root-cell K(+) permeability and growth rate in K(+)-limiting conditions of sos1 but not wild-type plants. Increasing K(+) availability partially rescued the sos1 growth phenotype. Therefore, it appears that in the presence of Na(+), the SOS1 Na(+)-H(+) antiporter is necessary for protecting the K(+) permeability on which growth depends. The hypothesis that the elevated cytoplasmic Na(+) levels predicted to result from loss of SOS1 function impaired the K(+) permeability was tested by introducing 10 mm NaCl into the cytoplasm of a patch-clamped wild-type root cell. Complete loss of AKT1 K(+) channel activity ensued. AKT1 is apparently a target of salt stress in sos1 plants, resulting in poor growth due to impaired K(+) uptake. Complementary studies showed that akt1 seedlings were salt sensitive during early seedling development, but skor seedlings were normal. Thus, the effect of Na(+) on K(+) transport is probably more important at the uptake stage than at the xylem loading stage.

Identification of nine sucrose nonfermenting 1-related protein kinases 2 activated by hyperosmotic and saline stresses in Arabidopsis thaliana

Several calcium-independent protein kinases were activated by hyperosmotic and saline stresses in Arabidopsis cell suspension. Similar activation profiles were also observed in seedlings exposed to hyperosmotic stress. One of them was identified to AtMPK6 but the others remained to be identified. They were assumed to belong to the SNF1 (sucrose nonfermenting 1)-related protein kinase 2 (SnRK2) family, which constitutes a plant-specific kinase group. The 10 Arabidopsis SnRK2 were expressed both in cells and seedlings, making the whole SnRK2 family a suitable candidate. Using a family-specific antibody raised against the 10 SnRK2, we demonstrated that these non-MAPK protein kinases activated by hyperosmolarity in cell suspension were SnRK2 proteins. Then, the molecular identification of the involved SnRK2 was investigated by transient expression assays. Nine of the 10 SnRK2 were activated by hyperosmolarity induced by mannitol, as well as NaCl, indicating an important role of the SnRK2 family in osmotic signaling. In contrast, none of the SnRK2 were activated by cold treatment, whereas abscisic acid only activated five of the nine SnRK2. The probable involvement of the different Arabidopsis SnRK2 in several abiotic transduction pathways is discussed.

Proteomic identification of annexins, calcium-dependent membrane binding proteins that mediate osmotic stress and abscisic acid signal transduction in Arabidopsis

Comparative proteomic analysis of the Arabidopsis thaliana root microsomal fraction was performed to identify novel components of salt stress signaling. Among the salt-responsive microsomal proteins, two spots that increased upon salt treatment on a two-dimensional gel were identified as the same protein, designated annexin 1 (AnnAt1). Annexins comprise a multigene family of Ca2+-dependent membrane binding proteins and have been extensively studied in animal cells. AnnAt1 is strongly expressed in root but rarely in flower tissue. In this study, the results suggest that salt stress induces translocation from the cytosol to the membrane and potential turnover of existing protein. This process is blocked by EGTA treatment, implying that AnnAt1 functions in stress response are tightly associated with Ca2+. T-DNA insertion mutants of annAt1 and a different isoform, annAt4, displayed hypersensitivity to osmotic stress and abscisic acid (ABA) during germination and early seedling growth. The results collectively suggest that AnnAt1 and AnnAt4 play important roles in osmotic stress and ABA signaling in a Ca2+-dependent manner.

Proteomics of calcium-signaling components in plants

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

Gene expression analysis in cucumber leaves primed by root colonization with Pseudomonas chlororaphis O6 upon challenge-inoculation with Corynespora cassiicola

Root colonization by Pseudomonas chlororaphis O6, a non-pathogenic rhizobacterium, induced systemic resistance in cucumber against target leaf spot caused by Corynespora cassiicola. A cDNA library was constructed using mRNA extracted from cucumber leaves 12 h after inoculation with C. cassiicola, using plants colonized by O6. To identify genes involved in O6-mediated induced systemic resistance (ISR), we employed a subtractive hybridization method using mRNAs extracted from pathogen-challenged cucumber leaves of plants lacking colonization. Differential screening of the cDNA library led to the isolation of six distinct genes encoding a GTP binding protein, a 60S ribosomal protein, a hypersensitive-induced reaction protein, a ubiquitin extension protein, a pyridine nucleotide-disulfide oxidoreductase, and a signal recognition particle receptor. Expression of these genes was not induced by O6 colonization alone. Rather, transcript accumulation of these genes increased significantly faster and stronger in the O6 colonized than in non-colonized plants after challenge infection. Therefore, O6-mediated ISR may be associated with an enhanced capacity for the rapid and effective activation of cellular defence responses after challenge inoculation.

Osmotically induced cell swelling versus cell shrinking elicits specific changes in phospholipid signals in tobacco pollen tubes

Pollen tube cell volume changes rapidly in response to perturbation of the extracellular osmotic potential. This report shows that specific phospholipid signals are differentially stimulated or attenuated during osmotic perturbations. Hypo-osmotic stress induces rapid increases in phosphatidic acid (PA). This response occurs starting at the addition of 25% (v/v) water to the pollen tube cultures and peaks at 100% (v/v) water. Increased levels of PA were detected within 30 s and reached maximum by 15 to 30 min after treatment. The pollen tube apical region undergoes a 46% increase in cell volume after addition of 100% water (v/v), and there is an average 7-fold increase in PA. This PA increase appears to be generated by phospholipase D because concurrent transphosphatidylation of n-butanol results in an average 8-fold increase in phosphatidylbutanol. Hypo-osmotic stress also induces an average 2-fold decrease in phosphatidylinositol phosphate; however, there are no detectable changes in the levels of phosphatidylinositol bisphosphates. In contrast, salt-induced hyperosmotic stress from 50 to 400 mm NaCl inhibits phospholipase D activity, reduces the levels of PA, and induces increases in the levels of phosphatidylinositol bisphosphate isomers. The pollen tube apical region undergoes a 41% decrease in cell volume at 400 mm NaCl, and there is an average 2-fold increase in phosphatidylinositol 3,5-bisphosphate and 1.4-fold increase in phosphatidylinositol 4,5-bisphosphate. The phosphatidylinositol 3,5-bisphosphate increase is detected within 30 s and reaches maximum by 15 to 30 min after treatment. In summary, these results demonstrate that hypo-osmotic versus hyperosmotic perturbation and the resultant cell swelling or shrinking differentially activate specific phospholipid signaling pathways in tobacco (Nicotiana tabacum) pollen tubes.

Binding of sulfonylurea by AtMRP5, an Arabidopsis multidrug resistance-related protein that functions in salt tolerance

Recently, a new member of the ABC transporter superfamily of Arabidopsis, AtMRP5, was identified and characterized. In the present work, we found that AtMRP5 can bind specifically to sulfonurea when it is expressed in HEK293 cells. We also present evidence for a new role of AtMRP5 in the salt stress response of Arabidopsis. We used reverse genetics to identify an Arabidopsis mutant (atmrp5-2) in which the AtMRP5 gene was disrupted by transferred DNA insertion. In root-bending assays using Murashige and Skoog medium supplemented with 100 mm NaCl, root growth of atmrp5-2 was substantially inhibited in contrast to the almost normal growth of wild-type seedlings. This hypersensitive response of the atmrp5-2 mutant was not observed during mannitol treatment. The root growth of the wild-type plant grown in Murashige and Skoog medium supplemented with the MRP inhibitor glibenclamide and NaCl was inhibited to a very similar extent as the root growth of atmrp5-2 grown in NaCl alone. The Na(+)-dependent reduction of root growth of the wild-type plant in the presence of glibenclamide was partially restored by diazoxide, a known K+ channel opener that reverses the inhibitory effects of sulfonylureas in animal cells. Moreover, the atmrp5-2 mutant was defective in 86Rb+ uptake. When seedlings were treated with 100 mm NaCl, atmrp5-2 seedlings accumulated less K+ and more Na+ than those of the wild type. These observations suggest that AtMRP5 is a putative sulfonylurea receptor that is involved in K+ homeostasis and, thus, also participates in the NaCl stress response.

Regulation of Vacuolar Na+/H+ Exchange in Arabidopsis thaliana by the Salt-Overly-Sensitive (SOS) Pathway

For plants growing in highly saline environments, accumulation of sodium in the cell cytoplasm leads to disruption of metabolic processes and reduced growth. Maintaining low levels of cytoplasmic sodium requires the coordinate regulation of transport proteins on numerous cellular membranes. Our previous studies have linked components of the Salt-Overly-Sensitive pathway (SOS1-3) to salt tolerance in Arabidopsis thaliana and demonstrated that the activity of the plasma membrane Na+/H+ exchanger (SOS1) is regulated by SOS2 (a protein kinase) and SOS3 (a calcium-binding protein). Current studies were undertaken to determine if the Na+/H+ exchanger in the vacuolar membrane (tonoplast) of Arabidopsis is also a target for the SOS regulatory pathway. Characterization of tonoplast Na+/H+ exchange demonstrated that it represents activity originating from the AtNHX proteins since it could be inhibited by 5-(N-methyl-N-isobutyl)amiloride and by anti-NHX1 antibodies. Transport activity was selective for sodium (apparent Km=31 mm) and electroneutral (one sodium ion for each proton). When compared with tonoplast Na+/H+-exchange activity in wild type, activity was significantly higher, greatly reduced, and unchanged in sos1, sos2, and sos3, respectively. Activated SOS2 protein added in vitro increased tonoplast Na+/H+-exchange activity in vesicles isolated from sos2 but did not have any effect on activity in vesicles isolated from wild type, sos1, or sos3. These results demonstrate that (i) the tonoplast Na+/H+ exchanger in Arabidopsis is a target of the SOS regulatory pathway, (ii) there are branches to the SOS pathway, and (iii) there may be coordinate regulation of the exchangers in the tonoplast and plasma membrane.

QTLs for Na+ and K+ uptake of the shoots and roots controlling rice salt tolerance

An F2 and an equivalent F3 population derived from a cross between a high salt-tolerance indica variety, Nona Bokra, and a susceptible elite japonica variety, Koshihikari, were produced. We performed QTL mapping for physiological traits related to rice salt-tolerance. Three QTLs for survival days of seedlings (SDSs) under salt stress were detected on chromosomes 1, 6 and 7, respectively, and explained 13.9% to 18.0% of the total phenotypic variance. Based on the correlations between SDSs and other physiological traits, it was considered that damage of leaves was attributed to accumulation of Na+ in the shoot by transport of Na+ from the root to the shoot in external high concentration. We found eight QTLs including three for three traits of the shoots, and five for four traits of the roots at five chromosomal regions, controlled complex physiological traits related to rice salt-tolerance under salt stress. Of these QTLs, the two major QTLs with the very large effect, qSNC-7 for shoot Na+ concentration and qSKC-1 for shoot K+ concentration, explained 48.5% and 40.1% of the total phenotypic variance, respectively. The QTLs detected between the shoots and the roots almost did not share the same map locations, suggesting that the genes controlling the transport of Na+ and K+ between the shoots and the roots may be different.

Regulation of ion homeostasis under salt stress

When under salt stress, plants maintain a high concentration of K(+) and a low concentration of Na(+) in the cytosol. They do this by regulating the expression and activity of K(+) and Na(+) transporters and of H(+) pumps that generate the driving force for transport. Although salt-stress sensors remain elusive, some of the intermediary signaling components have been identified. Evidence suggests that a protein kinase complex consisting of the myristoylated calcium-binding protein SOS3 and the serine/threonine protein kinase SOS2 is activated by a salt-stress-elicited calcium signal. The protein kinase complex then phosphorylates and activates various ion transporters, such as the plasma membrane Na(+)/H(+) antiporter SOS1.

Vacuolar cation/H+ exchange, ion homeostasis, and leaf development are altered in a T-DNA insertional mutant of AtNHX1, the Arabidopsis vacuolar Na+/H+ antiporter

The function of vacuolar Na+/H+ antiporter(s) in plants has been studied primarily in the context of salinity tolerance. By facilitating the accumulation of Na+ away from the cytosol, plant cells can avert ion toxicity and also utilize vacuolar Na+ as osmoticum to maintain turgor. As many genes encoding these antiporters have been cloned from salt-sensitive plants, it is likely that they function in some capacity other than salinity tolerance. The wide expression pattern of Arabidopsis thaliana sodium proton exchanger 1 (AtNHX1) in this study supports this hypothesis. Here, we report the isolation of a T-DNA insertional mutant of AtNHX1, a vacuolar Na+/H+ antiporter in Arabidopsis. Vacuoles isolated from leaves of the nhx1 plants had a much lower Na+/H+ and K+/H+ exchange activity. nhx1 plants also showed an altered leaf development, with reduction in the frequency of large epidermal cells and a reduction in overall leaf area compared to wild-type plants. The overexpression of AtNHX1 in the nhx1 background complemented these phenotypes. In the presence of NaCl, nhx1 seedling establishment was impaired. These results place AtNHX1 as the dominant K+ and Na+/H+ antiporter in leaf vacuoles in Arabidopsis and also suggest that its contribution to ion homeostasis is important for not only salinity tolerance but development as well.

A novel domain in the protein kinase SOS2 mediates interaction with the protein phosphatase 2C ABI2

SOS2 (salt overly sensitive 2) is a serine/threonine protein kinase required for salt tolerance in Arabidopsis thaliana. In this study, we identified the protein phosphatase 2C ABI2 (abscisic acid-insensitive 2) as a SOS2-interacting protein. Deletion analysis led to the discovery of a novel protein domain of 37 amino acid residues, designated as the protein phosphatase interaction (PPI) motif, of SOS2 that is necessary and sufficient for interaction with ABI2. The PPI motif is conserved in protein kinases of the SOS2 family (i.e., protein kinase S, PKS) and in the DNA damage repair and replication block checkpoint kinase, Chk1, from various organisms including humans. Mutations in the conserved amino acid residues in the PPI motif abolish the interaction of SOS2 with ABI2. We also identified a protein kinase interaction domain in ABI2 and examined the interaction specificity between PKS and the ABI phosphatases. We found that some PKSs interact strongly with ABI2 whereas others interact preferentially with ABI1. The interaction between SOS2 and ABI2 was disrupted by the abi2-1 mutation, which causes increased tolerance to salt shock and abscisic acid insensitivity in plants. Our results establish the PPI motif and the protein kinase interaction domain as novel protein interaction domains that mediate the binding between the SOS2 family of protein kinases and the ABI1/2 family of protein phosphatases.

Sodium influx and accumulation in Arabidopsis

Arabidopsis is frequently used as a genetic model in plant salt tolerance studies, however, its physiological responses to salinity remain poorly characterized. This study presents a characterization of initial Na+ entry and the effects of Ca2+ on plant growth and net Na+ accumulation in saline conditions. Unidirectional Na+ influx was measured carefully using very short influx times in roots of 12-d-old seedlings. Influx showed three components with distinct sensitivities to Ca2+, diethylpyrocarbonate, and osmotic pretreatment. Pharmacological agents and known mutants were used to test the contribution of different transport pathways to Na+ uptake. Influx was stimulated by 4-aminobutyric acid and glutamic acid; was inhibited by flufenamate, quinine, and cGMP; and was insensitive to modulators of K+ and Ca2+ channels. Influx did not differ from wild type in akt1 and hkt1 insertional mutants. These data suggested that influx was mediated by several different types of nonselective cation channels. Na+ accumulation in plants grown in 50 mM NaCl was strongly reduced by increasing Ca2+ activity (from 0.05-3.0 mM), and plant survival was improved. However, plant biomass was not affected by shoot Na+ concentration, suggesting that in Arabidopsis Na+ toxicity is not dependent on shoot Na+ accumulation. These data suggest that Arabidopsis is a good model for investigation of Na+ transport, but may be of limited utility as a model for the study of Na+ toxicity.

The Arabidopsis CDPK-SnRK superfamily of protein kinases

The CDPK-SnRK superfamily consists of seven types of serine-threonine protein kinases: calcium-dependent protein kinase (CDPKs), CDPK-related kinases (CRKs), phosphoenolpyruvate carboxylase kinases (PPCKs), PEP carboxylase kinase-related kinases (PEPRKs), calmodulin-dependent protein kinases (CaMKs), calcium and calmodulin-dependent protein kinases (CCaMKs), and SnRKs. Within this superfamily, individual isoforms and subfamilies contain distinct regulatory domains, subcellular targeting information, and substrate specificities. Our analysis of the Arabidopsis genome identified 34 CDPKs, eight CRKs, two PPCKs, two PEPRKs, and 38 SnRKs. No definitive examples were found for a CCaMK similar to those previously identified in lily (Lilium longiflorum) and tobacco (Nicotiana tabacum) or for a CaMK similar to those in animals or yeast. CDPKs are present in plants and a specific subgroup of protists, but CRKs, PPCKs, PEPRKs, and two of the SnRK subgroups have been found only in plants. CDPKs and at least one SnRK have been implicated in decoding calcium signals in Arabidopsis. Analysis of intron placements supports the hypothesis that CDPKs, CRKs, PPCKs and PEPRKs have a common evolutionary origin; however there are no conserved intron positions between these kinases and the SnRK subgroup. CDPKs and SnRKs are found on all five Arabidopsis chromosomes. The presence of closely related kinases in regions of the genome known to have arisen by genome duplication indicates that these kinases probably arose by divergence from common ancestors. The PlantsP database provides a resource of continuously updated information on protein kinases from Arabidopsis and other plants.

Na+/H+ exchange activity in the plasma membrane of Arabidopsis

In plants, Na+/H+ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na+ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na+/H+ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na+/H+ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mm NaCl and increased with prolonged salt exposure up to 8 d. H+-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na+/H+ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent Km) of the transporter for Na+ is 22.8 mm. Data from two experimental approaches supports electroneutral exchange (one Na+ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na+/H+ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na+/H+ exchanger and its relative contribution to the maintenance of cellular Na+ homeostasis during plant growth in salt.

The Arabidopsis CDPK-SnRK superfamily of protein kinases

The CDPK-SnRK superfamily consists of seven types of serine-threonine protein kinases: calcium-dependent protein kinase (CDPKs), CDPK-related kinases (CRKs), phosphoenolpyruvate carboxylase kinases (PPCKs), PEP carboxylase kinase-related kinases (PEPRKs), calmodulin-dependent protein kinases (CaMKs), calcium and calmodulin-dependent protein kinases (CCaMKs), and SnRKs. Within this superfamily, individual isoforms and subfamilies contain distinct regulatory domains, subcellular targeting information, and substrate specificities. Our analysis of the Arabidopsis genome identified 34 CDPKs, eight CRKs, two PPCKs, two PEPRKs, and 38 SnRKs. No definitive examples were found for a CCaMK similar to those previously identified in lily (Lilium longiflorum) and tobacco (Nicotiana tabacum) or for a CaMK similar to those in animals or yeast. CDPKs are present in plants and a specific subgroup of protists, but CRKs, PPCKs, PEPRKs, and two of the SnRK subgroups have been found only in plants. CDPKs and at least one SnRK have been implicated in decoding calcium signals in Arabidopsis. Analysis of intron placements supports the hypothesis that CDPKs, CRKs, PPCKs and PEPRKs have a common evolutionary origin; however there are no conserved intron positions between these kinases and the SnRK subgroup. CDPKs and SnRKs are found on all five Arabidopsis chromosomes. The presence of closely related kinases in regions of the genome known to have arisen by genome duplication indicates that these kinases probably arose by divergence from common ancestors. The PlantsP database provides a resource of continuously updated information on protein kinases from Arabidopsis and other plants.

Na+ tolerance and Na+ transport in higher plants

Tolerance to high soil [Na(+)] involves processes in many different parts of the plant, and is manifested in a wide range of specializations at disparate levels of organization, such as gross morphology, membrane transport, biochemistry and gene transcription. Multiple adaptations to high [Na(+)] operate concurrently within a particular plant, and mechanisms of tolerance show large taxonomic variation. These mechanisms can occur in all cells within the plant, or can occur in specific cell types, reflecting adaptations at two major levels of organization: those that confer tolerance to individual cells, and those that contribute to tolerance not of cells per se, but of the whole plant. Salt-tolerant cells can contribute to salt tolerance of plants; but we suggest that equally important in a wide range of conditions are processes involving the management of Na(+) movements within the plant. These require specific cell types in specific locations within the plant catalysing transport in a coordinated manner. For further understanding of whole plant tolerance, we require more knowledge of cell-specific transport processes and the consequences of manipulation of transporters and signalling elements in specific cell types.

Phospholipid-based signaling in plants

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.