Expression of a Suaeda salsa Vacuolar H+/Ca2+ Transporter Gene in Arabidopsis Contributes to Physiological Changes in Salinity

A cDNA (SsCAX1) encoding a tonoplast-localised Ca²⁺/H⁺ exchanger was isolated from a C₃ halophyte Suaeda salsa (L.). To clarify the role of SsCAX1 in plant salt tolerance, Arabidopsis plants expressing SsCAX1 were treated with NaCl. Transgenic Arabidopsis plants displayed decreased salt tolerance. Although Na⁺ content was close to wild-type plants, transgenic plants accumulated more Ca²⁺ and retained less K⁺ in leaves than the wild-type plants in salinity. Furthermore, transgenic lines held higher leaf membrane leakage than wild-type lines under NaCl treatment. In addition, transgenic plants showed a 23% increase in vacuolar H⁺-ATPase activity compared with wild-type plants in normal condition. But the leaf V-H⁺-ATPase activity had subtle changes in transgenic plants, while significantly increased in wild-type plants under saline condition. These results suggested that regulated expression of Ca²⁺/H⁺ antiport was critical for maintenance of cation homeostasis and activity of V-H⁺-ATPase under saline condition.     

Wheat V-H+-ATPase Subunit Genes Significantly Affect Salt Tolerance in Arabidopsis thaliana

Genes for V-H⁺-ATPase subunits were identified and cloned from the salt-tolerant wheat mutant RH8706-49. Sequences of these genes are highly conserved in plants. Overexpression of these genes in Arabidopsis thaliana improved its salt tolerance, and increased the activities of V-H⁺-ATPase and Na⁺/H⁺ exchange, with the largest increase in plants carrying the c subunit of V-H⁺-ATPase. Results from quantitative RT-PCR analysis indicated that the mRNA level of each V-H⁺-ATPase subunit in the Arabidopsis increased under salt stress. Overall, our results suggest that each V-H⁺-ATPase subunit plays a key role in enhancing salt tolerance in plants.     

NaCl-induced changes in vacuolar H<Superscript>+</Superscript>-ATPase expression and vacuolar membrane lipid composition of two shrub willow clones differing in their response to salinity

It has been suggested that vacuolar H⁺-ATPase (V-H⁺-ATPase) plays a pivotal role in salt stress, and salt stress could modulate the expression and enzyme activity of V-H⁺-ATPase. In this work, salt modulation of V-H⁺-ATPase and tonoplast fatty acid compositions were evaluated in two shrub willow clones differing in salt tolerance after 3, 6 and 12 days of treatment. The results showed that the activity of V-H⁺-ATPase was regulated in tissue and clone specifically under NaCl stress. In the leaves of salt-tolerant clone 2345, treatment with 100 mM NaCl increased V-H⁺-ATPase activity first and then decreased it at day 12, while V-H⁺-ATPase activity was stimulated in the roots by NaCl during the treatment time. In contrast, V-H⁺-ATPase activity reached the highest value at day 3 in the leaves of salt-sensitive clone 2367 and then it decreased. Accumulation of Na⁺ in the vacuole was observed in parallel with increase in V-H⁺-ATPase activity. Western blot and immunofluorescence analysis of V-H⁺-ATPase subunit E revealed that the protein content varied in parallel with V-H⁺-ATPase activity. Moreover, a decreased unsaturated fatty acids ratio to saturated ones together with an increased V-H⁺-ATPase activity was detected in the roots of salt-tolerant clone 2345 at day 12. Altogether, it suggested that the induction of V-H⁺-ATPase expression and increase in the saturation of tonoplast fatty acids as a homeostatic mechanism for shrub willow to cope with salt stress.     

Relationship between expression of the PM H<Superscript>+</Superscript>-ATPase,growth and ion partitioning in the leaves of salt-treated <Emphasis Type="Italic">Medicago</Emphasis> species

The role of the plasma membrane (PM) H⁺-ATPase (E.C. 3.6.1.3) in the plants response to salt stress was studied in the perennial leguminosae forage Medicago arborea L. and its close relative Medicago citrina (Font-Quer) Greuter, a species exposed to saline conditions in its original habitat. Plants were solution cultured for 8 days in 1 or 100 mM NaCl. Leaf growth and CO₂ assimilation were more inhibited by salt in M. arborea than in M. citrina. Both species were able to osmoregulate, and salt-treated plants maintained turgor potentials, with no differences between species. Contrasting ion distribution patterns showed that M. citrina was able to exclude Na⁺ from the leaves more selectively, while M. arborea had a greater buildup of leaf blade Na⁺. Isolation of purified PM and quantification of H⁺-ATPase protein by Western blot analysis against the 46E5B11D5 or AHA3 antibodies showed an increase in response to salt stress in the expanding (92%) and expanded leaves (87%) of M. citrina, while no differences were found in the corresponding leaves of M. arborea. The assay of H⁺-ATPase specific activity of the two leaf types in salinized M. citrina confirmed this increase, as activities increased with 55% and 104% for the expanded and expanding leaves, respectively, while no significant differences were found for either leaf type of salinized M. arborea. A possible role of the increased expression of the PM H⁺-ATPase for leaf expansion and ion exclusion in salt-stressed plants is discussed.     

Effects of salt stress and exogenous Ca2+ on Na+ compartmentalization,ion pump activities of tonoplast and plasma membrane in Nitraria tangutorum Bobr. leaves

Nitraria tangutorum Bobr. is a typical halophyte with superior tolerance to salinity. However, little is known about its physiological adaptation mechanisms to the salt environment. In the present study, N. tangutorum seedlings were treated with different concentrations of NaCl (100, 200, 300 and 400 mmol L^?1) combined with five levels of Ca²⁺ (0, 5, 10, 15 and 20 mmol L^?1) to investigate the effects of salt stress and exogenous Ca²⁺ on Na⁺ compartmentalization and ion pump activities of tonoplast and plasma membrane (PM) in leaves. Na⁺ and Ca²⁺ treatments increased the fresh weight and dry weight of N. tangutorum seedlings. The absorption of Na⁺ in roots, stems and leaves was substantially increased with the increases of NaCl concentration, and Na⁺ was mainly accumulated in leaves. Exogenous Ca²⁺ reduced Na⁺ accumulation in roots but promoted Na⁺ accumulation in leaves. The absorption and transportation of Ca²⁺ in N. tangutorum seedlings were inhibited under NaCl treatments. Exogenous Ca²⁺ promoted Ca²⁺ accumulation in the plant. Na⁺ contents in apoplast and symplast of leaves were also significantly increased, and symplast was the main part of Na⁺ intracellular compartmentalization. The tonoplast H⁺-ATPase and H⁺-PPase activities were significantly promoted under salt stress (NaCl concentrations ≤300 mmol L^?1). PM H⁺-ATPase activities gradually increased under salt stress (NaCl concentrations ≤200 mmol L^?1) followed by decreases with NaCl concentration increasing. The tonoplast H⁺-ATPase, H⁺-PPase and PM H⁺-ATPase activities increased first with the increasing exogenous Ca²⁺ concentration, reached the maximums at 15 mmol L^?1 Ca²⁺, and then decreased. The tonoplast and PM Ca²⁺-ATPase activities showed increasing trends with the increases of NaCl and Ca²⁺ concentration. These results suggested that certain concentrations of exogenous Ca²⁺ effectively enhanced ion pump activities of tonoplast and PM as well as promoted the intracellular Na⁺ compartmentalization to improve the salt tolerance of N. tangutorum.     

Photosynthesis and ion content of leaves and isolated chloroplasts of salt-stressed spinach

Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K⁺ was decreased but Na⁺ and Cl⁻ were greatly increased.

The potential photosynthetic capacity of the leaves was measured at saturating CO₂ to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.

Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO₂-dependent O₂ evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K⁺ in the chloroplast decreased by 50% in the salt-stressed plants, Na⁺ increased by 70%, and Cl⁻ increased by less than 20% despite large increases in leaf Na⁺ and Cl⁻.

It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na⁺ and Cl⁻ in the chloroplast.

     

Role of Tonoplast Proton Pumps and Na+/H+ Antiport System in Salt Tolerance of Populus euphratica Oliv.

Populus euphratica has been used as a plant model to study resistance against salt and osmotic stresses, with recent studies having characterized the tonoplast and the plasma membrane ATPases, and two Na⁺/H⁺ antiporters, homologs of the Arabidopsis tonoplast AtNHX1, were published in databases. In the present work we show that P. euphratica suspension-cultured cells are highly tolerant to high salinity, being able to grow with up to 150 mM NaCl in the culture medium without substantial modification of the final population size when compared to the control cells in the absence of salt. At a salt concentration of 300 mM, cells were unable to grow but remained highly viable up to 17 days after subculture. The addition of a 1-M-NaCl pulse to unadapted cells did not promote a significant loss in cell viability within 48 h. In tonoplast vesicles purified from cells cultivated in the absence of salt and from salt-stressed cells, vacuolar H⁺-pyrophosphatase (V-H⁺-PPase) seemed to be the primary tonoplast proton pump; however, there appears to be a decrease in V-H⁺-PPase activity with exposure to NaCl, in contrast to the sodium-induced increase in the activity of vacuolar H⁺-ATPase (V-H⁺-ATPase). Despite reports that in P. euphratica there is no significant difference in the concentration of Na⁺ in the different cell compartments under NaCl stress, in the present study, confocal and epifluorescence microscopic observations using a Na⁺-sensitive probe showed that suspension-cultured cells subject to a salt pulse accumulated Na⁺ in the vacuole when compared with control cells. Concordantly, a tonoplast Na⁺/H⁺ exchange system is described whose activity is upregulated by salt and, indirectly, by a salt-mediated increase of V-H⁺-ATPase activity.     

Salt accumulation and depletion in the root-zone of the halophyte Atriplex nummularia Lindl.: influence of salinity,leaf area and plant water use

Background and aims

Salt is known to accumulate in the root-zone of Na⁺ excluding glycophytes under saline conditions. We examined the effect of soil salinity on Na⁺ and Cl^? depletion or accumulation in the root-zone of the halophyte (Atriplex nummularia Lindl).

Methods

A pot experiment was conducted in soil to examine Na⁺ and Cl^? concentrations adjacent to roots at four initial NaCl treatments (20, 50, 200 or 400 mM NaCl in the soil solution). Plant water use was manipulated by leaving plants with all leaves intact, removing approximately 50 % of leaves, or removing all leaves. Daily evapotranspiration was replaced by watering undrained pots to weight with deionised water. After 35-38 days, samples were taken of the bulk soil and of soil loosely- and closely-adhering to the roots.

Results

In plants with leaves intact grown with 200 and 400 mM NaCl, average Na⁺ and Cl^? concentrations in the closely adhering soil were about twice the concentrations of the bulk soil. Ion accumulation increased with final leaf area and with cumulative transpiration over the duration of the trial. By contrast, in plants grown with the lowest salinity treatment (20 mM NaCl), Na⁺ and Cl^? concentrations decreased in the closely adhering soil with increasing leaf area and increasing cumulative water use.

Conclusions

Our data show that Na⁺ and Cl^? are depleted from the root-zone of A. nummularia at low salinity but accumulate in the root-zone at moderate to high salinity, and that the ions are drawn towards the plant in the transpiration stream.     

海岸不同生态断带植物根叶抗逆生理变化与其Na+含量的关系

以烟台海岸生态断带滨麦(Leymus mollis)和肾叶打碗花(Calystegia soldanella)为材料,在远离高潮线不同位置上取土样和植物材料,通过测定土壤Na+和两植物根叶Na+含量、丙二醛(MDA)含量、抗氧化酶(SOD、POD、CAT)活性和渗透调节物含量,以揭示滨麦和肾叶打碗花根叶中Na+在其适应海岸盐环境中的生理调控机制。结果表明,在高潮线土壤Na+含量最高,滨麦根叶Na+含量较高,两植物根叶中MDA和水分含量、抗氧化酶活力均较低,但渗透调节物含量均较高。随远离高潮线土壤Na+含量下降,滨麦根叶Na+含量下降,而肾叶打碗花根中Na+含量上升,其根叶Na+含量较滨麦分别高637%和319%。同时两植物根叶MDA含量、叶片含水量增加;两植物根中POD和SOD活力增加;两植物根叶可溶性糖和脯氨酸含量下降。但不同生态断带滨麦叶片平均含水量相对较低,MDA含量、POD和CAT和SOD活力、脯氨酸和可溶性糖含量相对较高。在盐土环境中滨麦通过降低Na+的吸收和提高抗氧化酶活力和有机渗透调节物含量维持氧自由基代谢平衡和水分平衡。而肾叶打碗花是泌盐植物,在不同生态断带其叶片Na+含量、平均含水量相对较高,叶MDA含量、POD和CAT活力、脯氨酸和可溶性糖含量均相对较低。泌盐植物的肾叶打碗花依赖根叶中积累的Na+作为无机渗透调节剂维护其离子平衡和水分平衡及正常生长。因此,积累在根叶中的Na+离子既作为无机渗透调节剂维护细胞离子平衡和水分平衡,又引发细胞生理干旱促进有机渗透调节物合成;另外还作为氧自由基诱发剂促使活性氧自由基(ROS)积累,通过积累的ROS激活抗氧化保护酶系统抑制膜脂过氧化、维护氧自由基代谢平衡。海岸沙地土壤中高浓度Na+是海滨滨麦和肾叶打碗花能长期在盐土环境中生存的依靠元素,其对植物的生理调控作用可能是滨麦和肾叶打碗花适应盐土生存的重要生理适应机理。     

Selection of a NaCl-tolerant Citrus plant

Summary The objective of the work was to select Citrus plants more tolerant to elevated NaCl concentrations in the irrigation water. For this purpose, unfertilized Troyer citrange ovules treated with a chemical mutagenic agent (ethyl methane sulphonate) were cultured in vitro. Whole plants were regenerated from embryos developed in the nucellar tissue of the ovule. The screening for salt tolerance was achieved by irrigating these plants with a nutrient solution containing 45 mM NaCl.Plants obtained from vegetative propagation of the selected plant show faster growth, less leaf damage and lower concentrations of Cl^– and Na⁺ in leaves than the original clone with increasing NaCl levels in the nutrient solution. In addition, the selected plant accumulated more Na⁺ in shoots and roots and this was accompanied by a significant reduction in K⁺concentration. It can be concluded that the selected plant restricts the accumulation of Na⁺ in leaves by concentrating Na⁺ in shoots and roots.Abbreviations EMS ethyl-methane-sulphonate - IBA indole 3-butyric acid     

Peroxidase activities of two rice cultivars differing in salinity tolerance as affected by proline and NaCl

Proline content, ion accumulation, cell wall and soluble peroxidase activities were determined in control and salt-treated calli (150 nM NaCl) and whole plants (30 mM NaCl) of two rice cultivars (salt sensitive cv. IKP and salt tolerant cv. Aiwu). Under salinity, the highest accumulation of Na⁺, Cl^? and proline occurred in calli, roots and younger leaves of cv. IKP, coupled with the highest decrease in K⁺ content; accumulations of Na⁺ and Cl^? were restricted to older leaves in cv. Aiwu. Relative growth rates of calli and roots or shoots from both cultivars were not linked to peroxidase activities. High concentrations (1 M) of exogenously applied glycerol did not inhibitin vitro activities of soluble peroxidase extracted from control and salt-treated calli or plants. Conversely, 35–55% (in cv. IKP) or 60–80% (in cv. Aiwu) of soluble peroxidase activities were found in presence of isosmotic proline concentration. There were no differences between proline and glycerol effects onin vitro cell wall peroxidase activities.     

Ornamental characters,ion accumulation and water status in Arbutus unedo seedlings irrigated with saline water and subsequent relief and transplanting

Arbutus unedo seedlings were grown in a greenhouse and submitted to three irrigation treatments (salinity period) using solutions with an EC of 0.85 dS m^?1 (control treatment), 5.45 dS m^?1 (S1) and 9.45 dS m^?1 (S2). After 16 weeks, growth and ornamental characters, leaf water potentials, gas exchange and ion concentrations were determined. After the salinity period, plants were exposed to a relief period for 1 month, whereby half of the plants were transplanted to field conditions and the other half into 24 cm diameter plastic pots. Salinity induced a significant decrease in shoot biomass and leaf area but root/shoot ratio was increased. Plant height was significantly inhibited by salinity. The ornamental characters were affected in the treated plants, with symptoms of salt injury, such as burning of leaf margin. Leaf water potentials decreased with increasing salinity, more significantly at predawn than at midday. The relationship between net photosynthesis (P_n) and leaf conductance (g_l) was linear for all treatments and the same values of P_n are associated with lower values of g_l for the saline treatments than for control treatment. The concentration of Cl^? in leaves increased with increasing salinity and was higher than the corresponding concentration of Na⁺. Na⁺ and Cl^? contents were higher in the leaves than in the roots in both saline treatments. The K⁺ and Ca²⁺ levels were lower in the treated plants than in control plants and applied salinity reduced the K⁺/Na⁺ ratio in leaves, stems and roots, the decrease being much greater for leaves than for roots. The Ca²⁺/Na⁺ ratio fell with salinity in all parts of the plants. At the end of the relief period leaf water potentials were recovered mainly in field conditions. S2 treatment showed lower values of P_n and g_l than control and S1 treatments in pot conditions and in field conditions S1 showed the lowest values for P_n and g_l.     

The physiological response of Artemisia annua L. to salt stress and salicylic acid treatment

Salinity has a great influence on plant growth and distribution. A few existing reports on Artemisia annua L. response to salinity are concentrated on plant growth and artemisinin content; the physiological response and salt damage mitigation are yet to be understood. In this study, the physiological response of varying salt stresses (50, 100, 200, 300, or 400 mM NaCl) on A. annua L. and the effect of exogenous salicylic acid (0.05 or 0.1 mM) at 300-mM salt stress were investigated. Plant growth, antioxidant enzyme activity, proline, and mineral element level were determined. In general, increasing salt concentration significantly reduced plant growth. Superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were stimulated by salt treatment to a higher enzyme activity in treated plants than those in untreated plants. Content of proline had a visible range of increment in the salt-treated plants. Distribution of mineral elements was in inconformity: Na⁺ and Ca²⁺ were mainly accumulated in the roots; K⁺ and Mg²⁺ were concentrated in leaves and stems, respectively. Alleviation of growth arrest was observed with exogenous applications of salicylic acid (SA) under salt stress conditions. The activity of SOD and POD was notably enhanced by SA, but the CAT action was suppressed. While exogenous SA had no discernible effect on proline content, it effectively inhibited excessive Na⁺ absorption and promoted Mg²⁺ absorption. Ca²⁺ and K⁺ contents showed a slight reduction when supplemented with SA. Overall, the positive effect of SA towards resistance to the salinity of A. annua will provide some practical basis for A. annua cultivation.     

Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl

Abstract. Propagules of the mangrove, Rhizophora mangle L., were precultivated for 9 months in a greenhouse. The young plants were transferred into unaerated nutrient solutions without and with 200 mol m ³ NaCl and subsequently their growth, their water relations and the photosynthetic properties of their leaves were studied. Growth of the salttreated plants was significantly increased, while the control plants gradually died off after finishing the experiments. The shoot water potential and the stomatal resistance of the leaves were lowered while the chlorophyll contents and the chlorophyll a/b ratio in the leaves of salt-treated plants were increased by NaCl, the net result being an enhanced rate of CO₂ assimilation. The leaves of both sets of plants showed diurnal fluctuations in malic acid concentration which were more pronounced in the leaves of salt treated plants which, additionally, were more succulent. However, the plants showed no net CO₂ fixation at night, indicating that Rhizophora mangle is a CAM-cycling plant. After 200 d of cultivation without or with NaCl, the Na⁺, Cl and K⁺ concentrations in tissues and vacuoles were measured. Energy-dispersive X-ray microprobe analyses on root vacuoles of control plants reveal Na⁺ preference, on those of salt treated plants a strong K⁺ preference. Vacuolar K⁺ concentrations are neither affected by NaCl nor do they vary across the root radius. High vacuolar Na⁺ and Cl concentrations are found in the hypodermis followed by a stepwise decrease towards the inner root cortex cells. Ion concentrations of the photosynthetically active leaf tissues seem to be regulated by (1) radial filtration across the root cortex: (2) ion exchange of the xlem parenchyma cells: and (3) sequestration of Na⁺ and Cl in the hypodermal water storage tissue of the leaves.     

Assessment of the preventive effect of vermicompost on salinity resistance in tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> cv. Ailsa Craig)

To determine the effects of vermicompost leachate (VCL) on resistance to salt stress in plants, young tomato seedlings (Solanum lycopersicum, cv. Ailsa Craig) were exposed to salinity (150 mM NaCl addition to nutrient solution) for 7 days after or during 6 mL L^??1 VCL application. Salt stress significantly decreased leaf fresh and dry weights, reduced leaf water content, significantly increased root and leaf Na⁺ concentrations, and decreased K⁺ concentrations. Salt stress decreased stomatal conductance (g_s), net photosynthesis (A), instantaneous transpiration (E), maximal efficiency of PSII photochemistry in the dark-adapted state (F_v/F_m), photochemical quenching (qP), and actual PSII photochemical efficiency (ΦPSII). VCL applied during salt stress increased leaf fresh weight and g_s, but did not reduce leaf osmotic potential, despite increased proline content in salt-treated plants. VCL reduced Na⁺ concentrations in leaves (by 21.4%), but increased them in roots (by 16.9%). VCL pre-treatment followed by salt stress was more efficient than VCL concomitant to salt stress, since VCL pre-treatment provided the greatest osmotic adjustment recorded, with maintenance of net photosynthesis and K⁺/Na⁺ ratios following salt stress. VCL pre-treatment also led to the highest proline content in leaves (50 µmol g^??1 FW) and the highest sugar content in roots (9.2 µmol g^??1 FW). Fluorescence-related parameters confirmed that VCL pre-treatment of salt-stressed plants showed higher PSII stability and efficiency compared to plants under concomitant VCL and salt stress. Therefore, VCL represents an efficient protective agent for improvement of salt-stress resistance in tomato.     

Nitric oxide enhances salt tolerance in maize seedlings through increasing activities of proton-pump and Na+/H+ antiport in the tonoplast

Nitric oxide (NO), an endogenous signaling molecule in animals and plants, mediates responses to abiotic and biotic stresses. Our previous work demonstrated that 100 μM sodium nitroprusside (SNP, an NO donor) treatment of maize seedlings increased K⁺ accumulation in roots, leaves and sheathes, while decreasing Na⁺ accumulation (Zhang et al. in J Plant Physiol Mol Biol 30:455–459, 2004b). Here we investigate how NO regulates Na⁺, K⁺ ion homeostasis in maize. Pre-treatment with 100 μM SNP for 2 days improved later growth of maize plants under 100 mM NaCl stress, as indicated by increased dry matter accumulation, increased chlorophyll content, and decreased membrane leakage from leaf cells. An NO scavenger, methylene blue (MB-1), blocked the effect of SNP. These results indicated that SNP-derived NO enhanced maize tolerance to salt stress. Further analysis showed that NaCl induced a transient increase in the NO level in maize leaves. Both NO and NaCl treatment stimulated vacuolar H⁺-ATPase and H⁺-PPase activities, resulting in increased H⁺-translocation and Na⁺/H⁺ exchange. NaCl-induced H⁺-ATPase and H⁺-PPase activities were diminished by MB-1. 1-Butanol, an inhibitor of phosphatidic acid (PA) production by phospholipase D (PLD), reduced NaCl- and NO-induced H⁺-ATPase activation. In contrast, applied PA stimulated H⁺-ATPase activity. These results suggest that NO acts as a signal molecule in the NaCl response by increasing the activities of vacuolar H⁺-ATPase and H⁺-PPase, which provide the driving force for Na⁺/H⁺ exchange. PLD and PA play an important role in this process.     

Abscisic acid does not mediate NaCl-induced accumulation of 70-kDa subunit tonoplast H+ -ATPase message in tomato

There is an increased accumulation of message for the catalytic (70-kDa) subunit of the tonoplast H⁺-ATPase in leaves of tomato (Lycopersicon esculentum L.) plants responding to NaCl. To determine if abscisic acid (ABA) mediates this response, message accumulation was examined in treatments designed to separate exposure to NaCl from increases in endogenous ABA. Under three different experimental conditions, salt-induced changes in the accumulation of 70-kDa message were unrelated to any change in endogenous ABA. The results were as follows: (i) under drought stress, plants accumulated levels of ABA similar to those measured in salt-treated plants; however, no increase in 70-kDa subunit message was observed; (ii) the ABA-deficient mutant sitiens exhibited an increased accumulation of message despite the absence of NaCl-induced accumulation of ABA; and (iii) the inhibitor of general isoprenoid biosynthesis, Lovastatin, blocked NaCl-induced accumulation of ABA but did not alter NaCl-induced accumulation of message. In addition to these three experimental responses, application of exogenous ABA increased endogenous ABA levels without any comparable increase in message accumulation. Based on these results, it is concluded that ABA does not mediate the NaCl-induced accumulation of 70-kDa subunit tonoplast H⁺ -ATPase message accumulation in tomato.     

New perspective on the mechanism of alleviating salt stress by spermidine in barley seedlings

Salt stress is considered to be a major limiting factor for plant growth and crop productivity. Salt injuries in plants are mostly due to excess Na⁺ entry. A possible survival strategy of plants under saline environments is the effective compartmentation of excess Na⁺ by sequestering Na⁺ in roots and inhibiting transport of Na⁺ from roots to shoots. Our previous study showed that exogenous application of polyamines (PAs) could attenuate salt injuries in barley plants. In order to further understand such protective roles of PAs against salt stress, the effects of spermidine (Spd) on sodium and potassium distribution in barley (Hordeum vulgare L.) seedlings under saline conditions were investigated. The results showed that exogenous application of Spd induced reductions in Na⁺ levels in roots and shoots with comparison of NaCl-treated plants, while no significant changes in K⁺ levels were observed. Correspondingly, the plants treated with Spd exogenously maintained high values of [K⁺]/[Na⁺] as compared with salt-stressed plants. Moreover, it was shown by X-ray microanalysis that K⁺ and Na⁺ accumulated mainly in the exodermal intercellular space and cortical cells of roots under salinity stress, and low accumulation was observed in endodermal cells and stelar parenchyma, indicating Casparian bands possibly act as ion transport barriers. Most importantly, Spd treatment further strengthened this barrier effects, leading to inhibition of Na⁺ transport into shoots. These results suggest that, by reinforcing barrier effects of Casparian bands, exogenous Spd inhibits Na⁺ transport from roots to shoots under conditions of high salinity which are beneficial for attenuating salt injuries in barley seedlings.