Salinity Stiffens the Epidermal Cell Walls of Salt-Stressed Maize Leaves: Is the Epidermis Growth-Restricting?

As a result of salt (NaCl)-stress, sensitive varieties of maize (Zea mays L.) respond with a strong inhibition of organ growth. The reduction of leaf elongation investigated here has several causes, including a modification of the mechanical properties of the cell wall. Among the various tissues that form the leaf, the epidermis plays a special role in controlling organ growth, because it is thought to form a rigid outer leaf coat that can restrict elongation by interacting with the inner cell layers. This study was designed to determine whether growth-related changes in the leaf epidermis and its cell wall correspond to the overall reduction in cell expansion of maize leaves during an osmotic stress-phase induced by salt treatment. Two different maize varieties contrasting in their degree of salt resistance (i.e., the hybrids Lector vs. SR03) were compared in order to identify physiological features contributing to resistance towards salinity. Wall loosening-related parameters, such as the capacity of the epidermal cell wall to expand, β-expansin abundance and apoplastic pH values, were analysed. Our data demonstrate that, in the salt-tolerant maize hybrid which maintained leaf growth under salinity, the epidermal cell wall was more extensible under salt stress. This was associated with a shift of the epidermal apoplastic pH into a range more favourable for acid growth. The more sensitive hybrid that displayed a pronounced leaf growth-reduction was shown to have stiffer epidermal cell walls under stress. This may be attributable to the reduced abundance of cell wall-loosening β-expansin proteins following a high salinity-treatment in the nutrient solution (100 mM NaCl, 8 days). This study clearly documents that salt stress impairs epidermal wall-loosening in growth-reduced maize leaves.     

Immunofluorescent Localization of Plasma Membrane H-ATPase in Barley Roots and Effects of K Nutrition

The plasma membrane H⁺-ATPase (PM-H⁺-ATPase) of barley (Hordeum vulgare L. cv Klondike) roots was assayed by cross-reaction on western blots and cryosections with an antibody against the PM-H⁺-ATPase from corn roots. Under conditions of reduced K availability, which have previously been shown to increase K influx by greater than 25-fold, there were only minor changes detected in PM-H⁺-ATPase levels. Antibody labeling of cryosections showed the relative distribution of PM-H⁺-ATPase among cell types in root tips and mature roots. Epidermal cells, both protoderm and mature root epidermis, including root hairs, had high levels of antibody binding. In mature roots, the stelar tissue showing the highest antibody binding was the companion cells of the phloem, followed by pericycle, xylem parenchyma, and endodermis.     

Apoplast Acidification in Growing Barley (Hordeum vulgare L.) Leaves

Apoplast acidification associated with growth is well documented in roots, coleoptiles, and internodes but not in leaves. In the present study, advantage was taken of the high cuticle permeability in the elongation zone of barley leaves to measure apoplast pH and growth in response to application of test reagents. The role of the plasma membrane H⁺-ATPase (PM-H⁺-ATPase) and K⁺ in this process was of particular interest. pH microelectrodes and an in vitro gel system with bromocresol purple as pH indicator were used to monitor apoplast pH. Growth was measured in parallel or in separate experiments using a linear variable differential transformer. Test reagents that blocked (vanadate) or stimulated (fusicoccin) PM-H⁺-ATPase or that reduced (Cs⁺, tetraethylammonium) K⁺ uptake were applied. Apoplast pH was lower in growing than in nongrowing leaf tissue and increased in the elongation zone with increasing apoplast K⁺. Vanadate increased apoplast pH and reduced growth, whereas fusicoccin caused the opposite effects. It is concluded that barley leaves exhibit acid-growth-type mechanisms in that apoplast pH is lower in elongating leaf tissue. Both growth and apoplast pH depend on the activity of the PM-H⁺-ATPase and K⁺ transport processes. However, not all of the growth displayed by leaves is dependent on a lower apoplast pH in the elongation zone; up to 50 % of growth is retained when apoplast pH in the elongation zone increases to a value observed in mature tissue.     

Salinity resistance in Zea mays: fluxes of potassium, sodium and chloride, cytoplasmic concentrations and microsomal membrane lipids

Abstract Cytoplasmic concentrations, fluxes of K⁺, Na⁺ and Cl and microsomal membrane lipids were investigated in a salt-sensitive and salt-resistant variety of Zea mays. The salt resistance of Protador relative to LG_H (salt-sensitive) appears to be related to higher K⁺ fluxes and cytoplasmic concentrations, and lower Na⁺ and Cl fluxes and cytoplasmic concentrations, when grown in NaCl. There were no apparent differences in the simple chemical composition of root microsomal membrane lipids between the two varicties, neither were these affected by salt.     

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.     

The Effect of Salinity on Growth, Cation Content, Na+-Uptake and Translocation in Salt-Sensitive and Salt-Tolerant Plantago Species

Growth of salt-sensitive Plantago media L. and salt-tolerant P. coronopus L. and P. maritima L. was followed under saline conditions. Growth was reduced according to the ecological features of these species: P. media was sensitive to 25 mM NaCl, while P, coronopus and P. maritima could grow in 150 mM and 300 mM NaCl, respectively. The three Plantago species accumulated Na⁺ in the shoot and maintained a relatively low Na⁺ level in the root. K⁺. Mg²⁺ and Ca²⁺ levels of both shoots and roots decreased with increasing salinity. The results suggest that the difference between salt-resistant and salt-sensitive species is located in the ion secretory system which is involved in the ion translocation from the root to the shoot rather than in the primary uptake process through the plasmalemma of the cortical cells.     

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.     

Effects of salt stress on growth, mineral nutrition and proline accumulation in relation to osmotic adjustment in rice (Oryza sativa L.) cultivars differing in salinity resistance

Salt-resistant rice cultivars Nona Bokra and IR 4630 exposed at the seedling stage during one or two weeks to 0, 20, 30, 40 or 50 mM NaCl accumulated less Na, Cl, Zn and proline and more K at root and shoot levels than salt-sensitive I Kong Pao and IR 31785. Aiwu, a moderately resistant genotype, exhibited an intermediate behaviour. P transport from root to shoot was inhibited in the most sensitive cultivar IR 31785. Accumulation of Na and Cl and decrease in K content at the shoot level were restricted to the oldest leaves in salt-resistant genotypes while proline accumulated in the youngest leaves in all cultivars. In the presence of NaCl, the osmotic potentials of the roots and of the oldest and youngest leaves were lower in the salt-resistant than in the salt-sensitive genotypes, differences among genotypes increasing with stress intensity. Proline did not appear to be involved in osmotic adjustment in salt-stressed rice plants and the significance of its accumulation is discussed in relation to salinity resistance.Abbreviations cv(s). cultivar(s) - EC electrical conductivity - IKP I Kong Pao - J rate of ion transport - MCW methanol-chloroform-water - PAR photon flux density - Pc partitioning coefficient - RGR mean relative growth rate - RI resistance index - s osmotic potential     

NaCl胁迫对不同耐盐性玉米自交系萌动种子和幼苗离子稳态的影响

盐胁迫影响植物组织的离子分布,不同品种间存在差异。以玉米耐盐自交系81162和8723及盐敏感自交系P138为材料,研究了不同浓度(0、60、140、220 mmol/L)Na Cl胁迫下萌动期种子和幼苗的不同部位中Na+、K+、Ca2+含量以及K+/Na+和Ca2+/Na+比值的变化,旨在探讨不同自交系耐盐性差异的原因。结果表明,在萌动种子中,3个玉米自交系中的Na+积累量表现为种皮胚胚乳,K+累积表现为胚种皮胚乳;幼苗中,Na+积累表现为根茎叶。随着Na Cl浓度的增加,3个玉米自交系萌动种子和幼苗中的Na+含量逐渐升高,但是萌动种子中耐盐自交系81162和8723的Na+增加幅度小于盐敏感自交系P138,Na+含量小于盐敏感自交系P138;幼苗中耐盐自交系81162和8723的Na+增加幅度大于盐敏感自交系P138,幼苗根中Na+含量大于盐敏感自交系P138;茎叶中的Na+含量小于盐敏感自交系P138。随着Na Cl浓度的增加,萌动种子和幼苗中的K+和Ca2+含量逐渐降低。K+离子在耐盐自交系81162和8723萌动种子和幼苗中的降低幅度小于盐敏感自交系P138;Ca2+离子在耐盐自交系81162和8723幼苗中的降低幅度小于盐敏感自交系P138;而在萌动种子中3个自交系Ca2+的流失差异不大。耐盐自交系81162和8723萌动种子和幼苗中K+含量都大于盐敏感自交系P138。耐盐自交系81162和8723的萌动种子和幼苗根中Ca2+含量都大于盐敏感自交系P138;幼苗叶片中则小于盐敏感自交系P138。萌动种子和幼苗中K+/Na+和Ca2+/Na+均随着Na Cl浓度的升高而降低,K+/Na+比值表现为耐盐自交系81162和8723大于盐敏感自交系P138。耐盐自交系81162和8723通过调节离子平衡维持萌动种子和幼苗中较高的K+/Na+比值从而提高耐盐性。     

Salt resistance is determined by osmotic adjustment and abscisic acid in newly developed maize hybrids in the first phase of salt stress

This study investigated the mechanisms of salt resistance of four maize (Zea mays L.) hybrids [cultivar (cv.) Pioneer 3906 and newly developed hybrids SR03, SR12 and SR13] during the first phase of salt stress. Plants were grown in aerated nutrient solutions at 1 mM Na+ (control) and 100 mM Na+ (salt stress). Stress was imposed in 25 mM steps and plants were harvested after 2 days at 100 mM Na+. At 100 mM Na+ the area of the fourth leaf, which developed under salt stress, did not change significantly in SR03 and SR12 whereas significant reductions were observed in cv. Pioneer 3906 and SR13. Concentrations of assimilates (i.e. glucose, fructose and sucrose) in the shoot sap were significantly greater under salt stress in SR03 and SR12. However, the greater assimilate supply was not responsible for their salt resistance as there were no significant reductions in assimilate concentrations even in the other two genotypes. Shoot turgor and growth were maintained in SR03 and SR12 at 100 mM Na+ through significant increases in osmolality of the shoot sap. Concentrations of free ABA and ABA-glucose esters (ABA-GE) in the growing region of the fourth leaf increased significantly under salt stress in all genotypes. Leaf area at 100 mM Na(+), expressed as a percentage of that at 1 mM, showed significant positive relationships with free ABA (R(2) = 0.62) and the sum of free ABA and ABA-GE (R(2) = 0.65). Results of this study indicate clearly that a combination of partial osmotic adjustment, a possible reduction of the sensitivity of leaf growth under salt stress to increased ABA concentrations and a growth-promoting function regulated by ABA is responsible for salt resistance in the first phase of salt stress. Genotypic variation in these mechanisms can be utilized to breed salt-resistant genotypes in maize.     

Salinity and drought interaction in wheat (Triticum aestivum L.) is affected by the genotype and plant growth stage

Salinity and drought are important agro-environmental problems occurring separately as well as together with the combined occurrence increasing with time due to climate change. Screening of bread wheat genotypes against salinity or drought alone is common; however, little information is available on the response of wheat genotypes to a combination of these stresses. This study investigates the response of a salt-resistant (SARC-1) and a salt-sensitive (7-Cerros) wheat genotype to drought at different growth stages under non-saline (ECe 2.1 dS m^?1) and saline soil (ECe 15 dS m^?1) conditions. Drought was applied by withholding water for 21 days at a particular growth stage viz. tillering, booting, and grain filling stages. At booting stage measurements regarding water relations, leaf ionic composition and photosynthetic attributes were made. At maturity grain yield and different yield, components were recorded. Salinity and drought significantly decreased grain yield and different yield components with a higher decrease in the case of combined stress of salinity × drought. The complete drought treatment (drought at tillering + booting + grain filling stages) was most harmful for wheat followed by drought at booting stage and grain filling–tillering stages, respectively. The salt-resistant wheat genotype SARC-1 performed better than the salt-sensitive genotype 7-Cerros in different stress treatments. A decrease in the water and turgor potentials, photosynthetic and transpiration rates, stomatal conductance, leaf K⁺, and increased leaf Na⁺ were the apparent causes of growth and yield reduction of bread wheat due to salinity, drought, and salinity × drought.     

Aspects of Salt Tolerance in a NaCl-Selected Stable Cell Line of Citrus sinensis

A NaCl-tolerant cell line which was selected from ovular callus of `Shamouti' orange (Citrus sinensis L. Osbeck) proved to be a true cell line variant. This conclusion is based on the following observations. (a) Cells which have been removed from the selection pressure for at least four passages retain the same NaCl tolerance as do cells which are kept constantly on 0.2 molar NaCl. (b) Na⁺ and Cl⁻ uptake are considerably lower in salt-tolerant cells (R-10) than in salt-sensitive cells (L-5) at a given external NaCl concentration. (c) Growth of salt-tolerant cells is markedly suppressed upon replacement of NaCl by KCl, whereas the growth of salt-sensitive cells is only slightly affected. Accumulation of K⁺ and Cl⁻ accompanies the inhibition of growth. Experiments carried out with sodium and potassium sulfate suggest that the toxic effect is due to the accumulated Cl⁻. (d) Removal of Ca²⁺ from the growth medium severely inhibits the growth of salt-tolerant cells in the presence of NaCl, while it has a minor effect on growth of salt-sensitive cells in the presence of NaCl. (e) Electron micrographs show that the salt-tolerant cells have very big vacuoles when exposed to salt, while the size of the vacuoles of the salt-sensitive cells does not change.     

Differential regulation of Na+,K+-ATPase and the Na+-coupled glucose transporter in hypertensive rat kidney

Several Na⁺ transporters are functionally abnormal in the hypertensive rat. Here, we examined the effects of a high-salt load on renal Na⁺,K⁺-ATPase and the sodium-coupled glucose transporter (SGLT1) in Dahl salt-resistant (DR) and salt-sensitive (DS) rats. The protein levels of Na⁺,K⁺-ATPase and SGLT1 in the DS rat were the same as those in the DR rat, and were not affected by the high-salt load. In the DS rat, a high-salt load decreased Na⁺,K⁺-ATPase activity, and this decrease coincided with a decrease in the apparent Mechaelis constant (K_m) for ATP, but not with a change of maximum velocity (V_max). On the contrary, a high-salt load increased SGLT1 activity in the DS rat, which coincided with an increase in the V_max for α-methyl glucopyranoside. The protein level of phosphorylated tyrosine residues in Na⁺,K⁺-ATPase was decreased by the high-salt load in the DS rat. The amount of phosphorylated serine was not affected by the high-salt load in DR rats, and could not be detected in DS rats. On the other hand, the amount of phosphorylated serine residues in SGLT1 was increased by the high-salt load. However, the phosphorylated tyrosine was the same for all samples. Therefore, we concluded that the high-salt load changes the protein kinase levels in DS rats, and that the regulation of Na⁺,K⁺-ATPase and SGLT1 activity occurs via protein phosphorylation.     

The response of Mo-hydroxylases and abscisic acid to salinity in wheat genotypes with differing salt tolerances

The differential responses of the wheat cultivars Shi4185 and Yumai47 to salinity were studied. The higher sensitivity of Yumai47 to salinity was linked to a greater growth reduction under salt stress, compared to more salt-tolerant Shi4185. Salinity increased the Na⁺, proline and superoxide anion radical (O₂ ^?) contents in both cultivars. Leaf Na⁺ content increased less in the more salt-tolerant cultivar Shi4185 than salt-sensitive Yumai47. The proline content increased more significantly in Shi4185 than Yumai47; on the contrary, superoxide anion radical content increased less in Shi4185 than Yumai47. This data indicated that wheat salinity tolerance can be increased by controlling Na⁺ transport from the root to shoot, associated with higher osmotic adjustment capability and antioxidant activity. Although salinity increased aldehyde oxidase (AO) activity and abscisic acid (ABA) content in the leaves and roots of both cultivars following the addition of NaCl to the growth medium, AO and ABA increased more in the salt-sensitive cultivar Yumai47 than the more salt-tolerant cultivar Shi4185. Xanthine dehydrogenase (XDH) activity in the leaves of both cultivars increased with increasing concentrations of NaCl; however, leaf XDH activity increased more significantly in Yumai47 than Shi4185. Root XDH activity in Shi4185 decreased with increasing NaCl concentrations, whereas salinity induced an increased root XDH activity in Yumai47. The involvement of AO and XDH enzymatic activities and altered ABA content in the response mechanisms of wheat to salinity are discussed herein.     

Growth cycle stage-dependent NaCl induction of plasma membrane H+-ATPase mRNA accumulation in de-adapted tobacco cells

A cDNA clone encoding an isoform of the plasma membrane H⁺-ATPase was isolated from Nicotiana tabacum. The steady-state plasma membrane H⁺-ATPase message levels were the same in unadapted tobacco cells and tobacco cells adapted to 428 mol m⁻³ NaCl. When cells adapted to 428 mol m⁻³ NaCl maintained in the absence of NaCl (deadapted) for an excess of 100 passages were exposed to 400 mol m⁻³ NaCl for 24 h, there was an increased accumulation of plasma membrane H⁺-ATPase message. The NaCl responsiveness of the deadapted cells was dependent upon the growth cycle stage. Alterations in the levels of plasma membrane FT-ATPase message during the growth cycle support a role for the H⁺-ATPase in cell growth. These results document the induction by NaCl of plasma membrane FT-ATPase message accumulation in tobacco cells, and suggest that enhanced expression of the plasma membrane FT-ATPase has a role in the short term response of cells of NaCl, but is not necessarily involved in long-term adaptation.