Osmotic solute responses of mycorrhizal citrus (<Emphasis Type="Italic">Poncirus trifoliata</Emphasis>) seedlings to drought stress

This study investigated the accumulation of osmotic solutes in citrus (Poncirus trifoliata) seedlings colonized by Glomus versiforme subjected to drought stress or kept well watered. Development of mycorrhizae was higher under well watered than under drought-stressed conditions. Arbuscular mycorrhizal (AM) seedlings accumulated more soluble sugars, soluble starch and total non-structural carbohydrates in leaves and roots than corresponding non-AM seedlings regardless of soil-water status. Glucose and sucrose contents of well-watered and drought-stressed roots, fructose contents of well-watered roots and sucrose contents of drought-stressed leaves were notably higher in AM than in non-AM seedlings. K⁺ and Ca²⁺ levels in AM leaves and roots were greater than those in non-AM leaves and roots, while AM symbiosis did not affect the Mg²⁺ level. AM seedlings accumulated less proline than non-AM seedlings. AM symbiosis altered both the allocation of carbohydrate to roots and the net osmotic solute accumulations in response to drought stress. It is concluded that AM colonization enhances osmotic solute accumulation of trifoliate orange seedlings, thus providing better osmotic adjustment in AM seedlings, which did not correlate with proline but with K⁺, Ca²⁺, Mg²⁺, glucose, fructose and sucrose accumulation.     

Effect of calcium on growth of submerged <Emphasis Type="Italic">Terfezia boudieri</Emphasis> mycelium

The effect of Ca²⁺ on mycelial growth in Terfezia boudieri was studied. Terfezia boudieri Chatin (Ascomycotinae) occurs in mycorrhizal association with Helianthemum shrubs in deserts with calcareous soils. External Ca²⁺ stimulated mycelial growth in both liquid media and solidified substrates. The response to Ca²⁺ was very faint in well-aerated culture but pronounced in mycelia immersed in the medium, indicating dependence on mycelial aeration. 2,4-Dinitrophenol (DNP), an inhibitor of oxidative phosphorylation, and succinate, a potential cytoplasm acidifier, inhibited mycelial growth but enhanced the stimulatory effect of Ca²⁺. This effect was reduced by the Ca²⁺ channel blocker verapamil.     

Effects of NaCl and mannitol induced stress on sugarcane (<Emphasis Type="Italic">Saccharum</Emphasis> sp.) callus cultures

The effects of NaCl and mannitol iso-osmotic stresses on calli issued from sugarcane cultivars (cvs.) R570, CP59-73 and NCo310 were investigated in relation to callus growth, water content, ion and proline concentrations. Callus growth and water content decreased under both stresses with the highest reduction under mannitol-induced osmotic stress. The ion concentration was drastically affected after exposure to NaCl and mannitol. Salt stress induced an increase in Na⁺ and Cl⁻ accumulation and a decrease in K⁺ and Ca²⁺ concentrations. Under mannitol-induced osmotic stress, K⁺ and Ca²⁺ concentrations decreased significantly while Na⁺ and Cl⁻ concentrations remained unchanged. Free proline accumulation occurred under both stresses and was more marked in stress-sensitive cv. than in stress-resistant one. Our results indicated that the physiological mechanisms operating at the plant cell level in response to salt- and osmotic-induced stress in sugarcane cvs. are different. Among the cvs., we concluded that the stress resistance is closely related to the maintain of an adequate water status and a high level of K⁺ and Ca²⁺ under both stresses and a low level of Na⁺ concentration in the presence of NaCl. Thus, sugarcane (Saccharum sp.) can be regarded as a Na⁺ excluder. We also provided evidence that proline accumulation is a stress-sensitive trait rather than a stress resistance marker.     

Cyanobacteria MT gene <Emphasis Type="Italic">SmtA</Emphasis> enhance zinc tolerance in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Zinc is essential but toxic in excess. A bacterial metallothionein, SmtA from Synechococcus PCC 7942, has high affinity for Zn²⁺ and the intracellular exclusively handling of Zn²⁺. In this study, we report a functional analysis of SmtA in Arabidopsis thaliana and its response to zinc stress. After high zinc stress, the transgenic plants over-expressing SmtA showed higher survival rate than the wild type. We also found that over-expression of SmtA in Arabidopsis increased the activities of SOD and POD, and enhanced the tolerance to zinc stress. Together, our results indicate that SmtA may play an important role in the response to zinc stress in Arabidopsis.     

Calmodulin-like Proteins from <Emphasis Type="Italic">Arabidopsis</Emphasis> and Tomato are Involved in Host Defense Against <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">tomato</Emphasis>

Complex signal transduction pathways underlie the myriad plant responses to attack by pathogens. Ca²⁺ is a universal second messenger in eukaryotes that modulates various signal transduction pathways through stimulus-specific changes in its intracellular concentration. Ca²⁺-binding proteins such as calmodulin (CaM) detect Ca²⁺ signals and regulate downstream targets as part of a coordinated cellular response to a given stimulus. Here we report the characterization of a tomato gene (APR134) encoding a CaM-related protein that is induced in disease-resistant leaves in response to attack by Pseudomonas syringae pv. tomato. We show that suppression of APR134 gene expression in tomato (Solanum lycopersicum), using virus-induced gene silencing (VIGS), compromises the plant’s immune response. We isolated APR134-like genes from Arabidopsis, termed CML42 and CML43, to investigate whether they serve a functionally similar role. Gene expression analysis revealed that CML43 is rapidly induced in disease-resistant Arabidopsis leaves following inoculation with Pseudomonas syringae pv. tomato. Overexpression of CML43 in Arabidopsis accelerated the hypersensitive response. Recombinant APR134, CML42, and CML43 proteins all bind Ca²⁺ in vitro. Collectively, our data support a role for CML43, and APR134 as important mediators of Ca²⁺-dependent signals during the plant immune response to bacterial pathogens. This work was supported by a research grant (WAS) and postgraduate scholarships (DC, SLD) from the Natural Science and Engineering Research Council of Canada, the National Science Foundation (IBN-0109633; GBM), and the Swedish Research Council (SKE).     

Exogenous Ca<Superscript>2+</Superscript> alleviates nitrogen and water deficit,and improves growth of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) seedlings exposed to high temperature

This study was designed to examine whether exogenous Ca²⁺ would improve nitrogen nutrition, water status and growth of high temperature (HT)-stressed wheat (Triticum aestivum) seedlings. Wheat plants were exposed to 35/30 and 25/20°C as temperature control. Some of HT-stressed plants were simultaneously treated with 4 mM Ca²⁺. External Ca²⁺ could obviously improve growth of HT-exposed wheat seedlings indicated by the biomass. Compared with Ca²⁺-untreated plants, total nitrogen content showed a significant increase in Ca²⁺-treated plants under HT stress, this primarily resulted from enhanced nitrate reductase activity and depressed loss of ammonium through photorespiration. External Ca²⁺ application could also increase leaf relative water content and alleviate osmotic stress via increased K⁺ ion and water-soluble carbohydrates in HT-stressed plants. Whereas free proline content showed remarkable decline in Ca²⁺-treated plants at HT stress.     

Drought-induced oxidative damage and antioxidant responses in peanut (<Emphasis Type="Italic">Arachis</Emphasis><Emphasis Type="Italic">hypogaea</Emphasis> L.) seedlings

Two cultivars of peanut (Arachis hypogaea L.) which were designated as resistant (Florispan) and sensitive (Gazipasa) according to their growth retardation under drought stress conditions were compared for their oxidative damage and antioxidant responses. Sixteen days-old peanut seedlings were subjected to PEG-6000 solutions of two different osmotic potentials; −0.4 and −0.8 MPa, and various growth parameters, photosystem II activity, changes in malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and proline levels, activities of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POX) and gluthatione reductase (GR) enzymes were determined. Both cultivars exhibited water deficit at −0.8 MPa osmotic potential of PEG-6000 and H₂O₂ levels significantly increased during exposure to −0.4 MPa osmotic potential. However, H₂O₂ levels were under control in both cultivars at exposure to −0.8 MPa osmotic potential. Significant proline accumulation was observed in the tissues of cv. Florispan at −0.8 MPa osmotic potential, whereas proline accumulation did not appear to be an essential part of the protection mechanism against drought in cv. Gazipasa. No significant variation in chlorophyll fluorescence values were detected in neither of the cultivars. Enzyme activity measurements revealed that Gazipasa copes well with lesser magnitudes of drought stress by increasing the activity of mainly APX, and during harsh stress conditions, only APX maintains its activity in the tissues. In cultivar Florispan, GR activity appears to take role in lesser magnitudes of drought stress, whereas CAT and APX activities appear to be very crucial antioxidative defenses during intense stress conditions. The results indicate that, the level of proline and activities of the enzymes CAT and APX are important mechanisms for the maintenance of drought tolerance in peanut plants.     

<Emphasis Type="SmallCaps">l</Emphasis>-Proline nutrition and catabolism in <Emphasis Type="Italic">Staphylococcus saprophyticus</Emphasis>

Staphylococcus saprophyticus strains ATCC 15305, ATCC 35552, and ATCC 49907 were found to require l-proline but not l-arginine for growth in a defined culture medium. All three strains could utilize l-ornithine as a proline source and contained l-ornithine aminotransferase and Δ¹-pyrroline-5-carboxylate reductase activities; strains ATCC 35552 and ATCC 49907 could use l-arginine as a proline source and had l-arginase activity. The proline requirement also could be met by l-prolinamide, l-proline methyl ester, and the dipeptides l-alanyl-l-proline and l-leucyl-l-proline. The bacteria exhibited l-proline degradative activity as measured by the formation of Δ¹-pyrroline-5-carboxylate. The specific activity of proline degradation was not affected by addition of l-proline or NaCl but was highest in strain ATCC 49907 after growth in Mueller–Hinton broth. A membrane fraction from this strain had l-proline dehydrogenase activity as detected both by reaction of Δ¹-pyrroline-5-carboxylate with 2-aminobenzaldehyde (0.79 nmol min⁻¹ mg⁻¹) and by the proline-dependent reduction of p-iodonitrotetrazolium (20.1 nmol min⁻¹ mg⁻¹). A soluble fraction from this strain had Δ¹-pyrroline-5-carboxylate dehydrogenase activity (88.8 nmol min⁻¹ mg⁻¹) as determined by the NAD⁺-dependent oxidation of dl-Δ¹-pyrroline-5-carboxylate. Addition of l-proline to several culture media did not increase the growth rate or final yield of bacteria but did stimulate growth during osmotic stress. When grown with l-ornithine as the proline source, S. saprophyticus was most susceptible to the proline analogues L-azetidine-2-carboylate, 3,4-dehydro-dl-proline, dl-thiazolidine-2-carboxylate, and l-thiazolidine-4-carboxylate. These results indicate that proline uptake and metabolism may be a potential target of antimicrobial therapy for this organism.     

Heterologous expression of <Emphasis Type="Italic">Arabidopsis</Emphasis><Emphasis Type="Italic">NPR1</Emphasis> (<Emphasis Type="Italic">AtNPR1</Emphasis>) enhances oxidative stress tolerance in transgenic tobacco plants

In Arabidopsis, NPR1 (non-expressor of pathogenesis related genes 1, AtNPR1) functions downstream of salicylic acid (SA) and modulates the SA mediated systemic acquired resistance. It is also involved in a cross talk with the jasmonate pathway that is essential for resistance against herbivores and necrotrophic pathogens. Overexpression of AtNPR1 in transgenic plants resulted in enhanced disease resistance. Recently, tobacco transgenic plants expressing AtNPR1 were shown to be tolerant to the early instars of Spodoptera litura (Meur et al., Physiol Plant 133:765–775, 2008). In this communication, we show that the heterologous expression of AtNPR1 in tobacco has also enhanced the oxidative stress tolerance. The transgenic plants exhibited enhanced tolerance to the treatment with methyl viologen. This tolerance was associated with the constitutive upregulation of PR1, PR2 (glucanase), PR5 (thaumatin like protein), ascorbate peroxidase (APX) and Cu²⁺/Zn²⁺ superoxide dismutase (SOD). This is the first demonstration of the novel function of heterologous expression of AtNPR1 in oxidative stress tolerance in transgenic tobacco.     

Increased tolerance to salt stress in the phosphate-accumulating <Emphasis Type="Italic">Arabidopsis</Emphasis> mutants <Emphasis Type="Italic">siz1</Emphasis> and <Emphasis Type="Italic">pho2</Emphasis>

High salinity is an environmental factor that inhibits plant growth and development, leading to large losses in crop yields. We report here that mutations in SIZ1 or PHO2, which cause more accumulation of phosphate compared with the wild type, enhance tolerance to salt stress. The siz1 and pho2 mutations reduce the uptake and accumulation of Na⁺. These mutations are also able to suppress the Na⁺ hypersensitivity of the sos3-1 mutant, and genetic analyses suggest that SIZ1 and SOS3 or PHO2 and SOS3 have an additive effect on the response to salt stress. Furthermore, the siz1 mutation cannot suppress the Li⁺ hypersensitivity of the sos3-1 mutant. These results indicate that the phosphate-accumulating mutants siz1 and pho2 reduce the uptake and accumulation of Na⁺, leading to enhanced salt tolerance, and that, genetically, SIZ1 and PHO2 are likely independent of SOS3-dependent salt signaling.     

<Emphasis Type="Italic">N</Emphasis>-Acetyltransferase Mpr1 confers ethanol tolerance on <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> by reducing reactive oxygen species

N-Acetyltransferase Mpr1 of Saccharomyces cerevisiae can reduce intracellular oxidation levels and protect yeast cells under oxidative stress, including H₂O₂, heat-shock, or freeze-thaw treatment. Unlike many antioxidant enzyme genes induced in response to oxidative stress, the MPR1 gene seems to be constitutively expressed in yeast cells. Based on a recent report that ethanol toxicity is correlated with the production of reactive oxygen species (ROS), we examined here the role of Mpr1 under ethanol stress conditions. The null mutant of the MPR1 and MPR2 genes showed hypersensitivity to ethanol stress, and the expression of the MPR1 gene conferred stress tolerance. We also found that yeast cells exhibited increased ROS levels during exposure to ethanol stress, and that Mpr1 protects yeast cells from ethanol stress by reducing intracellular ROS levels. When the MPR1 gene was overexpressed in antioxidant enzyme-deficient mutants, increased resistance to H₂O₂ or heat shock was observed in cells lacking the CTA1, CTT1, or GPX1 gene encoding catalase A, catalase T, or glutathione peroxidase, respectively. These results suggest that Mpr1 might compensate the function of enzymes that detoxify H₂O₂. Hence, Mpr1 has promising potential for the breeding of novel ethanol-tolerant yeast strains.     

Responses to nonaeration and/or salinity stress in hydroponically cultured <Emphasis Type="Italic">Populus nigra</Emphasis> and <Emphasis Type="Italic">Populus alba</Emphasis> cuttings

Growth, photosynthesis, and Na⁺, K⁺, Ca²⁺, and Mg²⁺ distributions were examined in two-year-old hydroponically cultured Populus nigra and Populus alba cuttings exposed to salt stress (0, 50, or 100 mM NaCl) for four or six weeks and to nonaeration stress for one or three weeks, followed by a three-week aeration period in 2/5 Hoagland solution. Salt stress with 100 mM NaCl totally inhibited height increase in P. nigra cuttings. Combined salinity and nonaeration inhibited height increase to a greater degree than either stress alone in both species. Simple salt stress did not affect diameter increase in P. alba, whereas combined high salinity (100 mM NaCl) and nonaeration inhibited diameter increase. Growth and biomass accumulation were more sensitive to salt stress in P. nigra cuttings than in P. alba, although P. alba showed a more rapid decrease in photosynthesis in response to nonaeration stress. Ion distributions in the leaves and roots differed between species. P. alba was superior to P. nigra in terms of Na⁺ exclusion capacity, such that most of the absorbed Na⁺ was confined to the root system, with little reaching the leaves. The distributions of K⁺, Ca²⁺, and Mg²⁺ in the leaves and roots of each species under the two stressors were also analyzed. The lower Na⁺/K⁺ ratio in leaves indicated that P. alba was more tolerant to salt stress than P. nigra.     

Elicitation of diosgenin production in <Emphasis Type="Italic">Trigonella foenum</Emphasis>-<Emphasis Type="Italic">graecum</Emphasis> L. seedlings by heavy metals and signaling molecules

Seedlings of Trigonella foenum-graecum were treated with four heavy metal salts (CdCl_2, CoCl₂, K₂Cr₂O₇ and NiCl₂) to study the effect of heavy metals on growth and diosgenin production. It was found that CdCl₂ increased diosgenin production up to 40-fold and CoCl₂ increased diosgenin production up to 41-fold at concentrations which did not affect growth significantly. But K₂Cr₂O₇ and NiCl₂ were toxic to growth and inhibited diosgenin production. Effect of exogenously applied methyl jasmonate (MeJa) and calcium (Ca²⁺) on diosgenin production in seedlings of T. foenum-graecum was also investigated. MeJa enhanced the production of diosgenin. Maximum increase (10.5-fold) was found at 100 μL L^−l concentration of MeJa. To study the role of Ca²⁺ on diosgenin production, seedlings of T. foenum-graecum were treated with a promoter of Ca²⁺ influx (calcium ionophore A23187), calcium depleted medium, Ca²⁺ channel blocker (verapamil) and antagonist (LaCl₃), a divalent cation chelator (EGTA) and modulator of calcium release (caffeine). All the treatments were compared with a control containing 220 mg L^−l concentration of CaCl₂. The results suggest that the increase in cytosolic Ca²⁺ has an inhibitory role on diosgenin production. However, a calcium chelator or Ca²⁺ channel inhibitors could be used to elicit diosgenin production in this plant.     

Examination of the putative Ca<Superscript>2+</Superscript>-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium <Emphasis Type="Italic">Thermochromatium</Emphasis><Emphasis Type="Italic">tepidum</Emphasis>

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915 nm for the Q _y transition, and is highly stable when copurified with reaction center (RC) in a LH1–RC complex form. In previous studies, we demonstrated that the calcium ions are involved in both the large red shift and the enhanced thermal stability, and possible Ca²⁺-binding sites were proposed. In this study, we further examine the putative binding sites in the LH1 polypeptides using purified chromatophores. Incubation of the chromatophores in the presence of EDTA revealed no substantial change in the absorption maximum of LH1 Q _y transition, whereas further addition of detergents to the chromatophores-EDTA solution resulted in a blue-shift for the LH1 Q _y peak with the final position at 892 nm. The change of the LH1 Q _y peak to shorter wavelengths was relatively slow compared to that of the purified LH1–RC complex. The blue-shifted LH1 Q _y transition in chromatophores can be restored to its original position by addition of Ca²⁺ ions. The results suggest that the Ca²⁺-binding site is exposed on the inner surface of chromatophores, corresponding to the C-terminal region of LH1. An Asp-rich fragment in the LH1 α-polypeptide is considered to form a crucial part of the binding network. The slow response of LH1 Q _y transition upon exposure to EDTA is discussed in terms of the membrane environment in the chromatophores.     

Osmotic stress response in <Emphasis Type="Italic">C</Emphasis>. <Emphasis Type="Italic">glutamicum</Emphasis>: impact of channel- and transporter-mediated potassium accumulation

Potassium accumulation is an essential aspect of bacterial response to diverse stress situations; consequently its uptake plays a pivotal role. Here, we show that the Gram-positive soil bacterium Corynebacterium glutamicum which is employed for the large-scale industrial production of amino acids requires potassium under conditions of ionic and non-ionic osmotic stress. Besides the accumulation of high concentrations of potassium contributing significantly to the osmotic potential of the cytoplasm, we demonstrate that glutamate is not the counter ion for potassium under these conditions. Interestingly, potassium is required for the activation of osmotic stress-dependent expression of the genes betP and proP. The Kup-type potassium transport system which is present in C. glutamicum in addition to the potassium channel CglK does not contribute to potassium uptake at conditions of hyperosmotic stress. Furthermore, we established a secondary carrier of the KtrAB type from C. jeikeium in C. glutamicum thus providing an experimental comparison of channel- and carrier-mediated potassium uptake under osmotic stress. While at low potassium availability, the presence of the KtrAB transporter improves both potassium accumulation and growth of C. glutamicum upon osmotic stress, at proper potassium supply, the channel CglK is sufficient.     

The <Emphasis Type="Italic">Arabidopsis</Emphasis><Emphasis Type="Italic"> cax3</Emphasis> mutants display altered salt tolerance,pH sensitivity and reduced plasma membrane H<Superscript>+</Superscript>-ATPase activity

Perturbing CAX1, an Arabidopsis vacuolar H⁺/Ca²⁺ antiporter, and the related vacuolar transporter CAX3, has been previously shown to cause severe growth defects; however, the specific function of CAX3 has remained elusive. Here, we describe plant phenotypes that are shared among cax1 and cax3 including an increased sensitivity to both abscisic acid (ABA) and sugar during germination, and an increased tolerance to ethylene during early seedling development. We have also identified phenotypes unique to cax3, namely salt, lithium and low pH sensitivity. We used biochemical measurements to ascribe these cax3 sensitivities to a reduction in vacuolar H⁺/Ca²⁺ transport during salt stress and decreased plasma membrane H⁺-ATPase activity. These findings catalog an array of CAX phenotypes and assign a specific role for CAX3 in response to salt tolerance.     

Effects of Calcium and Calmodulin Antagonists on Chilling Stress-Induced Proline Accumulation in <Emphasis Type="Italic">Jatropha curcas</Emphasis> L.

Regulation of proline accumulation in plants under chilling stress remains unclear. In this paper, we treated Jatropha curcas seedlings under chilling stress with exogenous calcium chloride (CaCl₂), the plasma membrane Ca²⁺-channel blocker lanthanum chloride (LaCl₃), calmodulin antagonists, chlorpromazine (CPZ), and trifluoperazine (TFP) and investigated the effects of calcium and calmodulin (CaM) on proline accumulation and chilling tolerance. The results showed that CaCl₂ treatment significantly enhanced chilling stress-induced proline accumulation. CaCl₂ also induced an almost immediate and rapid increase of Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and glutamate dehydrogenase activities, the key enzymes in the glutamate pathway of proline biosynthesis, and up-regulated P5CS expression, but it decreased the activity of proline dehydrogenase (ProDH), a key enzyme of proline degradation, and inhibited ProDH expression. Treatment with LaCl₃, CPZ, and TFP exhibited the opposite effects to those by CaCl₂ treatment. Moreover, CaCl₂, LaCl₃, CPZ, and TFP had little effect on the activities of ornithine aminotransferase and arginase, the key enzymes in the ornithine pathway of proline biosynthesis. These results indicated that Ca²⁺-CaM might be involved in signal transduction events, leading to proline accumulation in J. curcas seedlings under chilling stress, and that Ca²⁺-induced proline accumulation is a combined result of the activation of the glutamate pathways of proline biosynthesis and the simultaneous inhibition of the proline degradation pathway. In addition, CaCl₂ treatment increased tissue vitality, decreased the content of the lipid peroxidation product malondialdehyde (MDA), and alleviated electrolyte leakage in J. curcas seedlings under chilling stress, indicating that exogenous Ca²⁺ can enhance chilling tolerance, and proline might be a key factor in this increased chilling tolerance.     

Enhanced intracellular Ca<Superscript>2+</Superscript> concentrations in <Emphasis Type="Italic">Escherichia coli</Emphasis> and <Emphasis Type="Italic">Bacillus subtilis</Emphasis> after addition of oligosaccharide elicitors

Elicitation can lead to overproduction of secondary metabolites in plants and microbes. Potential changes in cytosolic Ca²⁺ levels in bacteria were studied in response to elicitation. We report, for the first time, the effect of oligosaccharide elicitors on intracellular Ca²⁺ levels. The apoaequorin gene was cloned into Escherichia coli DH5α and Bacillus subtilis 1604 cultures. Addition of elicitors, oligoguluronate and mannan oligosaccharides, to the cultures caused up to 11-fold increase in cytosolic Ca²⁺ in E. coli and tenfold increase in B. subtilis. These increases in Ca²⁺ levels could therefore contribute to the enhancement of secondary metabolite levels.     

Improved salt tolerance in a wheat stay-green mutant <Emphasis Type="Italic">tasg1</Emphasis>

Salt stress inhibited the growth of both tasg1 and wild-type (WT) wheat seedlings, but the inhibition in tasg1 plants was relatively weaker than that of WT. Compared to the WT, the chlorophyll content, thylakoid membrane polypeptides, Hill reaction activity, actual photochemical efficiency of PSII (ΦPSII), and Mg²⁺- and Ca²⁺-ATPase activities were higher in tasg1 under salt stress. At the same time, the photosynthetic activity of the tasg1 was significantly higher than that of WT. In addition, tasg1 plants displayed relatively less accumulation of reactive oxygen species and oxidative damage accompanied by higher activity of some antioxidant enzymes, and the up-regulation of antioxidant genes further demonstrated the improvement of antioxidant activity in tasg1 under salt stress. Furthermore, tasg1 plants also showed relatively weaker Na⁺ fluorescence and lower Na⁺ content, but relatively higher content of K⁺ in their roots and shoots, and then, the roots of tasg1 plants enhanced net outward Na⁺ flux and a correspondingly increased net inward K⁺ flux during salt stress. This might be associated with the relatively higher activity of H⁺-ATPase in tasg1 plants. These results suggest that the improved antioxidant competence and Na⁺/K⁺ ion homeostasis play an important role in the enhanced salinity tolerance of tasg1 plants.