Severe Salt Stress Research Articles

Expression of animal anti-apoptotic gene Ced-9 enhances tolerance during Glycine max L.-Bradyrhizobium japonicum interaction under saline stress but reduces nodule formation.

The mechanisms by which the expression of animal cell death suppressors in economically important plants conferred enhanced stress tolerance are not fully understood. In the present work, the effect of expression of animal antiapoptotic gene Ced-9 in soybean hairy roots was evaluated under root hairs and hairy roots death-inducing stress conditions given by i) Bradyrhizobium japonicum inoculation in presence of 50 mM NaCl, and ii) severe salt stress (150 mM NaCl), for 30 min and 3 h, respectively. We have determined that root hairs death induced by inoculation in presence of 50 mM NaCl showed characteristics of ordered process, with increased ROS generation, MDA and ATP levels, whereas the cell death induced by 150 mM NaCl treatment showed non-ordered or necrotic-like characteristics. The expression of Ced-9 inhibited or at least delayed root hairs death under these treatments. Hairy roots expressing Ced-9 had better homeostasis maintenance, preventing potassium release; increasing the ATP levels and controlling the oxidative damage avoiding the increase of reactive oxygen species production. Even when our results demonstrate a positive effect of animal cell death suppressors in plant cell ionic and redox homeostasis under cell death-inducing conditions, its expression, contrary to expectations, drastically inhibited nodule formation even under control conditions.

Growth, sodium uptake and antioxidant responses of coastal plants differing in their ecological status under increasing salinity

AbstractIn the present study, we compared the response to salinity of three plants from Brittany coast with contrasted ecological status: Limonium latifolium (salt marshes), Matricaria maritima (beach tops and sand dunes) and Crambe maritima (fixed dunes). Under controlled glasshouse conditions, the growth of the three plants decreased with increasing external salinity. L. latifolium and C. maritima exhibited the highest and lowest resistance to severe salt stress (400 mM), respectively. M. maritima could be considered as an intermediate species, since it tolerated salinity up to 200 mM. The same observation could be made with sodium absorption and acuumulation in plant tissues, the most tolerant species (L. latifolium being the least Na accumulator. Hydrogen peroxide (H2O2) and malondialdehyde (MDA), commonly produced in conditions of stress, accumulated significantly in salt treated C. maritima and M. maritima while not in the tolerant L. latifolium. The latter used glutathione reductase to maintain constant H2O2 levels under salt stress while peroxidases were very low and ascorbate peroxidase did not respond to salinity stimulation. The medium tolerant halophyte M. maritima used peroxidases to protect from NaCl-induced H2O2, while the sensitive C. maritima failed to detoxify H2O2 despite a sharp increase in catalase activity. Results showed that the three coastal species differ in resistance to salinity. They also suggested that the level of plant resistance to salinity could be attributed to differing mechanisms to manage the accumulation of sodium and cope with the oxidative damages.

Insertion of a Specific Fungal 3′-phosphoadenosine-5′-phosphatase Motif into a Plant Homologue Improves Halotolerance and Drought Tolerance of Plants

Soil salinity and drought are among the most serious agricultural and environmental problems of today. Therefore, investigations of plant resistance to abiotic stress have received a lot of attention in recent years. In this study, we identified the complete coding sequence of a 3′-phosphoadenosine-5′-phosphatase protein, ApHal2, from the halotolerant yeast Aureobasidium pullulans. Expression of the ApHAL2 gene in a Saccharomyces cerevisiae hal2 mutant complemented the mutant auxotrophy for methionine, and rescued the growth of the hal2 mutant in media with high NaCl concentrations. A 21-amino-acids-long region of the ApHal2 enzyme was inserted into the Arabidopsis thaliana homologue of Hal2, the SAL1 phosphatase. The inserted sequence included the META motif, which has previously been implicated in increased sodium tolerance of the Hal2 homologue from a related fungal species. Transgenic Arabidopsis plants overexpressing this modified SAL1 (mSAL1) showed improved halotolerance and drought tolerance. In a medium with an elevated salt concentration, mSAL1-expressing plants were twice as likely to have roots in a higher length category in comparison with the wild-type Arabidopsis and with plants overexpressing the native SAL1, and had 5% to 10% larger leaf surface area under moderate and severe salt stress, respectively. Similarly, after moderate drought exposure, the mSAL1-expressing plants showed 14% increased dry weight after revitalisation, with no increase in dry weight of the wild-type plants. With severe drought, plants overexpressing native SAL1 had the worst rehydration success, consistent with the recently proposed role of SAL1 in severe drought. This was not observed for plants expressing mSAL1. Therefore, the presence of this fungal META motif sequence is beneficial under conditions of increased salinity and moderate drought, and shows no drawbacks for plant survival under severe drought. This demonstrates that adaptations of extremotolerant fungi should be considered as a valuable resource for improving stress-tolerance in plant breeding in the future.

EFFECT OF ALKALINE POTASSIUM AND SODIUM SALTS ON GROWTH, PHOTOSYNTHESIS, IONS ABSORPTION AND SOLUTES SYNTHESIS OF WHEAT SEEDLINGS

SUMMARYPotassium (K) is an essential nutrient and abundant cation in plant cells. The application of K+could alleviate abiotic stress. However, it was reported that the alleviation of K+on salt-stressed plants only happened when K+concentration was low. Most studies were focused on effects of sodium salts on plants in salty soils, and little information was reported about potassium salts, especially a higher level of potassium in alkaline salts. To explore the effects of K+in alkaline salts on plant growth, and whether it had a same destructive impact as Na+, we mixed two alkaline sodium salts (ASS) (NaHCO3:Na2CO3= 9:1) and two alkaline potassium salts (APS) (KHCO3:K2CO3= 9:1) to treat 10-day-old wheat seedlings. Effects of ASS and APS on growth, photosynthesis, ions absorption and solutes accumulation were compared. Results indicated that effects of potassium salts in soil on plants growth were related to K+concentration. Both growth and photosynthesis of wheat seedlings decreased, and the reduction was higher in APS treatment than in ASS treatment at 40 mM alkalinity. ASS treatment absorbed Na+, competing with K+and free Ca2+, and inhibited the absorption of inorganic anions. APS treatments accumulated K+and reduced the absorption of anions, with no competition with other cations. Both APS and ASS treatments promoted free Mg2+accumulation and inhibited H2PO4−uptake. The reduction of H2PO4−promoted organic acid synthesis indirectly. Soluble sugar and proline accumulation were also related to the alkaline condition and extra K+addition. In conclusion, excess potassium ions in soil, especially in alkaline soils, were harmful to plants. APS was another severe salt stress, intensity of which was higher than ASS. The growth and physiological response mechanisms of wheat seedlings to APS were similar to ASS. Both inorganic ions and organic solutes took part in the osmotic adjustment. Differences for APS depended on K+, but ASS on Na+.

Proline and its metabolism enzymes in cucumber cell cultures during acclimation to salinity

Proline is an important osmolyte appearing as the result of salt stress response of plants. In the present study, we measured the proline concentration, activities of pyrroline-5-carboxylate synthetase (P5CS), pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (PDH) key regulatory enzymes in the biosynthesis and degradation of proline in the acclimated (AC20) and the non-acclimated (NAC) cucumber cell suspension cultures subjected to moderate (150 mM NaCl; AC20-150, NAC-150, respectively) and severe (200 mM NaCl; AC20-200, NAC-200, respectively) salt stress. The data showed that salt stress brought about a linear increase in proline content in both types of cultures. However, in the acclimated culture proline accumulation was observed earlier, in third hour after stress. Only in the acclimated culture moderate and severe stresses up-regulated P5CS activity throughout the experiment, whereas the activity of P5CR grew in response to both NaCl concentrations only in 24th and 48th hour. The severe salt stress resulted in decrease in P5CR in NAC-200 cultures. In response to salt stress, both types of cell suspension cultures reacted with decline in PDH activity below the spectrophotometrically detected level. Cell cultures vigor correlated with salt concentration and time of exposure to the stress factor. Both NaCl concentrations caused linear decline in vigor of the non-acclimated culture up to 80-90 % at the end of the experiment, whereas in the acclimated culture significant decrease by about 30-40 % was reached in 24th hour after stress. The presented data suggest that acclimation to salt stress up-regulated proline synthesis enzyme activity and caused intensive accumulations of proline by inhibiting its oxidation.

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na+ and Cl− content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.

Physiological and proteomic characterization of salt tolerance in a mangrove plant, Bruguiera gymnorrhiza (L.) Lam

Salinity is a major abiotic stress that is responsible for growth reduction in most higher plants. Bruguiera gymnorrhiza (L.) Lam., a mangrove plant, is a halophyte and is one of the most salt-tolerant plant species. Physiological and proteomic characteristics of B. gymnorrhiza were investigated under three NaCl concentrations (0, 200 and 500 mM) in this study. Maximum seedling growth occurred at 200 mM NaCl. Leaf osmotic potential was more negative as salt levels increased further. Physiological results revealed that inorganic ions (especially Na(+) and Cl(-)) played a key role in osmotic adjustment of B. gymnorrhiza leaves under salinity treatments. Comparative proteomic analysis revealed 23 salt-responsive proteins in B. gymnorrhiza leaves, which were differentially expressed under salt treatment compared with control. Ten protein spots were analyzed by liquid chromatography-tandem mass spectrometry, leading to identification of proteins involved in photosynthesis, antioxidation, protein folding, cell organization and metabolism. Salt-responsive mechanism was different between 200 and 500 mM NaCl-treated plants on the basis of the physiological and proteomic analyses. Salt tolerance under 200 mM NaCl treatment was due to effective osmotic adjustment, accumulation of inorganic ions (especially Na(+) and Cl(-)) as well as increased expression of photosynthesis-related proteins and antioxidant enzymes, which improved the salt tolerance of B. gymnorrhiza, and furthermore promoted plant growth. On the other hand, 500 mM NaCl reduced the growth of B. gymnorrhiza, which appears to have been caused by the accumulation of NaCl (ionic effect) and energy consumption by organic solute synthesis. Moreover, the repressed expression of photosynthesis-related proteins and antioxidant enzymes led to the reduction of growth. Protein folding and degradation-related proteins and cell organization-related protein were up-regulated and played important roles in salt tolerance of B. gymnorrhiza under severe salt stress.

Difference in Sodium Spatial Distribution in the Shoot of Two Canola Cultivars Under Saline Stress

Among different mechanisms of salt resistance, regulation of ion distribution among various tissues and intracellular compartmentation are of great importance. In this study, we investigated the effects of salt stress on growth, photosynthesis, and Na(+) accumulation and distribution in leaf apoplast and symplast of two canola (Brassica napus L.) cultivars (NYY 1 and BZY 1). The results showed that the declines in shoot dry mass, leaf water potential and net photosynthetic rate of BZY 1 (salt sensitive) were higher than those of NYY 1 (salt resistant) in response to salt stress. Stomatal limitation to photosynthesis was mainly affected under moderate salinity, whereas the reduction in assimilation rate under severe salt stress was due to both stomatal and non-stomatal limitations. We also found that more Na(+) was distributed to leaf veins in NYY 1 than in BZY 1; simultaneously, less Na(+) accumulated in the leaf blade in NYY 1 than in BZY 1. The percentage of Na(+) in the leaf symplast in NYY 1 was markedly lower than that in BZY 1. Also, Na(+) diffusion in leaves through apoplastic and symplastic pathways of BZY 1 was stronger than that in NYY 1, and the transpiration rate in BZY 1, especially at the leaf edges, decreased more than in NYY 1. Our results showed that NYY 1 accumulated less Na(+) in the shoot, especially in leaf blades, and confined Na(+) to the apoplast to avoid leaf salt toxicity, which could be one reason for the higher resistance of NYY 1 than BZY 1 plants to salt stress.

Redox state of low-molecular-weight thiols and disulphides during somatic embryogenesis of salt-treated suspension cultures of Dactylis glomerata L.

The tripeptide antioxidant γ-L-glutamyl-L-cysteinyl-glycine, or glutathione (GSH), serves a central role in ROS scavenging and oxidative signalling. Here, GSH, glutathione disulphide (GSSG), and other low-molecular-weight (LMW) thiols and their corresponding disulphides were studied in embryogenic suspension cultures of Dactylis glomerata L. subjected to moderate (0.085 M NaCl) or severe (0.17 M NaCl) salt stress. Total glutathione (GSH + GSSG) concentrations and redox state were associated with growth and development in control cultures and in moderately salt-stressed cultures and were affected by severe salt stress. The redox state of the cystine (CySS)/2 cysteine (Cys) redox couple was also affected by developmental stage and salt stress. The glutathione half-cell reduction potential (EGSSG/2 GSH) increased with the duration of culturing and peaked when somatic embryos were formed, as did the half-cell reduction potential of the CySS/2 Cys redox couple (ECySS/2 Cys). The most noticeable relationship between cellular redox state and developmental state was found when all LMW thiols and disulphides present were mathematically combined into a ‘thiol–disulphide redox environment’ (Ethiol–disulphide), whereby reducing conditions accompanied proliferation, resulting in the formation of pro-embryogenic masses (PEMs), and oxidizing conditions accompanied differentiation, resulting in the formation of somatic embryos. The comparatively high contribution of ECySS/2 Cys to Ethiol–disulphide in cultures exposed to severe salt stress suggests that Cys and CySS may be important intracellular redox regulators with a potential role in stress signalling.

Selective transport capacity for K+ over Na+ is linked to the expression levels of PtSOS1 in halophyte Puccinellia tenuiflora.

The plasma membrane Na+/H+ antiporter (SOS1) was shown to be a Na+ efflux protein and also involved in K+ uptake and transport. PtSOS1 was characterised from Puccinellia tenuiflora (Griseb.) Scribn. et Merr., a monocotyledonous halophyte that has a high selectivity for K+ over Na+ by roots under salt stress. To assess the contribution of PtSOS1 to the selectivity for K+ over Na+, the expression levels of PtSOS1 and Na+, K+ accumulations in P. tenuiflora exposed to different concentrations of NaCl, KCl or NaCl plus KCl were analysed. Results showed that the expression levels of PtSOS1 in roots increased significantly with the increase of external NaCl (25-150mM), accompanied by an increase of selective transport (ST) capacity for K+ over Na+ by roots. Transcription levels of PtSOS1 in roots and ST values increased under 0.1-1mM KCl, then declined sharply under 5-10mM KCl. Under 150mM NaCl, PtSOS1 expression levels in roots and ST values at 0.1mM KCl was significantly lower than that at 5mM KCl with the prolonging of treatment time. A significant positive correlation was found between root PtSOS1 expression levels and ST values under various concentrations of NaCl, KCl or 150mM NaCl plus 0.1 or 5mM KCl treatments. Therefore, it is proposed that PtSOS1 is the major component of selective transport capacity for K+ over Na+ and hence, salt tolerance of P. tenuiflora. Finally, we hypothesise a function model of SOS1 in regulating K+ and Na+ transport system in the membrane of xylem parenchyma cells by sustaining the membrane integrity; it also appears that this model could reasonably explain the phenomenon of Na+ retrieval from the xylem when plants are exposed to severe salt stress.

Severe salt stress in Vaccinium myrtillus (L.) in response to Na + ion toxicity

NaCl is spread on the northern roads as a winter de-icing agent. Na + stress may occur in roadside forests especially during growing season, because snowmelt increases Na + concentrations in the roadside forest soil. To simulate summertime Na + stress and its effect on viability and anthocyanin concentrations, bilberry ( Vaccinium myrtillus L.) plants were subjected to NaCl (0, 6, 30 and 60 g m −2) in a boreal mesic heath forest in northern Finland (65°N). It was hypothesised that Na + stress decreases tissue water content (TWC) in below- and aboveground stems, where resulting water stress increases anthocyanin level for osmotic regulation. Uptake of Na + from the soil to the below- and aboveground stems was detected by a fast sequential atomic absorption spectrometry. Na + accumulated in belowground stems, but it was translocated into the aboveground stems to a lesser extent. At the end of the growing season, TWC increased in belowground stems and decreased in aboveground stems. Also anthocyanin concentrations decreased in aboveground stems. The viability of the aboveground stems decreased to 30% along with the Na + accumulation. Despite the strong viability response in aboveground stems, TWC was constant in the middle of the growing season and decreased relatively little at the end of the growing season. It is thus proposed that direct effect of Na + ions, rather than Na +-induced water stress, is the primary reason behind the strong viability response and a severe salt stress in bilberry.

Induced over-expression of the transcription factor OsDREB2A improves drought tolerance in rice

The DREB and CBF transcription factors play a critical role in plant development and abiotic stress responses and, therefore, represent attractive targets for a molecular plant breeding approach. In this study, the rice OsDREB2A gene was isolated and expressed under the control of a stress-inducible promoter (4ABRC) to improve the abiotic stress tolerance of japonica rice variety TNG67. T₂ and T₃ transgenic lines over-expressing OsDREB2A were found to have improved survival rates under severe drought and salt stress conditions relative to non-transgenic rice plants or rice plants transformed with the empty vector control. OsDREB2A expression was found to be markedly induced by drought and ABA treatment. The results indicate that the induced over-expression of OsDREB2A driven by the 4ABRC promoter in engineered rice plants may protect cells during stress.

Nitric oxide modulates antioxidant defense and the methylglyoxal detoxification system and reduces salinity-induced damage of wheat seedlings

The present study investigates the possible regulatory role of exogenous nitric oxide (NO) in antioxidant defense and methylglyoxal (MG) detoxification systems of wheat seedlings exposed to salt stress (150 and 300 mM NaCl, 4 days). Seedlings were pre-treated for 24 h with 1 mM sodium nitroprusside, a NO donor, and then subjected to salt stress. The ascorbate (AsA) content decreased significantly with increased salt stress. The amount of reduced glutathione (GSH) and glutathione disulfide (GSSG) and the GSH/GSSG ratio increased with an increase in the level of salt stress. The glutathione S-transferase (GST) activity increased significantly with severe salt stress (300 mM). The ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT) and glutathione peroxidase (GPX) activities did not show significant changes in response to salt stress. The glutathione reductase (GR), glyoxalase I (Gly I), and glyoxalase II (Gly II) activities decreased upon the imposition of salt stress, especially at 300 mM NaCl, with a concomitant increase in the H2O2 and lipid peroxidation levels. Exogenous NO pre-treatment of the seedlings had little influence on the non-enzymatic and enzymatic components compared to the seedlings of the untreated control. Further investigation revealed that NO pre-treatment had a synergistic effect; that is, the pre-treatment increased the AsA and GSH content and the GSH/GSSG ratio, as well as the activities of MDHAR, DHAR, GR, GST, GPX, Gly I, and Gly II in most of the seedlings subjected to salt stress. These results suggest that the exogenous application of NO rendered the plants more tolerant to salinity-induced oxidative damage by enhancing their antioxidant defense and MG detoxification systems.

Selenium-Induced Up-Regulation of the Antioxidant Defense and Methylglyoxal Detoxification System Reduces Salinity-Induced Damage in Rapeseed Seedlings

The present study investigates the regulatory role of exogenous selenium (Se) in the antioxidant defense and methylglyoxal (MG) detoxification systems in rapeseed seedlings exposed to salt stress. Twelve-day-old seedlings, grown in Petri dishes, were supplemented with selenium (25μM Na(2)SeO(4)) and salt (100 and 200mM NaCl) separately and in combination, and further grown for 48h. The ascorbate (AsA) content of the seedlings decreased significantly with increased salt stress. The amount of reduced glutathione (GSH) and glutathione disulfide (GSSG) increased with an increase in the level of salt stress, while the GSH/GSSG ratio decreased. In addition, the ascorbate peroxidase (APX) and glutathione S-transferase (GST) activity increased significantly with increased salt concentration (both at 100 and 200mM NaCl), while glutathione peroxidase (GPX) activity increased only at moderate salt stress (100mM NaCl). Glutathione reductase (GR) activity remained unchanged at 100mM NaCl, while it was decreased under severe (200mM NaCl) salt stress. Monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) activities decreased upon the imposition of salt stress, whereas a sharp decrease of these activities was observed under severe salt stress (200mM NaCl). Concomitant increases in the levels of H(2)O(2) and lipid peroxidation (MDA) were also measured. Exogenous Se treatment alone had little effect on the non-enzymatic and enzymatic components. However, further investigation revealed that Se treatment had a synergistic effect: in salt-stressed seedlings, it increased the AsA and GSH contents; GSH/GSSG ratio; and the activities of APX, MDHAR, DHAR, GR, GST, GPX, CAT, Gly I, and Gly II. As a result, addition of Se in salt-stressed seedlings led to a reduction in the levels of H(2)O(2) and MDA as compared to salt stress alone. These results suggest that the exogenous application of Se rendered the plants more tolerant to salt stress-induced oxidative damage by enhancing their antioxidant defense and MG detoxification systems.

Differential regulation of photorespiratory gene expression by moderate and severe salt and drought stress in relation to oxidative stress

The tolerant C3 plant Pancratium maritimum L. was used to investigate the contribution of photorespiration to the oxidative load under salt and drought stress. 7-weeks old plants were salt-stressed by 150 or 300mM NaCl or drought-stressed by withholding water for 11 or 21 days. The growth, photosynthesis (A) and transpiration rates (E) were reduced by all stress treatments proportionally to the severity of stress. The rate of photorespiration was remarkably higher under moderate stresses than under severe stresses as revealed by large increase in the photorespiratory indicators Gly/Ser ratio and glyoxylate content and as substantiated by higher expression levels of photorespiratory enzymes. Nonetheless, indicators of oxidative stress namely, malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents showed greater increase under severe stresses suggesting that the increase in the oxidative load under severe stresses is independent of photorespiration rate. The lower contents of MDA and H2O2 under moderate stresses (compared to these under severe stresses) in spite of the strong upregulation of photorespiration indicate efficient detoxification of photorespiration-generated H2O2 as shown by efficient upregulation of catalase (CAT) and peroxidase (POD). It is thus concluded that photorespiration may not be major contributor to the oxidative load under salt and drought stress.

COMPARISON OF OSMOTIC REGULATION IN DEHYDRATION- AND SALINITY-STRESSED SUNFLOWER SEEDLINGS

Plant dry matter accumulation rate (DMAR), relative water content (RWC), electrolyte leakage percentage (ELP), chlorophyll content, osmotic adjustment ability (OAA), and osmotica accumulation in leaves of sunflower (Helianthus annuus L.) seedlings under different levels of dehydration and salinity stress induced by iso-osmotic PEG (polyethylene glycol) or sodium chloride (NaCl) were evaluated. Plants were subjected to four stress treatments for 10 days: −0.44 MPa PEG6000, −0.44 MPa NaCl, −0.88 MPa PEG6000, −0.88 MPa NaCl. Results showed that PEG and NaCl treatments decreased the plant's DMAR and RWC, and NaCl treatments had more severe inhibitory effect on the plants than PEG treatments. Leaf ELP in sunflower seedlings increased after NaCl and PEG treatments. However, leaf ELP under salt stress was higher than that under dehydration stress (PEG treatment). All stress treatments increased OAA in plant leaves. Leaf OAA was enhanced significantly as PEG concentration increases, while leaf OAA was less enhanced at higher concentration of NaCl. OAA of sunflower leaves under dehydration stress was due to an increase in potassium (K+), calcium (Ca2+), amino acid, organic acid, magnesium (Mg2+), and proline content. OAA of sunflower leaves under moderate salt stress was owing to an increase in K+, chlorine (Cl−), amino acid, organic acid, sodium (Na+), and proline content, and was mainly due to an accumulation of K+, Cl−, Na+, and proline under severe salt stress.

Changes in Free,Conjugated and Bound Polyamine Content in Salt Adaptation of Vetiver Grass (Vetiveria zizanioides,Poaceae)

Polyamines (PAs) have been suggested to play roles in plant salt stress adaptation.In this study,alteration of free,conjugated and bound PAs [putrescine (Put),cadaverine (Cad),spermidine (Spd) and spermine (Spm)] in roots and leaves of vetiver grass (Vetiveria zizanioides) seedlings in response to salt stress were investigated.Under moderate salt stresses (100 and 200 mmol L-1 NaCl) for 9 days,vetiver grass grew at similar vigor compared with seedlings under normal growth conditions.However,growth was severely arrested when plants were treated with severe salt stress (300 mmol L-1 NaCl).Under three different concentrations of NaCl stress mentioned above,the contents of free Put,Cad,Spd,Spm and total free PA substantially decreased in roots and leaves,and more severe loss of free PAs was observed under higher NaCl concentration.Conjugated Put,Cad,Spd,Spm,and total conjugated PAs remarkably increased,and the extent of the increase after 300 mmol L-1 NaCl treatment was smaller than those after 100 or 200 mmol L-1 NaCl treatments.Bound Put,Cad and total bound PAs decreased in both roots and leaves under salt stress,moreover,bound Spd and Spm decreased in leaves while increased in roots in response to salt treatments,and the more obvious rise was displayed under moderate salt stresses.With the exception of the significant decreases of free (Spd+Spm)/Put ratio in roots and leaves after 300 mmol L-1 NaCl treatment,other ratios of free,conjugated,bound and the total (Spd+Spm)/Put increased in roots and leaves of vetiver grass seedlings after different salt treatments,and especially under moderate salt stress.These results indicated that maintaining homeostasis of total PAs content and high (Spd+Spm)/Put ratios could be an adaptation mechanism in vetiver grass to moderate saline environment.

Exogenous Proline Effects on Photosynthetic Performance and Antioxidant Defense System of Young Olive Tree

The ability of exogenous compatible solutes, such as proline, to counteract salt inhibitory effects in olive plants ( Olea europaea L. cv. Chemlali) was investigated. Two-year-old olive trees were subjected to different saline water irrigation levels supplied or not with exogenous proline. Leaf water relations (relative water content, water potential), photosynthetic activity, and leaf chlorophyll content decreased under either saline water level. The proline supplement mitigated the reduction of growth and photosynthetic activity under salt stress, and the mitigating effect of proline was different among treatments. The increment rate of leaf relative water content (RWC) in the presence of 25 and 50 mM proline was 4.45 and 6.67%, respectively, in comparison to values recorded in SS1-treated plants (plants irrigated with water containing 100 mM NaCl). In SS2 (200 mM NaCl) plus proline-treated plants, this increase was 1.14 times for 25 mM proline and 1.19 times for 50 mM proline higher than those recorded in severe salt stress treatment (SS2). In response to salt stress, Chemlali olive plants seem to activate a complex antioxidative defense system that was displayed via the increase of activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) and the decrease of polyphenol oxidase (PPO) under either salt stress treatment. The exogenous application of proline improved the antioxidative enzyme activities of salt-stressed olive plants. Indeed, in young or old leaf tissues, the highest levels of these antioxidant enzymes activities were recorded in (SS2 + P2)-treated plants (plants irrigated with water containing 200 mM NaCl plus 50 mM proline). In young leaves, this increase was 2.11, 2.96, and 2.76 times, respectively, for SOD, APX, and CAT enzyme activities in comparison to their respective activities in control plants (nonstressed plants irrigated with fresh water). In old leaves, this increase was 2, 2.41, and 2.48 times, respectively, for the various enzymes. If compared to high water salinity-treated plants (SS2), this increase was 1.1, 1.3, and 1.4 times in young leaves, respectively, for SOD, APX, and CAT activities. From these results, the proline supplements seem to improve olive salt tolerance by amelioration of some antioxidative enzyme activities, photosynthetic activity, and, so, plant growth and the preservation of a suitable plant water status under salinity conditions. More to the point, the decrease of soluble sugars contents in proline treated-plants revealed the important osmoprotectant effect played by the added proline in such a way that limited the need of salt-stressed plants for soluble sugars synthesis.

10.1007/s11183-008-2010-3

Nitrate is an essential nitrogen source for plant growth and development. The experiments on nitrate uptake by tomato (Solanum esculentum L., cv. L402) seedlings pretreated with 48-h nitrogen starvation and 24-h 75 mM NaCl stress were performed at four different NO3 concentrations. The results showed that salt stress decreased NO −3 uptake regardless of the nitrate concentration. In order to study the effect of salt stress on nitrate transporters (NRT), nitrate reductase (NR), and glutamine synthetase (GS) gene expression patterns in young tomato roots, we analyzed the transcript levels of LeNRT, LeNR, and LeGS1 under 75 mM NaCl stress by the technique of real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). The results indicated that transcripts of LeNRT1.1 and LeNRT1.2 were significantly decreased, suggesting that LeNRT1 may be at least partly responsible for the reduction in nitrate uptake. The transcription of LeNRT2.1 was also decreased; but the mRNA levels of LeNRT2.2 and LeGS1.1 were not influenced dramatically; the transcription of LeNRT2.3 was slightly increased after 12-h salt stress. LeNR transcription in tomato roots exhibited transient up-regulation at 4-h salt treatment. However, the transcription of LeGS1.2 was significantly decreased under the salt treatment. Our results suggest that the down-regulation of LeNRT1 gene expression may be mainly involved in the reduction of nitrate uptake under severe salt stress.

Phenotypic and Transcriptomic Analyses of Mildly and Severely Salt-Stressed Bacillus cereus ATCC 14579 Cells

Bacteria are able to cope with the challenges of a sudden increase in salinity by activating adaptation mechanisms. In this study, exponentially growing cells of the pathogen Bacillus cereus ATCC 14579 were exposed to both mild (2.5% [wt/vol] NaCl) and severe (5% [wt/vol] NaCl) salt stress conditions. B. cereus continued to grow at a slightly reduced growth rate when it was shifted to mild salt stress conditions. Exposure to severe salt stress resulted in a lag period, and after 60 min growth had resumed, with cells displaying a filamentous morphology. Whole-genome expression analyses of cells exposed to 2.5% salt stress revealed that the expression of these cells overlapped with the expression of cells exposed to 5% salt stress, suggesting that the corresponding genes were involved in a general salt stress response. Upregulation of osmoprotectant, Na(+)/H(+), and di- and tripeptide transporters and activation of an oxidative stress response were noticeable aspects of the general salt stress transcriptome response. Activation of this response may confer cross-protection against other stresses, and indeed, increased resistance to heat and hydrogen peroxide could be demonstrated after preexposure to salt. A temporal shift between the transcriptome response and several phenotypic responses of severely salt-stressed cells was observed. After resumption of growth, these cells showed cellular filamentation, reduced chemotaxis, increased catalase activity, and optimal oxidative stress resistance, which corresponded to the transcriptome response displayed in the initial lag period. The linkage of transcriptomes and phenotypic characteristics can contribute to a better understanding of cellular stress adaptation strategies and possible cross-protection mechanisms.