Salt-sensitive Rice Cultivar Research Articles

Astaxanthin application enhances salinity tolerance in rice seedlings by abating oxidative stress effects and enhancing Na+/K+ homeostatic balance

Since many cultivated plants, including rice, are susceptible to stress and salt stress, resulting in a tremendous reduction in yield, threatens food security worldwide. Strategies such as using biostimulants to ameliorate salt stress can reduce stress effects and sustain production. The effects of soaking Koshihikari (salt-sensitive) seeds in astaxanthin (AS) under salt stress were determined in the present study. In particular, the seeds of the rice cultivar were subjected to control, salt stress (50 mM NaCl), AS (50 µM), and AS + salt stress treatments for two weeks in hydroponic culture. Thereafter, the plants were harvested, and their growth, physiological, and molecular parameters were analyzed. The results showed that the growth of plants under salt stress was significantly reduced; however, the growth was restored to levels comparable to those of non-stressed plants treated with AS. Salt stress significantly increased the concentrations of malondialdehyde, hydrogen peroxide, and the electrolyte leakage ratio in untreated plants and significantly decreased their concentration in AS-treated plants under the same conditions, with corresponding increases in leaf catalase, peroxidase, and ascorbate peroxidase activities. Leaf Na+ concentration markedly increased under salt stress in non-treated plants, and AS treatment reduced the concentration. However, the difference was not statistically significant, which resulted in a significant decrease in the Na+/K+ ratio in AS-treated plants compared to that in non-treated plants. Salt stress and AS treatment did not alter the concentration of photosynthetic pigments but enhanced the expression of OsBHY, OsNHX1, OsSOS1, and OsHKT1;5 genes. Overall, soaking seeds in AS induced salt stress tolerance in the Koshihikari rice cultivar by reducing oxidative stress damage and enhancing shoot Na+/K+ balance. Therefore, seed-soaking methods using AS could serve as a good strategy for improving the cultivation of salt-sensitive rice cultivars in saline soils.

Jacalin-related lectin 45 (OsJRL45) isolated from ‘sea rice 86’ enhances rice salt tolerance at the seedling and reproductive stages

BackgroundRice (Oryza sativa L.) is one of the most widely cultivated grain crops in the world that meets the caloric needs of more than half the world’s population. Salt stress seriously affects rice production and threatens food security. Therefore, mining salt tolerance genes in salt-tolerant germplasm and elucidating their molecular mechanisms in rice are necessary for the breeding of salt tolerant cultivars.ResultsIn this study, a salt stress-responsive jacalin-related lectin (JRL) family gene, OsJRL45, was identified in the salt-tolerant rice variety ‘sea rice 86’ (SR86). OsJRL45 showed high expression level in leaves, and the corresponding protein mainly localized to the endoplasmic reticulum. The knockout mutant and overexpression lines of OsJRL45 revealed that OsJRL45 positively regulates the salt tolerance of rice plants at all growth stages. Compared with the wild type (WT), the OsJRL45 overexpression lines showed greater salt tolerance at the reproductive stage, and significantly higher seed setting rate and 1,000-grain weight. Moreover, OsJRL45 expression significantly improved the salt-resistant ability and yield of a salt-sensitive indica cultivar, L6-23. Furthermore, OsJRL45 enhanced the antioxidant capacity of rice plants and facilitated the maintenance of Na+-K+ homeostasis under salt stress conditions. Five proteins associated with OsJRL45 were screened by transcriptome and interaction network analysis, of which one, the transmembrane transporter Os10g0210500 affects the salt tolerance of rice by regulating ion transport-, salt stress-, and hormone-responsive proteins.ConclusionsThe OsJRL45 gene isolated from SR86 positively regulated the salt tolerance of rice plants at all growth stages, and significantly increased the yield of salt-sensitive rice cultivar under NaCl treatment. OsJRL45 increased the activity of antioxidant enzyme of rice and regulated Na+/K+ dynamic equilibrium under salinity conditions. Our data suggest that OsJRL45 may improve the salt tolerance of rice by mediating the expression of ion transport-, salt stress response-, and hormone response-related genes.

Comparative transcriptome analysis of gene responses of salt-tolerant and salt-sensitive rice cultivars to salt stress

Salt stress is one unfavorable factor of global climate change that adversely affects rice plant growth and yield. To identify novel salt-tolerant genes and new varieties of salt-tolerant rice, a better understanding of the molecular regulation mechanism of salt tolerance in rice is needed. In this study we used transcriptome analyses to examine changes in gene expression of salt-tolerant and salt-sensitive rice plants. The salt-tolerant cultivar HH11 and salt-sensitive cultivar IR29 were treated with 200 mM NaCl solution for 0 h, 6 h, 24 h and 48 h at the three leaf stage. Physiological parameters and transcriptome were measured and analyzed after each treatment. Activity of SOD and POD, as well as the MDA and protein content of the two rice cultivars generally increased with increasing time of exposure to NaCl. Meanwhile, the APX activity first increased, then decreased in both cultivars, with maximum values seen at 6 h for IR29 and at 24 h for HH11. The GR and GPX activity of HH11 were stronger than that of IR29 in response to salt stress. The H2O2 content first increased at 0–6 h, then decreased at 6–24 h, and then increased again at 24–48 h under salt stress. Compared with IR29, SOD, POD and APX activity of HH11 was more sluggish in response to salt stress, reaching the maximum at 24 h or 48 h. The MDA, H2O2 and proline content of HH11 was lower than that of IR29 under salt stress. Relative to untreated HH11 plants (0 h) and those exposed to salt for 6 h, 24 h, and 48 h (H0-H6, H0-H24 and H0-H48), 7462, 6363 and 6636, differentially expressed genes (DEGs), respectively, were identified. For IR29, the respective total DEGs were 7566, 6075 and 6136. GO and KEGG enrichment analysis showed that metabolic pathways related to antioxidative responses and osmotic balance played vital roles in salt stress tolerance. Sucrose and starch metabolism, in addition to flavonoid biosynthesis and glutathione metabolism, showed positive responses to salt stress. Expression of two SPS genes (LOC_Os01g69030 and LOC_Os08g20660) and two GST genes (LOC_Os06g12290 and LOC_Os10g38740) was up-regulated in both HH11 and IR29, whereas expression of LOC_Os09g12660, a glucose-1-phosphate adenylyltransferase gene, and two SS genes (LOC_Os04g17650 and LOC_Os04g24430) was up-regulated differential expression in HH11. The results showed that HH11 had more favorable adjustment in antioxidant and osmotic activity than IR29 upon exposure to salt stress, and highlighted candidate genes that could play roles in the function and regulation mechanism of salt tolerance in rice.

Combined transcriptomic and metabolomic analysis of alginate oligosaccharides alleviating salt stress in rice seedlings

BackgroundSalt stress is one of the key factors limiting rice production. Alginate oligosaccharides (AOS) enhance plant stress resistance. However, the molecular mechanism underlying salt tolerance in rice induced by AOS remains unclear. FL478, which is a salt-tolerant indica recombinant inbred line and IR29, a salt-sensitive rice cultivar, were used to comprehensively analyze the effects of AOS sprayed on leaves in terms of transcriptomic and metabolite profiles of rice seedlings under salt stress.ResultsIn this experiment, exogenous application of AOS increased SOD, CAT and APX activities, as well as GSH and ASA levels to reduce the damage to leaf membrane, increased rice stem diameter, the number of root tips, aboveground and subterranean biomass, and improved rice salt tolerance. Comparative transcriptomic analyses showed that the regulation of AOS combined with salt treatment induced the differential expression of 305 and 1030 genes in FL478 and IR29. The expressed genes enriched in KEGG pathway analysis were associated with antioxidant levels, photosynthesis, cell wall synthesis, and signal transduction. The genes associated with light-trapping proteins and RLCK receptor cytoplasmic kinases, including CBA, LHCB, and Lhcp genes, were fregulated in response to salt stress. Treatment with AOS combined with salt induced the differential expression of 22 and 50 metabolites in FL478 and IR29. These metabolites were mainly related to the metabolism of amino and nucleotide sugars, tryptophan, histidine, and β -alanine. The abundance of metabolites associated with antioxidant activity, such as 6-hydroxymelatonin, wedelolactone and L-histidine increased significantly. Combined transcriptomic and metabolomic analyses revealed that dehydroascorbic acid in the glutathione and ascorbic acid cycles plays a vital role in salt tolerance mediated by AOS.ConclusionAOS activate signal transduction, regulate photosynthesis, cell wall formation, and multiple antioxidant pathways in response to salt stress. This study provides a molecular basis for the alleviation of salt stress-induced damage by AOS in rice.

The Efficiency of Humic Acid for Improving Salinity Tolerance in Salt Sensitive Rice (Oryza sativa): Growth Responses and Physiological Mechanisms

High-yielding rice cultivars exhibit a great performance in non-saline fields; however, their growth and productivity are greatly reduced in salt-affected lands. Humic acid has a promising stress-mitigating potential and can be effective in improving salt tolerance in salinity sensitive rice cultivars. Herein, seeds of Giza 177 (high-yielding but salt-sensitive rice cultivar) were primed in 40 mg/l humic acid, sown, and maintained. Then growth and physiological responses of the humic acid-primed plants to increased levels of salinity (EC: 0.55, 3.40, 6.77, and 8.00 mS/cm) were evaluated at the reproductive stage. Increasing salinity induced a progressive retardation in plant height, leaf area, fresh and dry weights. Such retardation was associated with Na+ buildup in shoot and root, high electrolyte leakage and accumulation of malondialdehyde, total soluble sugars, sucrose, glucose, proline, total soluble proteins, flavonoids, and phenolics. In contrast, salinity reduced K+, K+/Na+ ratio, total carbohydrates, and the activity of catalase, peroxidase, and polyphenol oxidase. Humic acid enhanced growth under non-saline and saline conditions. The humic acid-induced improvement in salt tolerance was associated with the reduction of Na+ toxicity, increasing K+/Na+ ratio, regulating osmolytes concentration, and enhancing the activities of antioxidant enzymes and thus reduce the oxidative stress. These results indicate that humic acid successfully reduced the salinity-induced plant damage, improved metabolism, and maintained active growth of Giza 177 under saline irrigation.

Comparative ribosome profiling reveals distinct translational landscapes of salt-sensitive and -tolerant rice

BackgroundSoil salinization represents a serious threat to global rice production. Although significant research has been conducted to understand salt stress at the genomic, transcriptomic and proteomic levels, few studies have focused on the translatomic responses to this stress. Recent studies have suggested that transcriptional and translational responses to salt stress can often operate independently.ResultsWe sequenced RNA and ribosome-protected fragments (RPFs) from the salt-sensitive rice (O. sativa L.) cultivar ‘Nipponbare’ (NB) and the salt-tolerant cultivar ‘Sea Rice 86’ (SR86) under normal and salt stress conditions. A large discordance between salt-induced transcriptomic and translatomic alterations was found in both cultivars, with more translationally regulated genes being observed in SR86 in comparison to NB. A biased ribosome occupancy, wherein RPF depth gradually increased from the 5′ ends to the 3′ ends of coding regions, was revealed in NB and SR86. This pattern was strengthened by salt stress, particularly in SR86. On the contrary, the strength of ribosome stalling was accelerated in salt-stressed NB but decreased in SR86.ConclusionsThis study revealed that translational reprogramming represents an important layer of salt stress responses in rice, and the salt-tolerant cultivar SR86 adopts a more flexible translationally adaptive strategy to cope with salt stress compared to the salt susceptible cultivar NB. The differences in translational dynamics between NB and SR86 may derive from their differing levels of ribosome stalling under salt stress.

Characterization of Na+ exclusion mechanism in rice under saline-alkaline stress conditions

This study was designed to elucidate the physiological responses of two rice genotypes to different pH levels under high saline stress. A salt-tolerant cultivar, FL478, and a salt-sensitive cultivar, IR29, were exposed to saline-alkaline solutions supplemented with 50 mM Na at pH 9 (severe), pH 8 (moderate), and pH 7 (mild) for three weeks. The results indicated that FL478 is relatively saline-alkaline tolerant compared to IR29, and this was evident from its higher dry mass production, lower Na+ concentration in the leaf blades, and maintenance of water balance under both mild and moderate saline-alkaline stress conditions. In both cultivars, Na+ concentrations in the leaf blades were considerably higher at pH 8 than at pH 7, indicating that high alkaline stress promoted Na+ accumulation under highly saline conditions. FL478 plants had lower Na+/K+ ratios in leaf blades and leaf sheaths than IR29 plants under saline-alkaline stress at both pH 7 and pH 8. To understand the mechanisms behind the difference in saline-alkaline tolerance between the two rice genotypes, transcript levels of the genes encoding Na+ transport proteins were analyzed. In response to mild and moderate saline-alkaline stress conditions, salt-tolerant FL478 had highly induced expression of the OsHKT1;5 gene in the roots, corresponding to lower Na+ accumulation in the leaf blades. Induction of high expression of the OsSOS1 gene in the roots of FL478 implied that Na may be effectively exported from cytosols to apoplasts in the roots resulting in sequestration of Na+ to outside of the roots and loading Na+ in xylem transpiration stream. On the other hand, the salt-sensitive IR29 had lower expression of the genes related to Na+ transporters, such as the OsHKT1;5 gene and the OsSOS1 gene, in the roots, leading to higher Na+ accumulation in the shoots. Expression of the determinant genes for alkaline tolerance, such as K+ and Fe acquisition and acidification of the rhizosphere was highly induced in FL478, but not in IR29. Thus, molecular analysis suggested that genes encoding Na+ transport proteins are involved in regulating Na+ transport under saline-alkaline stress in both salt-tolerant and salt-sensitive rice cultivars, and this is useful information for improving saline-alkaline tolerance traits of rice in the future.

Inoculation of Brevibacterium linens RS16 in Oryza sativa genotypes enhanced salinity resistance: Impacts on photosynthetic traits and foliar volatile emissions

The emission of volatiles in response to salt stress in rice cultivars has not been studied much to date. Studies addressing the regulation of stress induced volatile emission by halotolerant plant growth promoting bacteria containing ACC (1-aminocyclopropane-1-carboxylate) deaminase are also limited. The objective of the present study was to investigate the salt alleviation potential of bacteria by regulating photosynthetic characteristics and volatile emissions in rice cultivars, and to compare the effects of the bacteria inoculation and salt responses between two rice genotypes. The interactive effects of soil salinity (0, 50, and 100 mM NaCl) and inoculation with Brevibacterium linens RS16 on ACC accumulation, ACC oxidase activity, carbon assimilation and stress volatile emissions after stress application were studied in the moderately salt resistant (FL478) and the salt-sensitive (IR29) rice (Oryza sativa L.) cultivars. It was observed that salt stress reduced foliage photosynthetic rate, but induced foliage ACC accumulation, foliage ACC oxidase activity, and the emissions of all the major classes of volatile organic compounds (VOCs) including the lipoxygenase pathway volatiles, light-weight oxygenated volatiles, long-chained saturated aldehydes, benzenoids, geranylgeranyl diphosphate pathway products, and mono- and sesquiterpenes. All these characteristics scaled up quantitatively with increasing salt stress. The effects of salt stress were more pronounced in the salt-sensitive genotype IR29 compared to the moderately salt resistant FL478 genotype. However, the bacterial inoculation significantly enhanced photosynthesis, and decreased ACC accumulation and the ACC oxidase activity, and VOC emissions both in control and salt-treated plants. Taken together, these results suggested that the ACC deaminase-containing Brevibacterium linens RS16 reduces the temporal regulation of VOC emissions and increases the plant physiological activity by reducing the availability of ethylene precursor ACC and the ACC oxidase activity under salt stress.

Brevibacterium linens RS16 confers salt tolerance to Oryza sativa genotypes by regulating antioxidant defense and H+ ATPase activity

Soil salinity is one of the major limitations that affects both plant and its soil environment, leading to reduced agricultural production. Evaluation of stress severity by plant physical and biochemical characteristics is an established way to study plant-salt stress interaction, but the halotolerant properties of plant growth promoting bacteria (PGPB) along with plant growth promotion is less studied till date. The aim of the present study was to elucidate the strategy, used by ACC deaminase-containing halotolerant Brevibacterium linens RS16 to confer salt stress tolerance in moderately salt-tolerant (FL478) and salt-sensitive (IR29) rice (Oryza sativa L.) cultivars. The plants were exposed to salt stress using 0, 50, and 100 mM of NaCl with and without bacteria. Plant physiological and biochemical characteristics were estimated after 1, 5, 10 days of stress application. H+ ATPase activity and the presence of hydroxyectoine gene (ectD) that is responsible for compatible solute accumulation were also analyzed in bacteria. The height and dry mass of bacteria inoculated plants significantly increased compared to salt-stressed plants, and the differences increased in time dependent manner. Bacteria priming reduced the plant antioxidant enzyme activity, lipid peroxidation and it also regulated the salt accumulation by modulating vacuolar H+ ATPase activity. ATPase activity and presence of hydroxyectoine gene in RS16 might have played a vital role in providing salt tolerance in bacteria inoculated rice cultivars. We conclude that dual benefits provided by the halotolerant plant growth promoting bacteria (PGPB) can provide a major way to improve rice yields in saline soil.

Different forms of osmotic stress evoke qualitatively different responses in rice

Drought, salinity and alkalinity are distinct forms of osmotic stress with serious impacts on rice productivity. We investigated, for a salt-sensitive rice cultivar, the response to osmotically equivalent doses of these stresses. Drought, experimentally mimicked by mannitol (single factor: osmotic stress), salinity (two factors: osmotic stress and ion toxicity), and alkalinity (three factors: osmotic stress, ion toxicity, and depletion of nutrients and protons) produced different profiles of adaptive and damage responses, both locally (in the root) as well as systemically (in the shoot). The combination of several stress factors was not necessarily additive, and we even observed cases of mitigation, when two (salinity), or three stressors (alkalinity) were compared to the single stressor (drought). The response to combinations of individual stress factors is therefore not a mere addition of the partial stress responses, but rather represents a new quality of response. We interpret this finding in a model, where the output to signaling molecules is not determined by their abundance per se, but qualitatively depends on their adequate integration into an adaptive signaling network. This output generates a systemic signal that will determine the quality of the shoot response to local concentrations of ions.

Biochemical and molecular characterisation of salt-induced poor grain filling in a rice cultivar.

Despite the prevalence of poor grain filling in rice (Oryza sativa L.) under abiotic stress, the reason for this is largely unexplored. Application of 0.75% NaCl to a salt-sensitive rice cultivar at late booting resulted in a >20% yield loss. Spikelets per panicle and the percentage of filled grain decreased significantly in response to NaCl application. The inhibitory effect of NaCl on grain filling was greater in basal than in apical spikelets. Sucrose synthase (SUS) activity was positively correlated with grain weight. The transcript levels of the SUS isoforms differed greatly: the levels of SUS2 increased significantly in response to salt; those of SUS4 decreased drastically. Gene expression studies of starch synthase and ADP-glucose pyrophosphorylase showed that the decreased transcript levels of one isoform was compensated by an increase in those of the other. Salt application also significantly increased the gene expression of the ethylene receptors and the ethylene signalling proteins. The increase in their transcript levels was comparatively greater in basal than in apical spikelets. Significant enhancement in the transcript levels of the ethylene receptors and the increase in the production of ethylene indicated that the salt-induced inhibition of grain filling might be mediated by ethylene. Additionally, the inhibition of chromosomal endoreduplication mediated by decreased transcript levels of B-type cyclin could explain poor grain filling under salt stress. A significant increase in the transcript levels of the ethylene-responsive factors in the spikelets during grain filling in response to salt indicated their possible protective role in grain filling under stress.

Differential competence of redox-regulatory mechanism under extremes of temperature determines growth performances and cross tolerance in two indica rice cultivars

The present study investigated the relationship between reactive oxygen species (ROS) accumulation (total and individual), antioxidant and radical scavenging capacity (total and individual), transcript abundance of some antioxidative genes and oxidative damages to membrane protein and lipid in germinating tissues of a salt resistant (SR26B) and salt sensitive (Ratna) rice cultivars under extremes of temperature to elucidate redox-regulatory mechanism governing differential oxidative stress tolerance associated with better growth and yield potential and identification of cross tolerance, if any. Imbibitional heat and chilling stress caused disruption of redox-homeostasis and oxidative damage to a newly assembled membrane system by increasing pro-oxidant/antioxidant ratio and by aggravating membrane lipid peroxidation and protein oxidation [measured in terms of accumulation of thiobarbituric acid reactive substances (TBARS), free carbonyl content (CO groups), and membrane protein thiol level (MPTL)]. A concomitant increase in accumulation of individual ROS (superoxide and hydrogen peroxide) and significant reduction of radical scavenging activity (assessed in terms of ABTS, FRAP and DPPH methods), non-enzymatic and enzymatic anti-oxidative defense [assessed in terms of total thiol content and activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), ascorbate peroxidase (EC 1.11.1.11), and glutathione reductase (EC 1.6.4.2)] are also noticed in both the salt sensitive (Ratna) and resistant (SR26B) germinating tissues of rice cultivars. When compared, salt resistant cultivar SR26B was found to suffer significantly less redox-imbalance and related oxidative damages to membrane protein and lipid as compared to salt sensitive cultivar Ratna. The salt tolerant cultivar SR26B resisted imbibitional chilling and heat stress due to its early preparedness to combat oxidative stress by up-regulation of gene expression of anti-oxidative enzymes and better capacity of redox-regulation and mitigation of oxidative damage to membrane protein and lipid as compared to salt sensitive cultivar Ratna, under the same magnitude of imbibitional heat and chilling stress. A model for redox-homeostasis in which the ROS-antioxidant interaction acts as a metabolic interface for up-regulation of gene expression necessary for cross tolerance is also proposed.

Liquid chromatography-tandem mass spectrometry-assisted identification of two salinity-inducible ascorbate peroxidases in a salt-sensitive rice cultivar (Oryza sativa L. cv. ‘IR-29’)

Ascorbate peroxidases (APXs) play an essential role in H2O2 scavenging by converting it into H2O in living cells, and eight APXs have been annotated in the rice genome (Oryza sativa L. cv. ‘Nipponbare’). We conducted mass spectrometric sequencing of a protein pool, obtained using an APX in-gel assay, to retrieve salt-inducible genes from a salt-sensitive rice cultivar (O. sativa L. cv. ‘IR-29’) using liquid chromatography–tandem mass spectrometry. We selected seven candidates from the peptide sequencing output, and performed reverse transcription-polymerase chain reaction to evaluate the candidates for inducible expression against salt treatment (100 mM NaCl). Two orthologs of OsAPX1 and OsAPX2 (OsAPX1-IR29 and OsAPX2-IR29, respectively) were found, and in vitro recombinant enzyme assays revealed that both OsAPX1-IR29 and OsAPX2-IR29 could convert H2O2 into H2O by using ascorbate as the electron donor, with 10- to 15-fold higher enzymatic activity than that of a crude extract. OsAPX2-IR29 transcripts were significantly more abundant than OsAPX1-IR29 transcripts in the shoot. However, after salt treatment, OsAPX1-IR29 transcripts were highly induced, depending on salt concentration (greater than threefold) and time (greater than sixfold). Furthermore, we observed a similar APX protein accumulation pattern using isoform-specific antibodies. Importantly, transcript levels markedly increased 5 days after salt treatment, and were consistently maintained until 7 days after treatment, although the seedlings sustained serious damage and showed no indication of viability. Our results suggest that OsAPX1-IR29 and OsAPX2-IR29 are the major APX isozymes, and play significant roles in scavenging H2O2 during salt stress in the IR-29 cultivar.

Expression and functional analysis of putative vacuolar Ca2+-transporters (CAXs and ACAs) in roots of salt tolerant and sensitive rice cultivars

Vacuolar Ca2+-transporters could play an important role for salt tolerance in rice (Oryza sativa L.) root. Here, we compared the expression profiles of putative vacuolar cation/H+ exchanger (CAX) and calmodulin-regulated autoinhibited Ca2+-ATPase (ACA) in rice roots of salt tolerant cv. Pokkali and salt sensitive cv. IR29. In addition to five putative vacuolar CAX genes in the rice genome, a new CAX gene (OsCAX4) has been annotated. In the present study, we isolated the OsCAX4 gene and showed that its encoded protein possesses a unique transmembrane structure and is potentially involved in transporting not only Ca2+ but also Mn2+ and Cu2+. These six OsCAX genes differed in their mRNA expression pattern in roots of tolerant versus sensitive rice cultivars exposed to salt stress. For example, OsCAX4 showed abundant expression in IR29 (sensitive) upon prolonged salt stress. The mRNA expression profile of four putative vacuolar Ca2+-ATPases (OsACA4-7) was also examined. Under control conditions, the mRNA levels of OsACA4, OsACA5, and OsACA7 were relatively high and similar among IR29 and Pokkali. Upon salt stress, only OsACA4 showed first a decrease in its expression in Pokkali (tolerant), followed by a significant increase. Based on these results, a role of vacuolar Ca2+ transporter for salt tolerance in rice root was discussed.

EXOGENOUS APPLICATION OF POTASSIUM NITRATE TO ALLEVIATE SALT STRESS IN RICE SEEDLINGS

□ Overall growth characteristics of many plant species cultivated in soil affected by salinity could be alleviated by the application of potassium nitrate (KNO3) to the soil. The aim of this research was to investigate salt-tolerance in a salt-sensitive rice cultivar, ‘Pathumthani 1’ (PT1), in response to the exogenous application of 11.8 mM KNO3, in comparison to ‘Homjan’ (HJ), a salt tolerant cultivar. Water potential (ψw) in both the roots and leaves of PT1 seedlings under salt stress dropped significantly, while it was maintained in PT1 pretreated with KNO3, and similarly in HJ. The reduction of leaf water potential was positively related to total chlorophyll degradation, leading to diminished chlorophyll fluorescence, directly affecting growth in plants exposed to salt stress. In salt-sensitive PT1, the application of 11.8 mM KNO3 improved salt-tolerance via the conservation of water use efficiency, the maintenance of photosynthetic pigments, enhancement of chlorophyll a fluorescence, and stimulation of growth characters.

An inductive pulse of hydrogen peroxide pretreatment restores redox-homeostasis and oxidative membrane damage under extremes of temperature in two rice cultivars

Imbibitional heat and chilling stress caused disruption of redox-homeostasis and oxidative damage to newly assembled membrane system by aggravating membrane lipid peroxidation and protein oxidation [measured in terms of thiobarbituric acid reactive substances (TBARS), free carbonyl content (C=O groups) and membrane protein thiol level (MPTL)] along with concomitant increase in accumulation of reactive oxygen species (superoxide and hydrogen peroxide) and significant reduction of antioxidative defense (assessed in terms of total thiol content and activities of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase) in both the salt sensitive (Ratna) and resistant (SR 26B) germinating tissues of rice cultivars. When compared, salt resistant cultivar SR 26B found to suffer significantly less oxidative membrane damage as compared to salt sensitive cultivar Ratna. Treatment with low titer of hydrogen peroxide caused significant reversal in oxidative damages to the newly assembled membrane system imposed by imbibitional heat and chilling stress (evident from the data of TBARS, C=O, MPTL, ROS accumulation, membrane permeability status, membrane injury index and oxidative stress index) in seedlings of both the cultivars of rice (Ratna and SR 26B). Imbibitional H2O2 pretreatment also caused up-regulation of antioxidative defense (activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and total thiol content) in the heat and chilling stress-raised seedlings of experimental rice cultivars. When the parameters of early growth performances were assessed (in terms of relative growth index, biomass accumulation and vigor index), it clearly exhibited significant improvement of early growth performances of both the rice cultivars. Better response towards H2O2-mediated acclamatory performances and restoration of redox- homeostasis under extremes of temperature were noticed in salt sensitive rice cultivar Ratna compared to salt resistant SR 26B. Taken as a whole, the result suggests the significance of the role of ‘inductive pulse’ of H2O2 in acclimatizing adverse temperature stress by restoration of redox-homeostasis and mitigation of oxidative membrane protein and lipid damages during the recovery phase of post-germination event.

Spermidine and abscisic acid-mediated phosphorylation of a cytoplasmic protein from rice root in response to salinity stress

Importance of higher polyamines, spermidine, and spermine, in relation to the mechanism and adaptation to combat abiotic stress has been well established in cereals. Owing to their polycationic nature at physiological pH, polyamines bind strongly to negative charges in cellular components such as nucleic acids, various proteins, and phospholipids. To study the physiological role of polyamine during salinity stress, phosphorylation study was carried out in cytosolic soluble protein fraction isolated from the roots of salt tolerant (Nonabokra) and salt sensitive (M-1-48) rice cultivars treated with none (control), NaCl (150 mM, 16 h), spermidine (1 mM, 16 h) or with abscisic acid (100 μM, 16 h). A calcium independent auto regulatory 42 kDa protein kinase was found to phosphorylate myelin basic protein and casein but not histone. Interestingly, this was the only protein to be phosphorylated in root cytosolic fraction during NaCl/abscisic acid/spermidine treatment indicating its importance in salinity mediated signal transduction. This is the first report of polyamine as well as abscisic acid induced protein kinase activity in rice root in response to salinity stress.

Modification of OsSUT1 gene expression modulates the salt response of rice Oryza sativa cv. Taipei 309

A metabolic depletion syndrome was discovered at early vegetative stages in roots of salt sensitive rice cultivars after prolonged exposure to 100 mM NaCl. Metabolite profiling analyses demonstrate that this syndrome is part of the terminal stages of the rice salt response. The phenotype encompasses depletion of at least 30 primary metabolites including sucrose, glucose, fructose, glucose-6-P, fructose-6P, organic- and amino-acids. Based on these observations we reason that sucrose allocation to the root may modify the rice response to high salt. This hypothesis was tested using antisense lines of the salt responsive OsSUT1 gene in the salt sensitive Taipei 309 cultivar. Contrary to our expectations of a plant system impaired in one component of sucrose transport, we find improved gas exchange and photosynthetic performance as well as maintenance of sucrose levels in the root under high salinity. Two independent OsSUT1 lines with an antisense inhibition similar to the naturally occurring salt induced reduction of OsSUT1 gene expression showed these phenomena but not a more extreme antisense inhibition line. We investigated the metabolic depletion syndrome by metabolomic and physiological approaches and discuss our results with regard to the potential role of sucrose transporters and sucrose transport for rice salt acclimation.

New changes in the plasma‐membrane‐associated proteome of rice roots under salt stress

To gain a better understanding of salt stress responses in plants, we used a proteomic approach to investigate changes in rice (Oryza sativa) root plasma-membrane-associated proteins following treatment with 150 mmol/L NaCl. With or without a 48 h salt treatment, plasma membrane fractions from root tip cells of a salt-sensitive rice cultivar, Wuyunjing 8, were purified by PEG aqueous two-phase partitioning, and plasma-membrane-associated proteins were separated by IEF/SDS-PAGE using an optimized rehydration buffer. Comparative analysis of three independent biological replicates revealed that the expressions of 18 proteins changed by more than 1.5-fold in response to salt stress. Of these proteins, nine were up-regulated and nine were down-regulated. MS analysis indicated that most of these membrane-associated proteins are involved in important physiological processes such as membrane stabilization, ion homeostasis, and signal transduction. In addition, a new leucine-rich-repeat type receptor-like protein kinase, OsRPK1, was identified as a salt-responding protein. Immuno-blots indicated that OsRPK1 is also induced by cold, drought, and abscisic acid. Using immuno-histochemical techniques, we determined that the expression of OsRPK1 was localized in the plasma membrane of cortex cells in roots. The results suggest that different rice cultivars might have different salt stress response mechanisms.

Effects of NaCl stress on photochemical activity and thylakoid membrane polypeptide composition of a salt-tolerant and a salt-sensitive rice cultivar

NaCl stress (200 mM) inhibited the electron transport activity of photosystem 2 (PS2) more than that of PS1. The degree of electron transport activity inhibition was lower in the salt-tolerant cultivar Pokkali than in the salt-sensitive cultivar Peta. The polypeptide composition of the thylakoid membrane and PS2 particles did not change after NaCl treatment but there was a difference in polypeptide compositions of thylakoid membrane and PS2 particles between the two cultivars. PS2 particles of cv. Pokkali contained more 33-kDa and 43-kDa polypeptides than cv. Peta. Additionally, PS2 particles after NaCl treatment showed deficiency of 23-kDa outside polypeptides of PS2.