Salt-stress induced modulation of chlorophyll biosynthesis during de-etiolation of rice seedlings

Metabolomics Reveals the Impact of Overexpression of Cytosolic Fructose-1,6-Bisphosphatase on Photosynthesis and Growth in Nannochloropsis gaditana

Nannochloropsis gaditana, a microalga known for its photosynthetic efficiency, serves as a cell factory, producing valuable biomolecules such as proteins, lipids, and pigments. These components make it an ideal candidate for biofuel production and pharmaceutical applications. In this study, we genetically engineered N. gaditana to overexpress the enzyme fructose-1,6-bisphosphatase (cyFBPase) using the Hsp promoter, aiming to enhance sugar metabolism and biomass accumulation. The modified algal strain, termed NgFBP, exhibited a 1.34-fold increase in cyFBPase activity under photoautotrophic conditions. This modification led to a doubling of biomass production and an increase in eicosapentaenoic acid (EPA) content in fatty acids to 20.78-23.08%. Additionally, the genetic alteration activated the pathways related to glycine, protoporphyrin, thioglucosides, pantothenic acid, CoA, and glycerophospholipids. This shift in carbon allocation towards chloroplast development significantly enhanced photosynthesis and growth. The outcomes of this study not only improve our understanding of photosynthesis and carbon allocation in N. gaditana but also suggest new biotechnological methods to optimize biomass yield and compound production in microalgae.

Sulfated Nutrition Modifies Nutrient Content and Photosynthetic Pigment Concentration in Cabbage under Salt Stress

Negative effects of salt stress may be counteracted by adequate management of sulfated nutrition. Herein, we applied 3.50, 4.25, and 5.00 mM SO42- in a nutrient solution to counteract salt stress induced by 75 and 150 mM NaCl in cabbage cv. Royal. The increase in NaCl concentration from 75 to 150 mM reduced the contents of macronutrients and micronutrients in the shoot. When increasing from 3.50 to 4.25 mM SO42-, the contents of nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) in shoots were enhanced, at both concentrations of NaCl. Increasing from 3.50 to 4.25 mM SO42- enhanced iron (Fe), zinc (Zn), manganese (Mn), and sodium (Na) concentrations with 75 mM NaCl. With 150 mM NaCl, the increase from 3.50 to 4.25 mM SO42- enhanced the contents of Cu and Mn, but also those of Na. Chlorophylls a, b, and total decreased as the concentration of SO42- increased in plants treated with 150 mM NaCl. With 75 mM NaCl, carotenoid concentration had a positive relationship with SO42-. Hence, the 4.25 mM SO42- concentration increased the contents of macronutrients and micronutrients in the presence of 75 mM NaCl, while, with 150 mM NaCl, it improved the contents of macronutrients except K. The chlorophyll a/chlorophyll b ratio remained close to 3 when the plants were treated with 5.00 mM SO42-, regardless of NaCl. Similarly, this level of SO42- increased the concentration of carotenoids, which translated into reductions in the total chlorophyll/carotenoid ratios, indicating a protective effect of the photosynthetic apparatus. It is concluded that higher doses of sulfur favor the accumulation of nutrients and increase the concentration of carotenoids under salt stress.

Exogenous zinc application mitigates negative effects of salinity on barley (Hordeum vulgare) growth by improving root ionic homeostasis

Detrimental effects of salinity could be mitigated by exogenous zinc (Zn) application; however, the mechanisms underlying this amelioration are poorly understood. This study demonstrated the interaction between Zn and salinity by measuring plant biomass, photosynthetic performance, ion concentrations, ROS accumulation, antioxidant activity and electrophysiological parameters in barley (Hordeum vulgare L.). Salinity stress (200mM NaCl for 3weeks) resulted in a massive reduction in plant biomass; however, both fresh and dry weight of shoots were increased by ~30% with adequate Zn supply. Zinc supplementation also maintained K+ and Na+ homeostasis and prevented H2 O2 toxicity under salinity stress. Furthermore, exposure to 10mM H2 O2 resulted in massive K+ efflux from root epidermal cells in both the elongation and mature root zones, and pre-treating roots with Zn reduced ROS-induced K+ efflux from the roots by 3-4-fold. Similar results were observed for Ca2+ . The observed effects may be causally related to more efficient regulation of cation-permeable non-selective channels involved in the transport and sequestration of Na+ , K+ and Ca2+ in various cellular compartments and tissues. This study provides valuable insights into Zn protective functions in plants and encourages the use of Zn fertilisers in barley crops grown on salt-affected soils.

Mycorrhizas Affect Physiological Performance, Antioxidant System, Photosynthesis, Endogenous Hormones, and Water Content in Cotton under Salt Stress

Saline-alkali stress seriously endangers the normal growth of cotton (Gossypium hirsutum). Arbuscular mycorrhizal fungi (AMF) could enhance salt tolerance by establishing symbiotic relationships with plants. Based on it, a pot experiment was conducted to simulate a salt environment in which cotton was inoculated with Paraglomus occultum to explore its effects on the saline-alkali tolerance of cotton. Our results showed that salt stress noticeably decreased cotton seedling growth parameters (such as plant height, number of leaves, dry weight, root system architecture, etc.), while AMF exhibited a remarkable effect on promoting growth. It was noteworthy that AMF significantly mitigated the inhibitory effect of salt on cotton seedlings. However, AMF colonization in root and soil hyphal length were collectively descended via salt stress. With regard to osmotic regulating substances, Pro and MDA values in roots were significantly increased when seedlings were exposed to salt stress, while AMF only partially mitigated these reactions. Salt stress increased ROS levels in the roots of cotton seedlings and enhanced antioxidant enzyme activity (SOD, POD, and CAT), while AMF mitigated the increases in ROS levels but further strengthened antioxidant enzyme activity. AMF inoculation increased the photosynthesis parameters of cotton seedling leaves to varying degrees, while salt stress decreased them dramatically. When inoculated with AMF under a salt stress environment, only partial mitigation of these photosynthesis values was observed. Under saline-alkali stress, AMF improved the leaf fluorescence parameters (φPSII, Fv'/Fm', and qP) of cotton seedlings, leaf chlorophyll levels, and root endogenous hormones (IAA and BR); promoted the absorption of water; and maintained nitrogen balance, thus alleviating the damage from salt stress on the growth of cotton plants to some extent. In summary, mycorrhizal cotton seedlings may exhibit mechanisms involving root system architecture, the antioxidant system, photosynthesis, leaf fluorescence, endogenous hormones, water content, and nitrogen balance that increase their resistance to saline-alkali environments. This study provide a theoretical basis for further exploring the application of AMF to enhance the salt tolerance of cotton.

Moving forward to understand the alteration of physiological mechanism by seed priming with different halo-agents under salt stress

Soil salinity hampers the survival and productivity of crops. To minimize salt-associated damages in plant, better salt management practices in agriculture have become a prerequisite. Seed priming with different halo-agents is a technique, which improves the primed plant's endurance to tackle sodium. Salt tolerance is achieved in tolerant plants through fundamental physiological mechanisms- ion-exclusion and tissue tolerance, and salt-tolerant plants may (Na+ accumulators) or may not (Na+ excluders) allow sodium movement to leaves. While Na+ excluders depend on ion exclusion in roots, Na+ accumulators are proficient Na+ managers that can compartmentalize Na+ in leaves and use them beneficially as inexpensive osmoticum. Salt-sensitive plants are Na+ accumulators, but their inherent tissue tolerance ability and ion-exclusion process are insufficient for tolerance. Seed priming with different halo-agents aids in 'rewiring' of the salt tolerance mechanisms of plants. The resetting of the salt tolerance mechanism is not universal for every halo-agent and might vary with halo-agents. Here, we review the physiological mechanisms that different halo-agents target to confer enhanced salt tolerance in primed plants. Calcium and potassium-specific halo-agents trigger Na+ exclusion in roots, thus ensuring a low amount of Na+ in leaves. In contrast, Na+-specific priming agents favour processes for Na+ inclusion in leaves, improve plant tissue tolerance or vacuolar sequestration, and provide the greatest benefit to salt-sensitive and sodium accumulating plants. Overall, this review will help to understand the underlying mechanism behind plant's inherent nature towards salt management and its amelioration with different halo-agents, which helps to optimize crop stress performance.

Etioplasts are more susceptible to salinity stress than chloroplasts and photosynthetically active etio-chloroplasts of wheat (Triticum aestivum L.)

High soil salinity is a global problem in agriculture that directly affects seed germination and the development of the seedlings sown deep in the soil. To study how salinity affected plastid ultrastructure, leaf segments of 11-day-old light- and dark-grown (etiolated) wheat (Triticum aestivum L. cv. Mv Béres) seedlings were floated on Hoagland solution, 600 mM KCl:NaCl (1:1) salt or isosmotic polyethylene glycol solution for 4 h in the dark. Light-grown seedlings were also treated in the light. The same treatments were also performed on etio-chloroplasts of etiolated seedlings greened for different time periods. Salt stress induced slight to strong changes in the relative chlorophyll content, photosynthetic activity, and organization of thylakoid complexes. Measurements of malondialdehyde contents and high-temperature thermoluminescence indicated significantly increased oxidative stress and lipid peroxidation under salt treatment, except for light-grown leaves treated in the dark. In chloroplasts of leaf segments treated in the light, slight shrinkage of grana (determined by transmission electron microscopy and small-angle neutron scattering) was observed, while a swelling of the (pro)thylakoid lumen was observed in etioplasts. Salt-induced swelling disappeared after the onset of photosynthesis after 4 h of greening. Osmotic stress caused no significant alterations in plastid structure and only mild changes in their activities, indicating that the swelling of the (pro)thylakoid lumen and the physiological effects of salinity are rather associated with the ionic component of salt stress. Our data indicate that etioplasts of dark-germinated wheat seedlings are the most sensitive to salt stress, especially at the early stages of their greening.

In vitro assessment of physiological traits and ROS detoxification pathways involved in tolerance of Damask rose genotypes under salt stress

Rosa damascena is one of the most important medicinal and ornamental plants in Iran which is tolerant of salinity to some extent. However, the selection of genotypes that are more tolerant to salinity will influence on Damask cultivation in salt stress-affected regions. For this purpose, a factorial experiment in a completely randomized design with three replicates was performed under in vitro conditions on four Damask rose genotypes (Atashi, Bi-Khar, Chahar-Fasl and Kashan) at 5 concentrations of NaCl (0, 25, 50, 75, and 100 mM), and the physico-chemical traits were measured 14 and 28 days after treatment.The results showed that Atashi genotype with high levels of Chl a, Chl b, total Chl content, carotenoids, relative leaf water content, proline, total soluble protein, TPC, TFC, TAA, and the highest increase in the activity of antioxidant enzymes such as GPX, APX, CAT, SOD, and POD as well as the lowest amount of hydrogen peroxide showed a better protection mechanism against oxidative damage than the other three genotypes (Bi-Khar, Chahar-Fasl and Kashan) in the 14th and 28th days by maintaining the constructive and induced activities of antioxidant enzymes, it was shown that Bi-Khar genotype had moderate tolerance and Kashan and Chahar-Fasl genotypes had low tolerance to salinity stress. In vitro selection methods can be used effectively for salt tolerant screening of Damask rose genotypes, although the same experiment should be conducted in open filed cultures to verify the in vitro experimental results.

Exogenous Hydrogen Sulfide Supplementation Alleviates the Salinity-Stress-Mediated Growth Decline in Wheat (Triticum aestivum L.) by Modulating Tolerance Mechanisms

The impact of the exogenous supplementation of hydrogen sulfide (20 and 50 µM HS) on growth, enzyme activity, chlorophyll pigments, and tolerance mechanisms was studied in salinity-stressed (100 mM NaCl) wheat. Salinity significantly reduced height, fresh and dry weight, chlorophyll, and carotenoids. However, the supplementation of HS (at both concentrations) increased these attributes and also mitigated the decline to a considerable extent. The exogenous supplementation of HS reduced the accumulation of hydrogen peroxide (H2O2) and methylglyoxal (MG), thereby reducing lipid peroxidation and increasing the membrane stability index (MSI). Salinity stress increased H2O2, MG, and lipid peroxidation while reducing the MSI. The activity of nitrate reductase was reduced due to NaCl. However, the supplementation of HS alleviated the decline with obvious effects being seen due to 50 µM HS. The activity of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) was assayed and the content of reduced glutathione (GSH) increased due to salt stress and the supplementation of HS further enhanced their activity. A decline in ascorbic acid due to salinity stress was alleviated due to HS treatment. HS treatment increased the endogenous concentration of HS and nitric oxide (NO) under normal conditions. However, under salinity stress, HS supplementation resulted in a reduction in HS and NO as compared to NaCl-treated plants. In addition, proline and glycine betaine increased due to HS supplementation. HS treatment reduced sodium levels, while the increase in potassium justified the beneficial role of applied HS in improving salt tolerance in wheat.

Overexpression of chloroplastic Zea mays NADP-malic enzyme (ZmNADP-ME) confers tolerance to salt stress in Arabidopsis thaliana

The C4 plants photosynthesize better than C3 plants especially in arid environment. As an attempt to genetically convert C3 plant to C4, the cDNA of decarboxylating C4 type NADP-malic enzyme from Zea mays (ZmNADP-ME) that has lower Km for malate and NADP than its C3 isoforms, was overexpressed in Arabidopsis thaliana under the control of 35S promoter. Due to increased activity of NADP-ME in the transgenics the malate decarboxylation increased that resulted in loss of carbon skeletons needed for amino acid and protein synthesis. Consequently, amino acid and protein content of the transgenics declined. Therefore, the Chl content, photosynthetic efficiency (Fv/Fm), electron transport rate (ETR), the quantum yield of photosynthetic CO2 assimilation, rosette diameter, and biomass were lower in the transgenics. However, in salt stress (150 mM NaCl), the overexpressers had higher Chl, protein content, Fv/Fm, ETR, and biomass than the vector control. NADPH generated in the transgenics due to increased malate decarboxylation, contributed to augmented synthesis of proline, the osmoprotectant required to alleviate the reactive oxygen species-mediated membrane damage and oxidative stress. Consequently, the glutathione peroxidase activity increased and H2O2 content decreased in the salt-stressed transgenics. The reduced membrane lipid peroxidation and lower malondialdehyde production resulted in better preservation, of thylakoid integrity and membrane architecture in the transgenics under saline environment. Our results clearly demonstrate that overexpression of C4 chloroplastic ZmNADP-ME in the C3 Arabidopsis thaliana, although decrease their photosynthetic efficiency, protects the transgenics from salinity stress.

Differential Gene Expression Responses to Salt and Drought Stress in Tall Fescue (Festuca arundinacea Schreb.)

Understanding gene expression kinetics and the underlying physiological mechanisms in stress combinations is a challenge for the purpose of stress resistance breeding. The novelty of this study is correlating the physiological mechanisms with the expression of key target genes in tall fescue under a combination of various salinity and osmotic stress treatments. Four drought- and salt-responsive genes belonging to different crucial pathways evaluated included one transcription factor FabZIP69, one for the cytosolic polyamine synthetase FaADC1, one for ABA signaling FaCYP707A1, and another one for the specific Na+/H+ plasma membrane antiporter FaSOS1 involve in osmotic homeostasis. FaSOS1, FaCYP707A1, and FabZIP69 were induced early at 6 h after NaCl treatment, while FaSOS1 and FaCYP707A1 were transcribed gradually after exposure to PEG. However, stress interactions showed a significantly increased expression in all genes. Expression of these genes was positively correlated to Pro, SSs, IL, DPPH, and antioxidant enzyme activity and negatively correlated with RWC, total Chl, and MSI. Chemical analyses showed that tall fescue plants exposed to the combination of stresses exhibited increased quantity of reactive oxygen species (H2O2), EL and DPPH, and higher levels of antioxidant enzyme activities (CAT, and SOD), Pro, and SSs content, compared with control seedlings. Under dual-stress conditions, the expression of FabZIP69 was effective in controlling the expression of FaSOS1 and FaADC1 genes differently.

Mechanism of growth inhibition mediated by disorder of chlorophyll metabolism in rice (Oryza sativa) under the stress of three polycyclic aromatic hydrocarbons

Photosynthesis mediated by chlorophyll metabolism is the basis for plant growth, and also the important regulatory mechanism of carbon pool in cropland ecosystems. Soil organic pollutants induced growth inhibition in crop plants, herein, we conducted an in-depth investigation on the effects of three representative polycyclic aromatic hydrocarbons (PAHs), including phenanthrene (PHE), pyrene (PYR), and benzo[a]pyrene (BaP) on rice (Oryza sativa) growth and photosynthesis. PAHs were absorbed via root uptake and accumulated in leaves, causing the swelling of thylakoids and the increase of osmiophilic granules in chloroplasts. The actual quantum efficiency of PSII was significantly decreased under the stress of PHE, PYR, and BaP by 29.9%, 11.9%, and 24.1% respectively, indicating the inhibition in photon absorption and transfer, which was consistent with the decrease of chlorophyll a (22.3%-32.2% compared to the control) in rice leaves. Twenty-two encoding genes involved in chlorophyll metabolism were determined and the results indicated that the expression of chlorophyll synthetases was downregulated by over 50% whereas the degradation process was promoted. Consequently, the production of carbohydrates and the carbon fixation were inhibited, which revealed by the downregulation of intermediate metabolites in Calvin cycle and the declined carboxylation rate. The disturbed photosynthesis resulted in the decrease of the biomasses of both roots (21.0%-42.7%) and leaves (6.4%-22.1%) under the tested PAH stresses. The findings of this study implied that the photosynthetic inhibition was possibly attributed to the disorder of chlorophyll metabolism, thus providing novel insights into the mechanism of growth inhibition induced by organic pollutants and theoretical basis for the estimation of cropland carbon sequestration potential.

Photosynthesis, Water Status and K+/Na+ Homeostasis of Buchoe dactyloides Responding to Salinity

Soil salinization is one of the most serious abiotic stresses restricting plant growth. Buffalograss is a C₄ perennial turfgrass and forage with an excellent resistance to harsh environments. To clarify the adaptative mechanisms of buffalograss in response to salinity, we investigated the effects of NaCl treatments on photosynthesis, water status and K⁺/Na⁺ homeostasis of this species, then analyzed the expression of key genes involved in these processes using the qRT-PCR method. The results showed that NaCl treatments up to 200 mM had no obvious effects on plant growth, photosynthesis and leaf hydrate status, and even substantially stimulated root activity. Furthermore, buffalograss could retain a large amount of Na⁺ in roots to restrict Na⁺ overaccumulation in shoots, and increase leaf K⁺ concentration to maintain a high K⁺/Na⁺ ratio under NaCl stresses. After 50 and 200 mM NaCl treatments, the expressions of several genes related to chlorophyll synthesis, photosynthetic electron transport and CO₂ assimilation, as well as aquaporin genes (BdPIPs and BdTIPs) were upregulated. Notably, under NaCl treatments, the increased expression of BdSOS1, BdHKT1 and BdNHX1 in roots might have helped Na⁺ exclusion by root tips, retrieval from xylem sap and accumulation in root cells, respectively; the upregulation of BdHAK5 and BdSKOR in roots likely enhanced K⁺ uptake and long-distance transport from roots to shoots, respectively. This work finds that buffalograss possesses a strong ability to sustain high photosynthetic capacity, water balance and leaf K⁺/Na⁺ homeostasis under salt stress, and lays a foundation for elucidating the molecular mechanism underlying the salt tolerance of buffalograss.

Effects of Salinity Stress on Growth and Physiological Parameters and Related Gene Expression in Different Ecotypes of Sesuvium portulacastrum on Hainan Island

We conducted a study to examine the growth and physiological changes in 12 different ecotypes of Sesuvium portulacastrum collected from Hainan Island in China. These ecotypes were subjected to different concentrations (0, 200, 400, and 600 mmol/L) of sodium chloride (NaCl) salt stress for 14 days. We also analyzed the expression of metabolic genes related to stress response. Under low salt stress, indicators such as plant height in region K (0 mmol/L: 45% and highest at 200 mmol/L: 80%), internode length (0 mmol/L: 0.38, 200 mmol/L: 0.87, 400 mmol/L: 0.25, and 600 mmol/L: 1.35), as well as leaf area, relative water content, fresh weight, and dry weight exhibited an overall increasing trend with the increase in salt concentration. However, as the salt concentration increased, these indicators showed a decreasing trend. Proline and malondialdehyde contents increased with higher salt concentrations. When the NaCl concentration was 400 mmol/L, MDA content in the leaves was highest in the regions E (196.23%), F (94.28%), J (170.10%), and K (136.08%) as compared to the control group, respectively. Most materials demonstrated a significant decrease in chlorophyll a, chlorophyll b, and total chlorophyll content compared to the control group. Furthermore, the ratio of chlorophyll a to chlorophyll b (Rab) varied among different materials. Using principal component analysis, we identified three ecotypes (L from Xinglong Village, Danzhou City; B from Shuigoupo Village, Lingshui County; and J from Haidongfang Park, Dongfang City) that represented high, medium, and low salt tolerance levels, respectively, based on the above growth and physiological indexes. To further investigate the expression changes of related genes at the transcriptional level, we employed qRT-PCR. The results showed that the relative expression of SpP5CS1, SpLOX1, and SpLOX1 genes increased with higher salt concentrations, which corresponded to the accumulation of proline and malondialdehyde content, respectively. However, the relative expression of SpCHL1a and SpCHL1b did not exhibit a consistent pattern. This study contributes to our understanding of the salt tolerance mechanism in the true halophyte S. portulacastrum, providing a solid theoretical foundation for further research in this field.

Exogenous Hemin alleviates NaCl stress by promoting photosynthesis and carbon metabolism in rice seedlings

It is widely known that salt stress restricts rice growth and productivity severely. However, little information is available regarding the stage of rice seedlings subjected to the Heme oxygenase 1 (HO-1) inducer, Hemin. This study aimed to investigate the effects of salt stress on two rice varieties (Huanghuazhan and Xiangliangyou 900) and the effect of Hemin in promoting photosynthesis, carbohydrate metabolism, and key enzymes under salt-stress conditions. At the stage of three leaves and one heart, Huanghuazhan (HHZ) and Xiangliangyou 900 (XLY900) were sprayed with 5 μmol·L^-1 Hemin and then subjected to 50 mM NaCl stress. The results showed that NaCl stress decreased the contents of chlorophyll a, chlorophyll b, and carotenoids. Furthermore, the net photosynthetic rate (P_n) decreased remarkably and the starch content was also lowered. However, NaCl treatment enhanced the concentration of sucrose and soluble sugar, simultaneously enhancing the sucrose metabolism. Nevertheless, the foliar spraying of exogenous Hemin mediated the increase in fructose and starch content, along with the activities of key enzymes' soluble acid invertase (SAInv), basic/neutral invertase (A/N-Inv), and sucrose synthase (SS) in rice leaves under NaCl stress. The sucrose phosphate synthase (SPS) in leaves decreased significantly, and the fructose accumulation in leaves increased. Hemin also mediated the increase of starch content and the α-amylase, total amylase, and starch phosphorylase (SP) activities under NaCl stress. Under stress conditions, the application of the Heme oxygenase 1 (HO-1) inhibitor, ZnPP failed to alleviate the damage to rice seedlings by NaCl stress. The ZnPP treatment showed similar tendency to the NaCl treatment on pigment content, gas exchange parameters and carbon metabolism related products and enzymes. However, ZnPP decreased carotenoids, fructose, starch content and enzyme activities related to starch metabolism. The regulation effect of Hemin on HuangHuaZhan was better than XiangLiangYou 900. These results indicate that Hemin improved the effects of salt stress on the photosynthesis and physiological characteristics of rice leaves as a result of enhanced carbohydrate metabolism. Thus, Hemin could alleviate the damage caused by salt stress to a certain extent.

Transcriptome-Wide Identification and Functional Characterization of CIPK Gene Family Members in Actinidia valvata under Salt Stress

Fruit plants are severely constrained by salt stress in the soil due to their sessile nature. Ca²⁺ sensors, which are known as CBL-interacting protein kinases (CIPKs), transmit abiotic stress signals to plants. Therefore, it is imperative to investigate the molecular regulatory role of CIPKs underlying salt stress tolerance in kiwifruit. In the current study, we have identified 42 CIPK genes from Actinidia. valvata (A.valvata). All the AvCIPKs were divided into four different phylogenetic groups. Moreover, these genes showed different conserved motifs. The expression pattern analysis showed that AvCIPK11 was specifically highly expressed under salt stress. The overexpression of AvCIPK11 in 'Hongyang' (a salt sensitive commercial cultivar from Actinidia chinensis) enhanced salt tolerance by maintaining K⁺/Na⁺ homeostasis in the leaf and positively improving the activity of POD. In addition, the salt-related genes AcCBL1 and AcNHX1 had higher expression in overexpression lines. Collectively, our study suggested that AvCIPK11 is involved in the positive regulation of salt tolerance in kiwifruit.

Exogenous 24-epibrassinolide promoted growth and nitrogen absorption and assimilation efficiency of apple seedlings under salt stress

Introduction

High salinity significantly hampers global agricultural productivity. Plants typically undergo lower nitrogen utilization efficiency (NUE) under salt stress. As an active byproduct from brassinolide biosynthesis, 24-epibrassinolide (EBR) is involved in regulating the stress-treated plant N absorption and assimilation. However, the exogenous EBR application effects' on N absorption and assimilation in apple exposed to the salt-stressed condition remains unclear.

Methods

We sprayed exogenous EBR (0.2 mg L^-1) on apple dwarf rootstock (M9T337) seedlings (growing hydroponically) under salt (NaCl) stress in a growth chamber. We analyzed the seedling development, photosynthesis and its-mediated C fixation, N ( NO 3 - ) absorption and assimilation in reponse to exogenous EBR application under salt stress.

Results

The findings demonstrated that NaCl stress greatly hampered seedlings' root growth and that exogenous EBR application obviously alleviated this growth suppression. Exogenous EBR-treated plants under NaCl stress displayed the more ideal root morphology and root activity, stronger salt stress tolerance and photosynthetic capacity as well as higher C- and N-assimilation enzyme activities, NO 3 - ion flow rate and nitrate transporter gene expression level than did untreated plants. Furthermore, the results of isotope labeling noted that exogenous EBR application also enhanced ¹³C-photoassimilate transport from leaves to roots and ¹⁵ NO 3 - transport from roots to leaves under NaCl stress.

Conclusion

Our findings imply that exogenous EBR application, through strengthening photosynthesis, C- and N-assimilation enzyme activities, nitrate absorption and transport as well as synchronized optimizing the distribution of seedlings' C and N, has a fundamental role in improving NUE in apple rootstock seedlings under salt stress.

Effect of exogenous melatonin on improvement of chlorophyll content and photochemical efficiency of PSII in mallow plants (Malva parviflora L.) treated with cadmium

Melatonin is a growth regulator that improves the growth and chlorophyll (chl) content in plants. This study aims to investigate the effect of melatonin pretreatment on chl synthesis and fluorescence parameters in Malva parviflora exposed to cadmium (Cd). The 42-day-old plants were transferred to nutrient solutions containing 50 μM melatonin. After two days, some plants were exposed to 50 μM Cd. Eight days after Cd treatment, some indicators related to chl fluorescence and some biochemical parameters were measured. In this study, melatonin increased chl content and chl a/pheophytin a (pheo a) ratio, chlorophyllide a (chlide a), porphyrin compounds, and 5-aminolevulinic acid (5-ALA) in the presence of Cd. However, it decreased chl a/chlide a ratio under these conditions. Whereas Cd treatment resulted in significant reductions in photochemical activity and electron transfer rate in PSII, melatonin improved photochemical efficiency of PSII by reducing the toxic effect of Cd on the activity of the oxygen evolving complex (OEC) on the electron donor site and reducing non-photochemical quenching (NPQ). Based on the results, it appears that melatonin can maintain the chl content of plants exposed to Cd by increasing the precursors of the chl biosynthesis pathway and reducing its degradation rate. These results may, at least in our experimental conditions, partly explain the reason for the improved yield and growth of Cd-exposed plants when pretreated with melatonin.

IAA Plays an Important Role in Alkaline Stress Tolerance by Modulating Root Development and ROS Detoxifying Systems in Rice Plants

Auxin regulates plant growth and development, as well as helps plants to survive abiotic stresses, but the effects of auxin on the growth of alkaline-stressed rice and the underlying molecular and physiological mechanisms remain unknown. Through exogenous application of IAA/TIBA, this study explored the physiological and molecular mechanisms of alkaline stress tolerance enhancement using two rice genotypes. Alkaline stress was observed to damage the plant growth, while exogenous application of IAA mitigates the alkaline-stress-induce inhibition of plant growth. After application of exogenous IAA to alkaline-stressed rice, dry shoot biomass, foliar chlorophyll content, photosynthetic rate in the two rice genotypes increased by 12.6-15.6%, 11.7-40.3%, 51.4-106.6%, respectively. The adventitious root number, root surface area, total root length and dry root biomass in the two rice genotypes increased by 29.3-33.3%, 26.4-27.2%, 42.5-35.5% and 12.8-33.1%, respectively. The accumulation of H₂O₂, MAD were significantly decreased with the application of IAA. The activities of CAT, POD, and SOD in rice plants were significantly increased by exogenous application of IAA. The expression levels of genes controlling IAA biosynthesis and transport were significantly increased, while there were no significant effects on the gene expression that controlled IAA catabolism. These results showed that exogenous application of IAA could mitigate the alkaline-stress-induced inhibition of plant growth by regulating the reactive oxygen species scavenging system, root development and expression of gene involved in IAA biosynthesis, transport and catabolism. These results provide a new direction and empirical basis for improving crop alkaline tolerance with exogenous application of IAA.

Transcription Factor SlAREB1 Is Involved in the Antioxidant Regulation under Saline–Alkaline Stress in Tomato

Basic leucine zipper (bZIP) transcription factors of the ABA-responsive element binding factor/ABA-responsive element binding proteins (ABF/AREB) subfamily have been implicated in abscisic acid (ABA) and abiotic stress responses in plants. However, the specific function of ABF/AREB transcription factors under saline–alkaline stress is unclear. Here, we identified four ABF/AREB transcription factors in tomato and found that SlAREB1 strongly responded to both ABA and saline–alkaline stress. To further explore the function of SlAREB1 under saline–alkaline stress, SlAREB1-overexpressing lines were constructed. Compared with wild-type plants, SlAREB1-overexpressing transgenic tomato plants showed reduced malondialdehyde content, increased the relative water content, and alleviated the degradation of chlorophyll under saline–alkaline stress. Importantly, SlAREB1 directly physically interacted with SlMn-SOD, which improved the activity of antioxidant enzymes and increased the scavenging of excess reactive oxygen species. Overall, the overexpression of SlAREB1 increased the antioxidant capacity of the transgenic tomato under saline–alkaline stress.

Sodium Chloride (NaCl)-Induced Physiological Alteration and Oxidative Stress Generation in Pisum sativum (L.): A Toxicity Assessment

Salinity stress has a deleterious impact on plant development, morphology, physiology, and biochemical characteristics. Considering the NaCl-induced phytotoxicity, current investigation was done to better understand the salt-tolerant mechanisms using Pisum sativum L. (pea) as a model crop. Generally, NaCl resulted in a progressive decrease in germinative attributes and physiological and biochemical parameters of P. sativum (L.). The 400 mM NaCl level had a higher detrimental effect and reduced the germination rate, plumule, radicle length, and seedling vigor index (SVI) by 78, 89, 84, and 77%, respectively, under in vitro. Furthermore, after 400 mM NaCl exposure, physiological and enzymatic profiles like root dry biomass (71%) chl-a (66%), chl-b (54%), total chlorophyll (45%), and nitrate reductase activity (NRA) (59%) of peas were decreased. In addition, a NaCl dose-related increase in soluble protein (SP) and sugar (SS), Na⁺ and K⁺ ions, and stressor metabolites was recorded. For instance, at 400 mM NaCl, SP, SS, Na⁺ ion, K⁺ ion, root proline, and malondialdehyde (MDA) contents were significantly and maximally elevated by 65, 33, 84, 79, 85, and 89%, respectively, compared to the control (0 mM NaCl). Data analysis indicated that greater doses of pesticides dramatically increased reactive oxygen species (ROS) levels and induced membrane damage through production of thiobarbituric acid reactive substances (TBARS), as well as increased cell injury. To deal with NaCl-induced oxidative stress, plants subjected to higher salinity stress showed a considerable build-up in antioxidant levels. As an example, ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were maximally and significantly (p ≤ 0.05) increased by 68, 80, 74, and 58%, respectively, after 400 mM NaCl exposure. The propidium iodide (PI)-stained and NaCl-treated plant roots corroborated the damaging effect of salinity-induced stress on the cell membrane, which was observed under a confocal laser microscope (CLSM). The cells exposed to 400 mM NaCl had maximum fluorescence intensity, indicating that higher level of salts can cause pronounced cell damage and reactive oxygen species (ROS) generation. The increases in superoxide ion (O₂ ^-) and hydrogen peroxide (H₂O₂) content in NaCl-treated plant tissues indicated the elevation of ROS with increasing salt levels. This finding revealed that salt stress can cause toxicity in plants by causing alteration in metabolic activity, oxidative injury, and damage to cell membrane integrity.

Bacillus aryabhattai enhanced proline content, stabilized membrane and improved growth of cowpea under NaCl-induced salinity stress

AIMS

Salinity stress affects the growth of cowpea particularly at the stages of seed germination and early vegetative growth. This study examined the potential of particular stress-tolerant rhizospheric bacteria to improve the growth of cowpea under conditions of salinity stress.

METHODS AND RESULTS

Two rhizobacillus genotypes, Bacillus filamentosus-C8 and Bacillus aryabhattai-C29 were evaluated for their potentials to protect cowpea under NaCl-induced salinity stress. At 200 mM of NaCl concentration, control (non-inoculated) cowpea was affected, C8 was not able to significantly (p ≤ 0.05) alleviate the effects of salinity stress on cowpea growth while C29 significantly (p ≤ 0.05) reduced leaf wilting, increased chlorophyll content and improved the growth of cowpea plant under stressed condition. Interestingly, C29 significantly (p ≤ 0.05) induced high proline content and stabilized membrane by loss of electrolytes.

CONCLUSION

Our results indicate that stabilized membrane and enhanced proline content by Bacillus aryabhattai-C29 supported the growth of cowpea under salinity stress condition.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that rhizospheric bacteria screened for salinity stress tolerant have potential to be used as an effective bioprotectant for sustainable growth of cowpea under salinity stress condition.

Evaluation of salt tolerance of oat cultivars and the mechanism of adaptation to salinity

Soil salinity is a threat to agricultural production worldwide. Oat (Avena sativa L.) is an irreplaceable crop in areas with fragile ecological conditions. However, there is a lack of research on salt tolerance evaluation of oat germplasm resources. Therefore, the purpose of this work was to evaluate the salt tolerance of oat cultivars and investigate the mechanism of salt-tolerant oat cultivars' adaptation to salinity. Salt tolerance of 100 oat cultivars was evaluated, and then two salt-tolerant cultivars and two salt-sensitive cultivars were used to compare their physiological responses and expression patterns of Na⁺- and K⁺-transport-related genes under salinity. Principal component analysis and membership function analysis had good predictability for salt tolerance evaluation of oat and other crops. The 100 oat cultivars were clustered into three categories, with three salt tolerance levels. Under saline condition, salt-tolerant cultivars maintained higher growth rate, leaf cell membrane integrity, and osmotic adjustment capability via enhancing the activities of antioxidant enzymes and accumulating more osmotic regulators. Furthermore, salt-tolerant cultivars had stronger capability to restrict root Na ⁺ uptake through reducing AsAKT1 and AsHKT2;1 expression, exclude more Na⁺ from root through increasing AsSOS1 expression, compartmentalize more Na ⁺ into root vacuoles through increasing AsNHX1 and AsVATP-P1 expression, and absorb more K⁺ through increasing AsKUP1 expression, compared with salt-sensitive cultivars. The evaluation procedure developed in this work can be applied for screening cereal crop cultivars with higher salt tolerance, and the elucidated mechanism of oat adaptation to salinity lays a foundation for identifying more functional genes related to salt tolerance.

Abscisic acid alleviates harmful effect of saline-alkaline stress on tomato seedlings

Saline-alkaline stress inhibits plant growth and reduces yield. Abscisic acid (ABA) is an important plant hormone in response to plant stress. However, the role of ABA under saline-alkaline stress is poorly understood. Therefore, the mechanisms of ABA accumulation and resistance improvement in tomato seedlings were studied under saline-alkaline stress. We investigated whether ABA accumulation improved the saline-alkaline stress resistance ability of tomato. Here, wild-type (Solanum lycopersicum cv. Ailsa Craig) and ABA-deficient mutant (notabilis) seedlings were used to determine the membrane lipid peroxidation, osmotic substance and chlorophyll contents. ABA synthesis and signal transduction changes and ABA roles regulating the antioxidation in tomato seedlings subject to saline-alkaline stress were further explored. Results showed that ABA synthesis and signal transduction were induced by saline-alkaline stress. Under saline-alkaline stress, tomato seedlings had decreased relative water content, increased relative electrical conductivity and malondialdehyde content, and these changes were alleviated by exogenous ABA treatment. Exogenous ABA alleviated the degradation of chlorophyll in the leaves of tomato seedlings caused by saline-alkaline stress, further promoted the accumulation of proline and soluble sugar, reduced the content of ROS and improved the ability of the antioxidant enzyme system. Moreover, notabilis appeared to be sensitive to saline-alkaline stress. Overall, ABA is involved in the resistance of tomato seedlings to saline-alkaline stress, and exogenous ABA improves the saline-alkaline tolerance of tomato seedlings.

Exogenous Myo-Inositol Alleviates Salt Stress by Enhancing Antioxidants and Membrane Stability via the Upregulation of Stress Responsive Genes in Chenopodium quinoa L

Myo-inositol has gained a central position in plants due to its vital role in physiology and biochemistry. This experimental work assessed the effects of salinity stress and foliar application of myo-inositol (MYO) on growth, chlorophyll content, photosynthesis, antioxidant system, osmolyte accumulation, and gene expression in quinoa (Chenopodium quinoa L. var. Giza1). Our results show that salinity stress significantly decreased growth parameters such as plant height, fresh and dry weights of shoot and root, leaf area, number of leaves, chlorophyll content, net photosynthesis, stomatal conductance, transpiration, and Fv/Fm, with a more pronounced effect at higher NaCl concentrations. However, the exogenous application of MYO increased the growth and photosynthesis traits and alleviated the stress to a considerable extent. Salinity also significantly reduced the water potential and water use efficiency in plants under saline regime; however, exogenous application of myo-inositol coped with this issue. MYO significantly reduced the accumulation of hydrogen peroxide, superoxide, reduced lipid peroxidation, and electrolyte leakage concomitant with an increase in the membrane stability index. Exogenous application of MYO up-regulated the antioxidant enzymes' activities and the contents of ascorbate and glutathione, contributing to membrane stability and reduced oxidative damage. The damaging effects of salinity stress on quinoa were further mitigated by increased accumulation of osmolytes such as proline, glycine betaine, free amino acids, and soluble sugars in MYO-treated seedlings. The expression pattern of OSM34, NHX1, SOS1A, SOS1B, BADH, TIP2, NSY, and SDR genes increased significantly due to the application of MYO under both stressed and non-stressed conditions. Our results support the conclusion that exogenous MYO alleviates salt stress by involving antioxidants, enhancing plant growth attributes and membrane stability, and reducing oxidative damage to plants.

Metabolomics and Physiological Insights into the Ability of Exogenously Applied Chlorogenic Acid and Hesperidin to Modulate Salt Stress in Lettuce Distinctively

Recent studies in the agronomic field indicate that the exogenous application of polyphenols can provide tolerance against various stresses in plants. However, the molecular processes underlying stress mitigation remain unclear, and little is known about the impact of exogenously applied phenolics, especially in combination with salinity. In this work, the impacts of exogenously applied chlorogenic acid (CA), hesperidin (HES), and their combination (HES + CA) have been investigated in lettuce (Lactuca sativa L.) through untargeted metabolomics to evaluate mitigation effects against salinity. Growth parameters, physiological measurements, leaf relative water content, and osmotic potential as well as gas exchange parameters were also measured. As expected, salinity produced a significant decline in the physiological and biochemical parameters of lettuce. However, the treatments with exogenous phenolics, particularly HES and HES + CA, allowed lettuce to cope with salt stress condition. Interestingly, the treatments triggered a broad metabolic reprogramming that involved secondary metabolism and small molecules such as electron carriers, enzyme cofactors, and vitamins. Under salinity conditions, CA and HES + CA distinctively elicited secondary metabolism, nitrogen-containing compounds, osmoprotectants, and polyamines.

Modulation of salinity impact on early seedling stage via nano-priming application of zinc oxide on rapeseed (Brassica napus L.)

Salinity stress negatively affects the plant's developmental stages through micronutrient imbalance. As an essential micronutrient, ZnO can substitute Na⁺ absorption under saline conditions. Therefore, nanoparticles as technological innovation, improve the plant growth efficiency under biotic and abiotic stresses. Nano-priming has become widely applicable in agricultural research during the last decade. The current study was conducted to highlight the impact of ZnONPs priming on seedling biological processes under 150 mM of NaCl using two rapeseed cultivars during the early seedling stage. All concentrations of ZnONPs increased the germination parameters i.e., FG%, GR, VI (I), and VI (II). Meanwhile, the high concentration (ZnO 100%) showed the highest increase in shoot length (9.60% and 25.63%), root length (41.64% and 48.17%) for Yang You 9 and Zhong Shuang 11 over hydro-priming, respectively, as well as biomass. Additionally, nano-priming improved the proline, soluble sugar, and soluble protein contents as a result of osmotic protection modulation. Moreover, nano-priming alleviated ROS and biosynthesis pigments through the reduction of accumulated (H₂O₂) and (O₂^-), and chlorophyll degradation, respectively, also enhanced antioxidant adjustment via improving the plant defense system. Nano-priming substituted the Na⁺ by Zn²⁺, K⁺, and Ca²⁺, and compensated the deficit of micronutrients, thus reduced the Na⁺ toxicity in the cell cytosol. To track the effects of priming during seed imbibition, it noticed that ZnO 100% and ZnO 100%+S increased the Linoleic and Linolenic acids among the studied fatty acids composition by 12.02%, 7.59%, 13.27%, and 10.38% (Yang You 9), 7.42%, 2.77%, 2.93%, and 1.49% (Zhong Shuang 11) over the hydro-priming, respectively. Moreover, the gene expression patterns of BnCAM and BnPER reflected the enhancement of germination levels, notably under the influence of ZnO 100% priming, which increased the level of BnCAM by 70.42% and 111.9% in Yang You 9 and Zhong Shuang 11, respectively. Consequently, ZnO nano-priming enhanced the seedling development through the biosynthesis of pigments, osmotic protection, reduction of ROS accumulation, adjustment of antioxidant enzymes, and improvement of the nutrient absorption, thus enhancing the economic yield under saline conditions.

Exogenous Nitric Oxide Reinforces Photosynthetic Efficiency, Osmolyte, Mineral Uptake, Antioxidant, Expression of Stress-Responsive Genes and Ameliorates the Effects of Salinity Stress in Wheat

Salinity stress is one of the major environmental constraints responsible for a reduction in agricultural productivity. This study investigated the effect of exogenously applied nitric oxide (NO) (50 μM and 100 μM) in protecting wheat plants from NaCl-induced oxidative damage by modulating protective mechanisms, including osmolyte accumulation and the antioxidant system. Exogenously sourced NO proved effective in ameliorating the deleterious effects of salinity on the growth parameters studied. NO was beneficial in improving the photosynthetic efficiency, stomatal conductance, and chlorophyll content in normal and NaCl-treated wheat plants. Moreover, NO-treated plants maintained a greater accumulation of proline and soluble sugars, leading to higher relative water content maintenance. Exogenous-sourced NO at both concentrations up-regulated the antioxidant system for averting the NaCl-mediated oxidative damage on membranes. The activity of antioxidant enzymes increased the protection of membrane structural and functional integrity and photosynthetic efficiency. NO application imparted a marked effect on uptake of key mineral elements such as nitrogen (N), potassium (K), and calcium (Ca) with a concomitant reduction in the deleterious ions such as Na⁺. Greater K and reduced Na uptake in NO-treated plants lead to a considerable decline in the Na/K ratio. Enhancing of salt tolerance by NO was concomitant with an obvious down-regulation in the relative expression of SOS1, NHX1, AQP, and OSM-34, while D2-protein was up-regulated.

The miR528-AO Module Confers Enhanced Salt Tolerance in Rice by Modulating the Ascorbic Acid and Abscisic Acid Metabolism and ROS Scavenging

The monocot lineage-specific miR528 was previously established as a multistress regulator. However, it remains largely unclear how miR528 participates in response to salinity stress in rice. Here, we show that miR528 positively regulates rice salt tolerance by down-regulating a gene encoding l-ascorbate oxidase (AO), thereby bolstering up the AO-mediated abscisic acid (ABA) synthesis and ROS scavenging. Overexpression of miR528 caused a substantial increase in ascorbic acid (AsA) and ABA contents but a significant reduction in ROS accumulation, resulting in the enhanced salt tolerance of rice plants. Conversely, knockdown of miR528 or overexpression of AO stimulated the expression of the AO gene, hence lowering the level of AsA, a critical antioxidant that promotes the ABA content but reduces the ROS level, and then compromising rice tolerance to salinity. Together, the findings reveal a novel mechanism of the miR528-AO module-mediated salt tolerance by modulating the processes of AsA and ABA metabolism as well as ROS detoxification, which adds a new regulatory role to the miR528-AO stress defense pathway in rice.

A. Mohamed I, Wang Z, Khatab A, Sherif A, Ahmad H, Khan MN, Hassan HM, Elrewainy IM, Kuai J, Zhou G, Wang B. Antioxidative and Metabolic Contribution to Salinity Stress Responses in Two Rapeseed Cultivars during the Early Seedling Stage

Measuring metabolite patterns and antioxidant ability is vital to understanding the physiological and molecular responses of plants under salinity. A morphological analysis of five rapeseed cultivars showed that Yangyou 9 and Zhongshuang 11 were the most salt-tolerant and -sensitive, respectively. In Yangyou 9, the reactive oxygen species (ROS) level and malondialdehyde (MDA) content were minimized by the activation of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) for scavenging of over-accumulated ROS under salinity stress. Furthermore, Yangyou 9 showed a significantly higher positive correlation with photosynthetic pigments, osmolyte accumulation, and an adjusted Na⁺/K⁺ ratio to improve salt tolerance compared to Zhongshuang 11. Out of 332 compounds identified in the metabolic profile, 225 metabolites were filtrated according to p < 0.05, and 47 metabolites responded to salt stress within tolerant and sensitive cultivars during the studied time, whereas 16 and 9 metabolic compounds accumulated during 12 and 24 h, respectively, in Yangyou 9 after being sown in salt treatment, including fatty acids, amino acids, and flavonoids. These metabolites are relevant to metabolic pathways (amino acid, sucrose, flavonoid metabolism, and tricarboxylic acid cycle (TCA), which accumulated as a response to salinity stress. Thus, Yangyou 9, as a tolerant cultivar, showed improved antioxidant enzyme activity and higher metabolite accumulation, which enhances its tolerance against salinity. This work aids in elucidating the essential cellular metabolic changes in response to salt stress in rapeseed cultivars during seed germination. Meanwhile, the identified metabolites can act as biomarkers to characterize plant performance in breeding programs under salt stress. This comprehensive study of the metabolomics and antioxidant activities of Brassica napus L. during the early seedling stage is of great reference value for plant breeders to develop salt-tolerant rapeseed cultivars.

A Review on Plant Responses to Salt Stress and Their Mechanisms of Salt Resistance

Nowadays, crop insufficiency resulting from soil salinization is threatening the world. On the basis that soil salinization has become a worldwide problem, studying the mechanisms of plant salt tolerance is of great theoretical and practical significance to improve crop yield, to cultivate new salt-tolerant varieties, and to make full use of saline land. Based on previous studies, this paper reviews the damage of salt stress to plants, including suppression of photosynthesis, disturbance of ion homeostasis, and membrane peroxidation. We have also summarized the physiological mechanisms of salt tolerance, including reactive oxygen species (ROS) scavenging and osmotic adjustment. Four main stress-related signaling pathways, salt overly sensitive (SOS) pathway, calcium-dependent protein kinase (CDPK) pathway, mitogen-activated protein kinase (MAPKs) pathway, and abscisic acid (ABA) pathway, are included. We have also enumerated some salt stress-responsive genes that correspond to physiological mechanisms. In the end, we have outlined the present approaches and techniques to improve salt tolerance of plants. All in all, we reviewed those aspects above, in the hope of providing valuable background knowledge for the future cultivation of agricultural and forestry plants.

The Bacillus zanthoxyli HS1 Strain Renders Vegetable Plants Resistant and Tolerant against Pathogen Infection and High Salinity Stress

Various management systems are being broadly employed to minimize crop yield loss resulting from abiotic and biotic stresses. Here we introduce a Bacillus zanthoxyli HS1 strain as a potent candidate for managing manifold stresses on vegetable plants. Considering 16S rDNA sequence and biochemical characteristics, the strain is closely related to B. zanthoxyli. The B. zanthoxyli HS1's soil-drench confers disease resistance on tomato and paprika plants against infection with Ralstonia solanacearum and Phytophthora capsici, respectively. Root and shoot growths are also increased in B. zanthoxyli HS1-treated cabbage, cucumber, and tomato plants, compared with those in mock-treated plants, after application of high salinity solution. Moreover, the pretreatment of B. zanthoxyli HS1 on cabbage plants inhibits the degradation of chloroplast pigments caused by high salinity stresses, whereas the inhibitory effect is not observed in cucumber plants. These findings suggest that B. zanthoxyli HS1 stain inhibits disease development and confers tolerance to salinity stress on vegetable plants.

Streptomyces hygroscopicus OsiSh-2-induced mitigation of Fe deficiency in rice plants

The limited availability of nutrient Fe severely impairs the health of almost all organisms. Endophytic actinobacteria can benefit the host plant in different ways. We previously inferred that the rice (Oryza) endophytic Streptomyces hygroscopicus OsiSh-2 possesses a highly efficient Fe-acquisition system. In this work, we first evaluated the effects of OsiSh-2 on the Fe-deficiency resilience of the host rice. The results demonstrated that the inoculation of OsiSh-2 considerably increased the plant biomass, Fe concentration and translocation factor, and chlorophyll content, and net leaf photosynthetic rate under Fe limiting condition. The expression of genes involved with Fe³⁺-reduction-related strategy in rice was up-regulated, while that involved with Fe³⁺-chelation-related strategy was down-regulated by OsiSh-2 treatment. Meanwhile, the OsiSh-2-rice symbiont showed enhancement of Fe³⁺-chelate reductase activity, total siderophore production, and acidification trend in the rhizosphere under Fe deficiency compared to plants without this endophyte. In conclusion, endophytic OsiSh-2 could protect plants against Fe-deficient stress by a sophisticated interaction with the host, including modulating Fe chelation, solubilization, reduction and translocation, ultimately leading to enhanced fitness of plant.

Long-term somatic memory of salinity unveiled from physiological, biochemical and epigenetic responses in two contrasting rice genotypes

Plants are constantly exposed to environmental fluctuations, that may occur in a single day or over longer periods. In many cases, abiotic stresses are transient and recurrent, impacting how plants respond in subsequent adverse conditions. Adaptation mechanisms may occur at the physiological, biochemical and molecular level, modifying transcriptional response, regulatory proteins, epigenetic marks or metabolites. Here, we aimed to uncover the different strategies that rice uses to respond to recurrent stress. We tested varieties with contrasting behavior towards salinity (tolerance or sensitivity) and imposed salt stress (150 mM NaCl) during 48 h at vegetative and/or reproductive stages. After 48 h of stress in reproductive stage, leaves and roots were harvested separately or otherwise the plants were submitted to a 24 h recovery, prior to sample harvesting. Plants submitted to a recurrent stress responded differently from those suffering a single stress event. In the case of the sensitive genotype, recurrent stress led to lower Na/K ratio in roots and lower hydrogen peroxide accumulation and lipid peroxidation in leaves, but maintenance of global DNA methylation levels. In the tolerant genotype, recurrent stress did neither affect the Na/K ratio nor the stomatal conductance, although the levels of superoxide anion and hydrogen peroxide accumulation were lower, as also observed for global levels of DNA methylation. Our work shows that a short pre-exposure to salt stress may improve rice tolerance to subsequent stress, trough biochemical, physiological and epigenetic processes, with more significant changes visible in the tolerant genotype.

Chlorophyll Metabolism and Gene Expression in Response to Submergence Stress and Subsequent Recovery in Perennial Ryegrass Accessions Differing in Growth Habits

Submergence-induced leaf senescence may alter chlorophyll metabolism. The objective of this study was to characterize chlorophyll biosynthesis and degradation in contrasting perennial ryegrass (Lolium perenne) in response to submergence stress and recovery. The light-green and fast-growing accession PI238938 and the darker-green and slow-growing cultivar BrightStar SLT were exposed to 0, 6 h, 1-, 3-, and 7-d of submergence stress and 1- and 5- d of de-submergence, respectively. Plant growth of PI238938 were more severely inhibited by submergence stress and recovery. Both accessions showed increased leaf malondialdehyde under stress and recovery, but reduced chlorophyll (Chl) concentrations were observed at 3- and 7-d of stress and at recovery. The reduction in Chl was more severe in BrightStar SLT at 7 d of stress. The concentration of 5-aminolevulenic acid was unaffected by stress but increased at 1d of recovery. Activities of 5-aminolevulinic acid dehydratase (ALAD) involved in Chl biosynthesis remained unchanged under stress and recovery, while the activities of Chl degrading enzymes chlorophyllase (CHL) and pheophytinase (PPH) increased at 3 d or 7 d of stress, and returned to the control level after recovery in both accessions. The downregulation of Chl-biosynthetic genes CHLI, POR, and CHLP and the upregulation of Chl-degrading genes CLH, PPH, and SGR were observed in both accessions under most of the stress periods. BrightStar SLT exhibited much lower expressions of the Chl-biosynthetic genes PBGD, CHS, and CHID under stress, while PI238938 had remarkably higher expressions of genes involved in Chl breakdown including CLH, PPH, PAO, RCCR, and SGR, and the expressions of these genes remained at a higher level at 1 d of recovery. The results indicated that submergence-induced leaf senescence and declines in Chl were associated with downregulation of more Chl-biosynthetic genes in slow-growing genotype and upregulation of more Chl-degrading genes in fast-growing genotype of perennial ryegrass.

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress

Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown. The present study investigated the impact of exogenously applied ACh (10 µm) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150 mm NaCl). Salinity reduced root hydraulic conductivity while ACh-treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin-IX, Mg-photoporphyrin-IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up-regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO2 concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity-induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio. Results of the present study indicate that ACh can be effective in ameliorating NaCl-induced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

Interactive role of exogenous 24 Epibrassinolide and endogenous NO in Brassica juncea L. under salinity stress: Evidence for NR-dependent NO biosynthesis

The present study unravels origin of nitric oxide (NO) and the interaction between 24-Epibrassinolide (EBL) and nitrate reductase (NR) for NO production in Indian mustard (Brassica juncea L.) under salinity stress. Two independent experiments were performed to check whether (i) Nitrate reductase or Nitric oxide synthase takes part in the biosynthesis of endogenous NO and (ii) EBL has any regulatory effect on NR-dependent NO biosynthesis in the alleviation of salinity stress. Results revealed that NR-inhibitor tungstate significantly (P ≤ 0.05) decreased the NR activity and endogenous NO content, while NOS inhibitor l-NAME did not influence NO biosynthesis and plant growth. Under salinity stress, inhibition in NR activity decreased the activities of antioxidant enzymes, increased H₂O₂, MDA, protein carbonyl content and caused DNA damage, implying that antioxidant defense might be related to NO signal. EBL supplementation enhanced the NR activity but did not influence NOS activity, suggesting that NR was involved in endogenous NO production. EBL supplementation alleviated the inhibitory effects of salinity stress and improved the plant growth by enhancing nutrients, photosynthetic pigments, compatible osmolytes, and performance of AsA-GSH cycle. It also decreased the superoxide ion accumulation, leaf epidermal damages, cell death, DNA damage, and ABA content. Comet assay revealed significant (P ≤ 0.05) enhancement in tail length and olive tail moment, while flow cytometry did not showed any significant (P ≤ 0.05) changes in genome size and ploidy level under salinity stress. Moreover, EBL supplementation increased the G6PDH activity and S-nitrosothiol content which further boosted the antioxidant responses under salinity stress. Taken together, these results suggested that NO production in mustard occurred in NR-dependent manner and EBL in association with endogenous NO activates the antioxidant system to counter salinity stress.

Transcriptome analysis reveals a positive effect of brassinosteroids on the photosynthetic capacity of wucai under low temperature

Background

Brassinosteroids (BRs) have a positive effect on many processes during plant growth and development, and in response to various abiotic stressors. Low-temperature (LT) stress constricts the geographic distribution, growth, and development of wucai (Brassica campestris L. ssp. chinensis var. rosularis Tsen). However, there is little information on the global gene expression of BRs under LT stress in wucai. In this study, the molecular roles of 24-epibrassinolide (EBR) after exogenously application, were explored by RNA sequencing under LT conditions.

Results

According to the Gene Ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses, photosynthesis was significantly enriched after spraying EBR under LT. The transcripts encoding the photosystem II (PSII) oxygen-evolving enhancer protein, photosystem I (PSI) subunit, light-harvesting chlorophyll protein complexes I and II, and ferredoxin were up-regulated after the application of EBR. Transcripts encoding several key enzymes involved in chlorophyll biosynthesis were also up-regulated, accompanied by significant differences in the contents of 5-aminolevulinic acid (ALA), porphobilinogen (PBG), protoporphyrin IX (Proto IX), Mg-protoporphyrin IX (Mg-proto IX), protochlorophyllide (Pchl), and photosynthetic pigments. Notably, transcriptional and physiological analyses revealed that under LT stress, plant responses to EBR involved a major reorientation of photosynthesis, as well as porphyrin and chlorophyll metabolism.

Conclusion

This study explored the role of EBR as an LT stress tolerance mechanism in wucai. At the transcription level, LT tolerance manifests as an enhancement of photosynthesis, and the amelioration of porphyrin and chlorophyll metabolism.

Spermine application alleviates salinity induced growth and photosynthetic inhibition in Solanum lycopersicum by modulating osmolyte and secondary metabolite accumulation and differentially regulating antioxidant metabolism

Influence of exogenously applied spermine (Spm) on growth and salinity stress tolerance in tomato was investigated. Salinity reduced growth, chlorophyll synthesis and mineral uptake leading to significant reduction in photosynthesis, however Spm application proved beneficial in alleviating the decline to considerable extent. Applied Spm improved nitrate reductase activity, δ-amino levulinic acid content and gas exchange parameters more apparently at 100 μM than 50 μM concentrations. Spm application enhanced the accumulation of compatible osmolytes including proline, glycine betaine and sugars leading to greater tissue water content and photosynthesis. Salinity stress induced oxidative effects were mitigated by Spm treatment reflected interms of reduced accumulation of reactive oxygen species and the activities of protease and lipoxygenase, hence leading to membrane strengthening and protection of their function. Differential influence of exogenous Spm was evident on the functioning of antioxidant system with SOD, GR and APX activities much higher in Spm treated seedlings than CAT and DHAR. Increased synthesis of GSH, AsA and tocopherol in Spm treated seedlings was obvious thereby helping in maintaining the redox homeostasis and the enzymatic antioxidant functioning. Interestingly Spm application maintained the nitric oxide levels higher than control under normal condition while as lowered its concentrations in salinity stressed seedlings depicting existence of probable interaction. Activities of polyamine metabolizing enzymes was up-regulated and the accumulation of secondary metabolites including phenols and flavonoids also increased due to Spm application. Further studies are required to understand the mechanisms clearly.

Potential roles of melatonin and sulfur in alleviation of lanthanum toxicity in tomato seedlings

Owing to the active use of rare-earth elements in many areas, it is necessary to study their behavior in the environment and their biological impact on plants. Despite the role of melatonin and sulfur in plant growth, development and abiotic stress tolerance; it is still not clear how they have a strong regulatory influence and synergistic effect on growth, physiological and biochemical characteristics of plants under different environmental stresses. Therefore, this study highlights how melatonin and sulfur together potentially involved in a reversal of lanthanum-inhibited photosynthetic and growth responses in tomato seedlings. Here, we reported that seedlings grown in a medium containing 150 μM lanthanum exhibited increased overproduction of reactive oxygen species (ROS) and lipid peroxidation together with increased Chlorophyll degradation, and activity of chlorophyllase, proline dehydrogenase and glycolate oxidase (GOx), and decreased photosynthesis and growth. However, the application of melatonin and sulfur showed significant responses on tomato seedlings, although the response of their combined treatment was more effective by further increasing photosynthesis and growth under lanthanum toxicity. Melatonin supplied with sulfur suppressed ROS formation, lipid peroxidation and activity of GOx, and increased photosynthesis by upregulating activities of carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase. Also, sulfur supplementation with melatonin to seedlings resulted in an elevation in the accumulation of Chl and proline by increasing δ-aminolevulinic acid and activity of δ-aminolevulinic acid dehydratase and Δ¹-pyrroline-5-carboxylate synthetase activity. The administration of melatonin with sulfur substantially induced upregulation of enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) activities involved in the antioxidant system, thereby mitigating ROS-induced oxidative damage. Thus, this study provides strong evidence that melatonin and sulfur have strong regulatory influence and synergistic role in alleviating the adverse effect of lanthanum-toxicity by increasing photosynthesis and growth.

Exogenous melatonin-mediated modulation of arsenic tolerance with improved accretion of secondary metabolite production, activating antioxidant capacity and improved chloroplast ultrastructure in rosemary herb

Arsenic (As) recognized as a group I human carcinogen additionally poses a threat to plants which limit growth, metabolic activity, and productivity. Melatonin (MEL) is a naturally occurring compound in plants that have been recognized to mediate numerous morphological, physiological and molecular processes. Conversely, the role of MEL in inducing As-tolerance remains inexpressible and the plausible mechanisms in inducing As tolerance have remained largely unknown. The present investigation was designed to understand the protective role of MEL concentrations in rosemary herbs cultivated under As contamination. Arsenic evoked a deleterious decline on herb productivity, photosynthetic pigment, ion concentration, water status, ascorbic acid, essential oil (EO) yield and induced malformation of the chloroplast. Alternatively, increased organic osmolytes, oxidative impairment criteria, additionally antioxidant enzymes, phenol, flavonoid, anthocyanin, and EO%. Exogenous application of MEL with or without As, considerably increased growth, photosynthetic pigment, ion concentration, organic osmolytes as well as EO yield regarding polluted or non-polluted treatment respectively. Moreover, MEL treatment stabilized the cell membrane integrity, suppressed oxidative impairment criteria, and enhanced antioxidant capacity, additionally upregulation antioxidant enzymes. Plant treated with As showed a significant increase in As contamination and a bioconcentration factor in both root and shoot system. MEL supplementation under normal or As concentration, reduced As accumulation and bioconcentration factors, in either shoot or root systems. Additionally As decrease transfer factor, however, supplementation of MEL further decreased it. Application of 50 μM MEL might help the herbs to withstand As stress by strengthening their antioxidant machinery and osmoregulation capacity.

Modulation of salt tolerance in Thai jasmine rice (Oryza sativa L. cv. KDML105) by Streptomyces venezuelae ATCC 10712 expressing ACC deaminase

1-Aminocyclopropane-1-carboxylate (ACC) deaminase is a plant growth promoting (PGP) trait found in beneficial bacteria including streptomycetes and responsible for stress modulation. The ACC deaminase gene, acdS, of S. venezuelae ATCC 10712 was cloned into an expression plasmid, pIJ86, to generate S. venezuelae/pIJ86-acdS. Expression of acdS and production of ACC deaminase of S. venezuelae/pIJ86-acdS were significantly higher than the unmodified strain. The ACC deaminase-overexpressing mutant and the wild type control were inoculated into Thai jasmine rice (Oryza sativa L. cv. KDML105) under salt stress conditions. S. venezuelae on its own augmented rice growth and significantly increased more tolerance to salinity by reduction of ethylene, reactive oxygen species (ROS) and Na+ contents, while accumulating more proline, total chlorophyll, relative water content (RWC), malondialdehyde (MDA), and K+ than those of uninoculated controls. The overproducer did not alter chlorophyll, RWC, or MDA further-while it did boost more shoot weight and elongation, and significantly regulated salt tolerance of rice by increasing proline and reducing ethylene and Na+ contents further than that of the wild type. This work is the first illustration of the beneficial roles of S. venezuelae to enhance plant fitness endophytically by promotion of growth and salt tolerance of rice.

iTRAQ-Based Protein Profiling and Biochemical Analysis of Two Contrasting Rice Genotypes Revealed Their Differential Responses to Salt Stress

Salt stress is one of the key abiotic stresses causing huge productivity losses in rice. In addition, the differential sensitivity to salinity of different rice genotypes during different growth stages is a major issue in mitigating salt stress in rice. Further, information on quantitative proteomics in rice addressing such an issue is scarce. In the present study, an isobaric tags for relative and absolute quantitation (iTRAQ)-based comparative protein quantification was carried out to investigate the salinity-responsive proteins and related biochemical features of two contrasting rice genotypes—Nipponbare (NPBA, japonica) and Liangyoupeijiu (LYP9, indica), at the maximum tillering stage. The rice genotypes were exposed to four levels of salinity: 0 (control; CK), 1.5 (low salt stress; LS), 4.5 (moderate salt stress; MS), and 7.5 g of NaCl/kg dry soil (high salt stress, HS). The iTRAQ protein profiling under different salinity conditions identified a total of 5340 proteins with 1% FDR in both rice genotypes. In LYP9, comparisons of LS, MS, and HS compared with CK revealed the up-regulation of 28, 368, and 491 proteins, respectively. On the other hand, in NPBA, 239 and 337 proteins were differentially upregulated in LS and MS compared with CK, respectively. Functional characterization by KEGG and COG, along with the GO enrichment results, suggests that the differentially expressed proteins are mainly involved in regulation of salt stress responses, oxidation-reduction responses, photosynthesis, and carbohydrate metabolism. Biochemical analysis of the rice genotypes revealed that the Na⁺ and Cl⁻ uptake from soil to the leaves via the roots was increased with increasing salt stress levels in both rice genotypes. Further, increasing the salinity levels resulted in increased cell membrane injury in both rice cultivars, however more severely in NPBA. Moreover, the rice root activity was found to be higher in LYP9 roots compared with NPBA under salt stress conditions, suggesting the positive role of rice root activity in mitigating salinity. Overall, the results from the study add further insights into the differential proteome dynamics in two contrasting rice genotypes with respect to salt tolerance, and imply the candidature of LYP9 to be a greater salt tolerant genotype over NPBA.

Modulation of δ-Aminolevulinic Acid Dehydratase Activity by the Sorbitol-Induced Osmotic Stress in Maize Leaf Segments

Osmotic stress induced with 1 M sorbitol inhibited δ-aminolevulinic acid dehydratase (ALAD) and aminolevulinic acid (ALA) synthesizing activities in etiolated maize leaf segments during greening; the ALAD activity was inhibited to a greater extent than the ALA synthesis. When the leaves were exposed to light, the ALAD activity increased for the first 8 h, followed by a decrease observed at 16 and 24 h in both sorbitol-treated and untreated leaf tissues. The maximum inhibition of the enzyme activity was observed in the leaf segments incubated with sorbitol for 4 to 8 h. Glutamate increased the ALAD activity in the in vitro enzymatic preparations obtained from the sorbitol-treated leaf segments; sorbitol inhibited the ALAD activity in the preparations from both sorbitol-treated and untreated leaves. It was suggested that sorbitol-induced osmotic stress inhibits the enzyme activity by affecting the ALAD induction during greening and regulating the ALAD steady-state level of ALAD in leaf cells. The protective effect of glutamate on ALAD in the preparations from the sorbitol-treated leaves might be due to its stimulatory effect on the enzyme.

5-aminolevulinic acid improves salt tolerance mediated by regulation of tetrapyrrole and proline metabolism in Brassica napus L. seedlings under NaCl stress

5-aminolevulinic acid (ALA), a key biosynthetic precursor of tetrapyrroles, is vital for plant growth and adaptation to stress environments. Although exogenous ALA could enhance photosynthesis and biomass accumulation in plants under stress conditions, the underlying physiological and molecular mechanisms governed by ALA in promoting salt tolerance in Brassica napus L. are not yet clearly understood. In the present study, exogenous ALA with the concentration of 30 mg L^-1 was applied to the leaves of B. napus seedlings subjected to 200 mM NaCl. The results showed that NaCl stress decreased the photosynthesis, biomass accumulation, and levels of chlorophyll and heme with the reduction of the concentrations of intermediates including ALA, protoporphyrin IX (Proto IX), Mg-Proto IX, and Pchlide in the tetrapyrrole (chlorophyll and heme) biosynthetic pathway. The transcript levels of genes encoding ALA-associated enzymes and genes encoding Mg-chelatase in the chlorophyll biosynthetic branch were down-regulated, while the expression levels of genes encoding Fe-chelatase in the heme branch were not significantly altered by NaCl stress. Foliar application with ALA enhanced the aboveground biomass, net photosynthetic rate, activities of antioxidant enzymes, accumulation of chlorophyll and heme, and concentrations of intermediates related to chlorophyll and heme biosynthesis in B. napus under 200 mM NaCl. The expression of most genes mentioned above remained constant in ALA-treated plants in comparison with non-ALA-treated plants under NaCl stress. Additionally, exogenous ALA synchronously induced the proline concentration and up-regulated the expression of genes P5CS and ProDH encoding proline metabolic enzymes in the NaCl treatment. These findings suggested that ALA improved salt tolerance through promoting the accumulation of chlorophyll and heme resulting from the increase of intermediate levels in the tetrapyrrole biosynthetic pathway, along with enhancing the proline accumulation in B. napus.

Overexpression of plastidic maize NADP-malate dehydrogenase (ZmNADP-MDH) in Arabidopsis thaliana confers tolerance to salt stress

The plastidic C4 Zea mays NADP-malate dehydrogenase (ZmNADP-MDH), responsible for catalysis of oxaloacetate to malate, was overexpressed in Arabidopsis thaliana to assess its impact on photosynthesis and tolerance to salinity stress. Different transgenic lines were produced having ~3-6-fold higher MDH protein abundance and NADP-MDH enzyme activity than vector control. The overexpressors had similar chlorophyll, carotenoid, and protein content as that of vector control. Their photosynthetic electron transport rates, carbon assimilation rate, and consequently fresh weight and dry weight were almost similar. However, these overexpressors were tolerant to salt stress (150 mM NaCl). In saline environment, the Fv/Fm ratio, yield of photosystem II, chlorophyll, and protein content were higher in ZmNADP-MDH overexpressor than vector control. Under identical conditions, the generation of reactive oxygen species (H₂O₂) and production of malondialdehyde, a membrane lipid peroxidation product, were lower in overexpressors. In stress environment, the structural distortion of granal organization and swelling of thylakoids were less pronounced in ZmNADP-MDH overexpressing plants as compared to the vector control. Chloroplastic NADP-MDH in consort with cytosolic and mitochondrial NAD-MDH plays an important role in exporting reducing power (NADPH) and exchange of metabolites between different cellular compartments that maintain the redox homeostasis of the cell via malate valve present in chloroplast envelope membrane. The tolerance of NADP-MDH overexpressors to salt stress could be due to operation of an efficient malate valve that plays a major role in maintaining the cellular redox environment.

Photo-modulation of programmed cell death in rice leaves triggered by salinity

In this paper we provide evidence for involvement of chloroplast as alternate organelle for initiating PCD in plants under light and abiotic stress. In animals, mitochondria are the major source of reactive oxygen species (ROS) and key executioner of programmed cell death (PCD). In plants, however, the primary site of generation of ROS is chloroplast and yet its involvement in PCD has not been worked out in details. We found by Evans blue staining that salt (150 mM NaCl)-treated protoplasts obtained from green seedlings had higher rate of cell death than protoplasts obtained from etiolated seedlings. This indicated that cell death induced by NaCl is accentuated by light. Imposition of salt-stress to green protoplasts generated H₂O₂. Known hallmarks of PCD i.e., blebbing of cell membrane, loabing in nucleus, nick in DNA were observed in light-exposed salt-treated protoplasts and seedlings. TUNEL-FACS assay demonstrate several DNA nicks in the salt-treated green protoplasts exposed to light. Conversely, salt-treated etiolated protoplasts kept in dark had only a few TUNEL-positive nuclei. Similarly, a substantial numbers of TUNEL positive nuclei were observed in green seedlings due to salt treatment in light. However, salt-treated etiolated seedlings kept in dark had very few TUNEL positive nuclei. Addition of Caspase 3 inhibitor (DAVD-CHO) rescued (~50 %) green protoplasts from salt-stress induced cell death suggesting an involvement of apoptosis like PCD (AL-PCD). Ultra structure studies of chloroplast, mitochondria and nucleus from the leaves obtained from salt treated seedlings at the time point that showed PCD signature, resulted to severe granal de-stacking in chloroplasts while structural integrity of mitochondria was maintained. These studies demonstrate the photo-modulation of salinity-induced PCD in photosynthetic tissues is mainly executed by chloroplasts.

Transcriptomic and metabolic responses of Calotropis procera to salt and drought stress

BACKGROUND

Calotropis procera is a wild plant species in the family Apocynaceae that is able to grow in harsh, arid and heat stressed conditions. Understanding how this highly adapted plant persists in harsh environments should inform future efforts to improve the hardiness of crop and forage plant species. To study the plant response to droμght and osmotic stress, we treated plants with polyethylene glycol and NaCl and carried out transcriptomic and metabolomics measurements across a time-course of five days.

RESULTS

We identified a highly dynamic transcriptional response across the time-course including dramatic changes in inositol signaling, stress response genes and cytokinins. The resulting metabolome changes also involved sharp increases of myo-inositol, a key signaling molecule and elevated amino acid metabolites at later times.

CONCLUSIONS

The data generated here provide a first glimpse at the expressed genome of C. procera, a plant that is exceptionally well adapted to arid environments. We demonstrate, through transcriptome and metabolome analysis that myo-inositol signaling is strongly induced in response to drought and salt stress and that there is elevation of amino acid concentrations after prolonged osmotic stress. This work should lay the foundations of future studies in adaptation to arid environments.

Proteome dynamics and physiological responses to short-term salt stress in Leymus chinensis leaves

Salt stress is becoming an increasing threat to global agriculture. In this study, physiological and proteomics analysis were performed using a salt-tolerant grass species, Leymus chinensis (L. chinensis). The aim of this study is to understand the potential mechanism of salt tolerance in L. chinensis that used for crop molecular breeding. A series of short-term (<48 h) NaCl treatments (0 ~ 700 mM) were conducted. Physiological data indicated that the root and leaves growth were inhibited, chlorophyll contents decreased, while hydraulic conductivity, proline, sugar and sucrose were accumulated under salt stress. For proteomic analysis, we obtained 274 differentially expressed proteins in response to NaCl treatments. GO analysis revealed that 44 out of 274 proteins are involved in the biosynthesis of amino acids and carbon metabolism. Our findings suggested that L. chinensis copes with salt stress by stimulating the activities of POD, SOD and CAT enzymes, speeding up the reactions of later steps of citrate cycle, and synthesis of proline and sugar. In agreement with our physiological data, proteomic analysis also showed that salt stress depress the expression of photosystem relevant proteins, Calvin cycle, and chloroplast biosynthesis.