The combined effect of salinity and heat reveals a specific physiological,biochemical and molecular response in tomato plants

Many studies have described the response mechanisms of plants to salinity and heat applied individually; however, under field conditions some abiotic stresses often occur simultaneously. Recent studies revealed that the response of plants to a combination of two different stresses is specific and cannot be deduced from the stresses applied individually. Here, we report on the response of tomato plants to a combination of heat and salt stress. Interestingly, and in contrast to the expected negative effect of the stress combination on plant growth, our results show that the combination of heat and salinity provides a significant level of protection to tomato plants from the effects of salinity. We observed a specific response of plants to the stress combination that included accumulation of glycine betaine and trehalose. The accumulation of these compounds under the stress combination was linked to the maintenance of a high K⁺ concentration and thus a lower Na⁺/K⁺ ratio, with a better performance of the cell water status and photosynthesis as compared with salinity alone. Our findings unravel new and unexpected aspects of the response of plants to stress combination and provide a proposed list of enzymatic targets for improving crop tolerance to the abiotic field environment.     

Comparative physiological analysis in the tolerance to salinity and drought individual and combination in two cotton genotypes with contrasting salt tolerance

Soil salinity and drought are the two most common and frequently co‐occurring abiotic stresses limiting cotton growth and productivity. However, physiological mechanisms of tolerance to such condition remain elusive. Greenhouse pot experiments were performed to study genotypic differences in response to single drought (4% soil moisture; D) and salinity (200 mM NaCl; S) stress and combined stresses (D + S) using two cotton genotypes Zhongmian 23 (salt‐tolerant) and Zhongmian 41 (salt‐sensitive). Our results showed that drought and salinity stresses, alone or in combination, caused significant reduction in plant growth, chlorophyll content and photosynthesis in the two cotton genotypes, with the largest impact visible under combined stress. Interestingly, Zhongmian 23 was more tolerant than Zhongmian 41 under the three stresses and displayed higher plant dry weight, photosynthesis and antioxidant enzymes activities such as superoxide dismutase (SOD), peroxidase (POD) catalase (CAT) and ascorbate peroxidase (APX) activities compared to control, while those parameters were significantly decreased in salt‐stresses Zhongmian 41 compared to control. Moreover, Na⁺/K⁺‐ATPase activity was more enhanced in Zhongmian 23 than in Zhongmian 41 under salinity stress. However, under single drought stress and D + S stress no significant differences were observed between the two genotypes. No significant differences were detected in Ca²⁺/Mg²⁺‐ATPase activity in Zhongmian 41, while in Zhongmian 23 it was increased under salinity stress. Furthermore, Zhongmian 23 accumulated more soluble sugar, glycine‐betaine and K⁺, but less Na⁺ under the three stresses compared with Zhongmian 41. Obvious changes in leaf and root tips cell ultrastructure was observed in the two cotton genotypes. However, Zhongmian 23 was less affected than Zhongmian 41 especially under salinity stress. These results give a novel insight into the mechanisms of single and combined effects of drought and salinity stresses on cotton genotypes.     

Influence of salinity and <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> as the stress factors on morpho-physiological and yield attributes in onion

Occurrence of salt stress with the soil borne fungus Fusarium oxysporum f. sp. cepa (FOC) are potential threat to the crop yield. This investigation reports effect of the concurrent stresses (salinity and FOC) on morpho-physiological and yield attributes in onion. In vitro growth tests revealed proliferation of FOC biomass at different levels of salinity (2–8 dS m^?1). A greenhouse pot experiment with the proposed levels of salinity (2.5, 3.5 and 4.5 dS m^?1) in combination with FOC inoculation showed more drastic effect of combined stress on disease severity, plant growth and bulb as compared to the individual stress. In general, osmotic potential, total chlorophyll content, membrane stability index and total protein content of the leaf were decreased, while total phenolics were increased due to the given stress/s. Total sugar content decreased due to effect of the  individual stress of FOC, while it increased under the individual stress of salinity and in combination with FOC. FOC infection did not change activity of polyphenol oxidase (PPO), while it improved peroxidase (POX) and phenylalanine ammonia lyase (PAL) and decreased catalase (CAT) activity. Activities of POX and PPO increased, however PAL and CAT declined under individual as well as simultaneous stress of salinity ?and ?FOC. The research work concluded that FOC will be a more severe disease threat for onion cultivation in saline soils.     

Plastidial starch phosphorylase is highly associated with starch accumulation process in developing squash (Cucurbita sp.) fruit

Potassium (K⁺) is a macronutrient known for its high mobility and positive charge, which allows efficient and fast control of the electrical balance and osmotic potential in plant cells. Such features allow K⁺ to remarkably contribute to plant stress adaptation. Some agricultural lands are deficient in K⁺, imposing a stress that reduces crop yield and makes fertilization a common practice. However, individual stress conditions in the field are rare, and crops usually face a combination of different stresses. As plant response to a stress combination cannot always be deduced from individual stress action, it is necessary to gain insights into the specific mechanisms that connect K⁺ homeostasis with other stress effects to improve plant performance in the context of climate change. Surprisingly, plant responses to environmental stresses under a K⁺‐limiting scenario are poorly understood. In the present review, we summarize current knowledge and find substantial gaps regarding specific outcomes of K⁺ deficiency in addition to other environmental stresses. In this regard, combined nutrient deficiencies of K⁺ and other macronutrients are covered in the first part of the review and interactions arising from K⁺ deficiency with salinity, drought and biotic factors in the second part. Information available so far suggests a prominent role of potassium and nitrate transport systems and their regulatory proteins in the response of plants to several stress combinations. Thus, such molecular pathways, which are located at the crossroad between K⁺ homeostasis and environmental stresses, could be considered biotechnological targets in future studies.     

Expression of Na+/H+ antiporter gene in response to water and salinity stress in grapevine rootstocks

Grapevine rootstocks belonging to different species of Vitis differ in their response to stress. Vines of rootstocks 110R and 1613C were subjected to different salt and water stresses individually and in combination. Expression of VvNHX1, a Na⁺/H⁺ antiporter gene, was analyzed at 7 and 21 d of stress treatment. In 110R, the expression of VvNHX1 gene increased in response to both salinity and water stress already after 7 d. Under salinity and combined stress enhanced expression of this gene was observed also after 21 d whereas expression decreased under water stress. In 1613C, expression of this gene did not increase under salinity stress. There was delayed response to water and combined stress and expression increased several fold after 21 d of stress. The stressed vines of 110R maintained lower sodium content and higher K⁺/Na⁺ ratio as compared to rootstock 1613C.     

Contrasting Physiological Responses of Jatropha curcas Plants to Single and Combined Stresses of Salinity and Heat

In this study we evaluated the contrasting major physiological responses of Jatropha curcas L. to salinity alone and in combination with high temperature. The plants were subjected to salinity (100 mM NaCl) before and after exposure to 43 °C (heat stress) for 6 h. The effects of salinity were more harmful than heat stress, and the effects of salt stress were increased when both stress factors were combined. The negative effects of the combined treatments included strong impairment of the CO₂ assimilation rate and stomata conductance and increased Na⁺ and Cl^? accumulation in the leaves associated with increased membrane damage and lipid peroxidation. Heat favorably stimulated the accumulation of glycine betaine and chlorophyll in the salt-stressed leaves. Treatments with salt, heat, and their combination stimulated the antioxidant enzymatic defense system, that is, the expression of ascorbate peroxidase (APX) and superoxide dismutase (SOD), whereas the expression of catalase (CAT) was stimulated through treatments with salt alone and in combination with heat; treatment with heat alone did not affect CAT expression. The ascorbate redox state was decreased under salinity stress alone and in combination with heat but remained unaffected when treated with heat alone. Overall, the leaf H₂O₂ concentration did not change in response to these stresses, but lipid peroxidation and membrane damage was increased. Moreover, high temperature increases the negative effects of salt stress on key physiological processes, but treatment with heat alone is favorable for several metabolic indicators of young J. curcas plants. In contrast with heat, these plants exhibit higher physiological disturbances under isolated salinity stress.     

Effect of NaCl-induced changes in growth,photosynthetic characteristics,water status and enzymatic antioxidant system of <Emphasis Type="Italic">Calligonum caput-medusae</Emphasis> seedlings

Calligonum caput-medusae is known to grow well when irrigated with water containing NaCl. The aim of this study was to investigate ecophysiological responses of C. caput-medusae to different NaCl concentrations. In our study, we examined the effect of 0, 50, 100, 200, and 400 mM NaCl. Our results demonstrated that maximum seedling growth occurred at 50 mM NaCl. Photosynthetic parameters, such as the photosynthetic pigment content and gas exchange parameters, correlated with growth response. High salinity (≥ 100 mM NaCl) resulted in a significant reduction of the plant growth. Similarly, marked declines in the pigment content, maximal efficiency of PSII photochemistry, net photosynthetic rate, transpiration rate, and stomatal conductance were also detected. However, intercellular CO2 concentration showed a biphasic response, decreasing with water containing less than 200 mM NaCl and increasing with NaCl concentration up to 400 mM. Water-use efficiency and intrinsic water-use efficiency exhibited the opposite response. The reduction of photosynthesis at the high NaCl concentration could be caused by nonstomatal factors. High salinity led also to a decrease in the relative water content and water potential. Correspondingly, an accumulation of soluble sugars and proline was also observed. Na⁺ and Cl^? concentrations increased in all tissues and K⁺ concentrations were maintained high during exposure to NaCl compared with the control. High salinity caused oxidative stress, which was evidenced by high malondialdehyde and hydrogen peroxide contents. In order to cope with oxidative stress, the activity of antioxidative enzymes increased to maximum after 50 mM NaCl treatment. The data reported in this study indicate that C. caput-medusae can be utilized in mild salinity-prone environments.     

Effect of water stress and NaCl triggered changes on yield,physiology, biochemistry of broad bean (Vicia faba) plants and on quality of harvested pods

Salinity and drought are the most important abiotic stresses affecting crop yield. Broad bean was chosen as model plant for assessing the impact of salt stress and its interaction with drought in the field experiments. The factors examined in the experiments were the two irrigation rates (normal watering — NW with 3 L plant^?1 and drought — D) and three salinity rates imposed by foliar application (0, 50, 100 mg L^?1 NaCl). Highest NaCl level with normal water irrigation caused maximum reduction in plant height and production, which it was due to photosynthetic disturbances. Salt injuries were alleviated by increasing water stress. The control plants exposed to NaCl lost their ability over water control. The increased malondialdehyde (MDA) and H₂O₂ indicate the prevalence of oxidative stress due to salinity. The levels of proline and carbohydrates were higher under salinity alone than under simultaneous exposure to drought and NaCl. The protein concentration of immature and mature broad bean pods was more inhibited more by NaCl supply than by drought alone. The combination of drought and NaCl resulted in a significant increase in proteins, glucose, fructose and sucrose content. Overall, the ameliorative effect of drought under NaCl supply was quantified.     

Comparative physiological and biochemical evaluation of salt and nickel tolerance mechanisms in two contrasting tomato genotypes

Plant tolerance against a combination of abiotic stresses is a complex phenomenon, which involves various mechanisms. Physiological and biochemical analyses of salinity (NaCl) and nickel (Ni) tolerance in two contrasting tomato genotypes were performed in a hydroponics experiment. The tomato genotypes selected were proved to be tolerant (Naqeeb) and sensitive (Nadir) to both salinity and Ni stress in our previous experiment. The tomato genotypes were exposed to combinations of NaCl (0, 75 and 150 mM) and Ni (0, 15, and 20 mg l⁻¹) for 28 days. The results revealed that the tolerant and sensitive tomato genotypes showed similar response to NaCl and Ni stress; however, the level of response was significantly different in both genotypes. The tolerant tomato genotype showed less reduction in growth than the sensitive genotype against both NaCl and Ni stress. Root and shoot ionic analysis showed a decrease in Na and increase in K concentration by increasing Ni levels in the growth medium. Moreover, accumulation of Na and Ni in tissues showed a decrease in membrane stability index and an increase in malondialdehyde contents. The activity of superoxide dismutase, catalase, peroxidase and glutathione reductase under NaCl and Ni stress was significantly higher in the tolerant compared to the sensitive genotype. Enhanced activity of many antioxidant enzymes in Naqeeb under stress conditions is among the other mechanisms that enabled the genotype to better detoxify reactive oxygen species and therefore Naqeeb tolerated the stresses better than Nadir.     

Ectopic Expression of Plant RNA Chaperone Offering Multiple Stress Tolerance in <Emphasis Type="Italic">E. coli</Emphasis>

Members of the plant glycine-rich RNA-binding proteins (GR-RBPs) family have been reported in flowering, development, circadian rhythms, biotic and abiotic stresses. Particularly, GR-RBPs are reported to function as RNA chaperones, promoting growth and acclimation during cold shock. It is indispensable to further question the efficacy and mechanism of GR-RBPs under various environmental strains. Monitoring the expression of stress-regulated proteins under stress conditions has been a beneficial strategy to study their functional roles. In an effort to elucidate the NtGR-RBP1 function, stress markers such as salinity, drought, low temperature and heat stresses were studied. The NtGR-RBP1 gene was expressed in E. coli followed by the exposure to stress conditions. Recombinant E. coli expressing NtGR-RBP1 were more tolerant to stresses, e.g., salinity, drought, cold and heat shock. Recombinants exhibited higher growth rates compared to control in spot assays. The tolerance was further confirmed by monitoring the growth in liquid culture assays. Cells expressing NtGR-RBP1 under salt (500 mM NaCl), drought (20% PEG), cold (4 and 20 °C) and heat stresses (50 °C) had enhanced growing ability and better endurance. Our study supports the notion that the protective role of NtGR-RBP1 may contribute to growth and survival during diverse environmental stresses.     

Molecular cloning and functional characterization of a Cu/Zn superoxide dismutase gene (<Emphasis Type="Italic">CsCSD1</Emphasis>) from <Emphasis Type="Italic">Cucumis sativus</Emphasis>

Superoxide dismutase (SOD) proteins, which are widely present in the plant kingdom, play vital roles in response to abiotic stress. However, the functions of cucumber SOD genes in response to environmental stresses remain poorly understood. In this study, a SOD gene CsCSD1 was identified and functionally characterized from cucumber (Cucumis sativus). The CsCSD1 protein was successfully expressed in E. coli, and its overexpression significantly improved the tolerance of host E. coli cells to salinity stress. Besides, overexpression of CsCSD1 enhanced salinity tolerance during germination and seedling development in transgenic Arabidopsis plants. Further analyses showed that the SOD and CAT (catalase) activities of transgenic plants were significantly higher than those of wild-type (WT) plants under normal growth conditions as well as under NaCl treatment. In addition, the expression of stress-response genes RD22, RD29B and LEA4-5 was significantly elevated in transgenic plants. Our results demonstrate that the CsCSD1 gene functions in defense against salinity stress and may be important for molecular breeding of salt-tolerant plants.     

High-mass-resolution MALDI mass spectrometry imaging reveals detailed spatial distribution of metabolites and lipids in roots of barley seedlings in response to salinity stress

Introduction

Mass spectrometry imaging (MSI) is a technology that enables the visualization of the spatial distribution of hundreds to thousands of metabolites in the same tissue section simultaneously. Roots are below-ground plant organs that anchor plants to the soil, take up water and nutrients, and sense and respond to external stresses. Physiological responses to salinity are multifaceted and have predominantly been studied using whole plant tissues that cannot resolve plant salinity responses spatially.

Objectives

This study aimed to use a comprehensive approach to study the spatial distribution and profiles of metabolites, and to quantify the changes in the elemental content in young developing barley seminal roots before and after salinity stress.

Methods

Here, we used a combination of liquid chromatography–mass spectrometry (LC–MS), inductively coupled plasma mass spectrometry (ICP–MS), and matrix-assisted laser desorption/ionization (MALDI–MSI) platforms to profile and analyze the spatial distribution of ions, metabolites and lipids across three anatomically different barley root zones before and after a short-term salinity stress (150 mM NaCl).

Results

We localized, visualized and discriminated compounds in fine detail along longitudinal root sections and compared ion, metabolite, and lipid composition before and after salt stress. Large changes in the phosphatidylcholine (PC) profiles were observed as a response to salt stress with PC 34:n showing an overall reduction in salt treated roots. ICP–MS analysis quantified changes in the elemental content of roots with increases of Na⁺ and decreases of K⁺ content.

Conclusion

Our results established the suitability of combining three mass spectrometry platforms to analyze and map ionic and metabolic responses to salinity stress in plant roots and to elucidate tolerance mechanisms in response to abiotic stress, such as salinity stress.

     

Salt stress effects on growth,pigments, proteins and lipid peroxidation in <Emphasis Type="Italic">Salicornia persica</Emphasis> and <Emphasis Type="Italic">S. europaea</Emphasis>

The effects of NaCl stress on growth, water status, contents of protein, proline, malondialdehyde (MDA), various sugars and photosynthetic pigments were investigated in seedlings of Salicornia persica and S. europaea grown in vitro. Seeds were germinated under NaCl (0, 100, 200, 300, 400, 500 and 600 mM) on Murashige and Skoog medium for 45 d. The shoot growth of both species increased under low NaCl concentration (100 mM) and then decreased with increasing NaCl concentrations. In contrast to S. persica, root length in S. europaea reduced steadily with an increase in salinity. Proline content in S. persica was higher than in S. europaea at most NaCl concentrations. Proline, reducing saccharide, oligosaccharide and soluble saccharide contents increased under salinity in both species. In contrast, contents of proteins and polysaccharides reduced in both species under salt stress. MDA content remained close to control at moderate NaCl concentrations (100 and 200 mM) and increased at higher salinities. MDA content in S. europaea was significantly higher than S. persica at higher salinities. Salt treatments decreased K⁺ and P contents in seedlings of both species. Significant reduction in contents of chlorophylls and carotenoids due to NaCl stress was also observed in seedlings of both species. Some differences appeared between S. persica and S. europaea concerning proteins profile. On the basis of the data obtained, S. persica is more salt-tolerant than S. europaea.     

Role of Ca2+ in Drought Stress Signaling in Wheat Seedlings

Plants use complex signal transduction pathways to perceive and react to various biotic and/or abiotic stresses. As a consequence of this signaling, plants can modify their metabolism to adapt themselves to new conditions. One such change is the accumulation of proline in response to drought and salinity stresses. We have studied drought and salinity induced proline accumulation and the roles of Ca²⁺ (10 mM) and indoleacetic acid (IAA, 0.3 mM) in this response. Subjecting seedlings to both drought (6% polyethylene glycol, PEG) and salinity (150 mM NaCl) stress resulted in a dramatic increase in proline accumulation (7-fold higher than control level). However, the application of Ca²⁺ along with these stress factors had different effects. Unlike the salinity stress, Ca²⁺ prevented the drought induced proline accumulation indicating that these stress factors use distinct signaling pathways to induce similar responses. Experiments with IAA and EGTA (10 mM) supported this interpretation and suggested that Ca²⁺ and auxin participate in signaling mechanisms of drought-induced proline accumulation. Drought and salt stress-induced proline accumulation was compared on salt resistant (cv. Gerek 79) and salt sensitive (cv. Bezostaya) wheat varieties. Although proline level of the first was twofold lower than that of the second in control, relative proline accumulation was dramatically higher in the case of the salt resistant wheat variety under stress conditions.     

Effect of salinity and waterlogging on growth,anatomical and antioxidative responses in <Emphasis Type="Italic">Mentha aquatica</Emphasis> L.

Salinity and waterlogging are two stresses which in nature often occur simultaneously. In this work, effects of combined waterlogging and salinity stresses are studied on the anatomical alteration, changes of enzymatic antioxidant system and lipid peroxidation in Mentha aquatica L. plants. Seedlings were cultured in half-strength Hoagland medium 50 days after sowing, and were treated under combination of three waterlogging levels (well drained, moderately drained and waterlogging) and NaCl (0, 50, 100, 150 mM) for 30 days. Moderately drained and waterlogging conditions induced differently aerenchyma formation in roots of M. aquatica salt-treated and untreated plants. Moreover, stele diameter and endodermis layer were also affected by salt stress and waterlogging. Salt stress significantly decreased growth, relative water content (RWC), protein level, catalase (CAT) and polyphenol oxidase (PPO) activities, and increased proline content, MDA content, H₂O₂ level and activities of superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX). Waterlogging in salt-untreated plants increased significantly growth parameters, RWC, protein content, antioxidant enzyme activity, and decreased proline content, H₂O₂ and MDA levels. In salt-treated plant, waterlogging caused strong induction of antioxidant enzymes activities especially at severe stress condition. These results suggest M. aquatica is a waterlogging tolerant plant due to significant increase of antioxidant activity, membrane stability and growth under water stress. High antioxidant capacity under waterlogging can be a protective strategy against oxidative damage, and help to salt stress alleviation.