Plants exposed to salt stress conditions undergo changes in their environment. Around 20% of the world's cultivated area and nearly half of the world's irrigated lands are affected by salinity. In this paper, plant responses to salinity stress are reviewed with emphasis on physiological, and molecular mechanisms of salt tolerance. Additionally, with the advances in molecular biology and availability of advanced genetic tools considerable progress has been made to improving salt induced salt stress tolerance in plants. I found that, the synthesis of salt-tolerant species is correlated with stress induced enhancement of photorespiration, photosynthetic pathway, and biosynthesis of compatible solutes.
Nearly 20% of the world's cultivated area and nearly half of the world's irrigated lands are affected by salinity. In this paper we will focus on Plant physiological and molecular response to salinity. Thus, understanding the molecular and physiological basis will be helpful in developing selection strategies for improving salinity tolerance (Yoshida , 2002). Additionally, most of the regions in the world that contain soils resources which are too saline for most of the important economic crops, which affect plants through osmotic effects, oxidative stress and ion specific effects (Heidari et al., 2011). Salinity is mediated by the presence of excessive amounts of salts. Most commonly, high percentage of Na+ and Cl- cause the salt stress. Salt stress has three fold effects like: reducing water potential, causes ion imbalance or disturbances in ion homeostasis and toxicity. This altered water status leads to initial growth reduction and limitation of plant productivity. Thus the detrimental effects of high salinity on plants can be observed at the whole plant level as the death of plants or decreases in productivity. Many plants develop mechanisms either to tolerate its presence within the cells or to exclude salt from their cells. During the development of salt stress within a plant, all the major processes such as photosynthesis, protein synthesis, energy and lipid metabolism are affected. The earliest response is a reduction in the rate of leaf surface expansion. Growth resumes when the stress is relieved. Carbohydrates, are needed for cell growth, are supplied mainly through the process of photosynthesis, and photosynthesis rates are usually lower in plants exposed to salinity and especially to NaCl. In this review, I well covered organismal, physiological, and molecular mechanisms of salt tolerance with the salient features of salinity stress effects on plants (Parida et al.,2004). In this review, much research information about cellular, metabolic, molecular, and genetic processes associated with the response to salt stress, some of which presumably function to mediate salt tolerance, has been gathered. Screening of available local cultivars of crop plants for salinity tolerance has two major advantages, first the tolerant genotype thus made available can be used in breeding programs and second, a comparative analysis at physiological and molecular level of these cultivars can provide such ways in understanding and unraveling survival mechanisms ( Kumar et al., 2008). Thus, development of Transgenic plants are also gaining rapid attention from industry as natural bioreactors for the production of industrial and chemical products. Improved salt tolerance nowadays, is the most important goal, as this would make possible the cultivation of plants on salty land as well as enabling the use of seawater for irrigation.
1- Physiology and metabolism of plants under salinity stress:
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Salinity is a major environmental constraint that renders fields unproductive and limits plant growth and productivity. The adverse effects of salinity on plant growth are the result of changes in plant physiology which include ion toxicity, osmotic stress and nutrient deficiency. However, Roshandel and Flowers (2009) have shown that the changes induced by salinity are ionic rather than osmotic. This conclusion is drawn from their molecular studies in which they found that the changes in the expression of genes encoding proline rich proteins, senescence associated proteins and heat-shock proteins were responsive to the ionic rather than osmotic effects of salt in rice.
High salt stress causes accumulation of H2O2, and both salt and H2O2 induce the expression of specific protein, demonstrated an important role in osmoprotection and oxidative stress management under salt stress (Nazar et al.,2010).
1.1. Nutrients in salinity tolerance:
The presence of salinity adds a new level of complexity to the mineral nutrition of crops .There for, the uptake of nutrients by plants in saline environments are affected by many factors in the soil plant environment.
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The interaction between salinity, N nutrition and crop yield is a major concern in
improving crop production. Under salinity stress accumulation of excess Na+ and Clâˆ’ causes negative impacts on the acquisition and homeostasis of essential nutrients
making it more difficult for water and nutrients to move through the root membrane
decreasing their concentration in leaf. Salinity and N interactions have been reported to improve salinity tolerance. It was observation that tolerant alfalfa cultivars can absorb more K+ and Ca2+ ions under saline environment and prevent Na+ absorption and suggest the role of nutrients in salinity tolerance. Khorshidi et al. (2009) It has been shown that salinity increases the internal requirement for a specific nutrient, As an example, N in the halophyte Spartina alterniflora, P in tomato and K+ in spinach .Moreover, Phosphorus nutrition has been implicated in modifying the effects of saline conditions upon the growth of glycophytic plants ,as well as causes an induced enhancement of salinity tolerance (Nazar et al.,2010).
1.2. Polyamines (PAs):
Polyamines (PAs) are a component in all living organisms. Spermidine, putrescine and spermine are the major PAs present in plants. These poly cationic compounds are involved not only in fundamental cellular processes like growth, differentiation and morphogenesis, but also in various environmental stresses like salinity. In plants, the diamine putrescine is synthesized via ornithine by arginase and ornithine decarboxylase (Fig. 1) (Naka et al ,2010).
Fig. 1.Polyamine biosynthetic pathway in plants.
A-PA contents in various tissues of A. thaliana:
In two-week-old examined PA contents seedlings, mature rosette leaves, stems and flowers of WT Arabidopsis. The contents of all four PAs" putrescine, spermidine, thermospermine and spermine " were higher in flower organs than in mature leaves(Fig. 2A) .where the contents of putrescine, spermidine and spermine are higher in siliques, buds and flowers compared to the other organs (Naka et al ,2010).
Expression of Spms in young seedlings and flowers was rather lower relative to spermine contents (Fig. 2A and B). From figures suggested that spermine also has a
distinct role in stem development, differing from the ones of thermospermine which was indicated by the stem growth defect in the acl5 mutant gene.
1.3 - Sulfur :
Sulfur has a variety of vital functions within the plant. In fact, inorganic sulfur is converted to nutritionally and functionally important S containing compounds such as cysteine and methionine that are essential components of proteins, cofactors such as CoA and S-adenosylmethionine, sulfolipids and vitamins. Sulfur is important in the formation of sulfhydryl (S-H) and disulfide bonds (S-S). These bonds are important for the stabilization of protein structures making it ideally suitable for biological redox processes. Redox control regulates enzymes and protects against oxidative damage.
Fig.2. PA content(A) and ACL5 and Spms expression(B) of various tissues in A.thaliana.
1.3.A. Regulation of sulfur metabolism under salinity stress:
Once sulfate is within cells, it can be stored or can enter the metabolic stream. Metabolism of sulfate is initiated by its activation by the reaction of adenylation catalyzed by ATPS. GSH maintains cysteine, homocysteine, enzymatic protein, ascorbate (AsA) in active form and may regulate the thiol/disulfide ratio in proteins and protect cell membrane against H2O2 and free radicals. Both GSH and Cys contents are found to increase under salt stress, and GSH may play a protective role against salinity tolerance in plants. The effect of NaCl induced osmotic stress on GSH production, NaCl stress leads to an increase in intracellular Cys content and activities
of glutamylcysteine synthetase. The importance of sulfur in salinity tolerance is further evident by the regulation of different enzymes of S assimilation under salinity stress. Thus the sulfur assimilation enzymes increase, decrease or remain unaffected by salinity stress. Finally, Sulfur assimilates not only play key roles in the primary
metabolism of plants and provide structural components of essential cellular molecules, but also act as signaling molecules for cellular communication with the environment (Nazar et al.,2010).
2. Salt tolerance of plants:
Salt tolerance is the ability of plants to grow and complete their life cycle on a substrate that contains high concentrations of soluble salt. Plants that can
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survive on high concentrations of salt and grow well are called halophytes. According to their salt tolerating capacity, halophytes are either obligate or facultative.
Obligate characterized by low morphological and taxonomical diversity with relative growth rates increasing up to 50% sea water, while facultative are found in less saline habitats along the border between saline and nonsaline upland and characterized by broader physiological diversity which enables them to cope with saline and nonsaline conditions (Parida et al.,2004).
2.1. Mechanism of salt tolerance:
Such studies of biochemical and molecular mechanisms developed to cope with salt stress. Biochemical pathways leading to improve salt tolerance are likely to act additively and probably synergistically (Iyengar and Reddy, 1996). Biochemical strategies are many and its include (i) selective accumulation or exclusion of ions, (ii) control of ion uptake by roots and transport into leaves, (iii) compartmentalization of ions at the cellular and whole-plant levels, (iv) synthesis of compatible solutes, (v) change in photosynthetic pathway, (vi) alteration in membrane structure, (vii) induction of antioxidative enzymes, and (viii) induction of plant hormones. Moreover, Salt tolerance mechanisms are either low complexity or high complexity mechanisms. Low complexity mechanisms appear to involve changes in many biochemical
pathways. High-complexity mechanisms involve changes that protect major processes. such as photosynthesis and respiration (Parida et al.,2004).
2.2. Induced biosynthesis of compatible solutes:
To accommodate the ionic balance in the vacuoles, cytoplasm accumulates low molecular mass compounds termed compatible solutes because they do not interfere with normal biochemical. With accumulation proportional to the change of external osmolarity within species-specific limits, protection of structures and
osmotic balance supporting continued water influx or reduced efflux, which are accepted functions of osmolytes (Hasegawa et al., 2000). These compatible solutes
include mainly proline, polyols, sugars, and glycine betaine (GB) (Parida et al.,2004).
A number of nitrogen-containing compounds (NCC) accumulate in plants exposed to saline stress. Osmotic adjustment, storage of nitrogen , protection of cellular macromolecules, detoxification of the cells, maintenance of cellular pH, and scavenging of free radicals are proposed functions of these compounds under stress conditions. NCC accumulation is usually correlated with plant salt tolerance (Parida et al.,2004).
On the other hand, Many plants accumulate proline as a nontoxic and protective osmolyte under saline conditions. It has been reported that proline levels increase significantly in leaves of nonsecretor mangrove B. parviflora (Fig. 3) (Parida et al., 2002). Fig. 3. Effects of NaCl stress on proline level in
B. parviflora measured as a function of days of
NaCl treatment. Values are mean7SE (from
Parida et al., 2002).
Another study found a negative relationship between potassium and proline accumulation at vegetative (r2=-0.75) and reproductive stage (r2=-0.66) in
millet plants. They indicate that potassium has an important role in osmotic adjustment in plants under abiotic stress but it has not any role in proline metabolism and its accumulation in millet plants under salinity stress. So the application of potassium from 0 to 200 kg ha-1 had no effect on proline accumulation in millet plants under salinity stress at vegetative and reproductive stages (Figs. 4 and 5) below (Heidari et al., 2011).
Fig. 4 Effects of salinity and potassium on proline Fig. 5 Effects of salinity and potassium on proline content in content in leaves at vegetative stage. leaves at reproductive stage.
Thus, the compatible solutes have the capacity to preserve the activity of enzymes that are in saline solutions. These compounds have minimal effect on pH or charge balance of the cytosol or lumenal compartments of organelles. The synthesis of compatible osmolytes is often achieved by diversion of basic intermediary metabolites into unique biochemical reactions (Parida et al.,2004).
2.3. Induction of antioxidative enzymes activities:
Salt stress is complex and imposes a water deficit because of osmotic effects on a wide variety of metabolic activities. This water deficit leads to the formation of
reactive oxygen species (ROS) such as superoxide, hydrogen peroxide (H2O2), hydroxyl radical (OH) ,and singlet oxygen (O2) (Parida et al.,2004).The toxic effect of salinity is through oxidative stress caused by enhanced production of reactive oxygen species (ROS). The presence of high concentration of ROS can damage photosynthetic pigments, proteins, lipids and nucleic acids by oxidation.
In addition to the destructive role of ROS, they also serve as a stress signal molecule and activate acclimation and defense mechanisms that will in turn counteract stress-associated oxidative stress. Controlling ROS production and scavenging in the chloroplast is shown to be essential for tolerance to salinity in cabbage transgenic plants and in salinity tolerant cultivars.
Thus, in order to reduce salinity stress, plants speed up their rate of ROS production that sends signal to activate antioxidants for ROS scavenging. Plants containing high levels of antioxidants can detoxify ROS thereby contributing to increased salt
tolerance. Among the antioxidants involved in ROS scavenging and maintaining steady-state ROS level reduced GSH plays an important role. GSH is a tripeptide found abundantly in all cell compartments in its reduced form. The ratio of GSH to GSSG plays an important role in maintaining redox equilibrium in the cell during H2O2 degradation(Nazar et al.,2010).
Salt stress remarkably elevated the activities of those antioxidant enzymes at vegetative and reproductive stages (Figs. 6-9). Unlike those two enzymes,
by increasing salinity stress from 0 to 12 ds m-1, APX activity at vegetative stage decreased but at reproductive stage salinity elevated its activity in leaves of millet
plants (Figs. 10 and 11).
Fig. 6 Effects of salinity and potassium on catalase Fig. 7 Effects of salinity and potassium on catalase (CAT) (CAT) activity in leaves at vegetative stage. in leaves at reproductive stage.
Fig. 8 Effects of salinity and potassium on guaiacol Fig. 9 Effects of salinity and potassium on guaiacol peroxidase
(GPX) activity in leaves at reproductive stage. peroxidase (GPX) activity in leaves at vegetative stage.
Fig.10 Effects of salinity and potassium on ascorbate Fig.11 Effects of salinity and potassium on ascorbate peroxidase (APX) activity in leaves at vegetative stage. Peroxidase (APX) activity in leaves at reproductive stage.
Fig. 12 Effects of salinity and potassium on K+ Fig. 13 Effects of salinity and potassium on K+
and Na+ contents in leaves at vegetative stage. and Na+ contents in leaves at reproductive stage.
As a result, potassium content in leaves under salinity treatments significantly increased and sodium content significantly decreased with increasing potassium levels from 0 to 200 kg ha-1 (Heidari et al., 2011).
2.4. Change in photosynthetic pathway:
By reducing water potential, salt stress inhibits photosynthesis in plant . So the main aim of salt tolerance is to increase water use efficiency under salinity. To this goal,
facultative halophytic plants such as M. crystallinum shift their C3 mode of photosynthesis to CAM (Cushman et al., 1989). By that change the plant allows to reduce water loss by opening stomata at night, which decrease transpiratory water loss under prolonged salinity conditions. Moreover, there is also a shift from the C3 to the C4 pathway in response to salinity in salt-tolerant plant species such as Atriplex l entiformis (Parida et al.,2004).
2.5. Molecular mechanism of salt tolerance:
Physiologic or metabolic adaptations to salinity stress at the cellular level are the main responses to molecular analysis and have led to the identification of a large number of genes induced by salt. Most of the genes in the functional groups have been identified as salt inducible under stress conditions, which categorized into different functional groups which responsible for encoding salt-stress proteins: (i) genes for photosynthetic enzymes, (ii) genes for vacuolar-sequestering, (iii) enzymes genes for synthesis of compatible solutes, and (iv) genes for radical-scavenging enzymes. However, other physiological systems may be equally limiting under stress conditions and any mutants in these physiological pathways could lead to increased salt toxicity as well as, affecting survival in a negative manner (Parida et al.,2004).
It has been reported that salt stress and hyperosmotic stress have different effects on the cytoplasmic volume and gene expression in Synechocystis sp. PCC 6803 (Kanesaki et al., 2002). Moreover, DNA microarray analysis indicates that salt stress strongly induces the genes for some ribosomal proteins. As example, Hyperosmotic stress strongly induces the genes for 3-ketoacyl-acyl carrier protein reductase and rare lipoprotein A. Genes whose expression is induced both by salt and hyperosmotic stress include those for heat shock proteins and the enzymes for the synthesis of glucosylglycerol. However, each kind of stress induces a number of genes for proteins of unknown function (Parida et al.,2004). Findings of Kanesaki et al. (2002) estimated that Synechocystis sp.recognizes salt and hyperosmotic stress as
different stimuli, although mechanisms common to the responses to each form of stress might also contribute to gene expression.
3. Salinity effects on plants:
Salt stress has threefold effects, it reduces water potential and causes ion
imbalance or disturbances in ion homeostasis and toxicity. Thus, altered water status leads to limitation of plant productivity and initial growth reduction. Since salt
stress involves both osmotic and ionic stress , the growth suppression is directly related to osmotic potential of soil water or total concentration of soluble salts.
However ,Suppression of growth occurs in all plants, but their tolerance levels
and rates of growth reduction at lethal concentrations of salt vary among different plant species. Salt stress affects all the major processes such as growth, photosynthesis, protein synthesis, and energy and lipid metabolism. And these are discussed under separate headings (Parida et al.,2004).
3.1. Effects of salinity on growth:
Salt stress always results in a clear stunting of plants. The immediate response of salinity stress is reduction in the rate of leaf surface expansion leading to cessation of expansion as salt concentration increases. Moreover, salt stress results in a considerable decrease in the fresh and dry weights of leaves, roots, and stems. The optimum growth of plants is obtained at 50% seawater and declines with further increases in salinity in Rhizophora mucronata (Aziz and Khan, 2001). Dry and fresh weights of plants increase with an increase in salinity in Salicornia rubra while the optimal growth occurrs at 200mM NaCl .With further increase in salinity the growth were declines, ultimately resulting in plant death at 360mM NaCl, and maximum succulence is noted at 90mM NaCl. However, increasing salinity is accompanied by significant reductions in shoot weight, plant height, number of leaves per plant, root length, and root surface area per plant in tomato. Increased NaCl levels results in a significant decrease in root, shoot, and leaf growth biomass and increase in root/shoot ratio in cotton (Parida et al.,2004).
3.2. Effects of salinity on leaf anatomy:
Salinity causes increases in palisade cell length, epidermal thickness, palisade diameter, mesophyll thickness, and spongy cell diameter in leaves of cotton, bean, and Atriplex. In contrast both mesophyll thickness ,epidermal and intercellular spaces decrease the significantly in NaCl treated leaves of the mangrove B. parviflora. Salt stress causes four patterns in plant,(i) decrease in mitochondrial cristase and swelling of mitochondria, (ii) vacuolation development and partial swelling of endoplasmic reticulum, (iii) vesiculation and fragmentation of tonoplast , and (iv) degradation of cytoplasm by the mixture of cytoplasmic and vacuolar matrices in leaves of sweet potato (Fig. 14). In potato leaves ,salt stress causes smaller intercellular spaces, rounding of cells, and a reduction in chloroplast number. While in tomato plants salinity causes reduction of plant leaf area and stomatal density (Parida et al.,2004). Fig 14. Effects of NaCl stress on polyphenol level
in B. parviflora measured as a function of days of NaCl
treatment. Values are mean7SE .
3.3. Effects of salinity on lipids:
Lipids are the most effective sources of storage energy, they act as insulators of delicate internal organs and hormones. They play two useful roles ,as the structural constituents of most of the cellular membranes and also have vital roles in the tolerance to several physiological stressors in a variety of organisms including cyanobacteria. The mechanism of desiccation tolerance relies on phospholipid
bilayers, which are stabilized during water stress by sugars, especially by trehalose. Unsaturation of fatty acids also counteracts water or salt stress. Lipids are rich in these bonds and are a primary target for oxidative reactions. However, glycolipid shows a statistically significant increase in the total lipid as salinity in the increased and the content of plasma membrane phosphatidylcholine (PC) and phosphatidylethalomine (PE) decrease by salinity, but the PC/PE ratios are not affected by salinity (Parida et al.,2004).
3.4. Effects of salinity on chloroplast ultrastructure:
Electron microscopy shows that the thylakoidal structure of the chloroplasts becomes disorganized, the size and number of plastoglobuli increases, and their
starch content decreases in plants treated with NaCl. As example, in potato salt stress reduces the numbers and depth of the grana stacks ,causes a swelling of the thylakoid and starch grains become larger in the chloroplasts. However, chloroplast ultrastructural changes under salt stress are apparent in Eucalyptus microcorys;
these include the dilation of the thylakoid membranes, the presence of large starch grains, the near absence of grana, and the presence of enlarged mesophyll cells (Parida et al.,2004).
4- Biotechnology salinity stress tolerance:
High salinity conditions result in hyperosmotic damage to most plants, and elevated Na' concentration disrupt cellular processes by interfering with vital Na'-sensitive enzymes and by affecting essential ion transport. Most plants synthesize and accumulate osmolytes, socalled compatible solutes, as a response to high
salinity conditions (Yosida , 2002). However, the manipulation of signaling factors has produced the most impressive results arguably because they control a broad range of downstream events, which results in superior tolerance. Effective expression systems, including stress-inducible promoters and cell type-specific will be required to fine-tune the plant response to stress according to the time and circumstances for the onset of the environmental insult. Furthermore, the importance of cell type-specific processes is best exemplified by AtHKT1;1. Constitutive expression of AtHKT1;1 causes increased shoot accumulation of Na+ and reduced salinity tolerance, whereas specific transgene expression in the stele of roots has the opposite effect, that is Na+ exclusion from the shoot and enhanced salinity tolerance .As an evidence, Comparisons of unidirectional Na+ fluxes and rates of net accumulation of Na+ in roots indicate that 70-95% of the Na+ fluxed into the root symplast is extruded back to the apoplast, and that small differences in Na+ exclusion capacity lead to noticeable changes in the net accumulation of Na+.
As example, the plasma membrane Na+/H+ exchanger SOS1 in Arabidopsis, facilitates Na+ homeostasis by extruding the ion from root epidermal cells at the root soil interface. Additional evidence of the involvement of SOS1 in long distance, Na+ transport has been produced recently in the halophytic Arabidopsis relative the llungiella salsuginea and in tomato. Lower net Na+ flux was observed in the xylem sap of tomato plants with suppressed SOS1 activity and down regulation of ThSOS1 in The llungiella increased Na+ accumulation in the root tip and in the stele (Pardo, 2010).
Salinity effects and problems with regard to tolerance are discussed her. This seminar provides information on physiological, and molecular bases of salt tolerance. Efforts have been made to compare the relative sensitivity of various plant species to salt, and uptake of NaCl are considered with regard to phytotoxicity and their interactions with nutrients. Present knowledge in environmental and molecular biology offers some ways for increasing salt tolerance. In conclusion, salinity is the most serious threat to agriculture and to the environment in many regions of the world and key molecular factors that can be used for genetic engineering of salt-tolerant plants include over expression of specific transcription factors, overproduction of osmoprotectants, expression of water channel proteins ,characterization of dehydrin proteins, ion transporters expression and characterization of molecular chaperones.