Salt Stress

1. INTRODUCTION
Saline water occupies 71% of the Earth area. It is thought that even a quarter of the whole pedosphere is affected by salts (Glenn and O’Leary, 1985), amounting to 950 x 106 ha (Flowers and Yeo, 1995), while 23 % of the 1.5 x 109 ha cultivated land is considered as saline (Rhoades and Loveday, 1990). Furthermore, about a half of all the existing irrigation systems of the world (3 x 108 ha) are under the influence of secondary salinization, alkalization and waterlogging, and about 10 x 106 ha of irrigated land are abandoned each year because of the unfavorable effects of secondary salinization and alkalization (Szabolcs, 1987). Such unfavorable soils of low fertility are generally unsuitable for agricultural production, causing unacceptable yield reduction, and in some cases, being far from any reasonable utilization. Because of the increased need for food
production and increasing distribution of soils affected by salinity, research on plant responses to salinity has rapidly expanded in recent decades.
Studies of plant tolerance to salt stress cover many aspects of the influences
of salinity on plant behavior, including alterations at the morphological, physiological and molecular levels. Recently, investigations are focusing more on: biotechnology, transgenic plants, improvement of breeding and screening methodologies and modification of the genetic structure of existing crops aiming at enhanced adaptation to salinity conditions.
The alterations in physiological traits caused by salt stress have been frequently reviewed during the past decades (Waisel, 1972; Flowers et al., 1977; Greenway and Munns, 1980; Munns et al., 1983; Ungar; 1991, Munns, 2002). However, the progress of research methodologies and techniques has created a platform for better understanding of molecular aspects (Yeo, 1998; Hasegawa et al., 2000; Tester and Davenport,K.V. Madhava Rao, A.S. Raghavendra and K. Janardhan Reddy (eds.),Physiology and Molecular Biology of Stress Tolerance in Plants, © 2006 Springer. Printed in the Netherlands.2003) and genetic information related to this problem (Zhu, 2000, Yokoi et al., 2002a; Xiong and Zhu, 2002). The number of publications appearing on the effects of salinity on plants has continued to grow over the years, and now exceed 300 per annum (Flowers and Yeo, 1995).
Although salinity and sodicity are common phenomena for arid and semiarid regions of the world, salt-affected soils have been recorded in practically all the climatic regions, and in a wide range of altitudes. The term “salt-affected” refers to soils that are saline or sodic, and these cover nearly 10% of the total land surface (Pessarakli and Szabolcs, 1994). According to FAO Land and Plant Nutrition Management Service the
total world area under saline and soils is 397 x 106 and 434 x 106 ha, respectively, and of the almost 1500 x 106 ha of dryland agriculture about 2% are salt-affected. The regional distribution of salt-affected soils was evaluated by FAO Land and Plant Nutrition Management Service (http://www.fao.org/ag/agl/agll/ spush/topic2.htm) (Table 1).
Table 1. Regional distribution of salt-affected soils (in million hectares)
Regions Total area Salt-affected soils
Mha Mha %

Africa 1899
Asia and Australia 3107
Europe 2011
Latin America 2039
Near East 1802
North America 1924
Total
Medicago sativa L.
Prunus ducilis L.
Malus sylvestris L.
Prunus armeniaca L.
Asparagus officinalis L.
Hordeum vulgare L.
Beta vulgaris L.
Phaseolus vulgaris L.
2.0
1.5

1.6
4.1
8.0
4.0
1.0
Cynodon dactylon (L.) Pers. 6.9
Rubus sp.
Vicia faba L.
Brassica oleracea L.
Brassica napus L.
Daucus carota L.
Ricinus communis L.
Apium graveolens L.
Prunus avium L.
1.5
1.6
1.8

1.0

1.8
Cicer arietinum L.
Trifolium hybridum L.
Trifolium pratense L.
Melilotus sp. Mill.
Trifolium repens L.
Zea mays L.
Gossypium hirsutum L.
Cucumis sativus L.
Ribes sp. L.
Solanum melongena L.
Ficus carica L.
Linum usitatissimum L.
Allium sativum L.
Vitis sp. L.
Hibiscus cannabinus L.
Citrus limon L.
Lactuca sativa L.

1.5
1.5

1.5
1.7
7.7
2.5

1.1

1.7
1.7
1.5
8.1

1.3

Almond Apple Apricot Asparagus Barley Beet, red Bean Bermuda grass Blackberry Broad bean Cabbage Canola Carrot Castorbean Celery Cherry Chickpea Clover, alsike Clover, red Clover, sweet Clover, ladino Corn Cotton Cucumber Currant Eggplant Fig Flax Garlic Grape Kenaf Lemon Lettuce Salt Stress
58
Muskmelon

Cucumis melo L.
Avena sativa L.
1.0
Olea europaea L.
Allium cepa L.
Citrus sinensis L.
Pastinaca sativa L.
Pisum sativum L.
Prunus persica L.
Arachis hypogaea L.
Pyrus communis L.
Capsicum annuum L.
Prunus domestica L.
Punica granatum L.
Solanum tuberosum L.
Cucurbita pepo L.
Raphanus sativus L.
Rubus idaeus L.

1.2
1.7

3.4
1.7
3.2

1.5
1.5

1.7

1.2
Oryza sativa L.
Secale cereale L.
Lolium multiflorum Lam.
Lolium perenne L.

11.4

5.6

Oats Olive Onion Orange Parsnip Pea Peach Peanut Pear Pepper Plum Pomegranate Potato Pumpkin Radish Raspberry Rice Rye Rye grass, Italian Rye grass, perennial
Sorghum

Sorghum bicolor L.
Glycine max (L.) Merrill
Spinacia oleracea L.
Fragaria vesca L.
Beta vulgaris L.
Saccharum officinarum L.
Helianthus annuus L.
6.8
5.0
2.0
1.0
7.0
1.7
Phleum pratense L.
Lycopersicon esculentum L. 2.5
x Triticosecale Wittmack
Brassica rapa L.
Triticum eastivum L.
6.1
0.9
6.0

Soybean 20.0 MT
Spinach Strawberry Sugar beet Sugar cane Sunflower Timothy Tomato Turnip Wheat Table 3. Continued…
(adapted after Maas, 1986, 1990 and Francois and Maas, 1994; T- tolerant, MT- moderatelytolerant, MS-moderately sensitive, S- sensitive)
6.2. Nitrogen Fixation and Salt StressLegumes represent a very significant group of crops in agriculture, and therefore their responses to salt stress are described in several reports (e.g. Katerji et al., 2001, Lachaal et al., 2002). Tolerant varieties and accessions within the legumes have been revealed,such as for soybean (Essa, 2002) and among both cultivated and wild Phaseolus species (Bayuelo-Jimenez et al., 2002). Breeding and genetic engineering programs of legumes must be directed to optimise their nitrogen fixation and growth in saline conditions.
Establishment of symbiosis is highly sensitive to salt stress, whereas fully
developed nodules that had been formed under salt stress can continue to fix nitrogen (Singleton and Bohlool, 1984). Host tolerance was a major factor for nodulation and nitrogen fixation in genotypes of faba bean (Cordovilla et al., 1995). Rhizobia can survive under much higher salinity than its host legume (Nair et al., 1993). Bacterial ability to adapt to salt stress is important for the bacteroid nitrogen-fixing function inside the
legume nodule. Among several Rhizobium species tested for salt stress tolerance, R.meliloti has expressed a much higher ability to survive in the saline medium, comparing to R. leguminosarum and, especially R. japonicum (Bernard et al., 1986). Salt stress tolerance in rhizobia is at least partially associated with osmoregulation achieved by accumulation of compatible solutes (Imhoff, 1986). Inn conditions of salt stress, pea plants treated with boron and calcium exhibited enhanced cell and
tissue invasion by Rhizobium leguminosarum and increased nodule number (ElHamdaoui et al., 2003). Furthermore, enzymes involved in ammonium assimilation inroot nodules exhibited a significant sensitivity to salt stress and should not be considered as reliable criteria for selection of salt tolerance in faba bean (Cordovilla et al., 1995)
and pea (Cordovilla et al., 1999). Activity of nitrogenase, nodule number and dry matter
accumulation in soybean (Abdalla et al., 1998) and alfalfa (Serraj and Drevon, 1998)
were affected under salt stress.
Eight genes of Rhizobium tropici were involved in salt tolerance and establishment of symbiosis with beans. These were classified into three groups: a) two genes
responsible for regulation of gene expression and regulation of nitrogen metabolism, b)
genes related to synthesis or maturation of proteins, and c) genes associated withpotassium uptake and polysaccharide biosynthesis (Nogales et al., 2002).
7. WATER REGIME AND PHOTOSYNTHESIS UNDER SALT STRESS
The effects of drought and salt stress on plants are tightly related, since the firstresponses of plant cells under these stress conditions are induced by osmotic shock.
Thus, upon exposure to osmotic stress, plants exhibit many common adaptive reactions at the molecular, cellular and whole-plant level (Greenway and Munns, 1980; Yeo,
1998; Bohnert et al., 1995; Zhu et al., 1997). These include morphological and anatomical
alterations (life cycle, xeromorphic features, increased root/shoot ratio), and physiSalt Stresological traits associated with maintaining water relations and photosynthesis (e.g.
different pathways of carboxylation, such as C4, intermediate C3-CAM and CAM) (Dajic
et al., 1997a). Additionally, various metabolic changes, such as the maintenance of ion
and molecular homeostasis (e.g. synthesis of compatible solutes necessary for osmotic
adjustment), detoxification of harmful elements and growth recovery, which depends
mainly on various signaling molecules, occur under exposure to salt/drought stress
(Xiong and Zhu, 2002).
Increasing salinity in the growth medium decreases content of chlorophyll
and the net photosynthetic rate, which is expressed more conspicuously in salt-sensitive plants, such as alfalfa (Khavarinejad and Chaparzadeh, 1998) and canola (Qasim et
al., 2003). Under salinity treatment, two wheat cultivars expressed two phases of photosynthetic inhibition: in the first phase, photosynthetic reduction was gradual, whereas
in the second phase it was rapid and accompanied by a decline of the energy conversion efficiency in photosystem II, strongly related to adverse effects of salinity (Muranaka
et al., 2002). Reduction of net CO2 assimilation with salinity in tomato and sunflower
was related to decrease in stomatal conductance and stomatal density (Romeroaranda
et al., 2001; Rivelli et al., 2002b). The decrease was due to reduced CO2 assimilation
associated with a decline in stomatal conductance, water use efficiency and Rubiscoactivity, as well as slower electron transport of photosystem II under severe salt stress.
In many halophytic species regulation of the water regime is associated withthe type of CO2 fixation. Certain halophytes, originating from the tropics and subtropics, utilize the CAM (Crassulacean Acid Metabolism) pathway of carboxylation. Water
availability is the major selective factor for evolution of the CAM pathway in plants, where nocturnal CO
2 fixation saves loss of water by transpiration and increases wateruse efficiency (Larcher, 1995). Induction of the CAM pathway in the common ice plant(Mesembryanthemum crystallinum) under stress conditions is dependent on its biochemical machinery, which enables an increase in PEP-carboxylase and other CAM
enzyme activities (Michalowski et al., 1989, Thomas et al., 1992), as well as enzymesinvolved in synthesis of compatible solutes, particularly pinitol (Vernon and Bohnert,1992). The change from C3-photosynthesis to CAM in M. crystallinum is elicited bysalt stress and drought (Winter and Lüttge, 1979), and the kinetics of CAM induction
depends on the strength of the stress and the developmental stage of the plant (Cushman et al., 1990a). Moreover, the stress-induced switch from C3 to CAM may be linked wit the ABA-induced activity of vacuolar ATPase in adult plants, while vacuolar Na+compartmentation is regulated through ABA-independent pathways in M. crystallinum
(Barkla et al., 1999). The perennial cactus Cereus validus, having constitutive CAM,exhibits adaptations at the whole-plant level which differ from those of the annualCAM-inducible common ice plant, for example regulation of turgor and gas exchange,and metabolic adjustment at the cellular level and molecular level (Lüttge, 1993). Evaluation of signal transduction events involved in the induction of CAM in the common
ice plant revealed that transcript abundance of Ppc1, a gene encoding the CAM-specific isoform of phosphoenol pyruvate carboxylase, rapidly increased during osmoticstress (Taybi and Cushman, 1999).A significant number of halophytes are C4 species, which are characterized by
their higher requirements for sodium ions compared with C3 species (Brownell andCrossland, 1972). In conditions of osmotic stress and hightemperatures, C4 plants havean advantage in comparison with C3 plants, because of their ability to carry on photosynthesis when stomata are to a large extent closed, coupled with the absence of photorespiration in the mesophyll cells (Larcher, 1995). Photosynthetic responses tosalinity in the halophytic tribe Salsoleae (family Chenopodiaceae) have been reviewed,with particular attention paid to relations between the C4 NAD-ME (malic enzyme Salsoloid type of carboxylation and the chloroplast structure (Voznesenskaya et al.,1999).
Abscisic acid is well recognized as an important stress hormone. The concentration of ABA increases when water deficits occur, with its de novo synthesis beginning in the roots, in response to sensing an insufficient supply of water (Zhang et al.,1989). In halophytes, which grow in conditions of “physiological drought”, due to low
water potential in the root medium, the lowest concentrations of ABA were found under salinity concentrations optimum for growth (Clipson et al., 1988). Such case was reported for the highly tolerant halophytic species Suaeda maritima, which exhibited the lowest seasonal range of ABA contents (from 649.4 ng g-1 to 835.6 ng g-1 dry weight) in comparison with several other species, where higher ABA concentrations were correlated with increased sodium content of the shoot (Dajic et al., 1997a).
In glycophytes, salinity leads to the accumulation of ABA (Asch et al., 1995),
as in tomato (Chen and Plant, 1999; Yurekli et al., 2001) and wheat (Aldesuquy and Ibrahim, 2002). In bush bean plants exposed to 75 mM NaCl, inhibition of leaf expansion was mediated by ABA rather than by Na+ or Cl- toxicity, and the increase of ABA induced by a salt-pretreatment limited the accumulation of Na+ and Cl- in the leaves,
resulting in adaptation to salinity stress (Montero et al., 1997). Besides the significantrole of ABA, favorable effects of other hormones in plant responses to salinity, such as cytokinins (Kuiper and Steingrover, 1991) and gibberellins (Kaur et al., 1998; Ashraf etal., 2002) have been documented.
8. MOLECULAR BASIS OF SALT TOLERANCE
According to Hasegawa et al. (2000) determinants of salt stress tolerance include effector molecules that enable adaptive reactions and mechanisms of plants in saline environments and regulatory molecules that control these pathways. Effectors are proteins
and metabolites involved in ion homeostasis (membrane proteins involved in regulation of ionic transport), osmotic adjustment and water regime regulation (osmolytes)
and toxic radical scavenging (mainly enzymes), while regulatory molecules are cellular signal pathway components and transducers of long-distance response coordination (hormones, mediators, transcription factors and regulatory genes).
8.1. Biochemical Determinants of Salt Tolerance – Enzymes, Compatible Solutes an Protection FactorsThe cytotoxicity of sodium lies in the high charge/mass ratio of the sodium ion (compared with potassium), causing disruption in water structure and a decrease in hydrophobic interactions and hydrostatic forces within proteins (Pollard and Wyn Jones,
1979). Additionally, Na+ affects the activity of enzymes either by direct binding to inhibitory sites or by displacing K+ from activation sites. It has been suggested that more than 50 enzymes are activated by K+, and Na+ can’t be replaced in this function (Bhandal and Malik, 1988). Additionally, K+ is needed for protein synthesis, as binding of tRNA to ribosomes requires K+ (Blaha et al., 2000).
The effects of salts on enzymatic reactions are multiple and complex, although to a large extent, their influence is related to the change in cytosolic pH which strongly affects the activity of enzymes. It is generally accepted that enzymes exhibit slightly increased activity under low concentrations of ions, whereas they start to be inhibited
in the presence of NaCl concentrations higher than 100mM (Munns, 2002). For instance, the activity of DNAse and RNAse in alfalfa and lentil seedlings was inhibited in the presence of 100 mM NaCl (Yupsanis et al., 2001).
Enzymes of halophytes are, in general, just as sensitive as enzymes of
glycophytes (Greenway and Osmond, 1972; Flowers et al., 1977), but some salt tolerant plants exhibut in vitro tolerance of some enzymes to high concentrations of salts in (Flowers and Dalmond, 1992). However, the relevance of any assay under in vivo conditions is uncertain. Enzymes of cell wall compartment could be more salt-tolerant than
cytoplasmic enzymes of higher plants (Thiyagarajah et al., 1996).
The salt tolerance of plants, irrespective of the sensitivity of enzymes and
protein synthesis to high salt concentrations, is significantly related to the sequestration of salts into the vacuoles, which allows the normal activity of metabolic machinery in the cytoplasm. Salt-induced increases in the activity of enzymes involved in defense to oxidative stress are related to the reactive oxygen species scavenging pathway which takes place in the particular cell compartments, such as chloroplasts, peroxisomes, glyoxysomes and cytosol (Yeo, 1998; Rathinasabapathi; 2000, Xiong and Zhu,
2002), which, in difference to the vacuoles, do not accumulate the salts.
The cytosolic apparatus of both halophytes and glycophytes is very sensitive to osmotic and ionic effects of salts. Adverse effects of salts on the cell metabolism may be alleviated through synthesis and accumulation of compatible solutes and protection factors of macromolecules (mainly LEA proteins and chaperones). Accumulation of compatible solutes in response to salt stress is a metabolic adaptation, which primarily serves for osmotic adjustment and osmotic balance between vacuole and the cytosol. As found in a number of stress-tolerant species, there is a possibility of convergent evolution for this trait (Yancey et al., 1982; Rhodes and Hanson, 1993).
Compatible solutes are defined as organic osmolytes, which are compatible
with the cell’s metabolism, referring to protein/solute interactions and stabilization of macromolecules, irrespective of species and nature of the stress (Yeo, 1998; Nuccio et
al., 1999; Rathinasabapathi, 2000; Hasegawa et al., 2000; Huang et al., 2000). These Compatible solutes comprise a wide range of organic compounds, such as: simple sugars (fructose and glucose), sugar alcohols (glycerol and methylated inositols), complex sugars (trehalose, raffinose and fructans), polyols, quaternary ammonium compounds (proline, glycine betaine, â-alanine betaine, proline betaine) and tertiary sulfonium compounds (Rhodes and Hanson, 1993; Nuccio et al., 1999). As compatible solutes are hydrophilic, they can replace water at the surface of proteins, complex protein structures and membranes, which explains their action as osmoprotectants and as lowmolecular-weight chaperones (Hasegawa et al., 2000).
Within this group of molecules, glycine betaine is a ubiquitous protein-stabilizing osmolyte occurring in all organisms (Rhodes and Hanson, 1993). Glycine betaine is an amphoteric compound, electrically neutral over a wide range of pH and extremely soluble in water, allowing it to interact with both hydrophilic and hydrophobic regions of macromolecules (Sakamoto and Murata, 2002). Glycine betaine a accumulated in
many halophytic species, such as Suaeda maritima (Clipson et al., 1985), Atriplex nummularia, Spergularia marina, Salicornia europea (Stumpf, 1984) and Salsola soda (Manetas, 1990) in order to balance the osmotic potential difference between vacuole and the cytoplasm (Flowers et al., 1977). The accumulation of glycine betaine in
halophytes (e.g. Atriplex griffithii) is induced by salt stress and increases with a raise of salinity (Khan et al., 1998; 2000b) in the halophyte. Glycine betaine was the major organic osmolyte of non-halophytes, such as wheat (Saneoka et al., 1999), red-beet (Subbarao et al., 2001), and sorghum (Yang et al., 2003). Despite its wide presence in many speeies, glycine betaine is absent in some crops, (rice) and tobacco (Yeo, 1998). In higher plants the biosynthesis of glycine betaine is seeds as by two-step oxidation of choline (via the toxic intermediate betaine aldehyde) catalyzed by choline
monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively (Sakamoto and Murata, 2002). The activity of these enzymes is localized to the chloroplast stroma, although some BADH activity was found in the cytoplasm (Weigel et al.,1986). Two kinds of BADH found in the mangrove halophyte Avicennia marina were characterized by high efficiency in the oxidation of betaine aldehyde (Hibino et al.,
2001). Installing of genes involved in the synthesis of glycine betaine has had a certain success in improvement of salt tolerance in plants.
Polyols, such as glycerol, mannitol, sorbitol and sucrose, are osmoprotectants in algae and certain halophytic plants (Yancey et al., 1982). The biosynthesis and accumulation of specialized polyols (myo-inositol, D-ononitol and D-pinitol) in the cytosol
of the stress-tolerant common ice plant (Mesembryanthemum crystallinum) was reported to increase with salinity (Nelson et al., 1999).
The sugar alcohol mannitol may serve as a compatible solute in salinity conditions. The role of mannitol in salt stress tolerance was evaluated in the non-mannitol producer Arabidopsis by installing the M6PR gene (for synthesis of mannose-6-phosphatase reductase) from celery, and in the presence of NaCl, mature transgenic completed their normal development, including flowering and seed production even at salinity of 300 mM (Zhifang and Loescher, 2003). Enhanced salinity tolerance of eggplant was achieved through installing of the bacterial mt1D gene encoding the mannitol phosphodehydrogenase, an enzyme involved in the mannitol synthesis (Prabhavathiet al., 2002). Trehalose is a non-reducing disaccharide that functions as a compatible solute under abiotic stress in bacteria, fungi and invertebrates (Yeo, 1998). Until recently,
trehalose had only been found in a few resurrection (desiccation- tolerant) plants,indicating the role of this molecule in adaptations to water stress due its ability to act as a water substitute on the surface of macromolecules (Öko- Institut, http://www.plantstress.com/Articles/up_general_files/GE_Tol.pdf). Trehalose accumulates in plants in a very low concentrations, and might be involved in the ROS scavenging and signaling cascade (Flowers, 2004). The overexpression of Escherichia coli trehalose
biosynthetic genes (otsA and otsB) in transgenic rice resulted in sustained plant growth, lower photo-oxidative damage and a more favorable mineral balance under exposure tosalinity (Garg et al., 2002).
Proline is a significant compatible solute in many halophytic species (Stewartand Lee, 1974), as well as in glycophytes, like Medicago sativa (Fougere et al., 1991) and Sorghum bicolor (McCree, 1986). Accumulation of proline in rice is a symptom of salt stress injury, and is due increase of the ornithine delta-aminotransferase (OAT) and
its precursor glutamate (Lutts et al., 1999). Intermediates of proline biosynthesis and catabolism induced expression of several osmotically regulated genes in rice (Iyer and Caplan, 1998). The content of transcripts of two cDNA clones from alfalfa, encoding the first enzyme of proline biosynthesis pathway, MsP5Cs-1 and MsP5Cs-2, increased in
seedlings exposed to 90 mM NaCl (Ginzberg et al., 1999). Transgenic wheat plants producing proline due to the expression of genes for proline biosynthesis, transferred from Vigna aconitifolia, exhibited improved tolerance to salinity (Sawahel and Hassan,2002).
Different classes of proteins of uncertain biochemical function (possibly macromolecule protection factors) are synthesized under conditions of salt stress, such as:osmotins, dehydrins, late embryogenesis abundant proteins (LEA) and polyamines, primarily putrescine and spermine (Tester and Davenport, 2003). Under salinity treatments, a balance between content of the free and bound polyamines in roots of barley seedlings might be relevant for salt tolerance (Zhao et al., 2003). There is no much data
on the function of osmotins and dehydrins in conditions of salinity, but they may be involved in the maintenance of the protein structure (Campbell and Close, 1997). Thelate embryogenesis abundant-like proteins (LEA) accumulate in the vegetative tissues of all plant species in response to osmotic stress, caused by drought, salinity or cold
(Xiong and Zhu, 2002). They probably contribute to the preservation of the structural integrity of the cell (Winicov, 1998), acting as chaperones to prevent denaturation of proteins (Xiong and Zhu, 2002). Most of the LEA-like proteins of all organisms are involved in adaptations to osmotic stress, and interestingly, express features of ribosomal proteins that interact with RNA (Garay-Arroyo et al., 2000).
8.2. Ion Homeostasis and Regulation of Sodium Transport
Ion homeostasis in conditions of salt stress is related to the activity and regulation of intrinsic membrane transport proteins, such as ATPases, carrier transporters and ion
channels (Braun et al., 1986; Niu et al., 1993; Amtmann and Sanders, 1999; Krol and Trebacz, 2000; Blumwald et al., 2000; Tester and Davenport, 2003). There is no evidence for the existence of a Na+-ATPase (postulated to be a primary Na+ extrusion pump) in algae and higher plants, with the exceptions of the unicellular marine algae Tetraselmis
viridis and Heterosigma akashiwo (Gimmler, 2000).
Sodium transport from the environment into the plant cells is a passive process, since the negative electrical potential differences at the plasma membrane and low concentration of sodium ions in the cytosol are the major forces for sodium uptake (Blumwald et al., 2000). Sodium uptake thus, depends on the electrochemical gradient of Na+ and the presence of Na-permeable channels in the plasma membrane. Sodium ions
may accumulate in the cytoplasm to 100 times the external concentration. Such accumulation in salt-tolerant plants is prevented by control of influx (channel gating) and/or active export from the cytoplasm to the vacuoles, and also outside from the plant (Jacoby, 1994). This means that Na+ extrusion and vacuolar compartmentation are active processes. Active sodium transport in plant cells is performed by Na+/H+ transporters, which are ordinarily driven by an ATPase derived proton motive force.
8.2.1. Role of H+-ATPases Electrophoretic flux across the cell membranes and secondary active transport are facilitated by H+ pumps, including H+-ATPase of the plasma membrane and H+-ATPase and H+-pyrophosphatase of the tonoplast (Sze et al., 1999). The dominant ion pump at
the plasma membrane of higher plants is a H+- pumping ATPase, which provides the electrochemical potential difference for H+ ions across the plasma membrane. Several genes are known to be involved in encoding H+-ATPases (Michelet and Boutry, 1995).
Proton-translocating activity was doubled in root cells of the halophyte Atriplex nummularia exposed to 400mM NaCl, indicating a role for H+-ATPase in response to salt stress (Braun et al, 1986). Additionally, the NaCl responsiveness of A. nummularia to accumulate plasma membrane H+-ATPase mRNA was substantially greater than inthe salt-sensitive tobacco (Niu et al., 1993). That plasma membrane H+-ATPase may be a salt tolerance determinant was confirmed by mutation of the AHA4 gene controlling the Na+ flux across the endodermis (Vitart et al., 2001). Additionally, cDNA fragments corresponding to the plasma
membrane H+-ATPase from rice, designated as OSA1, OSA2 and OSA3, are involved in the control of salt uptake into root symplast and apoplast, and further translocation in the shoot (Zhang et al., 1999).
Vacuolar ATPase (V-ATPase) acidifies intracellular compartments and contributes to the H+-electrochemical gradient capable to drive the secondary transport of ions and metabolites across the tonoplast (Ratajczak and Wilkins, 2000). The V-ATPase holoenSalt Stress zyme has two main domains (peripheral and a membrane integral domain, Ratajczak,
2000) and three subunits: A, B and C, where subunit C transcripts increase in response to salinity (Chen et al., 2002). Increases in the tonoplast H+-ATPase activity under salinity conditions have been reported for different species (Mansour et al., 2003), in contrast to an inhibition of vacuolar H+-pyrophosphatase by increased NaCl concentrations (Blumwald et al., 2000). In the halophyte Sueada salsa the main strategy of salt tolerance seems to be up-regulation of vacuolar H+-ATPase activity, while H+-pyrophosphatase had a minor role (Wang et al., 2001). Nevertheless, an overexpression of
the AVP1 gene encoding the native vacuolar H+-translocating pyrophosphatase resulted in an increased salinity tolerance in Arabidopsis compared with the wild-type plants (Gaxiola et al., 2001). Thus, role and activity of H+-pyrophosphatase in responses to salinity is still uncertain.


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