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Introduction

Many arid and semi-arid regions in the world have soils and water resources that are too saline for most of the common conventional crop systems (Pitman and Lauchli 2002). Halophytes are plants that have been naturally selected in saline environments and are distinguishable from glycophytes by their capacity to cope with excessive levels of ions by various eco-physiological mechanisms. Some halophytes possess unique adaptations, such as salt glands or bladders that alleviate the deleterious effects of high ion concentrations. However, intrinsically cellular processes must make the major contribution to the capacity of plants for salt adaptation. At the molecular level, the higher salt-adaptive plasticity of halophytes may be due to constitutive expression of genes that encode salt-tolerance determinants (Casas et al. 1992) or the better aptitude to regulate the expression of these genes in response to salt. This hypothesis makes halophytes a source of exclusive genes or new genetic mechanisms that could be applied in genetic manipulation of crops. Cultivation of salt-tolerant crops, or halophytes, on saline soil has significant social and economical potential that needs to be further explored and developed (Debez et al. 2011).

Among the halophytes extensively used in physiological and molecular biological investigations is the Atriplex genus. The genus Atriplex (Chenopodiaceae) contains various species distinguishable by different morphology, biological cycles and ecological adaptations (Le Houérou 1992). Tolerance to salinity , drought , heavy metals and temperature are important characteristics of species of Atriplex . However, the value of certain Atriplex species has been recognized by their incorporation in the rangelands improvement programs in many salt-affected regions throughout the world. In this contribution, we review the literature regarding the ecological and agronomic importance of the plant genus Atriplex in arid and semi arid regions.

Geographical Distribution of the Genus Atriplex

Atriplex species constitute the largest and most diversified genus of the family Chenopodiaceae (Kadereit et al. 2010). Atriplex species (saltbushes) are dominant in many arid and semi-arid regions of the world, particularly in habitats that combine relatively high soil salinity with aridity (Ortíz-Dorda et al. 2005). Over 400 species of Atriplex have been found to be geographically distributed on all continents. Atriplex species are mainly found in the deserts and semi-deserts in North America, South Australia, South Central Asia, West and South East America, and the Mediterranean basin. A. nummularia, and A. halimus are the most widely distributed species of the genus Atriplex. A. halimus is a perennial native shrub of the Mediterranean region (Ortíz-Dorda et al. 2005). This species has two subspecies: the subsp. halimus, which is present on the northern shores of the Mediterranean basin and the subsp. schweinfurthii (Boiss.) common on the southern shores of the Mediterranean basin, North Africa and Near East. A. nummularia occurs naturally in the semi arid and arid zone of southern and central Australia where it was divided in three subspecies (subsp. nummularia; subsp. Omissa and subsp. Spathulata). Molecular genetic and taxonomic evidence suggests that Atriplex was transported to Australia during the late Miocene (Kadereit et al. 2010). A. nummularia is proposed to have evolved from a common octoploid ancestor A. paludosa ssp. moquiniana (Moq.) Parr-Smith in the coastal semi-arid fringe of southwestern Australia. Sampson and Byrne (2012) suggested that many species spread and diversified from this zone to exploit the arid and saline habitats that were increasingly becoming available as a result of changing climatic conditions through the Pliocene and Pleistocene in inland areas of Australia. There are two species that are considered to be closely related to A. nummularia that are found in the arid zones of Western and South Australia, respectively: A. amnicola Paul G. Wilson and A. incrassate F Muell (Sampson and Byrne 2012). A. breweri and A. cansecens are relatively close to A. halimus. Between 1920 and 1930, A. nummularia, A. semibaccata from Australia and A. canescens from USA were introduced to Tunisia and Morocco (Ben Salem et al. 2010 ). Only 13 species and subspecies are used for rangeland rehabilitation and fodder production: A. halimus subsp. halimus, A. halimus subsp. schweinfurthii, A. mollis, A. glauca, A. leucoclada, A. nummularia, A. canescens subsp. canescens, A. canescens subsp. linearis, A. amnicola, A. undulata, A. repanda, A. semibaccata, and A. barclayana (Ben Salem et al. 2010 ).

The Importance of Atriplex Species for Saline Soil Reclamation

Salt affected soils are widely spread in many arid and semi-arid regions of the world and increasingly threatening agricultural expansion and productivity. Yet, in many arid environments, high quality water in not available to support the establishment of plants for revegetation projects. The removal of sodium salts from saline soils by halophytes plants, as alternative for costly chemical amendments, has emerged as an efficient low cost technology (Gharaibeh et al. 2011). It is well known that Atriplex species actively accumulate soluble salts in leaves, especially sodium, in association with a drought tolerance mechanism. For this reason it is also considered as an excellent species for reducing soil salinity in drylands, if cut and collected (Ben Salem et al. 2005 ). It was found that the dehydrated A. halimus accumulated more Na+ than the control plants even without the addition of NaCl to the stressed plants (Martinez et al. 2003). Glenn and Brown (1998) concluded that tolerance of A. canescens to water and salt stress was linked through a common mechanism of accumulating Na+ for osmotic adjustment . By comparing salinity tolerance of three Atriplex species in well-watered and drying soil Glenn et al. (2012) found that A. hortensis was able to complete its life cycle on drying soil with a final salt content 85 g/l NaCl. A. lentiformis was able to survive on drying soils with salinities five times higher than seawater, whereas A. canescens had high survival on drying soils but was less salt tolerant than either A. hortensis or A. lentiformis. It has been demonstrated that there is a relationship between the habitat of the Mediterranean xero-halophyte species A. halimus and the strategy adopted for NaCl and osmotic stress resistance . The coastal (Monastir, salt-affected site) population is more tolerant of salinity than the inland (Sbikha, non saline semi-arid area) population and displays a higher ability to accumulate glycinebetaine (GB) in response to this constraint. In contrast, the inland population, exposed in its natural habitat to transient periods of drought , is more resistant to osmotic stress induced by 15 % PEG, and mainly accumulates proline in response to this treatment (Ben Hassine et al. 2008). Some Atriplex species grown under rangeland conditions has leaf ash concentrations of 13–27 % (Welch 1978; Hyder 1981; Berrett-Lennard 2002) and Atriplex species grown in saline soils can have leaf ash concentrations up to 39 % (Malcolm et al. 1988). Khan et al. (2000) reported that A. halimus was more salt-tolerant than A. calotheca and A. nitens, when grown at 750 mM NaCl (− 40, − 67 and − 80 % of biomass production) but interestingly, all three species were able to survive at this salt concentration. Recently, Benzarti et al. (2012) found that A. portulacoides was able to grow in medium containing 1,000 mM NaCl without displaying salt-induced toxicity symptoms. The salinity resistance of some Atriplex species is often attributed to the presence of vesiculated trichomes covering the leaf surface and containing large amounts of salt (Smaoui et al. 2011). These trichomes play a significant role in removing salt from the the leaf tissues, thereby preventing the accumulation of toxic salts in the parenchyma and vascular tissues. For many Atriplex species, more than 50 % of the salt transported to the shoots is excreted via these epidermic trichomes (Belkheiri and Mulas 2011). These attributes have led some workers to suggest that Atriplex species could be grown to remove salt from the soil (Barrett-Lennard 2002).

Atriplex species (A. canescens) has been especially recommended for arid zone restoration projects (Fitzsimmons et al. 1998). In a field experiment Chisci et al. (2001) demonstrated the use of A. halimus in improving physical characteristics of a clay soil in Italy and to provide environmental protection by controlling runoff and reducing soil erosion on slopes. Atriplex plant litter can modify the top soil salinity, along with other soil properties. Maganhotto de Souza Silva et al. (2008) found that soils cultivated with A. nummularia and irrigated with saline effluents, in semi-arid conditions in Brazil, improved their fertility (organic carbon, nitrogen and phosphorus contents) and microbiological properties (enzymes activity). Sameni and Soleimani (2007) studied the distribution of salinity and of some soil physico-chemical properties, observing significant changes in salinity and pH and found that A. nummularia may actually facilitate growth of plants under their canopy. The work by Zucca et al. (2011) in a study site in Morocco also confirmed that the significant relationship between soil properties and A. nummularia development can be mostly observed within the first 10 cm. A. nummularia is one of the most important species used for the revegetation of degraded land in low rainfall areas. Slavich et al. (1999) planted A. nummularia as a vegetative cover in a salt affected land in southeast Australia. Brown et al. (1999) showed that A. barclayana could be used as a biofilter to remove nutrients from saline aquaculture effluents. Gharaibeh et al. (2011) showed that amelioration of a calcareous saline sodic soil can be achieved efficiently by growing A. halimus without applying an amendment. Planting A. halimus reduced soil sodicity and electrical conductivity considerably to values comparable to that of gypsum treatments.

The Atriplex species may also be used for wildfire prevention purposes. The high salt concentration found in its leaves increases their moisture content, which makes this species behave as a fire retardant in the event of wildfire (Montgomery and Cheo 1969).

The Importance of Atriplex Species in Heavy Metal Phytoremediation

The contamination of soil by heavy metals is one of the most serious environmental problems and has significant implications for human health. The clean up of heavy metals contaminated soils is one of the most difficult tasks for environmental engineering. In most cases, traditional physiochemical methods are quite expensive and may lead to soil alterations (Gardea-Torresdey et al. 2005). Phytoremediation based on the use of plants to remove or degrade inorganic and organic pollutants, has been proposed as a promising, environmentally friendly and relatively cheap. The success of phytoremediation depends upon the identification of suitable plant species those hyperaccumulate heavy metals. The use of deep-rooting xero-halophyte species is may be of special interest in this context to remediate salty contaminated area, especially in arid and semi-arid regions. Several species of the genus Atriplex , which are naturally salt- and drought-tolerant have been also suggested as potential candidates for a phytoremediation approach (Table 16.1) because of their high biomass production associated with a deep root system. A. portulacoides was suggested as a suitable species for the phytoremediation owing to the high translocation rates of Cd and Cu towards the aboveground tissues (Reboreda and Caçador 2007). A. portulacoides can tolerate external Cu levels of up to 15 mmol/l (1,000 mg Cu/l) without suffering adverse physiological effects (Cambrollé et al. 2012a). Furthermore, despite the fact that Cu concentrations of between 20 and 100 mg Cu/kg DW in leaf tissue are generally considered excessive or toxic (Kabata-Pendias and Pendias 2001), growth parameters of this species are unaffected by leaf tissue concentrations as high as 80 mg Cu/kg DW. A. portulacoides is able to survive with external Cu levels of 35 mmol/l and can be found growing in sediments that contain 300–3,000 ppm Cu. Sousa et al. (2008) stated that compartmentalization and detoxification mechanisms are crucial to allow A. portulacoides to tolerate high levels of heavy metals , and found that this species is able to retain a considerable quantity of metals in the root cell wall. Cambrollé et al. (2012b) reported also that this salt-marsh shrub may represent a valuable tool in the restoration of Zn-polluted areas since the plant can tolerate high tissue concentrations of Zn without suffering adverse physiological effects, and can produce a significant amount of biomass while sequestrating high concentrations of this metal. Other species that belong to the Atriplex genus, such as Atriplex halimus, have already been studied for their level of resistance to heavy metals (Cd, Zn, Pb). These species have been recommended as a promising species for the phytoremediation of heavy-metal contaminated areas based on their high biomass production, deep root systems and ability to tolerate high concentrations of toxic elements (Lutts et al. 2004; Lefèvre et al. 2009; Manousaki and Kalogerakis 2009). Precipitation of heavy-metal with oxalate and/or its excretion into trichomes and increased synthesis of glycine betaine may contribute to the tolerance of A. halimus. Among the heavy metals frequently present in contaminated soils, mercury is arguably of the greatest environmental and public health concern. A. conodocarpa proved to be the most suitable candidates for mercury phytoextraction because of its ability to translocate mercury from roots to the above-ground tissues (Lomonte et al. 2010). Vickerman et al. (2002) evaluated 30 Atriplex lines for potential habitat improvement and phytoremediation of selenium contaminated sites. A. patula was found to be one of the top selenium accumulators and grew well in saline soil . A. atacamensis Phil has been proposed as possible candidates for phytoremediation of Ar (Vromman et al. 2011).

Table 16.1 Potential candidates for phytoremediation approach
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Atriplex Species Forage Production

The main limitations to animal production in the arid and semi-arid regions, is the scarcity of green forage. The use of native or introduced halophytes for livestock production is an important issue in many semi-arid and arid areas. Several halophytes plants have been used as fodder crops under saline conditions in order to produce green forage during the dry season (El Shaer 2010). These include planting perennial salt marsh plant species, mainly Atriplex species, in numerous regions. In several experiments, the Mediterranean Atriplex species has been the most successful shrub species in terms of establishment and productivity. Research conducted in north-east Morocco showed an average production of 920 Kg DM/ha (1,000 plants/ha density), with variations from 406 to 2,140 Kg DM/ha depending on the species studied. A. vesicaria, A. semibaccata, A. nummularia and A. paludosa scored the high levels of production. Other reports indicate that forage production from a 2-year old Atriplex plantation was 5 t under 150 and 200 mm rainfall.

The pastoral value of fodder shrubs depends not only upon their biomass but also upon their nutritive value and their palatability and digestibility (Salem et al. 2012). The low concentrations of metabolizable energy and high concentrations of soluble salt in herbage of Atriplex species as well as the presence of anti-nutritional compounds, including tannins, flavonoids, oxalate tends to reduce fodder palatability and feed intake of sheep and goats. Several methods have been devised to lessen adverse effects of phenols and to alleviate deleterious effects of sodium chloride found in tree and shrub fodder foliages. These include treatment with alkalis such as urea, ammonia and calcium hydroxide and oxidizing agents such as potassium dichromate. Addition of exogenous enzymes is also a method to improve the nutritive value of tree leaves. The study of Salem et al. (2012) showed that there are beneficial impacts of sun-drying and/or dietary exogenous enzyme addition for sheep fed A. halimus.

Despite the limited nutritional values, the use of Atriplex species as an important component of the diet should be considered in arid and semi-arid regions since this plant produces 2.5–20 t of dry matter per hectare per year and it is available most of the year. Given the low nutritional value of the Atriplex species, various authors have proposed supplementing them with other types of feed like barley grain, barly straw or spineless cactus so that the animals could obtain the energy from the hay and grain, and the protein and minerals from Atriplex species. Abu-Zanat (2005) reported that grazing a combination of salt-tolerant grasses, legumes and Atriplex species improved feeding value and maximize animal production (feed intake and growth rates) from saline land. Norman et al. (2008) reported that 170 g/day of grain was the minimum required to ensure sheep fed A. nummularia and A. amnicola maintained live weight. In the study of Ben Salem et al. (2005) lambs fed A. nummularia and supplemented with barley achieved a growth rate of 67 g/day over an 85-day period. Data presented by van der Baan et al. (2004) clearly demonstrate that supplementation with grains such as barley or maize significantly increases the digestibility of A. nummularia and this leads to an increase in growth rate of ruminant. Mixing alfalfa with Atriplex as green fodders to sheep may increase the palatability and consequently intake and utilization of Atriplex which lead to improvement of the performance of animal. Abu-Zanat (2005) reported that it is possible to replace up to 50 % of alfalfa hay by A. nummularia without negative effects on intake and digestibility of dry matter by Awassi lambs. Jacobs and Smith (1977) reported significant differences in chemical composition between (Atriplex nummularia, A. Canescens, A. Brewerii and A. Lentiformis) species and between seasons. Kandil and El-Shaer (1989) reported that Atriplex nummularia had higher nutritive value in spring and winter than in summer and autumn. Riasi et al. (2008) reported that A. dimorphostegia have more number of beneficial chemical nutritive components and digestible values than Suaeda arcuata as forage for ruminants. Farmers in some arid areas of the world have already begun to cultivate Atriplex as a salt-tolerant forage crop on lands where other crops are difficult to grow.

Characterization of Bioactive Compounds from Some Atriplex Species

Several salt marsh plants have traditionally been used for medical, nutritional, and even artisanal purposes. Currently, an increasing interest is granted to these species because of their high content in bioactive compounds (Ksouri et al. 2011). Various Atriplex species have medicinal values, e.g. A. semibaccata has been used as an antifungal agent and A. vestita in the traitement of bronchitis. A. hortensis has been regarded as a source of Vit A (Siddiqui et al. 1994). The arial parts of A. hortensis were used in flak medicine against diseases of respiratory tract, digestive and urinary systems, and due to their analgestic properties, in rheumatism (Bylka et al. 2001). A. halimus is effective in the treatment of type II diabetic patients (Bakhtiuary 2011) and it used in veterinary medicine to combat internal parasites. Extracts of A.confertifolia has significant bioactivity against human breast cancer cell lines; the bioactivity of A. confertifolia extract on these cells was compared to a FDA-approved cancer drug (Onxol®) and an industry-standard leukocyte control cell line. A dose-response curve of the extracts displayed significant cell death similar to Onxol® (Capua et al. 2010). Boughalleb et al. (2009) reported that many Atriplex species (A. inflate and A. portulacoides) may contain phytochemical compounds with fungicide properties.

Several studies attributed the anti-carcinogenic, anti-inflamatory, antifongique and antioxydants activities potential of plant extracts to their bioactive compounds compositions (Ksouri et al. 2011). Chemical investigation of the species of the genus Atriplex (Table 16.2) showed the presence of saponin glycosides, alkaloids , ascorbic acid and phytoecdysteroids (Keckeis et al. 2000). Benhammou et al. (2009) reported that A. halimus leaves and stems were characterized by the presence of the flavonoids, the tannins, the alkaloids and the sponins where the leaves exhibited the higher yields. Bylka et al. (2001) isolated from leaves of A. hortensis tow relatively rare sulphated flavonoids: kaempferol 3-O-sulphate-7-O-arabinopyranoside and quercetin 3-O-sulphate-7-O-arabinopyranoside (belong the group of compounds easily soluble in water) and a new acetylated flavonol glycoside from A. littoralis (Bylka 2004). Other earlier works suggested the presence the naringin, naringenin 7-O-glucoside, isorhamnetin 3-O-rhamnosyl (1–6) glucopyranoside and isorhamnetin 7-O-glucopyranoside in A. farinose (Al-Jaber et al. 1992). More recently, two new flavonoids, quercetin 6,4′-dimethoxy-3-fructo-rhamnoside and quercetin 4′-methoxy-3-fructo-rhamnoside in addition to another five known compounds were isolated from A. lentiformis (Torr.) S. Wats (Awaad et al. 2012). All of the extracts and the isolated compounds were tested for their antioxidant activity, the two new compounds were found to have the highest antioxidant activity with no side effect.

Table 16.2 Some Atriplex species and their isolated compounds
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Isolation and Characterization of Genes from Atriplex Species

Atriplex species are among the most salt tolerant higher plants. And the elucidation of its salt tolerance mechanisms is of significance for generating salt tolerant crops via selective breeding or genetic engineering . Studying the regulation of stress inducible genes can lead to understanding of the mechanisms by which halophytes maintain growth and thrive under abiotic stress . Several stress related genes have been isolated from these halophytes (Table 16.3). Both glycophytes and halophytes cannot tolerate large amounts of salt in the cytoplasm. Plants maintain a low concentration of Na+ in the cytosol by active exclusion of Na+ ions into the apoplast and vacuole by using specific plasma membrane and tonoplast Na+/H+ antiporter (NHX1) (Shi and Zhu 2002). In particular, the vacuolar Na+/H+ antiporter had been demonstrated to play a key role in salt tolerance of plants (Blumwald et al. 2000). The greater salt tolerance in Atriplex species is related to the transport to shoots of high quantities of Na+ concomitant to an efficient vacuolar compartmentation of this ion, which prevents the ionic damage to the cytoplasm. The vacuolar Na+/H+ antiporter genes has been characterized and identified from A. gmelili (AgNHX1) (Hamada et al. 2001). The analysis and comparison of these genes showed that they were highly homologous with similar structural and conserved domains. Ohta et al. (2002) demonstrated that transgenic rice plants overexpressing AgNHX1 gene could survive after short period of high concentration salt exposure (300 mM NaCl for 3 days). Evacuation of Na+ from the cytoplasm is energy-dependent. A partial sequence of an isoform of the plasma membrane PM-H+-ATPase was been isolated from A. nummularia. Increased H+-ATPase mRNA abundance was reported in A. nummularia when NaCl adapted (342 mM NaCl) cells were re-exposed to NaCl after having been grown in media without additional NaCl (Niu et al. 1993). Which provide evidence that enhanced H+-transport activity by NaCl in A. nummularia is mediated at least in part by transcriptional or post-transcriptional processes that result in higher mRNA accumulation.

Table 16.3 Genes isolated from some Atriplex species
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Exposure to saline and drought stress results in the accumulation in the cytosol of low-molecular mass compounds, termed as compatible solutes, which do not interfere with normal biochemical reactions. It has been frequently reported that GB acts as the main stress-induced agent involved in the osmotic adjustment and protection of cellular structure in plant species belonging to the Chenopodiaceae (Rhodes and Hanson 1993). GB facilitates osmotic adjustment by lowering the internal osmotic potential that contributes to the water stress tolerance ability. In addition, it stabilizes both PSII complex and RuBisCO during photosynthesis under stress conditions (Sakamoto and Murata 2000). Yang et al. (2007 ) reported that genetically engineered tobacco with the ability to accumulate GB showed a higher content of ascorbate and reduced glutathione as well as an increase in the activity of superoxide dismutase (SOD). The positive effect of exogenous glycine betaine application in plant growing under salinity stress has been proven. Plant cell could be protected from the adverse effect of salinity induced oxidative stress by the exogenous application of glycine betaine (Demiral and Türkan 2004). In A. nummularia GB play a major role in cytosol osmotic adjustment in both leaves and roots, regardless of NaCl presence (Silveira et al. 2009). In higher plants, the first and second steps in the biosynthesis of GB are catalyzed by a rate-limiting enzyme choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively (Sakamoto and Murata 2000). CMO gene from A. hortensis (AhCMO) has been isolated and used for GB production in tobacco (Shen et al. 2002 ) and cotton plants (Zhang et al. 2009 ) to improve its abiotic stress tolerance. CMO homologs have been also identified in A. prostrate (Wang and Showalter 2004) and A. nummularia (Tabuchi et al. 2005). The gene encoding the second enzyme, BADH, has been cloned from A. hortensis (AhBADH) and introduced into rice, wheat, and turf grass (Xiao et al. 1995; Guo et al. 1997, 2000) and improvement of salt tolerance in transgenic plants was observed during growth. Similar result was also achieved in transgenic trifoliate orange (Fu et al. 2011). The enhanced salt tolerance was correlated, at least in part, with reduced lipid peroxidation , greater abundance in photosynthetic proteins, stimulation of K+ uptake , and low Na+/K+ ratios. The BADH gene that originated from A. hortensis was also transformed into the most important forage crop alfalfa with Agrobacterium-mediated transformation method. The transgenic plants grew vigorous in salt stress condition, whereas the wild type plants was retarded and did not survive. The expression of BADH gene in alfalfa genome enhanced its salt tolerance through improved membrane protection as measured by relative electrical conductivity and malondialdehyde (MDA) content, scavenge of free radicals by increase of peroxidase (POD) and SOD activities, and the osmotic adjustment (Liu et al. 2011).

Transcription factor genes play important roles in stress survival by serving as master regulators of sets of downstream stress-responsive genes via binding to specific elements (cis-elements) in target genes. Functional analysis of the promoter regions of some of stress-inducible genes has led to identification of the cis-element DRE (Dehydration-responsive element), which is responsible for dehydration-inducible transcription (Yamaguchi-Shinozaki and Shinozaki 1994). Full-length DRE-binding transcription factor (AhDREB1) gene has been isolated from A. hortensis (Shen et al. 2003 ). In transgenic tobacco, AhDREB1 led to the accumulation of its putative downstream genes and these transgenic lines showed an increased stress tolerance, suggesting that the AhDREB1 protein functions as a DRE-binding transcription factor and play roles in the stress tolerant response of A. hortensis. DREB in A. halimus (AsDBRE) is regulated by the osmotic component but not by the ionic one of salt stress (Khedr et al. 2011). It seemed that DREB was not involved in the regulation of sodium manipulating genes like NHX1, SOS1 or H +-PPase. Moreover, DREB could be involved directly of indirectly in CMO regulation because of timing of induction. Also, DREB was the most up-regulated gene under salt (fivefold) and drought (twofold) conditions, which reinforced the importance of this gene in A. halimus tolerance to stress. Moreover, its constitutive expression under normal conditions also indicated its involvement in other growth and developmental programs (Khedr et al. 2011).

Microsatellites are widely used in population genetic studies and may prove to be useful in studies of closely related species to infer relationships when sequence variation is very low or there are few or no genome resources available. Ortíz-Dorda et al. 2005 has evaluated the genetic structure of 51 populations of A. halimus from the Mediterranean Basin using RAPD (random amplified polymorphic DNA)-PCR technique. The authors found that there are a clear intrapopulational diversity of A. halimus. Such heterogeneity could be exploited to select clones or develop synthetic populations with a combination of good traits such as high palatability, high edible biomass production, and good adaptability to environmental limiting factors in semi-arid Mediterranean environments. 12 polymorphic loci were isolated in A. nummularia (Byrne et al. 2008) which will be useful to describe levels of genetic variability across the range of the species and in a breeding programme.

Xu et al. (2011) investigate the physiological responses and differential gene expression caused by salinity exposure in A. centralasiatica plants grown from two different seed morphs. A. centralasiatica widely distributed in China produce tow type of seeds, yellow and brown seeds. Seedlings derived from yellow seeds showed a greater salt tolerance than those derived from brown seeds. By using suppression subtractive hybridization (SSH) and subsequent microarray and RT-PCR analysis to isolate and compare genes that were differentially expressed, the authors suggest a major contribution of gene regulation to the salt resistant phenotype of seedlings derived from yellow seeds. These genes encoded proteins related to osmotic and ionic homeostasis, redox equilibrium and signal transduction . This study clearly links physiological responses with differential gene expression in seedlings derived from dimorphic seeds. Such dimorphism offers the advantage to halophytic plants to survive in highly variable environments.

Conclusion and Future Perspective

The global distribution of the genus Atriplex in arid and semi-arid areas and its abiotic stress tolerance combined with the utility of Atriplex species for restoration, remediation and forage for livestock have helped these plants to rank among the most widely studied native halophytes species. The major limitation of use Atriplex in livestock production is its high salt concentration. Atriplex species are best considered as a supplement rather forage. The plant used for these purposes are primarily wild type and there are little information available on the nutritive value of Atriplex species growing in greenhouse and irrigated with different concentration of NaCl. Strategies need to be devised to minimize the salt contents in the Atriplex leaves.

Although the biochemistry of Atriplex species tends to establish that they may be a source of novel compounds along with providing a new source for many already know biologically active compounds. Data of chemical composition of Atriplex species is still not completed.

Atriplex species are well adapted to both salt and drought stress and can serve as model species to understand the mechanisms of tolerance in plant (Flowers and Colmer 2008). Very little research has been carried out to identify the molecular mechanisms directly responsible for the specific tolerance of Atriplex species to abiotic stress . In this way, Atriplex may serve as a particularly useful model plant for studies of regulatory mechanisms related to the activation of the GB biosynthetic pathway in response to environmental stress.

With over 400 species of this genus, a significant opportunity then exists to explore the potential of other locally adapted Atriplex species. Research is required to select and breed potentially useful plants and identify the best species combining nutritional, agronomic and environmental potential.