Osmotic Adjustment

Osmotic adjustment in response to abiotic stresses is an adaptive mechanism in the halophytes in order to maintain their water balance (Flowers and Colmer 2008). Besides the accumulation of inorganic ions and its sequestration in the vacuole, the osmotic balance between vacuole and cytoplasm is also maintained through the synthesis of organic solutes to retain the stability of the proteins in cells in response to drop in the water potential of the environment (Glenn and Brown 1999). Plant cells synthesize a variety of organic solutes such as proline, sucrose, polyols, trehalose and quaternary ammonium compounds (QACs) such as glycine betaine, alaninebetaine, prolinebetaine, choline-O-sulfate, hydroxyprolinebetaine, and pipecolatebetaine (Rhodes and Hanson 1993). These are low molecular weight, highly soluble compounds and are nontoxic even at high cellular concentrations (Ashraf and Foolad 2007) without disturbing intracellular biochemistry and cellular functions (Cushman 2001), protects subcellular structures, mitigate oxidative damage caused by free radicals (Attipali et al. 2004), maintains the enzyme activities under salt stress and protection of cellular components from dehydration injury (Ashraf and Foolad 2007). The osmolytes accumulation is frequently reported in glycophytes and halophytes being continuously exposed to abiotic stresses; however, synthesis of these osmo-lytes is an energy-dependent process which consumes large number of ATP molecules (Raven 1985), thus affecting the growth. Osmolytes synthesis and their overproduction in transgenic plants has been achieved in transgenic crop plants, however little success has been achieved on the desired protective levels of these osmolytes in plants. In contrast, some plants showed increased tolerance to abiotic stresses after exogenous application of these organic solutes (Ashraf and Foolad 2007). Although increased accumulation of these osmolytes by the plants exposed to abiotic stresses has been reported, not all plant species synthesize the all kinds of osmolytes at a time; some plant species synthesize and accumulate very low quantity of these compounds while some plant species not do so (Ashraf and Foolad 2007).

4.3.1 Proline

Similar to glycophytes, proline accumulation is a common adaptive response to various abiotic stresses. Several studies using transgenic plants or mutants demonstrated that proline metabolism has a complex effect on development and stress responses, and that proline accumulation is important for the tolerance to certain adverse environmental conditions (Hong et al. 2000; Miller et al. 2010). In plants, proline is mainly synthesized from glutamate using two important enzymes such as pyrroline-5-carboxylate synthetase (P5CS) and pyrroline-5-carboxylate reductase (P5CR). Proline is synthesized in cytoplasm; however in mitochondria, the catabolism occurs via sequential action of proline dehydro-genase (PDH) producing pyrroline-5-carboxylate (P5C) and its conversion to glutamate using P5C dehydrogenase (P5CDH) (Szabados and Savoure 2009). Halophytes have shown vast diversity for the accumulation of proline in response to abiotic stresses, wherein plants from the Aizoaceae family accumulate large quantities of proline showing its role in osmoprotection (Delauney and Verma 1993). Proline concentrated in the cytosol, chloroplast and vacuoles and compatible with enzyme activity in the cytoplasm showed its significant contribution to osmotic adjustment. Besides being an osmoprotectant, proline also has a role in detoxification of reactive oxygen species and act as an antioxidant, stabilization of proteins and protein complexes and as a signaling/regulatory molecule (Szabados and Savoure 2009) . It also function as a protein-compatible hydrotrope (Srinivas and Balasubramanian 1995), alleviating cytoplasmic acidosis, and maintaining appropriate NADP+/NADPH ratios compatible with metabolism (Hare and Cress 1997). Also, rapid breakdown of proline upon relief of stress provides sufficient reducing agents that support mitochondrial oxidative phosphorylation and generation of ATP for recovery from stress and repairing of stress-induced damages (Hare and Cress 1997). In halophytic plant species in response to abiotic stresses, proline accumulation in the cytosol has been shown to contribute substantially to cytoplasmic osmotic adjustment. For example, in cells of Distichlis spicata treated with 200 mM NaCl, the cytosolic proline concentration was estimated to be more than 230 mM (Ketchum et al. 1991). In Sesuviumportulacas-trum, Lokhande et al. (2010a, b, 2011a) found an extensive increase in proline content when the callus and axillary shoot cultures exposed to salt and drought stress alone or under iso-osmotic stress conditions of NaCl and PEG. Higher proline accumulation has also been shown in S. por-tulacastrum plants exposed to various abiotic constraints that include salinity, drought, and heavy metals (Messedi et al. 2004; Slama et al. 2008; Ghnaya et al. 2007; Moseki and Buru 2010; Lokhande et al. 2011b). Such an osmotic adjustment through proline accumulation is also evident in other species like Plantago crassiflora, Salicornia europaea, Atriplex halimus, A. halimus subsp. schweinfurthii, Avicennia marina, Hordeum maritimum, Ipomoea pes-caprae, Paspalum vaginatum, Phragmites australis, and Suaeda sps.

(Vicente et al. 2004; Reda et al. 2004; Nedjimi and Daoud 2009; Pagter et al. 2009; Lefevre et al. 2009; Sucre and Suarez 2010). Among different halophytic plants, S. portulacastrum has been reported as a high proline accumulator, with levels reaching 300 mmol g-1 leaf dry matter (Slama et al. 2008). Such a pronounced accumulation of proline and its physiological role in osmotic adjustment may have made the halophytes more successful to grow under adverse environmental stresses.

4.3.2 Glycine Betaine

Among the variety of quaternary ammonium compounds, glycine betaine (GB) is one of the most abundantly occurring and synthesized at higher concentrations in the plants exposing to dehydration stress due to adverse environmental calamities. GB is located in chloroplast where it plays an important role in osmotic adjustment and protection of thylakoid membrane, by maintaining the photosynthetic machinery in active state (Robinson and Jones 1986). GB is synthesized mainly from choline, which is converted to betaine aldehyde and then to GB through sequential enzymatic action of choline monooxyege-nase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively. Although other pathways such as direct N-methylation of glycine are also known, the pathway from choline to GB has been identified in all GB-accumulating plant species (Ashraf and Foolad 2007). It is widely believed that synthesis and accumulation of GB protects cytoplasm from ion toxicity, dehydration and temperature stress and helps normal functioning of the metabolic machineries in the cell during stressed conditions by stabilizing macromolecule structures, protecting chloro-plast and photosynthesis system II (PSII) by stabilizing the association of the extrinsic PSII complex proteins and indirectly interacting with phosphatidylcholine moieties of membranes to alter their thermodynamic properties (Subbarao et al. 2001). It has been shown that tolerant species are more amenable to accumulate higher GB in comparison to sensitive species as a response to abiotic stress imposition. Based on the GB and proline accumulation potential,

Tipirdamaz et al. (2006) categorized the halophytes from inland and salt marsh habitats of Turkey. The studies have shown that the species that behaved as GB accumulators appeared poor proline accumulators and vice versa. The GB accumulation reported in the halophytes is generally in the range of 1.5-400 mmol g-1 DW and some of the highest GB accumulating halo-phytes are members of the Chenopodiaceae (Halocnemum strobilaceum, Petrosimonia bra-chiata, Suaeda confuse), Compositae (Artemisia santonicum), and Frankeniaceae (Frankenia hir-suta( . Increased accumulation of GB has also been demonstrated in other halophytes such as Beta vulgaris (Subbarao et al. 2001), Spartina anglica (Mulholland and Otte 2002), Atriplex halimus (Martinez et al. 2005), A. Nummularia (Silveira et al. 2009), and S. portulacastrum (Lokhande et al. 2010a, b). Increased GB accumulation has also been correlated with increased betaine aldehyde dehydrogenase gene expression (BADHmRNA) in Salicornia europaea and Suaeda maritima leaves exposed to salt stress (Moghaieb et al. 2004). Considering the significance of GB in the osmotic balance of the halo-phytes under stressful environment, different methods can be derived to enhance the concentration of this compound in crop plants to increase their stress tolerance. The approaches can include breeding of sensitive cultivars with their tolerant relatives from halophytes with natural abilities to produce high levels of GB or genetically engineer the sensitive species through transformation of the genes responsible for GB synthesis. Although some progress has been made in introducing the genes for the production of these compounds in naturally accumulating or low-accumulating plant species, levels of these compounds' accumulation in transgenic plant have often been low or insufficient to the plant stress tolerance (Ashraf and Foolad 2007).

4.3.3 Soluble Sugars

(n general, modulations in the carbon metabolism and the levels of carbohydrates (sugars) are seen due to changes occurring in the process of photosynthesis and carbon partitioning of the plant at organ level and in whole plants exposing to abiotic stresses (Gonzalez et al. 2009). Soluble sugars function as metabolic resources and structural constituents of cells, besides acting as signals regulating various processes associated with plant growth and development. Such signaling can modulate stress pathways into a complex network to further orchestrate metabolic plant responses. A variety of sugar compounds such as sucrose, glucose, mannose, maltose, trehalose, and many other sugar alcohols have been studied in response to abiotic stresses (Briens and Larher 1982; Yuanyuan et al. 2009) and the accumulation of soluble sugars has been attributed as an important parameter of osmotic adjustment in the halophytes. Briens and Larher (1982) screened different organs of 16 halophyte species for soluble carbohydrates and other osmolytes and found that all the species accumulated sucrose, fructose and glucose whereas Plantago maritime, Juncus maritimus, Phrgamites communis and Scripus maritimus showed the highest accumulation of soluble sugars. The presence of higher amounts of soluble sugars has been reported as main contributors to osmotic adjustment in the Atriplex halimus plants exposed to PEG and NaCl stresses and it is correlated with the response of NaCl stress on soluble sugar synthesis (Martinez et al. 2005). The accumulation of total soluble sugars has also been correlated with the variations at genotypic level among two genotypes of Cakile maritime namely Jebra and Tabarka which showed differences in the total soluble carbohydrate concentrations. While the content of the sugars was unaffected in the leaves of Jerba plants at moderate salinity, the plants of the saltsensitive Tabarka showed a slight increase in soluble carbohydrate contents during leaf development. The contribution of this compatible solute group to the "osmotic pool" was found higher in the salt-tolerant Jerba than in the salt-sensitive Tabarka seedlings exposed to 400 mM NaCl stress (Megdichi et al. 2007) . Further, Sesuvium portulacastrum axillary shoots exposed to salinity stress showed optimum growth at 200 mM NaCl in comparison to control and exhibited increased synthesis of total soluble sugars over proline and glycine betaine (Lokhande et al. 2010b). Salinity-induced soluble sugar accumulation has also been observed in P. euphratica (Zhang et al. 2004). Accumulation of soluble sugars has been observed in plants undergoing drought, flooding, and water logging conditions (Chai et al. 2001; Munns 2002; Li and Li 2005). Chenopodium quinoa exposed to water deficit and water-logging stresses showed no changes in starch, sucrose, or fructose content but showed increased glucose and total soluble sugar content in stressed plants in comparison to control (Gonzalez et al. 2009) ; These studies in halo-phytes demonstrate that soluble sugars play a significant role besides other osmolytes in the osmotic adjustment.

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