Genes for Ionic Balance

In most saline soils, Na+ and Cl' are the predominant ions in the soil solution. At sufficiently high concentrations, both ions contribute to an unfavorable osmotic gradient between the soil solution and the plant roots. Both ions also cause ion- specific toxicity when accumulated in salt sensitive plants. And while it is clear that the exclusion of Na+ or Cl', or both, is correlated with improved salinity tolerance in some species (and the accumulation of both with others), the state knowledge of Na+ transport mechanisms is more advanced than that for Cl' transport (Teakle and Tyennan, 2010). Decreasing Na+ uptake in both glycophytes and halophytes, the net uptake of sodium into the roots is the sum of sodium influx and efflux. The negative electrical membrane potential difference at the plasma membrane of root cells (40 mV) favors the passive transport of sodium into root cells, and especially so when sodium concentrations increase in the soil solution.

The entry of sodium into root cells is mediated by uniporter or ion channel- type transporters, like HKT, LCT1 and NSCC (reviewed in Plett and Moller, 2010). The reduction of Na+ uptake might be accomplished by decreasing the number or activity of these transporters in the roots. Reduction of TaHKT2;l expression in wheat by antisense suppression resulted in lower net sodium uptake of transgenic roots and higher fresh weight of plants grown under salinity stress in controlled growth conditions (Laurie et al., 2002). Similarly, Arabidopsis T-DNA knockout mutants of AtCNGC3, a cyclic nucleotide gated channel which catalyses Na+uptake, had lower net influx of Na+ and were more tolerant to salinity at germination (Gobert et al, 2006). The efflux of sodium from the roots is an active process, which is presumed to be mediated by plasma membrane Na+/H+ antiporters. These secondary transporters use the energy of the proton gradient across the plasma membrane to drive the active efflux of sodium from the cytosol to the apoplast. The NaVH- antiporter, SOS1 (identified in a mutant screen as salt overly sensitive 1), is the only Na+/ efflux protein at the plasma membrane of plants characterized so far. The over expression of AtSOSl, a plasma membrane bound NaVLT antiporter, improved the ability of the Arabidopsis transgenic plants to glow in the presence of high NaCl concentrations (Shi et al., 2003). The rice orthologue, OsSOSl, is able to complement the Arabidopsis sosl mutant (Martinez- Atienza et al., 2007). The SOD2 (Sodium2) gene was identified in yeast, Schizosaccharomyces pombe, as a Na+/H+ antiporter on the plasma membrane involved in salt tolerance. Trans formation of rice with the SOD2 gene (under 35S promoter) resulted in accumulation of more K+, Ca2+, Mg2+and less Na+ in the shoots compared with wild type (Zhao et al, 2006b). The transgenic rice plants were able to maintain higher photosynthesis level and root proton exportation capacity, whereas reduced ROS generation. Although yield data not reported, the trials were conducted outdoors, which is the closest to field level study of a crop plant for this approach in the literature.

Decreasing Root to Shoot Translocation of Na+

The accumulation of sodium in shoots occurs via the translocation of sodium from the roots along the transpirational stream. The removal of sodium from the xylem, which reduces the rate of sodium tr ansfer to the shoot tissue, has been shown to be mediated by members of the HKT gene family (reviewed in Plett and Moller, 2010). AtHKTfl in Arabidopsis, OsHKTl;5 in rice, and HKT1;4 in wheat are all critical in reducing Na+ shoot concentrations by transporting Na|j from the xylem into the root stele (reviewed in Hauser and Horie, 2010). One strategy for improving salinity tolerance is to increase the expression of such genes to further reduce sodium concentrations in the xylem (Plett et al, 2010). The over expression of AtHKTl;l under the control of the constitutive promoter CaMV35S leads to increased salt sensitivity, presumably because NaJ) fluxes are increased in inappropriate cells and tissues (Moller et al, 2009). However, when expressed under the control of a promoter directing expression in root epidermal and cortical cells, both in rice and in Arabidopsis, HKT1;1 over expression causes an increase in root cortical sodium, a decrease in shoot sodium and a higher accumulation of fresh weight during the course of the experiment (Plett et al, 2010).

Sequestering Na+ The accumulation of Na+ ions into vacuoles through the operation of a vacuolar Na+- antiporter provided an efficient strategy to avert the deleterious effect of Na+in the cytosol and maintain osmotic balance by using Na+ (and Cl') accumulated in the vacuole to drive water into the cells (Apse et al, 1999; Apse and Blumwald, 2002). Transgenic plants over expressing an Aabidopsis vacuolar Na+ /H+ antiporter, AtNHXl, exhibited improved salt tolerance in Brassica napus (Zhang et al, 2001), tomato (Zhang and Blumwald, 2001), cotton (He et al, 2005), wheat (Xue et al, 2004), beet (Yang et al, 2005) and tall fescue (Zhao et al, 2007). The transformation of an orthologue gene (AgNHXl) from halophytic plant Atriplex gmelini into rice improved salt tolerance of the transgenic rice (Ohta et al, 2002). Maize plants over expressing rice OsNHXl gene accumulated more biomass, under 200 inM NaCl in greenhouse (Chen et al, 2007). Moreover, under field trail conditions, the transgenic maize plants produced higher grain yields than the wild-type plants. Transformation of another Na+ /H~antiporter family member, AtNHX3 (from Aabidopsis), in sugar beet (Beta vulgaris L.) resulted in increased salt accumulation in leaves, but not in the storage roots, with enhanced constituent soluble sugar contents under salt stress condition (Liu et al, 2008). The introduction of genes associated with the maintenance of ion homeostasis in halotolerant plant into crop plants confirmed salinity tolerance. The yeast gene HAL1 was introduced into tomato (Gisbert et al, 2000), watermelon (Citrullus lanatus (Thunb.); Ellul et al, 2003) and melon (Cucumis melo L.; Bordas et al., 1997), which confirmed higher level of salt tolerance, with higher cellular K+ to Na+ ratio under salt stress. Likewise, the introduction of the yeast HAL2 gene into tomato resulted in improved root growth under NaCl conditions, contributing to improved salt tolerance (Arrillaga et al., 1998). Over expression of HAL3 {S. cerevisiae) homologue NtHAL3 in tobacco increased proline biosynthesis and the enhancement of salt and osmotic tolerance in cultured tobacco cells (Yonamine et al., 2004). The electrochemical gradient of protons across the vacuolar membrane is generated by the activity of the vacuolar H+ translocating enzymes, H+ATPase and H+-pyrophosphatase. Increasing vacuolar H+ pumping might be required to provide the additional driving force for vacuolar accumulation via sodium/proton antiporters. A gene coding for a vacuolar ЕГ-pyrophosphatase proton pump (AVP1) from Arabidopsis was over expressed in tomato (Park et al, 2005), cotton (Pasapula et al, 2011) and rice (Zhao et al, 2006a) and induced improved growth during drought and salt stress. Interestingly, the over expressed AVP1 resulted in a more robust root system which could possibly improve the plants ability to absorb more water from the soil (Pasapula et al, 2011).

 
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