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  Section: Molecular Biology of Plant Pathways » Genetic Engineering for Salinity Stress Tolerance
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The Context of Salinity Stress

The considerable increase in worldwide crop production that occurred during the green revolution did not result in substantially greater land use but focused on adapting germplasms to respond to altered farm management practices (Trewavas, 2001). However, even with better adjustment of crops and increased production efficiency, the actual yield is less than the crop genetic potential. By now, the increased population in developing countries puts even more constraints on production as urban populations compete with agriculture for fresh water. In parts of the world, this has necessitated the use of less suitable irrigation water, often water of low quality that is unsuitable for high-yield agriculture with existing crops. This in turn makes it paramount to find ways that realize the genetic potential and yield capacity of crop genomes, even under moderate stresses, or to enhance them by transgenic means.

Significant among the abiotic stresses is salinity, which not only constraints crop production in a particular growing cycle but also leads to steady deterioration of soils and irrigation water that compounds the effects of salinity on subsequent crop generations. In many countries around the globe where water is already scarce and droughts are recurring, soil salinity is a major constraint to crop productivity that negatively affects much of the cultivated land and substantially reduces yield (Flowers and Yeo, 1995; Läuchli and Epstein, 1990; Maas, 1990; Munns, 1993). These facts, and the prospect of erratic rainfall patterns that could increase in the future, have led to efforts to improve yield stability of present-day crops by focusing on abiotic stress factors (Flowers, 2004; Flowers and Yeo, 1995; Serrano, 1996). Crop improvement strategies seek to develop more saltadapted or -adaptable germplasms by utilizing molecular genetic approaches and resources developed during the mid-1980s, notably marker-assisted breeding techniques, exploration of halophytic species, biotechnology, and genomics (Apse and Blumwald, 2002; Garciadeblas et al., 2003; Hasegawa et al., 2000b;Koyama et al., 2001; Loudet et al., 2003; Ribaut and Hoisington, 1998; Tuberosa et al., 2002; Zhu, 2002). Table 12.1 lists strategies that have been suggested, initially with respect to different classical breeding approaches, and more recently, including genome-anchored methods that can significantly enhance breeding because physical maps, mapped and phenotyped mutants, and genome sequences provide precision.

High salt in the root environment, salinity that typically appears in the form of increased NaCl in the soil, is a combination of ionic and hyperosmotic imbalance and secondary effects including pathologies that inhibit growth and can affect development or cause cell death (Hasegawa et al., 2000a; Zhu, 2001, 2002). Ionic and osmotic stress signals are sensed and decoded by all plants via distinct and interconnecting signal pathways that are response relays for the control of unique and stress-specific programs. These pathways are response relays that control genetic programs and coordinate determinants and processes required for adaptation. Both stress conditions constitute environmental perturbations that modulate normal cellular or developmental programs (Zhu, 2002, 2003). Consequently, salt adaptation involves determinants that establish ion homeostasis and/or osmolyte biosynthesis, termed osmotic adjustment. In cases where the severity of a stress condition exceeds the capacity of a species, ecotype or line to acclimate, this then precludes cell division, expansion and normal development and may result in death (Apse and Blumwald, 2002; Blumwald, 2000; Hasegawa et al., 2000b; Zhu, 2001, 2002). The capacity of species to adapt to salt stress distinguishes glycophytic species with a reduced capacity from halophytes that are to various degrees able to adapt well, or even grow better at slightly increased levels of sodium, and many of the latter may in fact use NaCl as a ‘‘cheap’’ osmoticum (Adams et al., 1998).

Evolutionary adaptations have resulted in species that exhibit different competence to tolerate or resist high salt and complete their life cycles. Although glycophytes and halophytes differ substantially in their capacity to tolerate salt, the cytosolic and organellar machineries of the two plant categories seem to be equally sensitive to Na+ and Cl- (Flowers, 2004; Greenway and Osmond, 1972; Hasegawa et al., 2000b; Jacoby, 1999; Serrano, 1996). Consequently, adaptation by plants in both groups requires cellular responses that attenuate the osmotic and ionic components of salt stress. The options are limited. They involve NaCl exclusion or compartmentalizing Na+ and Cl - into an ‘‘inert’’ compartment, vacuole, or tissue. Other response mechanisms, such as avoidance reactions, in essence induced dormancy, are not considered here. Simultaneously, mechanisms that confine salt must be accompanied by the accumulation of solutes that are compatible with cellular metabolism. Such osmolytes must increase in cytosol and organelles to achieve osmotic adjustment (Blumwald, 2000; Hasegawa et al., 2000a,b; Zhu, 2002, 2003). Both osmotic adjustment and the confinement of sodium in (pre)vacuoles have been shown in the single cell model, Saccharomyces cerevisiae (Gaxiola et al., 1999; Hohmann, 2002). That is, the mechanisms by which all plants achieve osmotic and ionic equilibria are mediated by orthologous mechanisms based on conserved biochemical and/or physiological functions that are inherently necessary for essential plant processes (Hasegawa et al., 2000a; Serrano et al., 1999; Van Camp, 2005; Van Camp et al., 1996; Zhu, 2000, 2001). This statement has been substantiated by the genomic DNA sequences of two glycophytes, Arabidopsis thaliana and Oryza sativa, which seem to include all components that have been researched as essential or necessary for plants to cope with salt stress in different model species and crops (Arabidopsis Genome Initiative, 2000; Goff et al., 2002; Yu et al., 2002).

What then, if the important stress tolerance components are ubiquitous, distinguishes glycophytes and halophytes? To solve this conundrum, research is directed into several areas. One is to determine if halophytic versions of salt adaptation determinants have greater innate operational capacity to facilitate survival, growth, and development in saline environments, that is, if halophytic versions of genes may represent an allele that encodes a more effective protein that functions in the presence of high salt (Waditee et al., 2002). An example supporting such a view may be the case of L-myo-inositol-1-phosphate synthase that distinguishes rice (O. sativa) from a wild relative (Porteresia coarctata). In Porteresia, the homodimeric enzyme retains its aggregation state in high salt, while the rice protein disintegrates into enzymatically inactive monomers at much lower salt concentrations. This may be due to a domain that discriminate the two forms of the enzyme, and, indeed, overexpression of the Porteresia enzyme enhances salt tolerance (Majee et al., 2004).

Alternatively, halophytes may control universal determinants in a manner that imparts to the species a preadapted state or a faster and superior ‘‘adaptive response capacity’’ when the saline environment becomes increasingly severe. A point in favor of such a scenario may be studies targeting the Arabidopsis relative Thellungiella halophila (salt cress), which is salt tolerant. Preliminary transcript profiling and analysis of expressed sequence tags (ESTs) seems to indicate that the salt cress constitutively shows high nonstress activities for a range of genes/ transcripts, and that induction of these transcripts is initiated at a higher stress level than in Arabidopsis (Inan et al., 2004; Taji et al., 2004). The fact that (eu) halophytes show increased growth at moderate concentrations of NaCl, higher than in fresh water, might be causally related to the high constitutive expression of stress response pathways.

Third, outlining a related hypothesis, it must also be considered that some halophytes have evolved specialized adaptations (e.g., salt glands for excretion or bladder cells for the storage of NaCl). It is therefore possible that such species also possess other unique determinants with specialized function to mediate adaptation that are missing from the genomes of glycophytes. Such uniqueness will only be revealed when we have identified the relevant genes in halophytic models. A variation of this theme is chromosomal context and genome size. The fusion of genomes during speciation or endoreduplication events that have been documented for many species in diverse plant families may have resulted in duplicated genes, paralogues of ubiquitous genes that further evolved in some species in response to changing environments (Arango et al., 2003). Such genes could, although they might not be superior in their biochemical function to those in glycophytes, impart higher tolerance to a species by their expression at constitutively higher levels, in a stress-inducible manner, in different compartments, or by being connected to altered or novel regulatory circuits. We will briefly review salt stress tolerance mechanisms and transgenic approaches that have begun to engineer ionic and osmotic tolerance mechanisms into model species. Subsequently, we will place emphasis on the regulatory circuits that control mechanisms of tolerance acquisition (Schachtman, 2000; Zhu, 2002).

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