High concentration of NaCl is the dominant salt stress globally because of its abundance in the environment (Hasegawa et al., 2000b). Although both Na+ and Cl- can be cytotoxic, best understood are effects of Na þ toxicity and the mechanisms by which homeostasis is established (Blumwald et al., 2000; Hasegawa et al., 2000b; Niu et al., 1995; Zhu, 2003). The capacity to maintain a high cytoplasmic K+ to Na+ ratio is essential. Apparently, halophytes have an inherently greater capacity to maintain this balance in saline environments (Flowers and Yeo, 1992; Flowers et al., 1986). Ca2+ facilitates K+ relative to Na+ selective uptake through mechanisms that control the uptake of both ions (Epstein, 1961; La¨uchli, 1996; La¨uchli and Epstein, 1990).
Vacuolar compartmentalization of Na+ is a salt-adaptive mechanism used by all plants and is a conserved process in organisms as taxonomically distant as yeasts (Blumwald et al., 2000; Flowers and Yeo, 1995; Flowers et al., 1986; Glenn et al., 1999; Hasegawa et al., 2000b; Pardo and Quintero, 2002). This process not only mitigates against toxic accumulation of ions in the cytoplasm, but it also is physiologically crucial for osmotic adjustment in saline environments, which is necessary for cell volume regulation and development (Hasegawa et al., 2000b). Although increased Na+ compartmentalization in the vacuole enhances salt tolerance (Apse et al., 1999; Blumwald et al., 2000; Hasegawa et al., 2000b; Zhang and Blumwald, 2001; Zhang et al., 2001). Na+ sequestration in this compartment is critically dependent on the regulation of net uptake at the plasma membrane (Hasegawa et al., 2000b; Ros et al., 1998). Functional disruption of the plasma membrane Na þ efflux system in Arabidopsis or yeast results in toxic levels of Na+ to accumulate in the cytosol, even when the vacuolar compartmentalization machinery is functional (Garciadeblas et al., 1993; Munns, 2002; Zhu, 2000).
Once loaded into the root xylem from the soil solution, ions move to the shoot in the transpirational stream (Flowers and Yeo, 1992; La¨uchli, 1996; Maathuis and Sanders, 1996; Munns et al., 2002). Controlling Na+ content in the root xylem thus regulates ion content in the shoot and leaf apoplast to a level where intracellular compartmentalizing processes allow cells to be ion sinks (Hasegawa et al., 2000a,b; Maathuis and Sanders, 1996; Munns et al., 2002). Ion homeostasis in planta requires coordination of cellular processes with those that function in intercellular, tissue, and organ ion regulation (Flowers et al., 1986; Hasegawa et al., 2000b; Maathuis and Sanders, 1996; Munns et al., 2002). For example, vacuolar compartmentalization allows interconnected root cells, from the epidermis to the xylem parenchyma, to function as at least temporary Na+ and Cl- sinks that substantially reduce ion content in the transpiration stream moving into the shoots.
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