Intracellular Compartmentation of Ions in Salt Adapted Tobacco Cells

Abstract

Na+ and Cl-are the principal solutes utilized for osmotic adjustment in cells of Nicotiana tabacum L. var Wisconsin 38 (tobacco) adapted to NaCi, accumulating to levels of 472 and 386 millimolar, respectively, in cells adapted to 428 millimolar NaCI. X-ray microanalysis of unetched frozen-hydrated cells adapted to salt indicated that Na+ and Cl-were compartmentalized in the vacuole, at concentrations of 780 and 624 millimolar, respectively, while cytoplasmic concentrations of the ions were maintained at 96 millimolar. The morphometric differences which existed between unadapted and salt adapted cells, (cytoplasmic volume of 22 and 45% of the cell, respectively), facilitated containment of the excited volume of the x-ray signal in the cytoplasm of the adapted cells. Confirmation of ion compartmentation in salt adapted cells was obtained based on kinetic analyses of 22NaI and 36CI-efflux from cells in steady state. These data provide evidence that ion compartmentation is a component of salt adaptation of glycophyte ceUs.A typical response of many plants to saline environments, particularly halophytes, is to accumulate high intracellular concentrations of Na+ and Cl- (9,11,23,26,28). Since the in vitro activities of enzymes isolated from glycophytes or halophytes are inhibited equally by NaCl (9, 11), it has been generally accepted that the accumulated ions are sequestered in the vacuole and 'compatible solutes,' such as sugars, proline, and glycinebetaine, function to balance the osmotic pressure of the cytoplasm (9,11,26). This ability to compartmentalize Na+ and Cl-has been considered to be a mechanism of tolerance of halophytes (26).Although salt tolerance of glycophytes, particularly agronomic species, is usually attributed to the ability to exclude Na+ and Cl-, especially from the shoot (8, 11), intracellular ion compartmentation also may occur in these species. Despite indications of negative correlations between ion accumulation and salt tolerance in glycophytes, such as tomato (22), rice (29), wheat (27), and maize (12), examination of the data reveals that, even in tolerant genotypes, the accumulated levels of Na+ and Clare substantial. In some instances whole cell Na+ and Cl-accumulation was similar for both tolerant and sensitive genotypes (22,27,29). Thus, the ability to compartmentalize Na+ and Clmay be an underlying determinant of the tolerance not only of halophytes but also of many crop species.Verification of ion compartmentation has been restricted by the difficulty in obtaining reliable measurements of subcellular ion concentrations (7,15,23,30). Despite this, cytoplasmic Na + concentration in leaf cells of Suaeda maritima was determined to be 165 mm by efflux analysis when the whole cell concentration was about 600 mM (28). Data obtained by x-ray microanalysis of frozen hydrated leaves of Atriplex spongiosa (23) revealed that cytoplasmic ion concentrations were considerably lower than those in the vacuole.Results from investigations of glycophytes exposed to salinity have been less conc...

Sodium as nutrient and toxicant

“…In addition to this well-recognized osmotic effect, a second parsimonious explanation may lie in the disruption of potassium homeostasis-one that may reasonably and fruitfully supplant alternative, to date ill-substantiated , hypotheses, such as those of "toxic" Na + fluxes or cytosolic K + /Na + ratios (Maathuis and Amtmann 1999;Davenport and Tester 2000;Yao et al 2010; see below). The decline in cytosolic [K + ] ([K + ] cyt ) under saline conditions is well documented (Hajibagheri et al 1987(Hajibagheri et al , 1988Binzel et al 1988;Schroeppel-Meier and Kaiser 1988;Speer and Kaiser 1991;Hajibagheri and Flowers 2001;Carden et al 2003;Kronzucker et al 2006; see also Fig. 1) and is attributable to sodium's effects on K + transport Kronzucker and Britto 2011).…”

Enhanced Net K+ Uptake Capacity of NaCl-Adapted Cells

“…Intracellular ion uptake is an integral component ofosmotic adjustment necessary for adaptation to salt (3,7,10). However, because of the deleterious effects of high ion concentrations on cytosolic physiology and biochemistry (7,10,28), it is essential that ion levels in this compartment be stringently controlled.…”

Proline Betaine Accumulation and Metabolism in Alfalfa Plants under Sodium Chloride Stress. Exploring Its Compartmentalization in Nodules

“…During osmotic stress, plants induce processes that regulate the osmotic adjustment and maintain sufficient cell turgor for growth to proceed (Zimmermann, 1978). Such adjustment requires the control of intracellular inorganic ions in the cytoplasm, via vacuolar sequestration, and accumulation of organic compounds compartmented mainly in the cytoplasm (Jeschke et al, 1986;Binzel et al, 1988;Bohnert et al, 1995). These organic solutes, termed osmolytes, compatible solutes, or osmoprotectants, are nontoxic low-M r molecules that raise osmotic pressure and protect some macromolecular structures against denaturation (Timasheff, 1992;Bourot et al, 2000).…”