Comparative Physiological Evidence that β-Alanine Betaine and Choline-O-Sulfate Act as Compatible Osmolytes in Halophytic Limonium Species

Abstract

The quaternary ammonium compounds accumulated in saline conditions by five salt-tolerant species of Limonium (Plumbaginaceae) were analyzed by fast atom bombardment mass spectrometry. Three species accumulated j-alanine betaine and choline-O-sulfate; the others accumulated glycine betaine and choline-C-sulfate. Three lines of evidence indicated that f,-alanine betaine and choline-0-sulfate replace glycine betaine as osmoregulatory solutes. First, tests with bacteria showed that 0-alanine betaine and choline-0-sulfate have osmoprotective properties comparable to glycine betaine. Second, when ,B-alanine betaine and glycine betaine accumulators were salinized, the levels of their respective betaines, plus that of choline-0-sulfate, were closely correlated with leaf solute potential. Third, substitution of sulfate for chloride salinity caused an increase in the level of choline-C-sulfate and a matching decrease in glycine betaine level. Experiments with '4C-labeled precursors established that B-alanine betaine accumulators did not synthesize glycine betaine and vice versa. These experiments also showed that 6-alanine betaine synthesis occurs in roots as well as leaves of fialanine betaine accumulators and that choline-C-sulfate and glycine betaine share choline as a precursor. Unlike glycine betaine, j-alanine betaine synthesis cannot interfere with conjugation of sulfate to choline by competing for choline and does not require oxygen. These features of #l-alanine betaine may be advantageous in sulfate-rich salt marsh environments.Considerable evidence indicates that the QAC2 glycine betaine accumulates in the cytoplasm and chloroplasts of species of the family Chenopodiaceae during osmotic stress and acts as a nontoxic or compatible osmolyte (for reviews, see refs. 15 and 28). Glycine betaine is likely to play the same role in the many other angiosperm families in which it accumulates in response to stress (27,30), although in these cases the '

Organic osmolytes as compatible, metabolic and counteracting cytoprotectants in high osmolarity and other stresses

“…There is evidence to support this, as illustrated by the following examples. (1) Growth of Escherichia coli in saline growth media can be improved with a variety of osmolytes, some not used naturally by the bacterium, added to the medium (Hanson et al, 1991). (2) One cultured line of mammalian kidney medullary cells (PAP-HT25) in hyperosmotic culture medium uses primarily sorbitol as an osmolyte.…”

Choline-O-Sulfate Biosynthesis in Plants (Identification and Partial Characterization of a Salinity-Inducible Choline Sulfotransferase from Species of Limonium (Plumbaginaceae)

“…Samples (50 or 100 mg) of freeze-dried, milled material were spiked with 1 pmol of ['H9]CS and 0.5 pmol of [*H9]Gly betaine and were extracted, fractionated, and analyzed by FABM! ; as described (Rhodes et al, 1987;Hanson and Gage, 1991) except that a mixed-bed column [Dowex-1 (OH~):BioRex-70 (H + ), 2:1, v/v] replaced Dowex-1 for CS isolation.…”

“…Deuterated ('H,) CS and Gly betaine standards were prepared as described (Hanson et al, 1991). Samples (50 or 100 mg) of freeze-dried, milled material were spiked with 1 pmol of ['H9]CS and 0.5 pmol of [*H9]Gly betaine and were extracted, fractionated, and analyzed by FABM!…”

“…In vivo radiotracer experiments with Limonium species (Hanson et al, 1991) showed that [14C]Cho was readily converted to [14C]CS. This suggests the presence of CST activity (EC 2.8.2.6), catalyzing the reaction:…”

“…CS accumulates in Limonium species (Hanson et al, 1991) and in all other species of Plumbaginaceae tested (Hanson et al, 1994). Because members of this family have salt glands that can secrete Na+ and C1-, but not SO4'- (Popp, 1984;Hanson and Gage, 1991), it has been hypothesized that CS accumulation may serve to detoxify part of the 504'-that builds up in these species (Hanson and Gage, 1991;Hanson et al, 1994). If CS synthesis can indeed function to detoxify SO4'-, it could be a valuable trait to transfer to crops because SO4'-salinity occurs in many agricultural soils (Epstein and Rains, 1987).…”