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

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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 beta-alanine betaine and choline-O-sulfate; the others accumulated glycine betaine and choline-O-sulfate. Three lines of evidence indicated that beta-alanine betaine and choline-O-sulfate replace glycine betaine as osmo-regulatory solutes. First, tests with bacteria showed that beta-alanine betaine and choline-O-sulfate have osmoprotective properties comparable to glycine betaine. Second, when beta-alanine betaine and glycine betaine accumulators were salinized, the levels of their respective betaines, plus that of choline-O-sulfate, were closely correlated with leaf solute potential. Third, substitution of sulfate for chloride salinity caused an increase in the level of choline-O-sulfate and a matching decrease in glycine betaine level. Experiments with (14)C-labeled precursors established that beta-alanine betaine accumulators did not synthesize glycine betaine and vice versa. These experiments also showed that beta-alanine betaine synthesis occurs in roots as well as leaves of beta-alanine betaine accumulators and that choline-O-sulfate and glycine betaine share choline as a precursor. Unlike glycine betaine, beta-alanine betaine synthesis cannot interfere with conjugation of sulfate to choline by competing for choline and does not require oxygen. These features of beta-alanine betaine may be advantageous in sulfate-rich salt marsh environments.