Glycinebetaine is thought to be the archetypal betaine in the plant kingdom (Weretilnyk et al, 1989; Hanson and Burnet, 1994; Hanson et al, 1994). Some species have apparently lost the capacity to accumulate large quantities of this compound, and others have evolved the capacity to synthesize alternative quaternary ammonium compounds, including choline-O-sulfate, B-alaninebetaine or prolinebetaine (Hanson and Gage, 1991; Hanson et al, 1991). This is particularly well illustrated in a consideration of the quaternary ammonium compounds accumulated by members of the Plumbaginaceae (Hanson et al, 1994). Ancestral sub-families of the Plumbaginaceae are glycinebetaine-accumulators (Hanson et al, 1994). In more advanced sub-families, however, glycinebetaine has been replaced in part or in whole by B-alaninebetaine or prolinebetaine (Hanson et al, 1994). In addition, all members of the Plumbaginaceae accumulate choline-O-sulfate. This compound is synthesized from choline by a salt stress-inducible choline sulfotransferase (Rivoal and Hanson, 1994).

(where PAPS = 3'-phosphoadenosine-5'phosphosulfate and PAP = 3'-phosphoadenosine-5'phosphate; see also PAPS under Sulfate uptake and assimilation).

The adaptive significance of choline-O-sulfate in the Plumbaginaceae is thought to reside in the fact that salt glands of these species can excrete chloride but not sulfate; thus conjugation of sulfate with choline is proposed to be a mechanism of sulfate detoxification, converting a normally deleterious anion to a compatible osmolyte (Hanson et al, 1994). Because choline-O-sulfate synthesis competes with glycinebetaine synthesis for available choline, this may have contributed to the evolution of alternative betaine biosynthesis pathways (B-alaninebetaine or prolinebetaine) which draw on substrates other than choline (Hanson et al, 1994; Hanson and Burnet, 1994).

Hanson AD, Burnet M 1994 Evolution and metabolic engineering of osmoprotectant accumulation in higher plants. In "Cell Biology: Biochemical and Cellular Mechanisms of Stress Tolerance in Plants", NATO ASI Series H (JH Cherry ed), Springer, Berlin, pp. 291-302.

Hanson AD, Gage DA 1991 Identification and determination by fast atom bombardment mass spectrometry of the compatible solute choline-O-sulfate in Limonium species and other halophytes. Austr. J. Plant Physiol. 18: 317-327.

Hanson AD, Rathinasabapathi B, Chamberlin B, Gage DA 1991 Comparative physiological evidence that beta-alanine betaine and choline-O-sulfate act as compatible osmolytes in halophytic Limonium species. Plant Physiol. 97: 1199-1205.

Hanson AD, Rathinasabapathi B, Rivoal J, Burnet M, Dillon MO, Gage DA 1994 Osmoprotective compounds in the Plumbaginaceae: A natural experiment in metabolic engineering of stress tolerance. Proc. Natl. Acad. Sci. U.S.A. 91: 306-310.

Rivoal J, Hanson AD 1994 Choline-O-sulfate biosynthesis in plants. Identification and partial characterization of a salinity-inducible choline sulfotransferase from species of Limonium (Plumbaginaceae). Plant Physiol. 106: 1187-1193.

Weretilnyk EA, Bednarek S, McCue KF, Rhodes D, Hanson AD 1989 Comparative biochemical and immunological studies of the glycine betaine synthesis pathway in diverse families of dicotyledons. Planta 178: 342-352.