In some higher plant species (notably members of the Labiatae, Asteraceae, Fabaceae, Bataceae, Rutaceae, Plumbaginaceae and Capparaceae) proline can be metabolized to prolinebetaine (stachydrine) (Naidu et al, 1987; Wood et al, 1991; Naidu et al, 1992; Hanson et al, 1994; Bonham et al, 1995; Gorham, 1995; Blunden et al, 1996; Nolte et al, 1997). The synthesis of prolinebetaine is thought to involve N-methylation of proline by SAM-dependent methyltransferases, via the intermediate N-methylproline (hygric acid) (Robertson and Marion, 1960; Essery et al, 1962), but the enzyme(s) have not yet been characterized.

Hydroxyprolinebetaine often (but not always) accumulates with prolinebetaine in these prolinebetaine-accumulating species (Wood et al, 1991; Hanson et al, 1994; Nolte et al, 1997). The proline and/or N-methylproline hydroxylase(s) putatively involved in hydroxyprolinebetaine synthesis have not yet been characterized.

Comparisons of related species with different capacities for accumulation of prolinebetaine and hydroxyprolinebetaine suggest that free proline pool size is inversely related to the total prolinebetaine + hydroxyprolinebetaine pool size (Hanson et al, 1994; Nolte et al, 1997). Prolinebetaine is a more potent osmoprotectant than proline per se (Hanson et al, 1994). Conversion of proline to prolinebetaine may therefore enhance osmotic stress resistance. In principle, increased flux of proline to N-methylated derivatives should alleviate P5CS from feedback regulation by proline by decreasing the pool size of free proline, simultaneously restricting proline oxidation and hence osmolyte catabolism (Samaras et al, 1995). One isomer of hydroxyprolinebetaine (trans-4-hydroxy-L-prolinebetaine) is a potent inhibitor of animal acetylcholineesterase (Friess et al, 1957), raising the possibility that the accumulation of this compound may serve a dual function, as a compatible osmolyte and an antifeedant affording protection against herbivores (Hanson and Burnet, 1994). In contrast, proline is a stimulant of insect herbivory (Haglund, 1980) (see also Proline, ornithine and arginine synthesis).

Blunden G, Yang M-H, Yuan Z-X, Smith BE, Patel A, Cegarra JA, Mathe IJr, Janicsak G 1996 Betaine distribution in the Labiatae. Biochem. Sys. Ecol. 24: 71-81.

Bonham CC, Wood KV, Yang W-J, Nadolska-Orczyk A, Samaras Y, Gage DA, Poupart J, Burnet M, Hanson AD, Rhodes D 1995 Identification of quaternary ammonium compounds by plasma desorption mass spectrometry. J. Mass Spectrom. 30: 1187-1194.

Essery JM, McCaldin DJ, Marion L 1962 The biogenesis of stachydrine. Phytochemistry 1: 209-213.

Friess SL, Patchett AA, Witkop B 1957 The acetylcholineesterase surface. VII. Interference with surface binding as reflected by enzymatic response to turicine, betonicine and related heterocycles. J. Am. Chem. Soc. 79: 459-462.

Gorham J 1995 Betaines in higher plants - biosynthesis and role in stress metabolism. In (RM Wallsgrove ed) "Amino acids and Their Derivatives in Higher Plants." Society for Experimental Biology Seminar Series, Vol 56, Cambridge University Press, Cambridge, pp. 173-203.

Haglund BM 1980 Proline and valine - cues which stimulate grasshopper herbivory during drought stress? Nature 288: 697-698.

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, 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.

Naidu BP, Jones GP, Paleg LG, Poljakoff-Mayber A 1987 Proline analogues in Melaleuca species: response of Melaleuca lanceolata and M. uncinata to water stress and salinity. Aust. J. Plant Physiol. 14: 669-677.

Naidu BP, Paleg LG, Jones GP 1992 Nitrogenous compatible solutes in drought-stressed Medicago spp. Phytochem. 31: 1195-1197.

Nolte KD, Hanson AD, Gage DA 1997 Proline accumulation and methylation to proline betaine in Citrus: implications for genetic engineering of stress resistance. J. Am. Soc. Hort. Sci. 122: 8-13.

Robertson AV, Marion L 1960 The biogenesis of alkaloids. XXV. The role of hygric acid in the biogenesis of stachydrine. Can. J. Chem. 38: 396-398.

Samaras Y, Bressan RA, Csonka LN, Garcia-Rios MG, Paino D'Urzo M, Rhodes D 1995 Proline accumulation during drought and salinity. In (N Smirnoff ed) "Environment and Plant Metabolism: Flexibility and Acclimation", Bios Scientific Publishers, Oxford, pp. 161-187.

Wood KV, Stringham KJ, Smith DL, Volenec JJ, Hendershot KL, Jackson KA, Rich PJ, Yang W-J, Rhodes D 1991 Betaines of alfalfa: characterization by fast atom bombardment and desorption chemical ionization mass spectrometry. Plant Physiol. 96: 892-897.