The sulfonium analog of B-alaninebetaine, 3-dimethylsulfoniopropionate (DMSP), is synthesized and accumulated to osmotically significant levels in a few higher plant species from diverse families; certain species of the Asteraceae (most notably Wollastonia biflora), and Poaceae (some [but not all] species of Spartina and Saccharum) (Paquet et al, 1994; 1995; James et al, 1995), and by many marine algae (see e.g. Blunden and Gordon, 1986; Dickson and Kirst, 1986). DMSP is as effective as glycinebetaine as an osmoprotectant for E. coli (Paquet et al, 1994), and is noted to be a highly effective cryoprotectant (Nishiguchi and Somero, 1992; Karsten et al, 1996).

In Wollastonia, DMSP is synthesized by methylation of methionine to S-methylmethionine (SMM) in the cytosol (Hanson et al, 1994; James et al, 1995ab), transport of SMM into the chloroplast, decarboxylation and deamination of SMM to DMSP-aldehyde (James et al, 1995b), and oxidation of the aldehyde to DMSP in the chloroplast (Trossat et al, 1996). The first enzyme of the pathway, SAM:L-methionine S-methyltransferase [EC 2.1.1.12] has been purified from Wollastonia leaves (James et al, 1995a). The final step in the pathway is catalyzed by a chloroplast localized DMSP-aldehyde dehydrogenase, which may be identical to the betaine aldehyde dehydrogenase (BADH) involved in glycinebetaine synthesis (James et al, 1995b; Trossat et al, 1996; 1997). The enzyme(s) responsible for metabolism of SMM to DMSP-aldehyde are not yet known. It is possible that SMM is first transaminated to the unstable 2-oxo acid, 4-dimethylsulfonio-2-oxobutyrate (DMSOB), which is then decarboxylated (Rhodes et al, 1997).

Salinity promotes accumulation of DMSP and its precursor, SMM, in chloroplasts of Wollastonia; approximately 70% of the DMSP is chloroplastic in salinized plants. DMSP may reach concentrations of 120 mM in the chloroplast stroma of salinized plants; such concentrations could contribute significantly to chloroplast osmotic adjustment (Trossat et al, 1998). The occurrence of both chloroplastic and extrachloroplastic pools of SMM is consistent with the finding that SMM is synthesized in the cytosol but converted to DMSP in the chloroplast. Because salinity raised the chloroplastic SMM level but lowered the cytosolic SMM it is inferred that salinity may increase the capacity or affinity of an SMM transporter in the chloroplast envelope (Trossat et al, 1998) (see also S-Methylmethionine under Sulfate uptake and assimilation).

The Spartina alterniflora pathway of DMSP synthesis differs from that found in Wollastonia in that DMSP-amine is an intermediate (Kocsis et al, 1998). The DMSP pathway in Spartina is proposed to involve the following route:

This implies that decarboxylation (rather than transamination) of SMM is the main route of conversion of SMM to DMSP in Spartina (Kocsis et al, 1998). Modeling of the DMSP pathway in Wollastonia (Kocsis et al, 1998) accurately predicted two enzymes of the pathway and their relative substrate affinities (Kocsis and Hanson, 2000).

Because DMSP does not contain N, the accumulation of DMSP could offer advantages over accumulation of nitrogenous onium compounds for osmotic adjustment in N-limiting environments; N-deficiency increases DMSP levels in Wollastonia (Hanson et al, 1994).

Blunden G, Gordon SM 1986 Betaines and their sulphonio analogues in marine algae. Prog. Phycol. Res. 4: 39-80.

Dickson DMJ, Kirst GO 1986 The role of beta-dimethylsulphoniopropionate, glycine betaine and homarine in the osmoacclimation of Platymonas subcordiformis. Planta 167: 536-543.

Hanson AD, Rivoal J, Paquet L, Gage DA 1994 Biosynthesis of 3-dimethylsulfoniopropionate in Wollastonia biflora (L.) DC.: Evidence that S-methylmethionine is an intermediate. Plant Physiol. 105: 103-110.

James F, Nolte KD, Hanson AD 1995a Purification and properties of S-adenosyl-L-methionine:L-methionine S-methyltransferase from Wollastonia biflora leaves. J. Biol. Chem. 270: 22344-22350.

James F, Paquet L, Sparace SA, Gage DA, Hanson AD 1995b Evidence implicating dimethylsulfoniopropionaldehyde as an intermediate in dimethylsulfoniopropionate biosynthesis. Plant Physiol. 108: 1439-1448.

Karsten U, Kuck K, Vogt C, Kirst GO 1996 Dimethylsulfoniopropionate production in phototrophic organisms and its physiological function as a cryoprotectant. In "Biological Chemistry of DMSP and Related Sulfonium Compounds" (RP Kiene, PT Visscher, MD Keller, GO Kirst eds), Plenum Press, New York, pp. 143-153.

Kocsis MG, Hanson AD 2000 Biochemical evidence for two novel enzymes in the biosynthesis of 3-dimethylsulfoniopropionate in Spartina alterniflora. Plant Physiol. 123: 1153-1162.

Kocsis MG, Nolte KD, Rhodes D, Shen T-L, Gage DA, Hanson AD 1998 Dimethylsulfoniopropionate biosynthesis in Spartina alterniflora: evidence that S-methylmethionine and dimethylsulfoniopropylamine are intermediates. Plant Physiol. 117: 273-281.

Nishiguchi MK, Somero GN 1992 Temperature- and concentration-dependence of compatibility of the organic osmolyte beta-dimethylsulfoniopropionate. Cryobiol. 29: 118-124.

Paquet L, Lafontaine PJ, Saini HS, James F, Hanson AD 1995 Discovery of the presence of 3-dimethylsulfonioproprionate in several species of angiosperms. Can. J. Bot. 73: 1889-1896.

Paquet L, Rathinasabapathi B, Saini H, Zamir L, Gage DA, Huang Z-H, Hanson AD 1994 Accumulation of the compatible solute 3-dimethylsulfoniopropionate in sugarcane and its relatives, but not other gramineous crops. Aust. J. Plant Physiol. 21: 37-48.

Rhodes D, Gage DA, Cooper AJL, Hanson AD 1997 S-Methylmethionine conversion to dimethylsulfoniopropionate: Evidence for an unusual transamination reaction. Plant Physiol. 115: 1541-1548.

Trossat C, Nolte KD, Hanson AD 1996 Evidence that the pathway of dimethylsulfoniopropionate begins in the cytosol and ends in the chloroplast. Plant Physiol. 111: 965-973.

Trossat C, Rathinasabapathi B, Hanson AD 1997 Transgenically expressed betaine aldehyde dehydrogenase efficiently catalyzes oxidation of dimethylsulfoniopropionaldehyde and omega-aminoaldehydes. Plant Physiol. 113: 1457-1461.

Trossat C, Rathinasabapathi B, Weretilnyk EA, Shen TL, Huang ZH, Gage DA, Hanson AD 1998 Salinity promotes accumulation of 3-dimethylsulfoniopropionate and its precursor S-methylmethionine in chloroplasts. Plant Physiol. 116: 165-171.