The synthesis of the quaternary ammonium and tertiary sulfonium compounds is dependent upon the activity of S-adenosylmethionine (SAM; AdoMet)-dependent N-methyltransferases for N-methylation of ethanolamine bases, B-alanine, proline and pipecolic acid. This implies that osmolyte accumulation is critically dependent on the supply of SAM as methyl donor (Hanson et al, 1995; Hanson and Roje, 2001; Bohnert and Jensen, 1996). When SAM is utilized as a methyl donor in a methyltransferase reaction, the resulting S-adenosylhomocysteine (SAHcy; AdoHcy) is recycled to homocysteine (Hcy) which can then accept a methyl group from the N⁵-methyltetrahydrofolate (N⁵-methyl-THF) pool to regenerate methionine, and hence SAM (Giovanelli et al, 1980):

The above cycle is known as the "activated methyl cycle" (Bohnert and Jensen, 1996). Consistent with an increased flux via the activated methyl cycle in response to osmotic stress, SAM synthetases are induced by salinity in tomato (Espartero et al, 1994).

N⁵-Methyl-THF serving as precursor of the methyl-group of methionine is derived from N⁵,N¹⁰-methylene-THF by the action of methylene tetrahydrofolate reductase (Cossins, 1987):

N⁵,N¹⁰-methylene-THF participates as an intermediate in the photorespiratory nitrogen cycle in the mitochondria of C₃ plants (Givan et al, 1988). Tetrahydrofolate (THF) allows transfer of the alpha-carbon of glycine (Gly) through glycine decarboxylase (GDC) [EC 1.4.4.2 and 2.1.2.10] activity for serine (Ser) biosynthesis by the serine hydroxymethyltransferase (SHMT) [EC 2.1.2.1] reaction in the photorespiratory nitrogen cycle of C₃ plants (Oliver, 1994; Prabhu et al, 1996):

In C₃ plants, the GDC activity is greater than that of SHMT activity in leaf mitochondria, which results in the maintenance of high levels of N⁵,N¹⁰-methylene-THF; thus, the equilibrium of the SHMT reaction is shifted in favor of serine synthesis, which enables recycling of THF for continuous operation of the GDC reaction during photorespiration (Prabhu et al, 1996).

Formate is a potential alternative single-carbon source for the production of the N⁵,N¹⁰-methylene-THF required for serine synthesis (Prabhu et al, 1996):

In most eukaryotes these three activities occur on a single polypeptide called C1-THF synthase; in plants, however, the synthetase activity occurs on a separate protein moiety from that of the cyclohydrolase and reductase, which occur on a bifunctional protein (Prabhu et al, 1996). Because the latter reactions are reversible, this allows an equilibrium between the pools of C1 units at the formyl and methylene levels of oxidation (Cossins, 1987). Therefore, serine, glycine, and formate can serve as the precursors for methionine synthesis (Cossins, 1987). ¹³C-Formate is metabolized to B-¹³C-serine in Arabidopsis in a manner consistent with C1-THF synthase and SHMT activities interacting with a common THF pool (Prabhu et al, 1996). Formate is a very effective precursor of the methyl groups of choline and glycinebetaine in glycinebetaine-accumulating plant species (Delwiche and Bregoff, 1958; Hanson and Nelsen, 1978). Formate can be derived from non-enzymic H₂O₂-mediated decarboxylation of glyoxylate, although Kleczkowski and Givan (1988) question whether this pathway is of physiological significance (see also pathways of glycine and serine synthesis discussed under Ammonia assimilation and recycling). Additional possible pathways of formate production in higher plants are described by Hourton-Cabassa et al (1998). These include synthesis from methanol (via formaldehyde), synthesis from glycolytic products, and the reverse of the reactions depicted above. Formate dehydrogenase (FDH) [EC 1.2.1.2] catalyzes the oxidation of formate to CO₂ and may thus play a key role in regulating formate levels. FDH is a major mitochondrial protein in potato, and is induced by formate, methanol, and various stresses, including anoxia (Hourton-Cabassa et al, 1998), and iron deficiency (Suzuki et al, 1998).

Methylthioadenosine (MTA) is converted to methylthioribose-1-P via the action of methylthioadenosine phosphorylase [EC 2.4.2.28] or methylthioadenosine nucleosidase [EC 3.2.2.16] plus methylthioribose kinase (Furfine and Abeles, 1988; Myers and Abeles,1990; Myers et al, 1993) in Klebsiella pneumoniae. The aminotransferase which catalyzes the final step in methionine recycling from methylthioadenosine, the conversion of alpha-ketomethiobutyrate (KMTB) to methionine, has been purified from Klebsiella pneumoniae and characterized. The enzyme was found to be a homodimer of 45-kDa subunits, and it catalyzed methionine formation primarily using aromatic amino acids and glutamate as the amino donors (Heilbronn et al, 1999). Histidine, leucine, asparagine, and arginine were also functional amino donors but to a lesser extent. The N-terminal amino acid sequence of the enzyme is almost identical to the N-terminal sequence of both the Escherichia coli and Salmonella typhimurium tyrosine aminotransferase [EC 2.6.1.5] (tyrB gene products) (Heilbronn et al, 1999).

Bohnert HJ, Jensen RG 1996 Strategies for engineering water-stress tolerance in plants. Tibtech 14: 89-97.

Cossins EA 1987 Folate biochemistry and the metabolism of one-carbon units. "The Biochemistry of Plants" (DD Davies ed), Vol. 11, Academic Press, New York, pp. 317-353.

Delwiche CC, Bregoff HM 1958 Pathway of betaine and choline synthesis in Beta vulgaris. J. Biol. Chem. 233: 430-433.

Espartero J, Pintor-Toro JA, Pardo JM 1994 Differential accumulation of S-adenosylmethionine synthetase transcripts in response to salt stress. Plant Mol. Biol. 25: 217-227.

Furfine ES, Abeles RH 1988 Intermediates in the conversion of 5'-S-methylthioadenosine to methionine in Klebsiella pneumoniae. J. Biol. Chem. 263: 9598-9606.

Giovanelli J, Mudd SH, Datko AH 1980 Sulfur amino acids in plants. In (BJ Miflin ed) "The Biochemistry of Plants", Vol 5, Academic Press, New York, pp. 453-505.

Givan CV, Joy KW, Kleczkowski LA 1988 A decade of photorespiratory nitrogen cycling. Trends Biochem. Sci. 13: 433-437.

Hanson AD, Nelsen CE 1978 Betaine accumulation and ¹⁴C-formate metabolism in water-stressed barley leaves. Plant Physiol. 62: 305-312.

Hanson AD, Rivoal J, Burnet M, Rathinasabapathi B 1995 Biosynthesis of quaternary ammonium and tertiary sulphonium compounds in response to water deficit. In "Environment and Plant Metabolism: Flexibility and Acclimation" (N Smirnoff ed), Bios Scientific, Oxford, pp. 189-198.

Hanson AD, Roje S 2001 One-carbon metabolism in higher plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2001 52: 119-137.

Heilbronn J, Wilson J, Berger BJ 1999 Tyrosine aminotransferase catalyzes the final step of methionine recycling in Klebsiella pneumoniae. J. Bacteriol. 181: 1739-1747.

Hourton-Cabassa C, Ambard-Bretteville F, Moreau F, de Virville JD, Remy R, des Francs-Small CC 1998 Stress induction of mitochondrial formate dehydrogenase in potato leaves. Plant Physiol. 116: 627-635.

Kleczkowski LA, Givan CV 1988 Serine formation in leaves by mechanisms other than the glycolate pathway. J. Plant Physiol. 132: 641-652.

Myers RW, Abeles RH 1990 Conversion of 5-S-methyl-5-thio-D-ribose to methionine in Klebsiella pneumoniae. Stable isotope incorporation studies of the terminal enzymatic reactions in the pathway. J. Biol. Chem. 265: 16913-16921.

Myers RW, Wray JW, Fish S, Abeles RH 1993 Purification and characterization of an enzyme involved in oxidative carbon-carbon bond cleavage reactions in the methionine salvage pathway of Klebsiella pneumoniae. J. Biol. Chem. 268: 24785-24791.

Oliver DJ 1994 The glycine decarboxylase complex from plant mitochondria. Annu. Rev. Plant Physiol. Plant Mol. Biol. 45: 323-337.

Prabhu V, Chatson KB, Abrams GD, King J 1996 ¹³C Nuclear magnetic resonance detection of interactions of serine hydroxymethyltransferase with C1-tetrahydrofolate synthase and glycine decarboxylase complex activities in Arabidopsis. Plant Physiol. 112: 207-216.

Suzuki K, Itai R, Suzuki K, Nakanishi H, Nishizawa NK, Yoshimura E, Mori S 1998 Formate dehydrogenase, an enzyme of anaerobic metabolism, is induced by iron deficiency in barley roots. Plant Physiol. 116: 725-732.