BIOSYNTHESIS OF 3-DIMETHYLSULFONIOPROPIONATE IN WOLLASTONIA-BIFLORA (L) DC - EVIDENCE THAT S-METHYLMETHIONINE IS AN INTERMEDIATE

The compatible solute 3-dimethylsulfoniopropionate (DMSP) is accumulated by certain salt-tolerant flowering plants and marine algae. It is the major biogenic precursor of dimethylsulfide, an important sulfur-containing trace gas in the atmosphere. DMSP biosynthesis was investigated in Wollastonia biflora (L.) DC. [=Wedelia biflora (L.) DC., Melanthera biflora (L.) Wild, Asteraceae]. After characterizing DMSP and glycine betaine accumulation in three diverse genotypes, a glycine betaine-free genotype was chosen for radiotracer and stable isotope-labeling studies. In discs from young leaves, label from [U-C-14]methionine was readily incorporated into the dimethylsulfide and acrylate moieties of DMSP. This establishes that DMSP is derived from methionine by deamination, decarboxylation, oxidation, and methylation steps, without indicating their order. Five lines of evidence indicated that methylation is the first step in the sequence, not the last. (a) In pulse-chase experiments with [C-14]methionine, S-methylmethionine (SMM) had the labeling pattern expected of a pathway intermediate, whereas 3-methylthiopropionate (MTP) did not. (b) [C-14]SMM was efficiently converted to DMSP but [C-14]MTP was not. (c) The addition of unlabeled SMM, but not of MTP, reduced the synthesis of [C-14]DMSP from [C-14]methionine. (d) The dimethylsulfide group of [(CH3)-C-13,(CH3)-H-2]SMM was incorporated as a unit into DMSP. (e) When [(CH3)-H-2,(CH3)-H-2]SMM was given together with [(CH3)-C-13]methionine, the main product was [(CH3)-H-2,(CH3)-H-2]DMSP, not [(CH3)-C-13,(CH3)-H-2]DMSP or [(CH3)-C-13,(CH3)-C-13]DMSP. The stable isotope labeling results also show that the SMM cycle does not operate at a high level in W. biflora leaves.