| Abstract: | 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-14C]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 14C]methionine, S-methylmethionine (SMM) had the labeling pattern expected of a pathway intermediate, whereas 3-methylthiopropionate (MTP) did not. (b) 14C]SMM was efficiently converted to DMSP but 14C]MTP was not. (c) The addition of unlabeled SMM, but not of MTP, reduced the synthesis of 14C]DMSP from 14C]methionine. (d) The dimethylsulfide group of 13CH3,C2H3]SMM was incorporated as a unit into DMSP. (e) When C2H3,C2H3]SMM was given together with 13CH3]methionine, the main product was C2H3,C2H3]DMSP, not 13CH3,C2H3]DMSP or 13CH3,13CH3]DMSP. The stable isotope labeling results also show that the SMM cycle does not operate at a high level in W. biflora leaves. |
