Characterization of S-adenosylmethionine synthetase genes and its expression is associated with ethylene synthesis in mustard (Brassica juncea)

S-Adenosylmethionine synthetase (SAMS, EC 2.5.1.6) catalyses the conversion of methionine to S-adenosylmethionine, which is a precursor of ethylene biosynthesis. To investigate whether regulation of ethylene synthesis is associated with transcript accumulation of SAMS, we report the isolation and characterization of one mustard genomic clone (MSAMS2) and two cDNAs (MSAMS3, and MSAMS4) that are highly homologous to plant SAMS. While both MSAMS2 and MSAMS3 encoded a single polypeptide of 393 amino acid residues, MSAMS4 encoded a protein of 390 residues. Accumulation of SAMS transcripts varied with mustard organs, in which transcripts were found to accumulate predominantly in root, flower bud, seedpod and leaf of the flowering plant but were less abundant in stem, petiole and leaf of the vegetative plant. SAMS transcripts could be upregulated in a gene-specific manner in response to exogenous applications of 1-aminocyclopropane-1-carboxylate, 2-chloroethylphosphonic acid, AgNO3, polyamines (putrescine, spermidine and spermine) and NaCl. Most treatments were stimulatory to MSAMS2 and MSAMS3 transcript accumulation but salt had no effect on MSAMS2 expression. With respect to MSAMS4, the expression was generally low in control leaves and was not affected by most treatments, but exogenous applications of putrescine and spermidine upregulated transcript accumulation. The level of ethylene produced in leaves was also increased significantly in response to various stimuli, but exogenous polyamines were inhibitory to ethylene production after 12 h of incubation. These results indicate that ethylene biosynthesis in mustard is regulated, at least in part, by SAMS at the transcript level in a gene-specific manner.