Characterization and expression of S-adenosylmethionine decarboxylase gene in transgenic plants

Abstract

The enzyme S-Adenosylmethionine decarboxylase (SAMDC) catalyzes the conversion of S-Adenosylmethionine (SAM) to decarboxylated SAM, which is the precursor for the synthesis of high polyamines (PAs), i.e. spermindine (Spd) and spermine (Spm). To elucidate the molecular basis of SAMDC in PA regulation and its role in shoot regeneration in vitro, four members of the SAMDC gene were cloned from mustard and shown to express differentially in response to stress and treatments with exogenous stimuli. In general, cold treatment was most stimulatory to SAMDC expression, which was inhibited by exogenous PAs. Similar expression was also observed in transgenic plants expressing the GUS gene driven by the upstream regulatory sequence of SAMDC, indicating that SAMDC expression is regulated transcriptionally. The presence of the 5a?? leader sequence, which possessed three introns and two tiny and small upstreame open reading frames (uORFs) was mandatory for gene expression in response to stress. Elimination of the introns attenuated gene expression. Both tiny and small uORFs were involved in post-transcriptional repression of SAMDC under stress and repression was associated with PAs. The cellular PA content could be modulated by overexpression and down-regulation of SAMDC cDNA. SAMDC overexpression resulted in Spd and Spm accumulation and decreased ethylene production in cultured tissues, which were highly regenerative. Comparatively, down-regulation of SAMDC expression showed a decrease in Spd and Spm and an increase in ethylene production and cultured tissues were poorly regenerative. These results support the precursor competition hypothesis. In which ethylene and PA biosynthesis is mutually regulated by competing for the same precursor SAM.