S-Adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50) is a key rate-limiting enzyme located in the polyamine biosynthesis pathway. When compared with other organisms, the plant SAMDC genes possess some distinct features because they are devoid of introns in the main open reading frame (ORF) but have an intron(s) in their 5# untranslated leader sequences, in which two overlapping tiny and small upstream ORFs (uORFs) are present. Our results show that the presence of the 5# leader sequence plays important roles in transcriptional and posttranscriptional regulation of SAMDC expression. This sequence may help to keep the transcript of its downstream cistron at a relatively low level and function together with its own promoter in response to external stimuli or internal changes of spermidine and spermine to initiate and regulate SAMDC expression. Under stress and high spermidine or spermine conditions, the tiny uORF shows the same function as its overlapping small uORF, which is involved in translational repression and feedback controlled by polyamines. The presence of introns is necessary for the SAMDC up-regulation process when the internal spermidine level is low. Our results suggest that plants have evolved one network to adjust SAMDC activity through their 5# leader sequences, through which transcriptional regulation is combined with an extensive posttranscriptional control circuit.S-Adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50) is a key enzyme in higher polyamine (PA) biosynthesis (Janne et al., 1978;Pegg et al., 1988). An understanding of how SAMDC genes are regulated is important for elucidating the molecular basis of PA biosynthesis and the role of PAs in plant growth and development.Plant SAMDC is initially synthesized as an inactive proenzyme and is autocatalytically processed to produce the mature form of the enzyme. This process is very rapid and, unlike the mammalian enzymes, is not regulated by the higher PA precursor putrescine (Put;Xiong et al., 1997). Characterization of SAMDC genes reveals the common feature of a long transcript leader sequence that carries upstream open reading frames (uORFs;Hill and Morris, 1992;Franceschetti et al., 2001). Evidence from several studies indicates that uORFs are involved in translational repression of mammalian SAMDC (Ruan et al., 1996;Pegg et al., 1998;Law et al., 2001). In plants, SAMDCs possess a highly conserved overlapping tiny and small uORFs, consisting of 3 and 52 to 53 codons, respectively (Franceschetti et al., 2001). In all cases, both the tiny and small uORFs are overlapped in such a manner that the last nucleotide A of the tiny uORF stop codon is the first nucleotide of the initiating ATG of the small uORF. The small uORF has also been shown to be responsible for the translational repression of the SAMDC gene (Hanfrey et al., 2002). Apart from the uORFs, the plant SAMDCs are not interrupted by introns through their main ORF but have an intron(s) in their untranslated 5# leader sequences, which is in contrast with SAMDCs from other organisms,...

S-Adenosylmethionine decarboxylase (SAMDC, EC 4.1.4.50) is a rate-limiting enzyme in the biosynthesis of polyamines (PAs) from putrescine. To gain more insight into the role of PAs in shoot organogenesis, a reverse genetic approach has been used to study in vitro shoot organogenesis by manipulating SAMDC expression in Arabidopsis. Up-and downregulation of SAMDC expression was achieved by transferring sense, antisense and double-stranded Arabidopsis SAMDC complementary DNA constructs back into Arabidopsis via Agrobacterium tumefaciens. Results show that the biosynthesis of PAs and ethylene is mutually antagonistic by manipulation of SAMDC expression. Further results demonstrate that increased shoot organogenesis seems to be directly related to PA accumulation. This effect of PA may be further enhanced with reduced ethylene. It also suggests that spermidine is involved in the process of acquiring organogenesis competence through downregulation of ethylene production and shoot organogenesis, which might result from the concerted action of PAs and plant hormones such as auxin, cytokinins and ethylene.

Polyamines (PAs) are low molecular weight metabolites involved in various physiological and developmental processes in eukaryotic and prokaryotic cells. The cellular PA level is regulated in part by the action of amine oxidases (AOs) including copper diamine oxidases (DAOs) and flavoprotein polyamine oxidases (PAOs). In this study, the isolation and characterization of flavin amine oxidases (FAOs) from Brassica juncea (BJFAO) and Arabidopsis (ATFAO1) are reported that were clustered in the same group as polyamine oxidases from maize (MPAO) and barley (BPAO1) and monoamine oxidases from mammalian species. ATFAO1 was temporally and spatially regulated in Arabidopsis and showed distinct expression patterns in response to different stress treatments. To investigate the in vivo function of FAO, transgenic Arabidopsis plants expressing sense, antisense, and double-stranded BJFAO RNAs were generated and those with altered activity of FAOs were selected for further characterization. It was found that the shoot regeneration response in transgenic plants was significantly affected by the modulated PA levels corresponding to FAO activities. Tissues that originated from transgenic plants with down-regulated FAO activity were highly regenerative, while those from transgenic plants with upregulated FAO activity were poorly regenerative. The shoot regeneration capacity in these transgenic plants was related to the levels of individual PAs, suggesting that FAO affects shoot regeneration by regulating cellular PAs. Furthermore, it was found that the effect of FAO activity on shoot regeneration was exerted downstream of the Enhancer of Shoot Regeneration (ESR1) gene, which may function in a branch of the cytokinin signalling pathway.

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