Identification of stress-induced mitochondrial proteins in cultured tobacco cells

S-Adenosylmethionine decarboxylase (SAMDC, E.C. 4.1.4.50) is a key enzyme involved in the polyamine (PA) biosynthetic pathway. 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. However, information regarding transcriptional regulation of SAMDC has been limited. In an attempt to address this question, we isolated four cDNAs from mustard (Brassica juncea), designated BJSAMDC1, BJSAMDC2, BJSAMDC3 and BJSAMDC4, encoding predicted SAMDC. A comparison of deduced amino acid sequence revealed that they are highly homologous to other plant SAMDCs. These proenzymes also possess the conserved cleavage domain and putative PEST sequence for SAMDC. Northern analysis showed that the SAMDC transcripts were most abundant in reproductive organs and roots but that the level was low in young leaves and petioles. Meanwhile, SAMDC expression in the leaf was up-regulated differentially in response to stress such as chilling and exogenous ACC. The effect of exogenous PAs on SAMDC expression appears to be divergent. While putrescine up-regulated the expression of BJSAMDC1, spermidine and spermine down-regulated its expression. Furthermore, mannitol was also shown to up-regulate SAMDC expression in a gene-specific manner, in which the BJSAMDC1 transcript increases but other SAMDC transcripts are not affected.