No abstract

The established pathways from serine to ethanolamine are indirect and involve decarboxylation of phosphatidylserine. Here we show that plants can decarboxylate serine directly. Using a radioassay based on ethanolamine (Etn) formation, pyridoxal 5-phosphatedependent L-serine decarboxylase (SDC) activity was readily detected in soluble extracts from leaves of diverse species, including spinach, Arabidopsis, and rapeseed. A putative Arabidopsis SDC cDNA was identified by searching GenBank TM for sequences homologous to other amino acid decarboxylases and shown by expression in Escherichia coli to encode a soluble protein with SDC activity. This cDNA was further authenticated by complementing the Etn requirement of a yeast psd1 psd2 mutant. In a parallel approach, a cDNA was isolated from a rapeseed library by its ability to complement the Etn requirement of a yeast cho1 mutant and shown by expression in E. coli to specify SDC. The deduced Arabidopsis and rapeseed SDC polypeptides are 90% identical, lack obvious targeting signals, and belong to amino acid decarboxylase group II. Recombinant Arabidopsis SDC was shown to exist as a tetramer and to contain pyridoxal 5-phosphate. It does not attack D-serine, L-phosphoserine, other L-amino acids, or phosphatidylserine and is not inhibited by Etn, choline, or their phosphoesters. As a soluble, pyridoxal 5-phosphate enzyme, SDC contrasts sharply with phosphatidylserine decarboxylases, which are membrane proteins that have a pyruvoyl cofactor.

To understand how plant cell changes gene expression during cell division and after termination of cell division, we analyzed the change of gene expression during the growth of tobacco BY-2 cell lines using a cDNA microarray, which contained about 9,200 expression sequence tag fragments and corresponded to about 7,000 genes. We found that log phase cells predominantly expressed DNA/chromosome duplication gene homologs. In addition, many genes for basic transcription and translation machineries, as well as proteasomal genes, were up-regulated at the log phase. About half of the kinesin homolog genes, but not myosin homolog genes, were predominantly expressed at the dividing phase as well. In contrast, stationary phase cells expressed genes for many receptor kinases, signal transduction machineries and transcription factors. Several hundreds of genes showed differential expression after incubation of stationary phase cells with medium containing either salicylic acid or abscisic acid. These findings suggested that BY-2 cells at the stationary phase express genes for perceiving extracellular signals.