Characterization of rice tryptophan decarboxylases and their direct involvement in serotonin biosynthesis in transgenic rice |
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Authors: | Sei Kang Kiyoon Kang Kyungjin Lee Kyoungwhan Back |
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Institution: | (1) Department of Molecular Biotechnology, Agricultural Plant Stress Research Center, Biotechnology Research Institute, Chonnam National University, Gwangju, 500-757, South Korea |
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Abstract: | l-Tryptophan decarboxylase (TDC) and l-tyrosine decarboxylase (TYDC) belong to a family of aromatic l-amino acid decarboxylases and catalyze the conversion of tryptophan and tyrosine into tryptamine and tyramine, respectively.
The rice genome has been shown to contain seven TDC or TYDC-like genes. Three of these genes for which cDNA clones were available were characterized to assign their functions using
heterologous expression in Escherichia coli and rice (Oryza sativa cv. Dongjin). The purified products of two of the genes were expressed in E. coli and exhibited TDC activity, whereas the remaining gene could not be expressed in E. coli. The recombinant TDC protein with the greatest TDC activity showed a K
m of 0.69 mM for tryptophan, and its activity was not inhibited by phenylalanine or tyrosine, indicating a high level of substrate
specificity toward tryptophan. The ectopic expression of the three cDNA clones in rice led to the abundant production of the
products of the encoded enzymes, tyramine and tryptamine. The overproduction of TYDC resulted in stunted growth and a lack
of seed production due to tyramine accumulation, which increased as the plant aged. In contrast, transgenic plants that produced
TDC showed a normal phenotype and contained 25-fold and 11-fold higher serotonin in the leaves and seeds, respectively, than
the wild-type plants. The overproduction of either tyramine or serotonin was not strongly related to the enhanced synthesis
of tyramine or serotonin derivatives, such as feruloyltyramine and feruloylserotonin, which are secondary metabolites that
act as phytoalexins in plants. |
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Keywords: | Tryptophan decarboxylase Tyrosine decarboxylase Tyramine Serotonin Tryptamine Transgenic rice |
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