Characterization of rice tryptophan decarboxylases and their direct involvement in serotonin biosynthesis in transgenic rice

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.

Beyond a neurotransmitter: The role of serotonin in plants

“…In transgenic rice (Oryza sativa L.) that lack the downstream enzyme serotonin N-acetyltransferase (SNAT), which is involved in the conversion of serotonin to melatonin, serotonin content was increased and was associated with increased coleoptile growth but slower overall seedling growth [37] . In comparison, an earlier study by Kang et al (2007) found that overexpression of TDC, which despite increasing serotonin levels by up to 25 fold, showed no difference in growth. This suggests that it is not purely carbon flux through the pathway, which is important but overall levels of each of the metabolites in the pathway [38] .…”

“…In comparison, an earlier study by Kang et al (2007) found that overexpression of TDC, which despite increasing serotonin levels by up to 25 fold, showed no difference in growth. This suggests that it is not purely carbon flux through the pathway, which is important but overall levels of each of the metabolites in the pathway [38] .…”

Metabolic engineering of the tryptophan and phenylalanine biosynthetic pathways in rice

“…In accordance with this, the simultaneous overexpression of a gene encoding a functional TDC and OASA1D resulted in the accumulation of serotonin at a high concentration (Dubouzet et al unpublished results). Kang et al (2007) similarly reported the accumulation of serotonin in the rice seedlings that express a TDC gene under the control of maize ubiquitin promoter.…”

Enzymatic features of serotonin biosynthetic enzymes and serotonin biosynthesis in plants

“…This hypothesis is consistent with results showing that TDC overexpression in rice does not result in a massive increase of tryptamine, in contrast to the huge increase of tyramine in tyrosine decarboxylase (TYDC) overexpression. 10 The small increase of tryptamine or/and serotonin in transgenic rice plants, despite the overexpression of TDC, has been ascribed to the absence of an increased tryptophan level. Therefore, serotonin synthesis seems to be regulated by two rate-limiting steps: tryptophan level and the induction of TDC in plants.…”