Os08g0140500

The degree of stunted growth and dark-brown color was proportional to the expression levels of TDC-3. The levels of tryptamine and serotonin accumulation in these transgenic rice lines were also directly correlated with the expression levels of TDC-3.

Annotated Information

Function

the over-expression of rice TDC-1 and TDC-3 increased the accumulation of serotonin, which affected plant growth. In addition, extended exposure of plants to sunlight (UV) triggered the formation of covalent serotonin dimer in the plants, resulting in brown phenotypes. Moreover, the deposition of brown material at pathogen infection sites could also resulted from the accumulation of the covalent serotonin dimer. A mutant M47286 with a stunted growth, low fertility and dark-brown phenotype was identified from a T-DNA-tagged rice mutant library. This mutant contained a copy of the T-DNA tag inserted at the location where the expression of two putative tryptophan decarboxlyase genes, TDC-1 and TDC-3, were activated. Enzymatic assays of both recombinant proteins showed tryptophan decarboxlyase activities that converted tryptophan to tryptamine, which could be converted to serotonin by a constitutively expressed tryptamine 5 0 hydroxylase (T5H) in rice plants. Over-expression of TDC-1 and TDC-3 in transgenic rice recapitulated the stunted growth, darkbrown phenotype and resulted in a low fertility similar to M47286. The degree of stunted growth and dark-brown color was proportional to the expression levels of TDC-1 and TDC-3. The levels of tryptamine and serotonin accumulation in these transgenic rice lines were also directly correlated with the expression levels of TDC-1 and TDC-3. A mass spectrometry assay demonstrated that the darkbrown leaves and hulls in the TDC-overexpressing transgenic rice were caused by the accumulation of serotonin dimer and that the stunted growth and low fertility were also caused by the accumulation of serotonin and serotonin dimer, but not tryptamine. These results represent the first evidence that over-expression of TDC results in stunted growth, low fertility and the accumulation of serotonin, which when converted to serotonin dimer, leads to a dark brown plant color.

Expression

Serotonin (5-hydroxytryptamine) is a well-known aromatic amine neurotransmitter that controls several important physiological functions such as mood, sleep and anxiety in animals and humans. Evolutionarily, serotonin existed in plants even before the appearance of animals, with the first report coming in the fruit of the cowhage (Mucuna pruriens) plant . Similar to the multiple roles played by serotonin in animal cells, serotonin has also been suggested to be involved in several physiological functions in plants, such as growth regulation, seed germination, flowering, ion permeability, adaptation to environmental changes and morphogenesis . Recently, serotonin has been reported to accumulate in rice leaves to delay senescence and at sites of infection as part of the defensive response . Serotonin is also incorporated into the cell wall at disease lesions infected withBipolaris oryzae andMagnaporthe grisea, suggesting that serotonin played an important role in controlling the strength of cell walls as the mechanical barrier against pathogens . Although multiple physiological functions of serotonin are currently known in plants, the effects of serotonin accumulation on plant growth and pigmentation have not been reported. In plants, serotonin can be synthesized by L-tryptophan decarboxylases (TDCs), which catalyze the decarboxylation of tryptophan to produce tryptamine, followed by the catalysis of tryptamine 5-hydroxylase (T5H) to form serotonin. The study of TDC has been extended to the staple food crop rice for their involvements in the serotonin biosynthesis and for their functions in pathogenic defense and delaying the senescence of rice leaves . In rice, TDC is expressed at very low or negligible levels, while T5H is constitutively expressed . Therefore, TDC serves as a bottleneck point regulating serotonin biosynthesis. Accordingly, the accumulation of serotonin can be achieved by over-expressing TDC in transgenic rice plants. For example, the ectopic over-expression of the TDC AK53 and AK31 genes in rice plants resulted in 7- and 25-fold increases in serotonin accumulation, respectively, in the leaves of 14-day-old seedlings. Therefore, the key enzyme TDC can be used to genetically modify the metabolic pathways for the production of the pharmaceutically important compounds serotonin and monoterpenoid indole alkaloids in an approach similar to the over-expression of TYDC to produce dopamine or octopamine and to serve as a starting point for benzylisoquinoline alkaloid biosynthesis in several plant species. Seven putativeTDCand TYDC-like genes have been identified in the rice genome. Among these genes, one TYDC (accession number AK065830, or AK30) and twoTDCs including AK31 (accession number AK069031, also named TDC-1) and AK53 (accession number AK103253, also namedTDC-2) have been characterized. In vitro enzymatic assays demonstrated that both AK31 and AK53 have tryptophan decarboxylase activity and revealed high substrate specificity to tryptophan . Over-expression ofTDCand TYDCgenes ectopically in transgenic rice plants have been previously reported, and the production of important compounds in these transgenic plants and their effects on plant growth have also been recorded . The ectopic over-expression of theTYDC gene (AK30) in rice plants resulted in an 80-fold increase in tyramine synthesis, and all of these transgenic rice plants showed stunted growth. A further study revealed that the accumulation of tyramine reduced cell division that caused the dwarf phenotype of theseTYDCoverexpression rice plants. Whereas the transgenic rice plants that over-expressed TDC genes accumulated 7- to 11-fold more tryptamine in 8-week-old leaves and up to 25-fold additional increase of serotonin in seedlings and both grew normally as the phenotype difference or detrimental growth effects were not observed.

Evolution

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Labs working on this gene

Institute of Molecular Biology, National Chung Hsing University, Taiwan Department of Agronomy, National Chung Hsing University, Taiwan Department of Chemistry, National Taiwan University, Taiwan

References

Parawee Kanjanaphachoat;Bi-Yin Wei;Shuen-Fang Lo;I-Wen Wang;Chang-Sheng Wang;Su-May Yu;Ming-Liang Yen;Sheng-Hsien Chiu;Chien-Chen Lai;Liang-Jwu Chen Serotonin accumulation in transgenic rice by over-expressing tryptophan decarboxlyase results in a dark brown phenotype and stunted growth Plant Molecular Biology, 2012, 78(6): 525-543