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.

3 mM. Unlike TDC, all downstream enzymes after TDC have low K m values for corresponding substrates. For example, for the stub-strate tryptamine, the K m of T5H is 20 μM. The high K m values of TDC enzymes may suggest that TDC functions actively in cells with high tryptophan accumulations. Furthermore, recombinant rice TDC is slightly tolerant of high temperatures, with maximum TDC activity at 45°C and 50% enzyme activity at 55°C (Fig. 1A). In addition , TDC activity is greatest at high pH levels, with peak activity at pH 7.5-8.5, but rapidly decreases to 50% at pH 6.5 (Fig. 1B). Regulation Between Tryptophan and Serotonin Biosynthesis Tryptophan biosynthesis is tightly feedback-regulated by anthra-nilate synthase (AS), the first enzyme of the biosynthetic pathway. Serotonin, a pineal hormone in mammals, is found in a wide range of plant species at detection levels from a few nanograms to a few milligrams, and has been implicated in several physiological roles, such as flowering, morphogenesis and adaptation to environmental changes. Serotonin synthesis requires two enzymes, tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H), with TDC serving as a rate-limiting step because of its high K m in relation to the substrate tryptophan (690 μM) and its undetectable expression level in control plants. However, T5H and downstream enzymes, such as serotonin N-hydroxycin-namoyl transferase (SHT), have low K m values with corresponding substrates. This suggests that the biosynthesis of serotonin or sero-tonin-derived secondary metabolites is restricted to cellular stages when high tryptophan levels are present. Serotonin is found in a broad range of plants and is abundant in reproductive organs, such as fruits and seeds. 1-3 Even though many physiological roles for serotonin in plants have been proposed, 2-7 its actual roles have yet to be examined in detail using molecular, biochemical and genetic approaches. In plants, serotonin is synthesized by two enzymes: tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H). TDC decarboxylates tryptophan into tryptamine, after which T5H hydroxylates tryptamine into serotonin. 8-10 TDC expresses at an undetectable level in rice leaves, whereas T5H expresses constitutively. TDC from Catharanthus roseus has a low K m for tryptophan (0.072 mM), but the K m of other TDC enzymes isolated from tomato, 13 Ophiorrhiza pumila 14 and rice 10 is at least tenfold higher than that of C. roseus. In particular, the K m of tomato TDC is

No abstract

[1] The baroclinic response of tide and tidal currents in the Yellow and East China Seas is investigated using a two-layer numerical model. Seasonal variability in the M 2 tide, especially the smaller summer amplitudes prevailing along the Korea/Tsushima Strait [Kang et al., 1995], is investigated by a series of numerical experiments with varying degrees of stratification specific to winter and summer. Model results show that the summer amplitudes of the M 2 tide around the southwestern tip of the Korean peninsula and Korea/Tsusima Strait decrease, with a peak decrease of about 14 cm off the southwestern tip of the Korean peninsula, while the summer amplitudes in other coastal regions tend to increase. This seasonal variability generally coincides with the observations. These models results indicate that seasonal stratification has several noticeable effects on the tides, including varying degrees of current shear, varying frictional dissipation, and varying barotropic energy flux. In particular, it drives complicated seasonal variability in the M 2 tide, with a peak amplitude modulation of nearly 5% off the southwestern tip of the Korean peninsula. The seasonal variation of barotropic M 2 energy flux through the eastern entrance of the Yellow Sea is thought to induce the corresponding variability in the M 2 amplitude in the Korea/Tsusima Strait, with smaller amplitudes found in the summer.

3 mM. Unlike TDC, all downstream enzymes after TDC have low K m values for corresponding substrates. For example, for the stubstrate tryptamine, the K m of T5H is 20 μM. The high K m values of TDC enzymes may suggest that TDC functions actively in cells with high tryptophan accumulations. Furthermore, recombinant rice TDC is slightly tolerant of high temperatures, with maximum TDC activity at 45°C and 50% enzyme activity at 55°C (Fig. 1A). In addition, TDC activity is greatest at high pH levels, with peak activity at pH 7.5-8.5, but rapidly decreases to 50% at pH 6.5 (Fig. 1B). Regulation Between Tryptophan and Serotonin BiosynthesisTryptophan biosynthesis is tightly feedback-regulated by anthranilate synthase (AS), the first enzyme of the biosynthetic pathway.Serotonin, a pineal hormone in mammals, is found in a wide range of plant species at detection levels from a few nanograms to a few milligrams, and has been implicated in several physiological roles, such as flowering, morphogenesis and adaptation to environmental changes. Serotonin synthesis requires two enzymes, tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H), with TDC serving as a rate-limiting step because of its high K m in relation to the substrate tryptophan (690 μM) and its undetectable expression level in control plants. However, T5H and downstream enzymes, such as serotonin N-hydroxycinnamoyl transferase (SHT), have low K m values with corresponding substrates. This suggests that the biosynthesis of serotonin or serotonin-derived secondary metabolites is restricted to cellular stages when high tryptophan levels are present.Serotonin is found in a broad range of plants and is abundant in reproductive organs, such as fruits and seeds. 1-3 Even though many physiological roles for serotonin in plants have been proposed, 2-7 its actual roles have yet to be examined in detail using molecular, biochemical and genetic approaches. In plants, serotonin is synthesized by two enzymes: tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H). TDC decarboxylates tryptophan into tryptamine, after which T5H hydroxylates tryptamine into serotonin. 8-10 TDC expresses at an undetectable level in rice leaves, whereas T5H expresses constitutively. 11,12 Enzymatic Features of Serotonin Biosynthetic EnzymesTDC from Catharanthus roseus has a low K m for tryptophan (0.072 mM), but the K m of other TDC enzymes isolated from tomato, 13 Ophiorrhiza pumila 14 and rice 10 is at least tenfold higher than that of C. roseus. In particular, the K m of tomato TDC is