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