Biosynthesis of indole-3-acetic acid from L-tryptophan in coleoptile tips of maize (Zea mays L)

Coleoptile tips (about 2.5 mm in length) were excised from 3-day-old dark-adapted maize (Zea mays L.) seedlings and incubated in darkness in potassium phosphate buffer that contained C-14-L-tryptophan (Trp). Subsequent analysis by gas chromatography-mass spectrometry indicated that a significant portion of endogenous indole-3-acetic acid (IAA) had been labeled with C-14. About 8% Of the IAA that diffused from the tissue into the medium during incubation from 0.5 to 2 h was labeled, and 12% of the IAA extracted from the tissue after a 2-h incubation was labeled. On the other hand, 30% of the Trp extracted from the tissue after a 2-h incubation was C-14-Trp, which was more than those determined for IAA. Since the experiments were carried out under the non-steady-state conditions in which the tissue content of C-14-Trp increased with time, and since the extracted Trp included the C-14-Trp in the apoplastic space, it seemed that synthesis de novo from Trp was the major means by which IAA was produced in the coleoptile tip. The conversion of Trp to IAA was not detected in sub-apical segments (5-7.5 mm from the top) that were incubated similarly, an indication that synthesis of IAA occurs specifically in the tip region. When intact seedlings were irradiated with a pulse of red light 2 h before excision of tips and the application of C-14-Trp, the amounts of extractable and diffusible IAA were reduced by 40-60% without a change in the rate of C-14 incorporation. This result indicated that the production of IAA from Trp is controlled by a red-light signal.