Vigorous synthesis of indole-3-acetic acid in the apical very tip leads to a constant basipetal flow of the hormone in maize coleoptiles

Coleoptile tips are the most likely candidates for the site of auxin (indole-3-acetic acid (IAA)) biosynthesis. Using a rapid, micro-scale method for IAA determination by gas chromatography-mass spectrometry, we investigated in detail the distribution and movement of free and conjugated IAA in maize (Zea mays L.) coleoptiles. The highest concentrations of IAA (approximately 250 ng/g fresh weight) were found within 0-0.5 mm of the apical tip. These amounts decreased steeply until 2 mm from the top and then gently decreased further away from the top, reaching about 13 ng/g fresh weight in the basal region. About 500 pg/h of IAA diffused from 2-3 mm tip sections into an agar block. This amount of diffusible IAA that accumulated during the 3.5 h incubation, was more than 6 times higher than that of free IAA detected in the 2 mm tip before incubation. During the incubation, while the free IAA in the tip decreased slightly, the amount of conjugated IAA rather increased. Treatment with N-1-naphthylphthalamic acid, an auxin transport inhibitor, caused a large amount of free IAA to accumulate in the 2 mm tip. These results provide definite evidence that the apical 0-2 mm region of maize coleoptiles has the potential to produce IAA de novo, which leads to a constant flow of IAA towards the lower parts.