Abstract
The phototropism has fascinated plant biologists since the work of Darwins using grass coleoptiles. This tropic response is the result of differential growth of the elongating shoot, explained by the IAA redistribution. Here, using detached coleoptile segments from maize seedlings, we confirmed the first pulse positive phototropism were induced by blue light. To reveal the precise sensing region for blue light, effects of partial irradiation on the curvature were tested. The result showed that the photo sensing at the apical 2mm region was required for phototropism. The formation of IAA redistribution was observed after 20min in the top 0-3mm region, which began to be recognized in the lower region after 30min. When NPA was treated at top 2mm, the phototropism was completely inhibited; even after auxin was simultaneously applied at 3-4mm region. These results showed that lateral IAA movement occurred only in top 0-3mm region, and the IAA redistribution was transmitted from the tip into basal region. Further, it suggests the presence of a key component(s) responsible for the photo-sensing or formation of IAA redistribution in the tip region. We therefore investigated expression level of several genes relating in phototropism. NPH3- and PGP-like genes were shown to be exclusively expressed at the apical 0-2mm region. These results indicate that these genes are possibly involved in converting the light-signal to lateral IAA transport at the tip region.
