The Phototropic Response is Locally Regulated Within the Topmost Light-Responsive Region of the Arabidopsis thaliana Seedling

Phototropism is caused by differential cell elongation between the irradiated and shaded sides of plant organs, such as the stem. It is widely accepted that an uneven auxin distribution between the two sides crucially participates in this response. Plant-specific blue-light photoreceptors, phototropins (phot1 and phot2), mediate this response. In grass coleoptiles, the sites of light perception and phototropic bending are spatially separated. However, these sites are less clearly distinguished in dicots. Furthermore, the exact placement of the action of each phototropic signaling factor remains unknown. Here, we investigated the spatial aspects of phototropism using spotlight irradiation with etiolated Arabidopsis seedlings. The results demonstrated that the topmost part of about 1.1 mm of the hypocotyl constituted the light-responsive region in which both light perception and actual bending occurred. In addition, cotyledons and the shoot apex were dispensable for the response. Hence, the response was more region autonomous in dicots than in monocots. We next examined the elongation rates, the levels of phot1 and the auxin-reporter gene expression along the hypocotyl during the phototropic response. The light-responsive region was more active than the non-responsive region with respect to all of those parameters.