Figure 1.

BL-Induced Phototropism in Higher Plants Requires the Establishment of a Differential Gradient of Auxin.

(A) Diagram of a hypocotyl exhibiting a phototropic response. Auxin synthesized in the apical portions of the stem is polarly transported toward the root predominately through the central vasculature and to a lesser extent via epidermal and subepidermal cell layers (downward pointing red arrows). While in dark-grown seedlings the amount of auxin transported through the outer cell layers is not appreciably different side-to-side across the hypocotyl (data not shown), in seedlings exposed to directional BL, a differential of downward auxin flow is established (see downward pointing outer red arrows). Directional BL also induces a lateral redistribution of auxin from the lit to shaded portion of the hypocotyl (trans-hypocotyl red arrows). Together, these actions result in the differential accumulation of auxin in the elongation zone of shaded versus lit sides of the seedling (yellow shading).

(B) Cross section within the elongation zone of an Arabidopsis seedling hypocotyl illustrating the gradients of phot1 activity (false-colored white to blue) and auxin accumulation (false-colored white to yellow). Gray dots are meant to represent nuclei within the outer two cell layers, regions particularly important to auxin-mediated transcriptional responses (see Figures 3 and 4). Cells outlined in green are endodermal cells.