Abstract
Although phototropism is regulated mainly by photoreceptors, phototropins, several studies suggested the modulation of phototropism by other photoreceptors, phytochromes. In our present study, we isolated the flabby mutant that shows phytochrome-inducible hypocotyl bending in Arabidopsis. The FLABBY gene encodes the ABC-type auxin transporter, PGP19, and its expression is suppressed by the activation of phytochromes. Our results indicate that the phytochromes have at least two effects upon the tropic responses of the hypocotyls in Arabidopsis; the enhancement of hypocotyl bending through the suppression of PGP19, and a PGP19-independent mechanism that induces hypocotyl bending. By the use of an auxin polar-transport assay and DR5:GUS expression analysis, we find that the phytochromes inhibit basipetal auxin-transport and induce the asymmetric distribution of auxin in the hypocotyls. These data suggest that the control of auxin transport by phytochromes is a critical regulatory component of hypocotyl growth in response to light.
