Arabidopsis Phytochrome D Is Involved in Red Light-Induced Negative Gravitropism of Hypocotyles

The phytochrome gene family, which is in Arabidopsis thaliana, consists of phytochromes A-E (phyA to phyE), regulates plant responses to ambient light environments. PhyA and phyB have been characterized in detail, but studies on phyC to phyE have reported discrepant functions. In this study, we show that phyD regulates the Arabidopsis gravitropic response by inhibiting negative gravitropism of hypocotyls under red light condition. PhyD had only a limited effect on the gravitropic response of roots in red light condition. PhyD also enhanced phyB-regulated gravitropic responses in hypocotyls. Moreover, the regulation of hypocotyl gravitropic responses by phyD was dependent upon the red light fluence rate.