Figure 9.

Model Depicting the Non-Cell-Autonomous Function of phyB in the Inhibition of Hypocotyl Negative Gravitropism.

Epidermal phyB inhibits hypocotyl negative gravitropism at least in part by promoting the degradation of endodermal PIFs in response to red light. This loss of endodermal PIFs results in a reduction in the size of endodermal starch granules. The physical separation of the epidermis and endodermis by the layers of the cortex suggests that epidermal phyB generates mobile signals that travel to the endodermis to promote PIF degradation. These mobile signals have not yet been identified.