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Title: Multiple Phytochrome-Interacting bHLH Transcription Factors Repress Premature Seedling Photomorphogenesis in Darkness

Author
item LEIVAR, PABLO - University Of California
item MONTE, ELENA - University Of California
item OKA, YOSHITO - Kyoto University
item LIU, TIFFANY - University Of California
item CARLE, CHRISTINE - University Of California
item CASTILLON, ALICIA - University Of Texas
item HUQ, ENAMUL - University Of Texas
item QUAIL, PETER - University Of California

Submitted to: Current Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2008
Publication Date: 12/9/2008
Citation: Leivar, P., Monte, E., Oka, Y., Liu, T., Carle, C., Castillon, A., Huq, E., Quail, P.H. 2008. Multiple Phytochrome-Interacting bHLH Transcription Factors Repress Premature Seedling Photomorphogenesis in Darkness. Current Biology. 18(23):1815-23.

Interpretive Summary: The success of terrestrial flowering plants in colonizing the land depended on the evolution of a developmental strategy, termed skotomorphogenesis. This enabled seedlings emerging from buried seed to grow strongly upward in the subterranean darkness toward the surface. We show that a central component of the mechanism underlying this strategy is the repression, by transcription factors called PIFs, of the converse developmental pathway, termed photomorphogenesis, familiar as the normal growth of fully green seedlings after emergence from the soil. The exposure of emerging seedlings to light is the trigger for the switch from skotomorphogenesis to photomorphogenesis. We show that the mechanism underlying this switch involves light-induced activation of the phyochrome photoreceptors which then bind to the PIFs inducing their degradation, thereby reversing their repressive action.

Technical Abstract: An important contributing factor to the success of terrestrial flowering plants in colonizing the land was the evolution of a developmental strategy, termed skotomorphogenesis, whereby postgerminative seedlings emerging from buried seed grow vigorously upward in the subterranean darkness toward the soil surface. Here we provide genetic evidence that a central component of the mechanism underlying this strategy is the collective repression of premature photomorphogenic development in dark-grown seedlings by several members of the phytochrome (phy)-interacting factor (PIF) subfamily of bHLH transcription factors (PIF1, PIF3, PIF4, and PIF5). Conversely, evidence presented here and elsewhere collectively indicates that a significant component of the mechanism by which light initiates photomorphogenesis upon first exposure of dark-grown seedlings to irradiation involves reversal of this repression by rapid reduction in the abundance of these PIF proteins, through degradation induced by direct interaction of the photoactivated phy molecule with the transcription factors. We conclude that bHLH transcription factors PIF1, PIF3, PIF4, and PIF5 act as constitutive repressors of photomorphogenesis in the dark, action that is rapidly abrogated upon light exposure by phy-induced proteolytic degradation of these PIFs, allowing the initiation of photomorphogenesis to occur.