A small subset of basic helix-loop-helix transcription factors called PIFs [phytochrome (phy)-interacting factors] act to repress seed germination, promote seedling skotomorphogenesis and promote shade-avoidance through regulated expression of over a thousand genes. Light-activated phy molecules directly reverse these activities by inducing rapid degradation of the PIF proteins. Here, we review recent advances in dissecting this signaling pathway and examine emerging evidence that indicates that other pathways also converge to regulate PIF activity, including the gibberellin pathway, the circadian clock and high temperature. The PIFs thus have broader roles than previously appreciated, functioning as a cellular signaling hub that integrates multiple signals to orchestrate regulation of the transcriptional network that drives multiple facets of downstream morphogenesis. The relative contributions of the individual PIFs to this spectrum of regulatory functions ranges from quantitatively redundant to qualitatively distinct. Phytochrome signal perception and transductionThe perception of light signals by the phytochrome (phy) family of sensory photoreceptors [phyA through phyE in Arabidopsis (Arabidopsis thaliana)] initiates an intracellular transduction process that culminates in the altered expression of nuclear genes that direct growth and developmental responses, termed photomorphogenesis, appropriate to the prevailing environment, throughout the plant life cycle [1,2]. Current data indicate that the transduction process involves rapid translocation of the light-activated photoreceptor molecule (the Pfr conformer) from the cytoplasm into the nucleus, where it induces transcriptional responses in target genes [3]. The pathway by which the signaling information is propagated to the transcriptional network involves direct, physical interaction of the translocated Pfr conformer with a small subset of constitutively nuclear, basic helixloop-helix (bHLH) transcription factors, designated Phytochrome-Interacting Factors (PIFs) [4,5]. Several comprehensive articles have examined various aspects of this overall process in recent years [6][7][8][9][10][11]. This review focuses predominantly on recent advances in defining the phy-PIF signaling mechanism and the function of the PIFs in regulating the primary,
SUMMARY Background 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 post-germinative seedlings emerging from buried seed grow vigorously upward in the subterranean darkness toward the soil surface. Results 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. Conclusions 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.
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