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Title: Multisite light-induced phosphorylation of the transcription factor PIF3 is necessary for both its rapid degradation and concomitant negative feedback modulation of photoreceptor phyB levels in Arabidopsis

Author
item NI, W - University Of California
item XU, S - University Of California
item CHALKLEY, R - University Of California
item PHAM, T - University Of California
item GUAN, S - University Of California
item MALTBY, D - University Of California
item BURLINGAME, A - University Of California
item WANG, Z - University Of California
item QUAIL, P - University Of California

Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2013
Publication Date: 7/31/2013
Citation: Ni, W., Xu, S.L., Chalkley, R.J., Pham, T.N., Guan, S., Maltby, D.A., Burlingame, A.L., Wang, Z.Y., Quail, P.H. 2013. Multisite light-induced phosphorylation of the transcription factor PIF3 is necessary for both its rapid degradation and concomitant negative feedback modulation of photoreceptor phyB levels in Arabidopsis. The Plant Cell. 25:2679-2698. PMCID: PMC3753391.

Interpretive Summary: Being rooted in soil, plants need to adjust their growth and development according to environmental cues. Among these cues, light is one of the most important factors, since it is plants’ only energy source. Plants have evolved different classes of photoreceptors to perceive light information, such as the quality (wavelength) and quantity (intensity) of the incoming signals. The phytochrome (phy) family perceives the red (R) and far-red (FR) light information to direct many aspects of plant growth, such as seed germination, seedling deetiolation, shade avoidance, and flowering. Experiments were carried out to dissect the molecular nature of the signal transfer process. The data provide insight into the intermolecular transaction underlying phytochrome activity.

Technical Abstract: Plants constantly monitor informational light signals using sensory photoreceptors, which include the phytochrome (phy) family (phyA to phyE), and adjust their growth and development accordingly. Following light-induced nuclear translocation, photoactivated phy molecules bind to and induce rapid phosphorylation and degradation of phy-interacting basic Helix Loop Helix (bHLH) transcription factors (PIFs), such as PIF3, thereby regulating the expression of target genes. However, the mechanisms underlying the signal-relay process are still not fully understood. Here, using mass spectrometry, we identify multiple, in vivo, light-induced Ser/Thr phosphorylation sites in PIF3. Using transgenic expression of site-directed mutants of PIF3, we provide evidence that a set of these phosphorylation events acts collectively to trigger rapid degradation of the PIF3 protein in response to initial exposure of dark-grown seedlings to light. In addition, we show that phyB-induced PIF3 phosphorylation is also required for the known negative feedback modulation of phyB levels in prolonged light, potentially through codegradation of phyB and PIF3. This mutually regulatory intermolecular transaction thus provides a mechanism with the dual capacity to promote early, graded, or threshold regulation of the primary, PIF3-controlled transcriptional network in response to initial light exposure, and later, to attenuate global sensitivity to the light signal through reductions in photoreceptor levels upon prolonged exposure.