Minisymposium 16: Clocks & Light

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Abs # M1602: PIF5: A Possible Node For Crosstalk Between Ethylene and Early Light Signaling Pathways

Presenter:	Khanna, Rajnish       Contact Presenter
Authors	Khanna, Rajnish  (A)   Shen, Yu  (A)   Marion, Colleen M. (A)   Carle, Christine M. (A)   Schaefer, Eberhard  (A) (B)  Quail, Peter H. (A)
Affiliations:	(A): University of California, Berkeley (B): University of Freiburg, Germany

Light triggers seedling de-etiolation by suppressing hypocotyl cell elongation, along with apical hook opening, cotyledon expansion and greening. Different members of the phytochrome family (phyA-E) act with differential, and partly overlapping, photosensory and/or physiological functions in this process. Phytochrome Interacting Factor 5 (PIF5), a bHLH-class transcription factor, binds specifically to the light activated (Pfr) form of phyB through the Active Phytochrome Binding (APB) motif. Our studies with pif5-mutant and PIF5-over-expressing (OX) seedlings have revealed that PIF5 is involved in an intricate interplay between ethylene and early light signaling pathways. In seedlings grown in darkness, PIF5-(OX) enhances the expression of ACS5 and ACS8, two key enzymes for ethylene biosynthesis. PIF5-(OX) seedlings carrying functional or mutated APB (mAPB) motifs exhibit a triple respons in the dark that can be rescued by treatment with Ag+ ions, a characteristic phenotype caused by elevated ethylene levels. In seedlings grown in red light (Rc) the PIF5 protein is rapidly degraded through the 26S proteasome pathway. Strikingly, phyB protein levels are also greatly reduced in Rc-treated PIF5-(OX) seedlings with the functional APB, but not in PIF5-(OX) with a mAPB. Treatment with MG132, an inhibitor of the 26S proteasome pathway, blocks the degradation of PIF5 and phyB in these seedlings. Together, these results indicate that PIF5 may regulate ethylene levels in dark grown seedlings, and that in the light phyB (Pfr) binds PIF5 through the APB motif possibly to facilitate PIF5 degradation and/or to modulate its activity. PIF5 may play a pivotal role during the dark to light transition, providing a mechanism of crosstalk between ethylene and early light-signaling pathways.