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JXB Advance Access published online on September 12, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm186
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© The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

FOCUS PAPER

Out of the dark: how the PIFs are unmasking a dual temporal mechanism of phytochrome signalling

Elena Monte1,*, Bassem Al-Sady2, Pablo Leivar3 and Peter H. Quail3

1Departament de Genètica Molecular Vegetal, CSIC-IRTA. Barcelona, Spain
2Department of Biochemistry and Biophysics, UC San Francisco, San Francisco, CA, USA
3Department of Plant and Microbial Biology, UC Berkeley, CA and USDA/UCB Plant Gene Expression Center, Albany, NY, USA

* To whom correspondence should be addressed. E-mail: emcgme{at}cid.csic.es

Following light-induced nuclear translocation, the phytochromes induce changes in gene expression to regulate plant development. PIF3 and other PIFs (phytochrome-interacting factors), members of the bHLH (basic helix–loop–helix) family of transcriptional regulators, interact specifically with the active Pfr conformer of the phytochrome molecule, suggesting that the PIFs are key components of phytochrome signal transduction. The mechanism by which the PIFs transduce phytochrome signals is not understood. After initial studies that suggested that PIF3 was a positive regulator of phytochrome signalling, mutant studies indicated that the PIFs primarily act as negative regulators in the pathway. Furthermore, in some cases they accumulate in the dark and are degraded upon illumination by the ubiquitin–26S proteasome system. At least for PIF3, the protein degradation depends on direct interaction with the phytochrome molecule and is preceded by protein phosphorylation. In this review, the current understanding of the role of the PIFs in phytochrome-mediated photomorphogenesis will be summarized, and recent findings suggesting an unanticipated dual mechanism of action of the PIFs will be discussed.

Key words: Arabidopsis, bHLH factors, phytochrome signalling, PIF3, protein degradation

Received 12 April 2007; Revised 28 June 2007 Accepted 16 July 2007


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